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Multifunctional Antennas
and Arrays for Wireless
Communication Systems

Multifunctional Antennas and Arrays for
Wireless Communication Systems
Edited by
Satish K. Sharma and Jia-Chi S. Chieh
San Diego State University
San Diego, CA, USA

This edition first published 2021
© 2021 John Wiley Sons, Inc.
All rights reserved. No part of this publication may be reproduced, stored in a retrieval system,
or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording
or otherwise, except as permitted by law. Advice on how to obtain permission to reuse material
from this title is available at http://www.wiley.com/go/permissions.
The right of Satish K. Sharma and Jia‐Chi S. Chieh to be identified as the author of the editorial
material in this work has been asserted in accordance with law.
Registered Office
John Wiley Sons, Inc., 111 River Street, Hoboken, NJ 07030, USA
Editorial Office
111 River Street, Hoboken, NJ 07030, USA
For details of our global editorial offices, customer services, and more information about Wiley
products visit us at www.wiley.com.
Wiley also publishes its books in a variety of electronic formats and by print‐on‐demand. Some
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While the publisher and authors have used their best efforts in preparing this work, they make
no representations or warranties with respect to the accuracy or completeness of the contents
of this work and specifically disclaim all warranties, including without limitation any implied
warranties of merchantability or fitness for a particular purpose. No warranty may be created
or extended by sales representatives, written sales materials or promotional statements for
this work. The fact that an organization, website, or product is referred to in this work as a
citation and/or potential source of further information does not mean that the publisher and
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or recommendations it may make. This work is sold with the understanding that the publisher
is not engaged in rendering professional services. The advice and strategies contained herein
may not be suitable for your situation. You should consult with a specialist where appropriate.
Further, readers should be aware that websites listed in this work may have changed or
disappeared between when this work was written and when it is read. Neither the publisher nor
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limited to special, incidental, consequential, or other damages.
Library of Congress Cataloging‐in‐Publication data applied for
ISBN: 9781119535058
Cover design by Wiley
Cover image: © Andrey Suslov/iStock.com
Set in 9.5/12.5pt STIXTwoText by SPi Global, Pondicherry, India
10 9 8 7 6 5 4 3 2 1

To our parents, teachers, and family members

vi
List of Contributors xi
Preface xii
Acknowledgements xv
1 Introduction 1
Satish K. Sharma and Jia-Chi S. Chieh
1.1
Introduction 1
1.2
Antenna: an Integral Component of Wireless Communications 1
1.3
Antenna Performance Parameters 2
1.4
Antenna Types 2
1.5
Multifunctional Antennas 3
1.6
Reconfigurable Antennas 6
1.7
Frequency Agile/Tunable Antenna 13
1.8
Antenna Measurements 17
1.9
Conclusion 18
References 18
2 Frequency Reconfigurable Antennas 19
Saeed I. Latif and Satish K. Sharma
2.1
Introduction 19
2.2
Mechanism of Frequency Reconfigurability 20
2.3
Types of FRAs 21
2.3.1 Frequency Reconfigurability by Switches/Tunable Components 21
2.3.1.1 Electrical Switches 22
2.3.1.2 Varactor Diodes 31
2.3.1.3 Micro-Electro-Mechanical-System (MEMS) Switches 40
2.3.1.4 Optical Switches 40
2.3.1.5 Ground Plane Membrane Deflection 43
2.3.2 Frequency Reconfigurability Using Special Materials 43
2.3.2.1 Liquid Crystals 45
2.3.2.2 Graphene 47
Contents

Contents vii
2.3.3 Frequency Reconfigurability by Mechanical Changes 49
2.3.3.1 Actuators 49
2.3.3.2 Motors 50
2.3.4 Frequency Reconfigurability Using Special Shapes 53
2.3.4.1 Origami Antennas 53
2.3.4.2 Fractal Shapes 54
2.4
FRAs in the Future: Applications in Emerging Technologies 58
2.5
Conclusion 59

References 59
3 Radiation Pattern Reconfigurable Antennas 67
Sima Noghanian and Satish K. Sharma
3.1
Introduction 67
3.2
Pattern Reconfigurable by Electronically Changing Antenna Elements 67
3.3
Pattern Reconfigurable by Electronically Changing Feeding Network 88
3.4
Mechanically Controlled Pattern Reconfigurable Antennas 90
3.5
Arrays and Optimizations 98
3.6
Reconfigurable Wearable and Implanted Antennas 110
3.7
Conclusion 119

References 119
4 Polarization Reconfigurable Antennas 122
Behrouz Babakhani and Satish K. Sharma
4.1
Introduction 122
4.2
Polarization Reconfiguration Mechanism Using RF Switches 124
4.3
Solid-State RF Switch-Based Polarization Reconfigurable Antenna 125
4.4
Mechanical and Micro-electro-mechanical (MEMS) RF Switch-Based
Antennas 140
4.5
Switchable Feed Network-Based Polarization Reconfiguration 148
4.6
Polarization Reconfigurable Antennas Using Metasurface 157
4.7
Other Methods to Create Polarization Reconfigurable Antennas 162
4.8
Conclusion 169

References 169
5 Liquid Metal, Piezoelectric, and RF MEMS-Based Reconfigurable
Antennas 172
Jia-Chi S. Chieh and Satish K. Sharma
5.1
Introduction 172
5.2
Liquid Metal – Frequency Reconfigurable Antennas 172
5.3
Liquid Metal – Pattern Reconfigurable Antennas 175
5.4
Liquid Metal – Directivity Reconfigurable Antennas 182
5.5
Piezoelectric – Pattern Reconfigurable Array 184
5.6
RF MEMS – Frequency Reconfigurable 189

Contents
viii
5.7
RF MEMS – Polarization Reconfigurable 191
5.8
RF MEMS – Pattern Reconfigurable 194
5.9
Conclusion 196

References 197
6 Compact Reconfigurable Antennas 198
Sima Noghanian and Satish K. Sharma
6.1
Introduction 198
6.2
Reconfigurable Pixel Antenna 199
6.3
Compact Reconfigurable Antennas Using Fluidic 209
6.4
Compact Reconfigurable Antennas Using Ferrite and
Magnetic Materials 213
6.5
Metamaterials and Metasurfaces 224
6.6
Conclusion 229

References 229
7 Reconfigurable MIMO Antennas 232
Kumud R. Jha and Satish K. Sharma
7.1
Introduction 232
7.2
Reconfigurable Antennas for MIMO Applications 234
7.3
Isolation Techniques in MIMO Antennas 237
7.3.1 Decoupling Network 237
7.3.2 Neutralization Lines 238
7.3.3 Using Artificial Material 240
7.3.4 Defected Ground Plane 241
7.4
Pattern Diversity Scheme 241
7.5
Reconfigurable Polarization MIMO Antenna 244
7.6
MIMO Antenna Performance Parameters 254
7.6.1 Envelope Correlation Coefficient (ECC) 254
7.6.2 Total Active Reflection Coefficient (TARC) 255
7.6.3 Mean Effective Gain (MEG) 256
7.6.4 Diversity Gain 257
7.7
Some Reconfigurable MIMO Antenna Examples 258
7.8
Conclusion 274

References 274
8 Multifunctional Antennas for 4G/5G Communications and MIMO
Applications 279
Kumud R. Jha and Satish K. Sharma
8.1
Introduction 279
8.2
MIMO Antennas in Multifunctional Systems 281
8.3
MIMO Antennas in Radar Systems 284
8.4
MIMO Antennas in Communication Systems 290

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Contents ix
8.5
MIMO Antennas for Sensing Applications 290
8.6
MIMO Antennas for 5G Systems 292
8.7
Massive MIMO Array 293
8.8
Dielectric Lens for Millimeter Wave MIMO 298
8.9
Beamforming in Massive MIMO 301
8.10
MIMO in Imaging Systems 303
8.11
MIMO Antenna in Medical Applications 306
8.11.1 Ex-VIVO Applications 306
8.11.2 MIMO Antenna for Medical Imaging 309
8.11.3 Wearable MIMO Antenna 309
8.11.4 MIMO Indigestion Capsule 310
8.11.5 Reconfigurable Antennas in Bio-Medical Engineering 313
8.12
Conclusion 316

References 317
9 Metamaterials in Reconfigurable Antennas 321
Saeed I. Latif and Satish K. Sharma
9.1
Introduction 321
9.2
Metamaterials in Antenna Reconfigurability 321
9.3
Metamaterial-Inspired Reconfigurable Antennas 322
9.3.1 Metamaterial-Based Frequency Reconfigurability 323
9.3.2 Metamaterial-Based Pattern Reconfigurability 325
9.3.3 Metamaterial-Based Polarization Reconfigurability 328
9.4
Metasurface-Inspired Reconfigurable Antennas 333
9.5
Conclusion 336

References 337
10 Multifunctional Antennas for User Equipments (UEs) 341
Satish K. Sharma and Sonika P. Biswal
10.1
Introduction 341
10.2
Lower/Sub-6 GHz 5G Band Antennas 342
10.3
5G mm-Wave Antenna Arrays 353
10.4
Collocated Sub-6 GHz and mm-Wave 5G Array Antennas 360
10.5
RF and EMF Exposure Limits 369
10.6
Conclusion 374
References 374
11 DoD Reconfigurable Antennas 378
11.1
Introduction 378
11.2
TACAN 378
11.2.1 TACAN Antenna 379
11.2.2 Course Bearing 382

Contents
x
11.2.3 Fine Bearing 382
11.3
Sea-Based X-Band Radar 1 (SBX-1) 383
11.4
The Advanced Multifunction RF Concept (AMRFC) 384
11.5
Integrated Topside (InTop) 390
11.5.1 Wavelength Scaled Arrays 390
11.5.2 Low-Cost Multichannel Microwave Frequency Phased Array Chipsets
on Si and SiGe 394
11.6
DARPA Arrays of Commercial Timescales (ACT) 400
11.7
AFRL Transformational Element Level Array (TELA) 405
11.8
Conclusion 406
References 408
12 5G Silicon RFICs-Based Phased Array Antennas 409
12.1
Introduction 409
12.2
Silicon Beamformer Technology 409
12.3
LO-Based Phase Shifting 413
12.4
IF-Based Phase Shifting 414
12.5
RF-Based Phase Shifting 415
12.6
Ku-Band Phased Arrays Utilizing Silicon Beamforming Chipsets 422
12.7
Ku-Band Phased Arrays on ROHACELL Utilizing Silicon
Beamforming Chipsets 425
12.8
Ku-Band Phased Arrays with Wide Axial Ratios Utilizing Silicon
Beamforming Chipsets 431
12.9
28 GHz Phased Arrays Utilizing Silicon Beamforming Chipsets 433
12.10
Phased Array Reflectors Utilizing Silicon Beamforming Chipsets 438
12.11
Conclusion 442

References 443
Index 445

xi
Behrouz Babakhani
Antenna and Microwave Lab (AML),
Department of Electrical and
Computer Engineering, San Diego
State University, San Diego, CA, USA
Sonika P. Biswal
Jia-Chi S. Chieh
Kumud R. Jha
Department of Electronics and
Communication Engineering, Shri
Mata Vaishno Devi University, SMVD
University, Katra, India
Saeed I. Latif
Computer Engineering, University of
South Alabama, Mobile, AL, USA
Sima Noghanian
Satish K. Sharma
List of Contributors

xii
Multifunctional antennas and arrays are the new trend in the field of antennas for
diversified applications such as wireless and satellite communications as well as
for radar applications. Reconfigurable antennas starting from frequency recon-
figuration, pattern reconfiguration to polarization reconfiguration and their com-
binations make these antennas not only multifunctional but also reduce space
requirements on the host communication devices. In the last two decades there
has been great efforts to design and realize these reconfigurable antennas and we
anticipate even more efforts to come in the near future. A wide range of sub‐topics
as they apply to multifunction antennas and arrays include the design and devel-
opment of the reconfigurable multiple‐input‐multiple‐output (MIMO) antennas,
liquid metal antennas, piezoelectric antennas, radio frequency (RF) micro‐elec-
tro‐mechanical‐systems (MEMS) based reconfigurable antennas, multifunctional
antennas for 4G/5G communications and MIMO applications, metamaterials
reconfigurable antennas, multifunctional antennas for user equipment (EUs),
reconfigurable antennas for the defense applications and phased array antennas
using 5G silicon RFICs.
The purpose of this book is to present in‐depth theory, as well as design and
development insight of these various multifunctional antennas and arrays. The
book is aimed for use by practicing antenna engineers and researchers in the
industry and academia. This book starts with an introduction to the antennas in
Chapter 1, which discusses the importance of antennas. It also provides an intro-
duction to antenna performance parameters, antenna types, multifunctional
antennas, reconfigurable antennas, and antenna measurements. Next in
Chapter 2, frequency reconfigurable antennas (FRAs) are detailed. This chapter
starts with discussion of the mechanism of frequency reconfigurability, types of
the FRAs using various switches and tunable components, FRAs by employing
mechanical changes such as ground plane membrane deflection, and FRAs by
using special materials and special shapes. Chapter 3 presents discussion on
the pattern reconfigurable antennas which includes the following: pattern
Preface

Preface xiii
reconfiguration by electronically changing antenna elements and feeding net-
works, mechanically controlled pattern reconfigurable antennas, pattern recon-
figurable arrays and optimizations, and reconfigurable wearable and implanted
antennas. In Chapter 4, we discuss the polarization reconfigurable antennas with
emphasis on the polarization reconfiguration mechanism using RF switches,
polarization reconfigurable antennas using solid‐state RF switches, mechanical
and micro‐electro‐mechanical‐system (MEMS) RF switches, switchable feed net-
works, usage of metasurfaces, as well as other methods. These chapters describe
the three main types of reconfigurable antennas and arrays as described in the
introduction.
Reconfigurable antennas using the liquid metal, piezoelectric and RF MEMS
are discussed in Chapter 5. This chapter specifically includes discussion on the
liquid metal based frequency, pattern, and directivity reconfigurable antennas,
piezoelectric based pattern reconfigurable arrays, and RF MEMS based frequency
and pattern reconfigurable antennas. Compact reconfigurable antennas are dis-
cussed in Chapter 6 with the main focus on the reconfigurable pixel antennas, and
reconfigurable antennas using fluidic, ferrite and magnetic materials, metamate-
rials and metasurfaces.
Reconfigurable MIMO antennas are presented in Chapter 7, which discusses
the following: reconfigurable antennas for MIMO applications, isolation tech-
niques in MIMO antennas, pattern diversity scheme, reconfigurable polarization
MIMO antennas, MIMO antenna performance parameters, and finally some
reconfigurable MIMO antenna examples. Chapter 8 offers discussion on the
MIMO antennas in multifunctional systems, MIMO antennas in Radar systems,
MIMO antennas in communication systems, MIMO antennas for sensing applica-
tions, MIMO antennas for 5G systems, massive MIMO arrays, dielectric lens for
millimeter wave MIMO, beamforming in massive MIMO, MIMO in imaging sys-
tems, and MIMO antenna in medical applications. Use of metamaterials in recon-
figurable antennas have been addressed in Chapter 9. This chapter focuses the
discussion on metamaterials in antenna reconfigurability, metamaterial‐inspired
reconfigurable antennas, and metasurface‐inspired reconfigurable antennas.
Chapter 10 provides detailed discussion on the multifunctional antennas for
user equipments (UEs) with emphasis on the lower/sub‐6 GHz 5G band anten-
nas, 5G mm‐wave antenna arrays, collocated sub‐6 GHz and mm‐Wave 5G array
antennas, and RF and electromagnetic fields (EMF) exposure limits. The depart-
ment of defense (DoD) related reconfigurable antennas are presented in
Chapter 11 with a focus on the tactical air navigation system (TACAN) antennas,
sea‐based X‐Band Radar 1 (SBX‐1) antennas, the advanced multifunction RF con-
cept (AMRFC) antennas, integrated topside (InTop) antennas, the Defense
Advanced Research Projects Agency (DARPA) arrays of commercial timescales
(ACT), and the Air Force Research Laboratory (AFRL) transformational element

Preface
xiv
level array (TELA). Finally, Chapter 12 discusses 5G silicon RFICs‐based phased
array antennas, which introduces silicon beamformer technology. It includes a
short discussion of three phase shifting topologies using local oscillator (LO)
based phase shifting, intermediate frequency (IF) based phase shifting and RF
based phase shifting for beam steering array antennas. Several flat panel phased
array antenna examples using the silicon beamforming chipsets both at Ku‐ and
Ka‐band with linear and circular polarizations are also presented.
We would like to mention that the slight overlap between the content in couple
of chapters is acknowledged. We have done this intentionally so that discussion is
complete in the respective chapters. While the contributors and authors have
made great effort to present details for each topic area, they are by no means com-
plete as the body of work in this field is large. They do represent the interpreta-
tions of each chapter’s contributors. As time progresses, further improvements
and innovations in the state‐of‐the‐art technologies in reconfigurable antennas is
anticipated. Therefore, it is expected that interested readers should continually
refresh their knowledge to follow the growth of communication technologies.
1 February 2021 Professor Satish K. Sharma, PhD
San Diego, CA, USA Jia‐Chi S. Chieh, PhD

xv
Acknowledgements
We would like to offer our sincere thanks to the chapter coauthors for their valu-
able contributions, patience and timely support throughout the development of
this book. We would also like to thank the Wiley team members especially, Brett
Kurzman, Victoria Bradshaw, Sarah Lemore, Sukhwinder Singh and most impor-
tantly S. M. Amudhapriya for their immense help throughout the completion of
this book.
Professor Satish K. Sharma will like to take this opportunity to thank his
research collaborators, past and present graduate students, post‐doctoral fellows,
visiting scholars, and undergraduate students at San Diego State University
(SDSU) who have been the continuous source for his research growth. He thanks
Dr. Jia‐Chi S. Chieh for agreeing to work on this book. He also thanks the funding
agencies: National Science Foundation (NSF) for the prestigious CAREER award,
the Office of Naval Research (ONR), the Naval Information Warfare Center‐Pacific
(NIWC‐PAC), the Space and Naval Warfare Systems Command (SPAWAR)‐San
Diego, and the SBIR/STTR Phase I and II research grants subcontracted through
the local industries, which have helped him pursue his research work. Finally, he
thanks his spouse Mamta Sharma (Author and Artist) and daughters Shiva Shree
Sharma (Doctoral Student in Material Science Engineering at University of
California, Riverside, California) and Shruti Shree Sharma (Undergraduate
Student in Electrical Engineering at University of California, Irvine, California)
who spared their valuable time to let him work on this book and offered their
unconditional love and support as always. He also thanks his pet dog and cat
Charlie Sharma and Razzle Sharma, respectively, for their unconditional love to
him. Lastly, he is grateful to his parents (Mr. Rama Naresh Sharma and Mrs.
Taravati Sharma), elders in his extended family, research advisors (Professors
L. Shafai, the University of Manitoba and B. R. Vishvakarma, Indian Institute of
Technology, Banaras Hindu University), teachers, colleagues, friends and the
almighty God for bestowing continuous blessings on him.

Acknowledgement
xvi
Dr. Jia‐Chi S. Chieh is grateful to his research group at the Naval Information
Warfare Center in San Diego for their tireless efforts in the development of low‐
cost phased array antennas over the last decade. He is also grateful for the research
collaboration opportunities he has had with Prof. Satish K. Sharma from San
Diego State University (SDSU), as well as his mentorship and friendship over the
years. He is thankful to his family for their love and support, and who have
allowed him to complete this work including his wife Kristine, and his two daugh-
ters Joanna and Audrey. Lastly, he is grateful to his parents (Dr. Shih‐Huang Chieh
and Mrs. Dolly Chieh), who taught him the importance of learning and to
never stop.

xvi Acknowledgements
Dr. Jia‐Chi S. Chieh is grateful to his research group at the Naval Information
Warfare Center in San Diego for their tireless efforts in the development of low‐
cost phased array antennas over the last decade. He is also grateful for the research
collaboration opportunities he has had with Prof. Satish K. Sharma from San
Diego State University (SDSU), as well as his mentorship and friendship over the
years. He is thankful to his family for their love and support, and who have
allowed him to complete this work including his wife Kristine, and his two daugh-
ters Joanna and Audrey. Lastly, he is grateful to his parents (Dr. Shih‐Huang Chieh
and Mrs. Dolly Chieh), who taught him the importance of learning and to
never stop.

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be produced by hands far less skilled than those required for pure
hand-work.
In the description given above of bottle-blowing by hand we
have already seen an example of the use of moulds in aiding the
blower to form his object to the desired size and shape. Much more
complicated and decorative objects can, however, be produced by
the use of moulds. Such objects as globes and shades for gas, oil
and electric lamps, when of a light substance and suitable shape, are
usually produced by blowing bulbs of glass into moulds, where they
acquire the general shape as well as the detailed decorated surface
configuration which they afterwards present. Here again the body
remains a closed vessel, and is only opened and trimmed to the final
shape at the end of the operation when all the blowing and
moulding have been done. Articles blown in this way very frequently
show “mould marks,” since the contact of the hot glass with the
relatively cold surface of the mould results in a certain crinkling or
roughening of the glass, much as in the process of rolling. This
effect can be minimised by dressing the interior surfaces of the
moulds with suitable greasy dressings, whose chief property should
be that they do not stick to the hot glass and leave little or no
residue when gradually burnt away in the mould; the proper care of
the moulds and their maintenance is in fact the first essential to
successful manufacture in this as well as in the pressed-glass
industry. Even under the most favourable conditions, however, the
surface of glass blown into moulds is not so good as that of hand-
blown articles which have never come into contact with cold
materials, and therefore retain undiminished the natural “fire polish”
which glass possesses when allowed to cool freely from the molten
state. An effort at producing a similar brilliance of surface on
moulded and pressed articles is often made by exposing them, after
they have attained their final form, to the heat of a furnace to such
an extent as to soften the surfaces and allow the glass to re-solidify
under the undisturbed influence of surface-tension much as it would
do in solidifying freely in the first place. Unfortunately this process
cannot be carried out without more or less softening the entire

article, so that skilful manipulation is required to prevent serious
deformation of the object, while a certain amount of rounding off in
all sharp corners and angles cannot be avoided.
The air-pressure required to bring the whole of the surfaces of a
large and possibly complicated piece of glass into contact with the
surfaces of the mould is sometimes very considerable, and the lung-
power of the blower is often insufficient for the purpose; in many
works, therefore, compressed air is supplied for the purpose,
arrangements being employed whereby the operative can quickly
connect the mouthpiece of his pipe with the air-main, while he can
accurately control the pressure by means of a suitable valve. The
Sievert process of moulding by the aid of steam pressure has already
been described.
Although the evolution of the industry scarcely followed this
path, it is not a large step to pass from a process in which air-
pressure is used to drive viscous glass into contact with a mould to a
process in which the pressure of the air is replaced by the pressure
of a suitably-shaped solid plunger, and this is essentially the widely-
used process of glass pressing. In the first instance this mode of
manufacture is obviously applicable to solid or flat and shallow
articles which could not be conveniently evolved from the spherical
bulb which stands as embryo of all blown glass; at first sight it would
seem in fact as though the process must be limited to articles of
such a shape that a plunger can readily enter and leave the concave
portions. By the ingenious device, however, of pressing two halves of
a closed or nearly closed vessel simultaneously in two adjacent
moulds and then pressing the two halves together while still hot
enough to unite, it has been made possible to produce by the press
alone such objects as water-jugs, for example, into which a plunger
could not possibly be introduced when finished. The process of
pressing being a purely mechanical one and requiring no very
elaborate plant and little skilled labour, has placed upon the market a
host of cheap and extremely useful articles, thus serving to widen
very considerably the useful applications of glass. On the other
hand, the process has been and is still used to some extent for the

production of articles intended to imitate the products of other
processes such as hand-blown and cut glass, with the result that a
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The essential feature of the process of glass pressing consists,
as already indicated, in forcing a layer of glass into contact with a
mould by the pressure of a mechanically actuated plunger. For this
purpose a suitable mould and plunger as well as a press for holding
the former and actuating the latter are required. The moulds are
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they are kept trimmed and dressed in much the same manner as the
moulds used for blowing (except that the latter are sometimes made
of wood). For the purpose of facilitating the removal of the finished
article, the moulds are generally made in several pieces which fit
into one another and can be separated by means of hinges. A very
important point about these moulds is that the various pieces should
fit accurately into one another, since otherwise a minute “fin” of
glass will be forced into every interstice, and the traces of these fins
will always remain visible on the finished article; the very perfect fit
required to entirely prevent the formation of such fins is, of course,
scarcely attainable in practice except in the case of new moulds, so
that the traces of fins are generally to be found on all pressed
articles, and serve as a ready means of identifying these products
when an attempt is made to imitate better classes of glass-ware by
their means. The presses used in this process are generally of the
hand-lever type; power presses could no doubt be used, but it is
contended that the hand-press has a very great advantage in
allowing the operator to judge by touch when sufficient pressure has
been exerted, and this is an important consideration, since an
excessive pressure would either force the glass out of the mould
altogether or would be liable to burst or injure the mould seriously.
The actual presses consist of vertical guides and levers for
controlling the movement of the plunger and a table for holding the
moulds, and in some cases a system of cranks and levers for

opening and closing the moulds. The process of pressing is
exceedingly simple. The proper quantity of glass is gathered from
the pot on a solid rod and dropped into the mould. The thread of
glass which remains between the glass in the mould and that
remaining on the iron is cut off with a pair of shears, and then the
plunger is lowered into the mould and allowed to remain there until
the glass has stiffened sufficiently to retain its shape, when the
plunger is withdrawn. In this proceeding it will be seen that the glass
is forced into intimate contact with the relatively cold surfaces of
mould and plunger, and while undergoing this treatment the glass
must remain sufficiently plastic to readily adapt itself to the
configuration of the mould. It is therefore not surprising to find that
the pressing process can only be used successfully with glass of a
kind specially adapted for it. Certain varieties of flint glass and some
barium glasses are used for this purpose, but the greater quantity of
pressed glass, particularly as produced on the Continent, is made of
a lime-alkali silicate containing considerable quantities of both soda
and potash and relatively little lime; while sufficiently resistant for
most purposes, this glass is particularly soft and adaptable while in
the viscous condition.
The deleterious effect produced upon glass surfaces when
brought into contact with relatively cold metal has already been
referred to above, and it only remains to add that this is the principal
difficulty with which the glass-pressing process has to contend. It is
overcome to some extent by the aid of the reheating process
described above; but this is only a partial remedy, and in the
majority of pressed glass products the surface is “covered” as far as
possible by the application of relief decorations such as grooves,
spirals, and ribbings. An attempt is sometimes made to imitate the
appearance of cut glass, but the rounding of the angles during the
reheating process destroys the sharpness of the effect and allows of
the ready detection of the imitation, while the cheapness of the
decoration when applied in the mould has frequently led
manufacturers to grossly over-decorate, and, therefore, destroy all
claim to beauty in their wares.

CHAPTER IX.
ROLLED OR PLATE-GLASS.
In the present chapter we propose to deal with all those
processes of glass manufacture in which the first stage consists in
converting the glass into a slab or plate by some process of rolling.
We have already considered the general character of the rolling
process, and have seen that, although hot, viscous glass lends itself
readily to being rolled into sheets or slabs, these cannot be turned
out with a smooth, flat surface. In practice the surface of rolled glass
is always more or less dimmed by contact with the minute
irregularities of table or roller, and larger irregularities of the surface
arise from the buckling that occurs at a great many places in the
sheet. These limitations govern the varieties of glass that can be
produced by processes that involve rolling, and have led to the
somewhat curious result that both the cheapest and roughest, as
well as the best and most expensive kinds of flat glass, are produced
by rolling processes. Ordinary rough “rolled plate,” such as that used
in the skylights of workshops and of railway stations, is the extreme
on the one hand, while polished plate-glass represents the other end
of the scale. The apparent paradox is, however, solved when it is
noted that in the production of polished plate-glass the character of
the surface of the glass as it leaves the rollers is of very minor
importance, since it is entirely obliterated by the subsequent
processes of grinding, smoothing, and polishing. Intermediate
between the rough “rolled” and the “polished” plate-glass we have a
variety of glasses in which the appearance of the rolled surface is

hidden or disguised to a greater or lesser extent by the application
of a pattern that is impressed upon the glass during the rolling
process; thus we have rolled plate having a ribbed or lozenge-
patterned surface, or the well-known variety of “figured rolled” plate,
sometimes known as “Muranese,” whose elaborate and deeply-
imprinted patterns give a very brilliant effect.
Rolled plate-glass being practically the roughest and cheapest
form of glazing, is principally employed where appearance is not
considered, and its chief requirement is, therefore, cheapness,
although both the colour and quality of the glass are of importance
as affecting the quantity and character of the light which it admits to
the building where the glass is used. On the ground of cheapness it
will be obvious from what we have said above (Chapter IV.), that
such glass can only be produced economically in large tank furnaces,
and these are universally used for this purpose. The requirements as
regards freedom from enclosed foreign bodies of small size and of
enclosed air-bells are not very high in such glass, and, therefore,
tanks of very simple form are generally used. No refinements for
regulating the temperature of various parts of the furnace in order to
ensure perfect fining of the glass are required, and the furnace
generally consists simply of an oblong chamber or tank, at one end
of which the raw materials are fed in, while the glass is withdrawn
by means of ladles from one or two suitable apertures at the other
end. For economical working, however, the furnace must be capable
of working at a high temperature, because a cheap glass mixture is
necessarily somewhat infusible, at all events where colour is
considered. This will be obvious if we remember that the fusibility of
a glass depends upon its alkali contents, and alkali is the most
expensive constituent of such glasses.
The actual raw materials used in the production of rolled plate-
glass are sand, limestone and salt-cake, with the requisite addition
of carbon and of fluxing and purifying materials. The selection of
these materials is made with a view to the greatest purity and
constancy of composition which is available within the strictly-set
limits of price which the low value of the finished product entails.

These materials are handled in very large quantities, outputs of from
60 to 150 tons of finished glass per week from a single furnace
being by no means uncommon; mechanical means of handling the
raw materials and of charging them into the furnace are therefore
adopted wherever possible.
The glass is withdrawn from the furnace by means of large iron
ladles. These ladles are used of varying sizes in such a way as to
contain the proper amount of glass to roll to the various sizes of
sheets required. The sizes used are sometimes very large, and ladles
holding as much as 180 to 200 lbs. of glass are used. These ladles,
when filled with glass, are not carried by hand, but are suspended
from slings attached to trolleys that run on an overhead rail. The
ladler, whose body is protected by a felt apron and his face by a
mask having view-holes glazed with green glass, takes the empty
ladle from a water-trough, in which it has been cooled, carries it to
the slightly inclined gangway that leads up to the opening in the
front of the furnace, and there introduces the ladle into the molten
glass, giving it a half-turn so as to fill it with a “solid” mass of glass.
By giving the ladle two or three rapid upward jerks, the operator
then detaches the glass in the ladle as far as possible from the
sheets and threads of glass which would otherwise follow its
withdrawal; then the part of the handle of the ladle near the bowl is
placed in the hook attached to the overhead trolley, and by bearing
his weight on the other end of the handle, the workman draws the
whole ladle up from the molten bath in the furnace and out through
the working aperture. This operation only takes a few seconds to
perform, but during this time the ladler is exposed to great heat, as
a more or less intense flame generally issues from the working
aperture, whence it is drawn upward under the hood of the furnace.
From the furnace opening, the ladler, generally aided by a boy, runs
the full ladle to the rolling table and there empties the ladle upon the
table just in front of the roller. In doing this, two distinctly different
methods are employed. In one, only the perfectly fluid portion of the
glass is poured out of the ladle by gradually tilting it, the chilled
glass next to the walls of the ladle being retained there and

ultimately returned to the furnace while still hot. In the other
method, the chilling of the glass is minimised as far as possible, and
the entire contents of the ladle are emptied upon the rolling table by
the ladler, who turns the entire ladle over with a rapid jerk which is
so arranged as to throw the coldest part of the glass well away from
the rest. When the sheet is subsequently rolled this chilled portion is
readily recognised by its darker colour, and since it lies entirely at
one end of the sheet it is detached before the sheet goes any
further. Neither method appears to present any preponderating
advantage.
Fig. 9.—Rolling table for rolled
plate-glass.
The rolling table used in the manufacture of rolled plate is
essentially a cast-iron slab of sufficient size to accommodate the
largest sheet which is to be rolled; over this slab moves a massive
iron roller which may be actuated either by hand or by mechanical
power—the latter, however, being now almost universal. The
thickness of the sheet to be rolled is regulated by means of slips of
iron placed at the sides of the table in such a way as to prevent the
roller from descending any further towards the surface of the table:
so long as the layer of glass is thicker than these slips, the entire

weight of the roller comes upon the soft glass and presses it down,
but as soon as the required thickness is attained, the weight of the
roller is taken by the iron slips and the glass is not further reduced in
thickness. The width of the sheet is regulated by means of a pair of
iron guides, formed to fit the forward face of the roller and the
surface of the table, in the manner indicated in Fig. 9. The roller, as
it moves forward, pushes these guides before it, and the glass is
confined between them. When the roller has passed over the glass,
the sheet is left on the iron table in a red-hot, soft condition, and it
must be allowed to cool and harden to a certain extent before it can
be safely moved. In this interval, the chilled portion—if any—is
partially severed by an incision made in the sheet by means of a
long iron implement somewhat like a large knife, and then the sheet
is loosened from the bed of the table by passing under it, with a
smooth rapid stroke, a flat-bladed iron tool. The sheet is next
removed to the annealing kiln or “lear,” being first drawn on to a
stone slab and thence pushed into the mouth of the kiln. At this
stage the chilled portion of the sheet is completely severed by a
blow which causes the glass to break along the incision previously
made.
The rolled-plate annealing kiln is essentially a long, low tunnel,
kept hot at one end, where the freshly-rolled sheets are introduced,
and cold at the other end, the temperature decreasing uniformly
down the length of the tunnel. The sheets pass down this tunnel at
a slow rate, and are thus gradually cooled and annealed sufficiently
to undergo the necessary operations of cutting, etc. Although thus
simple in principle, the proper design and working of these “lears” is
by no means simple or easy, since success depends upon the correct
adjustment of temperatures throughout the length of the tunnel and
a proper rate of movement of the sheets, while the manner of
handling and supporting the sheets is vital to their remaining flat
and unbroken. The actual movement of the sheets is effected by a
system of moving grids which run longitudinally down the tunnel.
The sheets ordinarily lie flat upon the stone slabs that form the floor
of the tunnel, and the grids are lowered into recesses cut to receive

them. At regular intervals the iron grid bars are raised just
sufficiently to lift the sheets from the bed of the kiln, and are then
moved longitudinally a short distance, carrying the sheets forward
with them and immediately afterwards again depositing them on the
stone bed. The grids return to their former position while lowered
into their recesses below the level of the kiln bed.
When they emerge from the annealing kiln or “lear” the sheets
of rolled plate-glass are carried to the cutting and sorting room. Here
the sheets are trimmed and cut to size. The edges of the sheets as
they leave the rolling table are somewhat irregular, and sometimes a
little “beaded,” while the ends are always very irregular. Ends and
edges are therefore cut square or “trimmed” by the aid of the
cutting diamond. For this purpose the sheet is laid upon a flat table,
the smoothest side of the sheet being placed upwards, and long cuts
are taken with a diamond—good diamonds of adequate size and
skilful operators being necessary to ensure good cutting on such
thick glass over long lengths. Strips of glass six or eight feet long
and half an inch wide are frequently detached in the course of this
operation, and the final separation is aided by slight tapping of the
underside of the glass just below the cut and—if necessary—by
breaking the strip off by the aid of suitable tongs.
No very elaborate “sorting” of rolled plate glass is required,
except perhaps that the shade of colour in the glass may vary
slightly from time to time, and it is generally preferable to keep to
one shade of glass in filling any particular order. Apart from this, the
rolled plate cutter has merely to cut out gross defects which would
interfere too seriously with the usefulness of the glass. As we have
already indicated, air-bells and minute enclosures of opaque matter
are not objectionable in this kind of glass, but large pieces of opaque
material must generally be cut out and rejected, not only because
they are too unsightly to pass even for rough glazing purposes, but
also because they entail a considerable risk of spontaneous cracking
of the glass—in fact, visible cracks are nearly always seen around
large “stones,” as these inclusions are called. These may arise from
various causes, such as incomplete melting of the raw materials, or

the contamination of the raw materials with infusible impurities, but
the most fruitful source of trouble in this direction lies in the
crumbling of the furnace lining, which introduces small lumps of
partially melted fire-clay into the glass. In a rolled plate tank furnace
which is properly constructed and worked, the percentage of sheets
which have to be cut up on account of such enclosures should be
very small, at all events until the furnace is old, when the linings
naturally show an increasing tendency to disintegrate.
Returning now to the rolling process, it is readily seen that a
very slight modification will result in the production of rolled plate-
glass having a pattern impressed upon one surface; this modification
consists in engraving upon the cast-iron plate of the rolling table in
intaglio any pattern that is to appear upon the glass in relief. As a
matter of fact only very simple patterns are produced in this way,
such as close parallel longitudinal ribbing and a lozenge-pattern, the
reason probably being that the cost of cutting an elaborate pattern
over the large area of the bed-plate of one of these tables would be
very considerable. Further, as these tables and their bed-plates are
so very heavy, they are not readily interchanged or left standing idle,
so that only patterns required in very great quantity could be
profitably produced in this way. These disadvantages are, however,
largely overcome by the double-rolling machine. In this machine,
into whose rather elaborate details we cannot enter here, the glass
is rolled out into a sheet of the desired size and thickness by being
passed between two rollers revolving about stationary axes, the
finished sheet emerging over another roller, and passing on to a
stone slab that moves forward at the same rate as the sheet is fed
down upon it. In this machine a pattern can be readily imprinted
upon the soft sheet as it passes over the last roller by means of a
fourth roller, upon which the pattern is engraved; this is pressed
down upon the sheet, and leaves upon it a clear, sharp and deep
impress of its pattern. The general arrangement of the rollers in this
machine is shown in the diagram of Fig. 10, which represents the
sectional elevation of the appliance. After leaving the rolling
machine, the course of the “figured rolled plate” produced in this

manner is exactly similar to that of ordinary rolled plate, except that
as a somewhat softer kind of glass is generally used for “figured,”
the temperature of the annealing kilns requires somewhat different
adjustment. The cutting of the glass also requires rather more care,
and it should be noted that such glass can only be cut with a
diamond on the smooth side; the side upon which the pattern has
been impressed in relief cannot be materially affected by a diamond.
This is one reason why it is not feasible to produce such glass with a
pattern on both sides.
Fig. 10.—Sectional diagram of machine for
rolling “figured rolled” plate-glass.
Figured rolled glass, being essentially of an ornamental or
decorative nature, is generally produced in either brilliantly white
glass or in special tints and colours, and the mixtures used for
attaining these are, of course, the trade property of the various
manufacturers; the whiteness of the glass, however, is only
obtainable by the use of very pure and, therefore, expensive
materials. As regards the coloured plate-glasses, a general account
of the principles underlying the production of coloured glass will be
found in Chapter XI.

The manufacture of polished plate-glass really stands somewhat
by itself, almost the only feature which it has in common with the
branches of manufacture just described being the initial rolling
process.
The raw materials for the production of plate-glass are chosen
with the greatest possible care to ensure purity and regularity;
owing to the very considerable thickness of glass which is sometimes
employed in plate, and also to the linear dimensions of the sheets
which allow of numerous internal reflections, the colour of the glass
would become unpleasantly obtrusive if the shade were at all
pronounced. The actual raw materials used vary somewhat from one
works to another; but, as a rule, they consist of sand, limestone,
and salt-cake, with some soda-ash and the usual additions of fluxing
and purifying material such as arsenic, manganese, etc. The glass is
generally melted in pots, and extreme care is required to ensure
perfect melting and fining, since very minute defects are readily
visible in this glass when finished, and, of course, detract most
seriously from its value.
The method of transferring the glass from the melting-pot to the
rolling table differs somewhat in different works. In many cases the
melting-pots themselves are taken bodily from the furnace and
emptied upon the bed-plate of the rolling machine, while in other
cases the glass is first transferred to smaller “casting” pots, where it
has to be heated again until it has freed itself from the bubbles
enclosed during the transference, and then these smaller pots are
used for pouring the glass upon the rolling slab. The advantage of
the latter more complicated method lies, no doubt, in the fact that
the large melting-pots, which have to bear the brunt of the heat and
chemical action during the early stages of melting, are not exposed
to the great additional strain of being taken from the hot furnace
and exposed for some time to the cold outside air. Apart from the
mechanical risks of fracture, this treatment exposes the pots to
grave risks of breakage from unequal expansion and contraction on
account of the great differences of temperature involved. Where
smaller special casting-pots are used, these are not exposed to such

prolonged heat in the furnace, and are never exposed to the
chemical action of the raw materials, so that these subsidiary pots
may perhaps be made of a material better adapted to withstand
sudden changes of temperature than the high-class fire-clay which
must be used in the construction of melting pots. On the other hand,
the transference of the glass from the melting to the casting-pots
involves a laborious operation of ladling and the refining of the glass,
with its attendant expenditure of time and fuel. Finally, the
production of plate-glass in tank furnaces could only be attempted
by the aid of such casting-pots in which the glass would have to
undergo a second fining after being ladled from the tank, and this
would materially lessen the economy of the tank for this purpose,
while it is by no means an easy matter to produce in tank furnaces
qualities of glass equal as regards colour and purity to the best
products of the pot furnace.
The withdrawal of the pots containing the molten glass from the
furnace is now universally carried out by powerful machinery. The
pots are provided on their outer surface with projections by which
they can be held in suitably-shaped tongs or cradles. A part of the
furnace wall, which is constructed each time in a temporary manner,
is broken down; the pot is raised from the bed or “siege” of the
furnace by the aid of levers, and is then bodily lifted out by means of
a powerful fork. The pot is then lifted and carried by means of
cranes until it is in position above the rolling table; there the pot is
tilted and the glass poured out in a steady stream upon the table,
care being taken to avoid the inclusion of air-bells in the mass during
the process of pouring. When empty, the pot is returned to the
furnace as rapidly as possible, the glass being meanwhile rolled out
into a slab by the machine. Except for the greater size and weight of
both table and roller, the plate-glass rolling table is similar to that
already described in connection with rolled plate. Of course, since
the glass is poured direct from the pot, there is no chilled glass to be
removed. Further, owing to the large size of sheets frequently
required, the bed of the rolling table cannot be made of a single slab
of cast-iron, a number of carefully jointed plates being, in fact,

preferable, as they are less liable to warp under the action of the hot
glass.
In arranging the whole of the rolling plant, the chief
consideration to be kept in mind is that it is necessary to produce a
flat sheet of glass of as nearly as possible equal thickness all over.
The final thickness of the whole slab when ground and polished into
a sheet of plate-glass must necessarily be slightly less than that of
the thinnest part of the rough rolled sheet. If, therefore, there are
any considerable variations of thickness, the result will be that in
some parts of the sheet a considerable thickness of glass will have to
be removed during the grinding process. This will arise to a still
more serious extent if the sheet as a whole should be bent or
warped so as to depart materially from flatness. The two cases are
illustrated diagrammatically in Fig. 11, which shows sectional views
of the sheets before and after grinding on an exaggerated scale.
Fig. 11.—Sectional diagram illustrating waste of glass
in grinding curved or irregular plate.
While it is evident that careful design of the rolling table will
avoid all tendency to the formation of sheets of such undesirable
form, it is a much more difficult matter to avoid all distortion of the

sheet during the annealing process and while the sheet is being
moved from the rolling table to the annealing kiln. Owing to the
great size of the slabs of glass to be dealt with, and still more to the
stringent requirement of flatness, the continuous annealing kiln, in
which the glass travels slowly down a tunnel from the hot to the cold
end, has not been adopted for the annealing of plate-glass, and a
form of annealing kiln is still used for that glass which is similar in its
mode of operation to the old-fashioned kilns that were used for
other kinds of glass before the continuous kiln was introduced.
These kilns simply consist of chambers in which the hot glass is
sealed up and allowed to cool slowly and uniformly during a more or
less protracted period. In the case of plate-glass, the slabs are laid
flat on the stone bed of the kiln. This stone bed is built up of
carefully dressed stone, or blocks of fire-brick bedded in sand in
such a way that they can expand freely laterally without causing any
tendency for the floor to buckle upwards as it would do if the blocks
were set firmly against one another. The whole chamber is
previously heated to the requisite temperature at which the glass
still shows a very slight plasticity. The hot glass slabs from the rolling
table are laid upon the bed of this kiln, several being usually placed
side by side in the one chamber, and the slabs in the course of the
first few hours settle down to the contour of the bed of the kiln,
from which shape and position they are never disturbed until they
are removed when quite cold. In modern practice the cooling of a
kiln is allowed to occupy from four to five days; even this rate of
cooling is only permissible if care is taken to provide for the even
cooling of all parts of the kiln, and for this purpose special air-
passages are built into the walls of the chamber and beneath the
bed upon which the glass rests, and air circulation is admitted to
these in such a way as to allow the whole of the kiln to cool down at
the same rate; in the absence of such special arrangements, the
upper parts of the kiln would probably cool much more rapidly than
the base, so that the glass would be much warmer on its under than
on its upper surface.

When the slabs of plate-glass are removed from the annealing
kilns they very closely resemble sheets of rolled plate in appearance,
and they are quite sufficiently transparent to allow of examination
and the rejection of the more grossly defective portions; the more
minute defects, of course, can only be detected after the sheets
have been polished, but this preliminary examination saves the
laborious polishing of much useless glass.
The process of grinding and polishing plate-glass consists of
three principal stages. In the first stage the surfaces of the glass are
ground so as to be as perfectly flat and parallel as possible; in order
to effect this object as rapidly as possible, a coarse abrasive is used
which leaves the glass with a rough grey surface. In the second
stage, that of smoothing, these rough grey surfaces are ground
down with several grades of successively finer abrasive until finally
an exceedingly smooth grey surface is left. In the third and final
stage, the smooth grey surface is converted into the brilliant
polished surface with which we are familiar by the action of a
polishing medium.
Originally the various stages of the grinding and polishing
processes were carried out by hand, but a whole series of ingenious
machines has been produced for effecting the same purpose more
rapidly and more perfectly than hand-labour could ever do. We
cannot hope to give any detailed account of the various systems of
grinding and polishing machines which are even now in use, but
must content ourselves with a survey of some of the more important
considerations governing the design and construction of such
machinery.
In the first place, before vigorous mechanical work can be
applied to the surface of a plate of glass, that plate must be firmly
fixed in a definite position relatively to the rest of the machinery, and
such firm fixing of a plate of glass is by no means readily attained,
since the plate must be supported over its whole area if local
fracture is to be avoided. While the surface of the plate is in the
uneven condition in which it leaves the rolling table, such a firm

setting of the glass can only be attained by bedding it in plaster, and
this must be done in such a manner as to avoid the formation of air-
bubbles between plaster and glass; if bubbles are allowed to form,
they constitute places where the glass is unsupported. During the
grinding and polishing processes these unsupported places yield to
the heavy pressure that comes upon them, and irregularities in the
finished polished surfaces result. The most perfect adhesion
between glass and plaster is attained by spreading the paste of
plaster on the up-turned surface of the slab of glass and lowering
the iron bed-plate of the grinding table down upon it, the bed-plate
with the adhering slab of glass being afterwards turned over and
brought into position in the grinding machine. When one side of the
glass has been polished, it is generally found sufficient to lay the
slab down on a bed of damp cloth, to which it adheres very firmly,
although sliding is entirely prevented by a few blocks fixed to the
table in such a way as to abut against the edges of the sheet. In
many works, however, the glass is set in plaster for the grinding and
polishing of the second side as well as of the first.
The process of grinding and polishing is still regarded in many
plate-glass works as consisting of three distinct processes, known as
rough grinding, smoothing and polishing respectively. Formerly these
three stages of the process were carried out separately; at first by
hand, and later by three different machines. In the most modern
practice, however, the rough and smooth grinding are done on the
same machine, the only change required being the substitution of a
finer grade of abrasive at each step for the coarser grade used in the
previous stage. For the polishing process, however, the rubbing
implements themselves must be of a different kind, for while the
grinding and smoothing is generally done by means of cast-iron
rubbers moving over the glass, the polishing is done with felt pads.
The table of the machine, to which the glass under treatment is
attached, is therefore made movable, and when the grinding and
smoothing processes are complete, the table with its attached glass
is moved so as to come beneath a superstructure carrying the

polishing rubbers, and the whole is then elevated so as to allow the
rubbers to bear on the glass.
The earliest forms of grinding machines gave a reciprocal motion
to the table which carries the glass, or the grinding rubbers were
moved backward and forward over the stationary table. Rotary
machines, however, were introduced and rapidly asserted their
superiority, until, at the present time, practically all plate-glass is
ground on rotating tables, some of these attaining a diameter of
over 30 ft. The grinding “rubbers” consist of heavy iron slabs, or of
wood boxes shod with iron, but of much smaller diameter than the
grinding table. The rubbers themselves are rotary, being caused to
rotate either by the frictional drive of the rotating table below them,
or by the action of independent driving mechanism, but the design
of the motions must be so arranged that the relative motion of
rubber and glass shall be approximately the same at all parts of the
glass sheets, otherwise curved instead of plane surfaces would be
formed. This condition can be met by placing the axes of the rubbers
at suitable points on the diameter of the table. The abrasive is fed
on to the glass in the form of a thin paste, and when each grade or
“course” has done the work required of it, the whole table is washed
down thoroughly with water and then the next finer grade is applied.
The function of the first or coarsest grade is simply to remove the
surface irregularities and to form a rough but plane surface. The
abrasive ordinarily employed is sharp sand, but only comparatively
light pressure can be applied, especially at the beginning of this
stage, since at that period the weight of the rubber is at times borne
by relatively small areas of glass that project here and there above
the general level of the slab. As these are ground away, the rubbers
take a larger and more uniform bearing, and greater pressure can be
applied. The subsequent courses of finer abrasives are only required
to remove the coarse pittings left in the surface by the action of the
first rough grinding sand; the finer abrasive replaces the deep pits of
the former grade by shallower pits, and this is carried on in a
number of steps until a very smooth “grey” surface is attained and
the smoothing process is complete. The revolving table or “platform”

is now detached from the driving mechanism, and moved along
suitably placed rails on wheels provided for that purpose, until it
stands below the polishing mechanism. Here it is attached to a fresh
driving mechanism, and it is then either raised so as to bring the
glass into contact with the felt-covered polishing rubbers, or the
latter are lowered down upon the glass. The polishing rubbers are
large felt-covered slabs of wood or iron which are pressed against
the glass with considerable force; their movement is very similar to
that of the grinding rubbers, but in place of an abrasive they are
supplied with a thin paste of rouge and water. The time required for
the polishing process depends upon the perfection of the smoothing
that has been attained; in favourable cases two or three hours are
sufficient to convert the “grey” surface into a perfectly polished one;
where, however, somewhat deeper pits have been left in the glass,
the time required for polishing may be much longer, and the polish
attained will not be so perfect. The mode of action of a polishing
medium such as rouge is now recognised to be totally different in
character from that of even the finest abrasive; the grains of the
abrasive act by their hardness and the sharpness of their edges,
chipping away tiny particles of the glass, so that the glass steadily
loses weight during the grinding and smoothing processes. During
the polishing process, however, there is little or no further loss of
weight, the glass forming the hills or highest parts of the minutely
pitted surface being dragged or smeared over the surface in such a
way as to gradually fill up the pits and hollows. The part played by
the polishing medium is probably partly chemical and partly physical,
but it results, together with the pressure of the rubber, in giving to
the surface molecules of the glass a certain amount of freedom of
movement, similar to that of the molecules of a viscid liquid; the
surface layers of glass are thus enabled to “flow” under the action of
the polisher and to smooth out the surface to the beautiful level
smoothness which is so characteristic of the surfaces of liquids at
rest. This explanation of the polishing process enables us to
understand why the proper consistency of the polishing paste, as
well as the proper adjustment of the speed and pressure of the
rubbers, plays such an important part in successful polishing; it also

serves to explain the well-known fact that rapid polishing only takes
place when the glass surface has begun to be perceptibly heated by
the friction spent upon it.
It has been estimated that, on the average, slabs of plate-glass
lose one-third of their original weight in the grinding and polishing
processes, and it is obvious that the erosion of this great weight of
glass must absorb a great amount of mechanical energy, while the
cost of the plant and upkeep is proportionately great. Every factor
that tends to diminish either the total weight of glass to be removed
per square yard of finished plate, or reduces the cost of removal,
must be of the utmost importance in this manufacture. The flatness
of the plates as they leave the annealing kiln has already been
referred to, and the reason why the processes of grinding and
polishing have formed the subject for innumerable patents will now
be apparent. The very large expansion of the use of plate-glass in
modern building construction, together with the steady reduction in
the prices of plate, are evidence of the success that has attended
the efforts of inventors and manufacturers in this direction.
At the present time, plate-glass is manufactured in very large
sheets, measuring up to 26 ft. in length by 14 ft. in width, and in
thickness varying from 3/16th of an inch up to 1½ in., or more, for
special purposes. At the same time the quality of the glass is far
higher to-day than it was at earlier times. This high quality chiefly
results from more careful choice of raw materials and greater
freedom from the defects arising during the melting and refining
processes, while a rigid process of inspection is applied to the glass
as it comes from the polishing machines. For this purpose the sheets
are examined in a darkened room by the aid of a lamp placed in
such a way that its oblique rays reveal every minute imperfection of
the glass; these imperfections are marked with chalk, and the plate
is subsequently cut up so as to avoid the defects that have thus
been detected.
Perhaps the most remarkable fact about the quality of modern
plate-glass is its relatively high degree of homogeneity. Glass, as we

have seen in Chapter I., is not a chemically homogeneous substance,
but rather a mixture of a number of substances of different density
and viscosity. Wherever this mixture is not sufficiently intimate, the
presence of diverse constituents becomes apparent in the form of
striæ, arising from the refraction or bending of light-rays as they
pass from one medium into another of different density. Except in
glass that has undergone elaborate stirring processes, such striæ are
never absent, but the skill of the glass-maker consists in making
them as few and as minute as possible, and causing them to assume
directions and positions in which they shall be as inconspicuous as
possible. In plate-glass this is generally secured in a very perfect
manner, and to ordinary observation no striæ are visible when a
piece of plate-glass is looked at in the ordinary way, i.e., through its
smallest thickness; if the same piece of glass be looked at
transversely, the edges having first been polished in such a way as
to render this possible, the glass will be seen to be full of striæ,
generally running in fine lines parallel with the polished surfaces of
the glass. This uniform direction of the striæ is partly derived from
the fact that the glass has been caused to flow in this direction by
the action of the roller when first formed into a slab, but this process
would not obliterate any serious inequalities of density which might
exist in the glass as it leaves the pot, so that successful results are
only attainable if great care is taken to secure the greatest possible
homogeneity in the glass during the melting process.
At the present time probably the greater bulk of plate-glass is
used for the purpose of glazing windows of various kinds, principally
the show windows of shops, etc. As used for this purpose the glass
is finished when polished and cut to size. The only further
manipulation that is sometimes required is that of bending the glass
to some desired curvature, examples of bent plate-glass window-
panes being very frequently seen. This bending is carried out on the
finished glass, i.e., after it has been polished; the glass is carefully
heated in a special furnace until softened, and is then gently made
to lie against a stone or metal mould which has been provided with
the desired curvature. It is obvious that during this operation there

are great risks of spoiling the glass; roughening of the surface by
contact with irregular surfaces on either the mould, the floor of the
kiln, or the implements used in handling the glass, can only be
avoided by the exercise of much skill and care, while all dust must
also be excluded since any particles settling on the surface of the
hot glass would be “burnt in,” and could not afterwards be detached.
Small defects can, of course, be subsequently removed by local
hand-polishing, and this operation is nearly always resorted to where
polished glass has to undergo fire-treatment for the purpose of
bending.
In addition to its use for glazing in the ordinary sense, plate-
glass is employed for a number of purposes; the most important and
frequent of these is in the construction of the better varieties of
mirrors. For this purpose the glass is frequently bevelled at the
edges, and sometimes a certain amount of cutting is also introduced
on the face of the mirror. Bevelling is carried out on special grinding
and polishing machines, and a great variety of these are in use at
the present time. The process consists in grinding off the corners of
the sheet of glass and replacing the rough perpendicular edge left by
the cutting diamond by a smooth polished slope running down from
the front surface to the lower edge at an angle of from 45 to 60
degrees. Since only relatively small quantities of glass have to be
removed, small grinding rubbers only are used, and in some of the
latest machines these take the form of rapidly-revolving emery or
carborundum wheels. These grinding wheels have proved so
successful in grinding even the hardest metals that it is surprising to
find their use in the glass industry almost entirely restricted to the
“cutting” of the better kinds of flint and “crystal” glass for table ware
or other ornamental purposes. The reason probably lies in the fact
that the use of such grinding wheels results in the generation of a
very considerable amount of local heat, this effect being intensified
on account of the low heat-conducting power of glass. If a piece of
glass be held even lightly against a rapidly-revolving emery wheel it
will be seen that the part in contact with the wheel is visibly red-hot.
This local heating is liable to lead to chipping and cracking of the

glass, and these troubles are those actually experienced when emery
or carborundum grinding is attempted on larger pieces of glass. In
the case of at least one modern bevel-grinding machine, however, it
is claimed that the injurious effects of local heating are avoided by
carrying out the entire operation under water.
For the purpose of use in mirrors, plate-glass is frequently
silvered, and this process is carried on so extensively that it has
come to constitute an entire industry which has no essential
connection with glass manufacture itself; for that reason we do not
propose to enter on the subject here, only adding that the nature
and quality of the glass itself considerably affects the ease and
success of the various silvering processes. Ordinary plate-glass, of
course, takes the various silvering coatings very easily and uniformly,
but there are numerous kinds of glass to which this does not apply,
although there are probably few varieties of glass which are
sufficiently stable for practical use, and to which a silvering coating
cannot be satisfactorily applied, provided that the most suitable
process be chosen in each case.
While there is little if any use for coloured glass in the form of
polished plate, entirely opaque plate-glass, coloured both black and
white, is used for certain purposes. Thus, glass fascias over shop-
fronts, the counters and shelves of some shops, and even
tombstones are sometimes made of black or white polished plate.
From the point of view of glass manufacture, however, these
varieties only differ from ordinary plate-glass in respect of certain
additions to the raw materials, resulting in the production of the
white or black opacity. The subsequent treatment of the glass is
identical with that of ordinary plate-glass, except that these opaque
varieties are rarely required to be polished on both sides, so that the
operations are simplified to that extent.
Certain limitations to the use of all kinds of plate-glass, whether
rough-rolled, figured or polished, were formerly set by the fact that
under the influence of fire, partitions of glass were liable to crack,
splinter and fall to pieces, thus causing damage beyond their own

destruction and leaving a free passage for the propagation of the
fire. To overcome these disadvantages, glass manufacturers have
been led to introduce a network or meshing of wire into the body of
such glass. Provided that the glass and wire can be made so as to
unite properly, then the properties of such reinforced or “wired”
glass should be extremely valuable. In the event of breakage from
any cause, such as fire or a violent blow, while the glass would still
crack, the fragments would be held together by the wire network,
and the plates of glass as a whole would remain in place, neither
causing destruction through flying fragments nor allowing fire or, for
the matter of that, burglar a free passage. The utility of such a
material has been readily recognised, but the difficulty lies in its
production. These difficulties arise from two causes. The most
serious of these is the considerable difference between the thermal
expansion of the glass and of the wire to be embedded in it. The
wire is necessarily introduced into red-hot glass while the latter is
being rolled or cast, and therefore glass and wire have to cool down
from a red heat together. During this cooling process the wire
contracts much more than the glass, and breakage either results
immediately, or the glass is left in a condition of severe strain and is
liable to crack spontaneously afterwards. An attempt has been made
to overcome this difficulty by using wire made of a nickel steel alloy,
whose thermal expansion is very similar to that of glass; but, as a
matter of fact, this similarity of thermal expansion is only known to
hold for a short range of moderate temperatures, and probably does
not hold when the steel alloy is heated to redness. In another
direction, greater success is to be attained by the use of wire of a
very ductile metal which should yield to the stress that comes upon
it during cooling; probably copper wire would answer the purpose,
but the great cost of copper is a deterrent from its use. A second
difficulty is met with in introducing wire netting into glass during the
rolling operation, and this lies in effecting a clean join between glass
and wire. Most metals when heated give off a considerable quantity
of gas, and when this gas is evolved after the wire has been
embedded in glass, numerous bubbles are formed, and these not
only render the glass very unsightly but also lessen the adhesion

between the wire and the glass. This difficulty, however, can be
overcome more readily than the first, since the surface of the metal
can be kept clean and the gas expelled from the interior of the wire
by preliminary heating. On the whole, however, wired glass is
perhaps still to be regarded as a product whose evolution is not yet
complete, and there can be no doubt that there are great
possibilities open to the material when its manufacture has been
more fully developed.

CHAPTER X.
SHEET AND CROWN GLASS.
In the preceding chapter we have dealt with the processes of
manufacture employed in the production of both the crudest and the
most perfect forms of flat glass as used for such purposes as the
glazing of window openings. The products now to be dealt with are
of an intermediate character, sheet-glass possessing many of the
properties of polished plate, but lacking some very important ones;
thus sheet-glass is sufficiently transparent to allow an observer to
see through it with little or no disturbance—in the best varieties of
sheet-glass the optical distortion caused by its irregularities is so
small that the glass appears nearly as perfect as polished plate—but
in the cheap glass that is used for the glazing of ordinary windows,
sheets are often employed which produce the most disturbing, and
sometimes the most ludicrous, distortions of objects seen through
them. It is a curious fact that even in good houses the use of such
inferior glass is tolerated without comment, the general public being,
apparently, remarkably nonobservant in this respect. In another
direction sheet-glass has the great advantage over plate-glass that it
is very much lighter, or can at least be produced of much smaller
weight and thickness, although this advantage entails the
consequent disadvantage that sheet-glass is usually much weaker
than plate, and can only be used in much smaller sizes. In recent
times the production of relatively thin plate-glass has, however,
made such strides that it is now possible to obtain polished plate-
glass thin enough and light enough for almost every architectural

purpose. Finally, the most important advantage of sheet-glass, and
the one which alone secures its use in a great number of cases in
preference to plate-glass, is its cheapness, the price of ordinary
sheet-glass being about one-fourth that of plate-glass of the same
size.
The raw materials for the manufacture of sheet-glass are sand,
limestone, salt-cake, and a few accessory substances, such as
arsenic, oxide of manganese, anthracite coal or coke, which differ
considerably according to the practice of each particular works. In a
general way these materials have already been dealt with in Chapter
III., and we need only add here that the sheet-glass manufacturer
must keep in view two decidedly conflicting considerations. On the
one hand the requirements made in the case of sheet-glass as
regards colour and purity render a rigorous choice of raw material
and the exclusion of anything at all doubtful very desirable; but on
the other hand the chief commercial consideration in connection with
this product is its cheapness, and in order to maintain a low selling
price at a profit to himself the manufacturer must rigorously exclude
all expensive raw materials. For this reason sheet-glass, works such
as those of Belgium and some parts of Germany, which have large
deposits of pure sand close at hand, possess a very considerable
advantage over those in less favoured situations, since sand in
particular forms so large a proportion of the glass, and the cost of
carriage frequently exceeds, and in many cases very greatly
exceeds, the actual price of the sand itself. The same considerations
will apply, although in somewhat lesser degree, to the other bulky
materials, such as limestone and salt-cake; but both these are more
generally obtainable at moderate prices than are glass-making sands
of adequate quality for sheet manufacture.
Ordinary “white” sheet-glass is now almost universally produced
in tank furnaces, and a very great variety of these furnaces are used
or advocated for the purpose. It would be beyond the scope of the
present book to enter in detail into the construction of these various
types of furnace or to discuss their relative merits at length. Only a

brief outline of the chief characteristics of the most important forms
of sheet-tank furnaces will therefore be given here.
Sheet tanks differ from each other in several important respects;
these relate to the sub-division of the tank into one, two, or even
three more or less separate chambers, to the depth of the bath of
molten glass and the height of the “crown” or vault of the furnace
chamber, to the shape and position of the apertures by which the
gas and air are admitted into the furnace, and the resultant shape
and disposition of the flame, and finally to the position and
arrangement of the regenerative appliances by which some of the
heat of the waste gases is returned into the furnace.
Taking these principal points in order, we find that in some sheet
tank furnaces the whole furnace constitutes a single large chamber.
In this type of furnace the whole process of fusion and fining of the
glass goes on in this single chamber, and an endeavour is made to
graduate the temperature of the furnace in a suitable manner from
the hot end where the raw materials have to be melted down to the
colder end where the glass must be sufficiently viscous to be
gathered on the pipes. It is obvious that this control of the
temperature cannot be so perfect in a furnace of the single chamber
type as in one that is sub-divided. Such sub-divided furnaces are, as
a matter of fact, much more frequent in sheet-glass practice; but
this practice differs widely as to the manner and degree of the sub-
division introduced. In the extreme form the glass practically passes
through three independent furnaces merely connected with one
another by suitable openings of relatively small area through which
the glass flows from one to the other. If it were possible to build
furnaces of materials that could resist the action of heat and of
molten glass to an indefinite extent, it is probable that this extreme
type would prove the best, since it gives the operator of the furnace
the means of controlling the flow of glass in such a way that no
unmelted material can leave the melting chamber and enter the
fining chamber, and that no insufficiently fined glass can leave the
fining chamber and find its way into the working chamber. But in
practice the fact that this extreme sub-division introduces a great

deal of extra furnace wall, exposed both to heat and to contact with
the glass, involves very serious compensating disadvantages—the
cost of construction, maintenance and renewal of the furnace is
greatly increased, while there is also an increased source of
contamination of the glass from the erosion of the furnace walls. It
is, therefore, in accordance with expectations to find that the most
successful furnaces for the production of sheet-glass are
intermediate in this respect between the simple open furnace and
the completely sub-divided one. In some cases the working chamber
is separated from the melting and fining chamber by a transverse
wall above the level of the glass, while fire-clay blocks floating in the
glass just below this cross wall serve to complete the separation and
to retain any surface impurities that may float down the furnace.
As regards the depth of glass in the tank, practice also varies
very much. The advantages claimed for a deep bath are that the
fire-clay bottom of the furnace is thereby kept colder and is
consequently less attacked, so that this portion of the furnace will
last for many years. On the other hand the existence of a great mass
of glass at a moderate heat may easily prove the source of
contamination arising from crystallisation or “devitrification”
occurring there and spreading into the hotter glass above. Also, if for
any reason it should become necessary to remove part or all of the
contents of the tank, the greater mass of glass in those with deep
baths becomes a formidable obstacle. On the whole, however,
modern practice appears to favour the use of deeper baths, depths
of 2 ft. 6 in. or even 3 ft. being very usual, while depths up to 4 ft.
have been used.
The question of the proper height of the “crown” or vault of the
furnace is of considerable importance to the proper working of the
tank. For the purpose of producing the most perfect combustion, it is
now contended that a large free flame-space is required. The earlier
glass-melting tanks, like the earlier steel furnaces, were built with
very low crowns, forcing the flame into contact with the surface of
the molten glass, the object being to promote direct heating by
immediate contact of flame and glass; the modern tendency,

however, is strongly in the direction of higher crowns, leaving the
heating of the glass to be accomplished by radiation rather than
direct conduction of heat. There can be little doubt that up to a
certain point the enlargement of the flame-space tends towards
greater cleanliness of working and a certain economy of fuel, but if
the height of a furnace crown be excessive there is a decided loss of
economy. Flame-spaces as high as 6 ft. from the level of the glass to
the highest part of the crown have been used, but the more usual
heights range from 2 ft. to 5 ft.
The “ports” or apertures by which pre-heated gas and air enter
the furnace chamber differ very widely in various furnaces. In some
cases the gas and air are allowed to meet in a small combustion
chamber just before entering the furnace itself, while in other cases
the gas and air enter the furnace by entirely separate openings, only
meeting in the furnace chamber. The latter arrangement tends to the
formation of a highly reducing flame, which is advantageous for the
reduction of salt-cake, but is by no means economical as regards
fuel consumption. On the other hand, by producing a perfect mixing
of the entering gas and air in suitable proportions, the other type of
ports can be made to give almost any kind of flame desired,
although their tendency is to form a more oxidising atmosphere
within the furnace. The latter type of ports, although widely varied in
detail, are now almost universally adopted in sheet tank furnaces.
All modern tank furnaces work on the principle of the recovery
of heat from the heated products of combustion as they leave the
furnace, and the return of this heat to the furnace by utilising it to
pre-heat the incoming gas and air; but the means employed to
effect the application of this “regenerative” principle differ
considerably in various types of plant. Perhaps the most widely-used
form of furnace is the direct descendant of the original Siemens
regenerative furnace, in which four regenerator chambers are
provided with means for reversing the flow of gas and air in such a
way that each pair of chambers serves alternately to absorb the heat
of the outgoing gases and subsequently to return this heat to the
incoming air that passes through one, and the incoming gas that

passes through the other of these chambers. In these furnaces, the
regenerator chambers themselves are generally placed underneath
the melting furnace, and they are built of fire-brick and filled with
loosely-stacked fire-bricks, whose function it is to absorb or deliver
the heat. In the most modern type of furnaces of this class, the gas-
regenerators are omitted entirely, the air only being pre-heated by
means of regenerators, while the gas enters the furnace direct from
the producer, thus carrying with it the heat generated in the
producer during the gasification of the fuel. While this arrangement
is undoubtedly economical, it has the serious disadvantage,
especially in the manufacture of sheet-glass, that the gas, rushing
direct from the producer into the furnace, carries with it a great deal
of dust and ash, which it has no opportunity of depositing, as in the
older types of furnace, in long flues.
The most serious disadvantages of the ordinary types of
regenerative furnaces are due to the considerable dimensions of the
regenerative apparatus, necessitating a costly form of construction
and occupying a large space, while the necessity of periodically
reversing the valves so as to secure the alternation in the flow of
outgoing and incoming gases requires special attention on the part
of the men engaged in operating the furnace, as well as the
construction and maintenance of valves under conditions of heat and
dirt that are not favourable to the life of mechanical appliances. It is
claimed that all these disadvantages are overcome to a considerable
extent in one or other of the various forms of furnace known as
“recuperative.” In these furnaces there is no alternation of flow, and
the regenerator chambers are replaced by the “recuperators.” These
consist of a large number of small flues or pipes passing through a
built-up mass of fire-brick in two directions at right-angles to one
another; through the pipes running in one direction the waste gases
pass out to the chimney, while the incoming gas and air pass
through the other set of pipes. A transference of heat between the
two currents of gas takes place by the conductivity of the fire-brick,
and thus the outgoing gases are continuously cooled while the
ingoing gases are heated—the transference of heat being somewhat

similar to that which takes place in the surface condenser of a steam
engine. Theoretically this is a much simpler arrangement than that
of separate regenerator chambers, and to some extent it is found
preferable in practice, but there are certain disadvantages associated
with the system which arise principally from the peculiar nature of
the material—fire-brick—of which the recuperators must be
constructed. In the first place, the heat-conductivity of fire-brick is
not very high, so that, in order to secure efficiency, the recuperators
must be large, and while the individual pipes must be of small
diameter, their area as a whole must be large enough to allow the
gases to pass through somewhat slowly. Next, owing to the
tendency of fire-brick to warp, shrink and crack under the prolonged
effects of high temperatures, it becomes difficult to prevent leakage
of gases from one set of pipes into the other. If this occurs to a
moderate extent its only effect will be to allow some of the
combustible gas to pass direct to the chimney, and at the same time
a dilution of the gases entering the furnace by an addition of
products of combustion from the waste-gas flues. This, of course,
will materially reduce the efficiency of the furnace and require a
higher fuel consumption if the temperature of the furnace is to be
maintained at its proper level. If, however, the leakage should
become more serious, a disastrous explosion might easily result,
particularly if the nature of the leakage were such as to allow the
incoming gas and air to mix in the flues. It follows from these
considerations that, although the recuperative furnace is somewhat
simpler and cheaper to construct, it requires, if anything, more
careful maintenance than the older forms of regenerative furnace.
Tank furnaces for the production of sheet-glass in this country
are generally worked from early on Monday morning until late on
Saturday night, glass-blowing operations being suspended during
Sunday, although the heat of the furnace must be maintained. On
the Continent, and especially in Belgium, the work in connection
with these furnaces goes on without any intermission on Sunday—a
difference which, however desirable the English practice may be, has
the effect of handicapping the output of a British furnace of equal

capacity by about 10 per cent. without materially lessening the
working cost.
The process of blowing sheet-glass in an English glassworks is
generally carried out by groups of three workmen, viz., a “pipe-
warmer,” a “gatherer” and a “blower,” although the precise division of
the work varies according to circumstances. The pipe-warmer’s work
consists in the first place in fetching the blowing-pipe from a small
subsidiary furnace in which he has previously placed it for the
purpose of warming up the thick “nose” end upon which the glass is
subsequently gathered. The sheet-blower’s pipe itself is an iron tube
about 4 ft. 6 in. long, provided at the one end with a wooden sleeve
or handle, and a mouthpiece, while the other end is thickened up
into a substantial cone, having a round end. Before introducing the
pipe into the opening of the tank furnace, the pipe-warmer must see
that the hot end of the pipe is free from scale or dirt and must test,
by blowing through it, whether the pipe is free from internal
obstructions. He then places the butt of the pipe in the opening of
the furnace and allows it to acquire as nearly as possible the
temperature of the molten glass. When this is the case the pipe is
either handed on to the gatherer, or the pipe-warmer, who is usually
only a youth, may take the process one step further before handing
it on to the more highly skilled workman. This next step consists in
taking up the first gathering of glass on the pipe. For this purpose
the hot nose of the pipe is dipped into the molten glass, turned
slowly round once or twice and then removed, the thread of viscous
glass that comes up with the pipe being cut off against the fire-clay
ring that floats in the glass in front of the working opening. A small
quantity of glass is thus left adhering to the nose of the pipe, and
this is now allowed to cool down until it is fairly stiff, the whole pipe
being meanwhile rotated so as to keep this first gathering nicely
rounded, while a slight application of air-pressure, by blowing down
the pipe, forms a very small hollow space in the mass of glass and
secures the freedom of the opening of the pipe. When the glass
forming the first gathering has cooled sufficiently, the gatherer
proceeds to take up the second gathering upon it. The pipe is again

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