The glass industry document discusses the glass manufacturing process and opportunities for advanced ceramics. It summarizes that the glass industry employs over 150,000 people and produces over 21 million tons of products annually. It then describes the four main steps in glass production: batching, melting, refining, and forming. The melting section highlights opportunities for advanced ceramics to improve furnace components, filters, sensors and heat exchangers to increase efficiency and lifespan while reducing emissions. The document also discusses using oxygen for combustion or preheating batch materials to further increase furnace output and performance.

1. CHAPTER SIX
G LASS
economy, employing more than 150,000 people in
skilled jobs and generating more than 21 million
metric tons of consumer products each year with
INDUSTRY
by Douglas W. Freitag
Glass is an important component of the U.S. • product development of innovative new uses
for glass.
The following sections describe current glass
manufacturing processes and how ceramic-based
an estimated value of $22 billion. Glass production materials might contribute to achieving the glass
is energy intensive and accounts for 12% of the industry vision. Materials commonly used in
total cost of sales. In theory, about 2.2 million Btu glass-processing equipment include fused silica,
of energy are required to melt a ton of glass. In graphite, precious metals, and water-cooled fer-
reality, it takes twice as much energy because of rous alloys. Ceramics are primarily used as refrac-
inefficiencies and loses. The glass industry consists tories, with ceramic coatings increasingly being
of four major segments: container glass (bottles, used for wear resistance. Advanced ceramics are
jars, etc.); flat glass (windows, windshields, rarely used because of their high cost. Moreover,
mirrors, etc.); fiberglass (building insulation and owing to the lack of suitable higher-temperature-
textile fibers); and specialty glass (cookware, flat capable materials, fluxes are commonly added to
panel displays, light bulbs, fiber optics, medical reduce glass-processing temperatures and permit
equipment, etc.). Flat glass accounts for 17% of U.S. the use of conventional materials.
production by weight, container glass 60%,
fiberglass 9% and specialty glass 4%. While the
container, fiber, and flat glass industries focus on
producing soda-lime glass for large, broad-based
markets, the specialty glass industry focuses on
higher temperature glasses and produces over
60,000 products. Examples of products
manufactured by the glass industry are shown in
Fig. 6.1. Key drivers for investment in the glass
industry include cost, quality, and increased
productivity.
The desired state of the glass industry in twenty
years has been formulated in a vision document
prepared cooperatively with the U.S. Department
of Energy (DOE). Future technology challenges
and research opportunities were identified by
comparing the vision of the future with the cur-
rent state of the glass industry. Technology chal-
lenges are broadly categorized into four areas:
• advances in melting and refining processes and
in fabricating/forming;
• technology development of new glass-making
techniques, processing controls, and computer
simulations to model new processes; Fig. 6.1. Examples of glass industry products.
• systems improvements in emission controls, Source: Marketing brochure from Praxair, Inc.,
recycling methods, and solid waste manage- Danbury, Conn.
ment; and
Page 6-1

2. Chapter 6
The discussion is organized according to the consumption, and lower temperatures above the
four primary operations for glass manufacturing: melt for reduced emissions.
batching, melting, refining, and forming. Batching,
melting, and refining operations are common to Combustion-Heated Furnaces
all glass manufacturing processes with some varia-
tion in furnace type. Various postprocesses are Combustion-heated furnaces can be further
used, depending on the end product. divided according to the method used to recover
exhaust waste heat and the way the fuel is burned
(with air or pure oxygen). Heat recovery is impor-
6.1 BATCHING tant because only 10% of the heat generated is ac-
tually used to melt the glass, whereas 67% is lost
Raw material selection is based on chemistry, through the exhaust. Recovery of exhaust waste
purity, uniformity, and particle size. Additional heat is performed with a regenerator or
organic, metallic, and ceramic contaminants can recuperator. Regenerator furnace designs repre-
be introduced throughout the delivery, storage, sent one of the oldest and most universal devices
mixing, and sizing processes, as well as through for saving energy in the glass industry.
the use of cullet (recycled glass). Because of the An example of a regenerative furnace is shown
effects of contamination, the use of cullet varies in Fig. 6.2. The batch introduced at one end
within the industry according to the desired end immediately begins to melt. As glass is extracted
product. Flat glass manufacturers recycle up to from the opposite end, the newly introduced glass
30% of their own cullet. Ceramic contaminants flows along the melt zone where it becomes com-
undergo little reaction with the glass melt and re- positionally homogeneous and bubble-free. After
side as inclusions (stones) in the finished product. leaving the melt zone, the molten glass enters the
Metal and organic contaminants create instabili- refining zone where it becomes thermally homo-
ties during glass processing (through reduction/ geneous. After leaving the refining zone, the glass
oxidation reactions) and degrade the quality of the enters the forehearth, where it can be distributed
glass. Organics present in the batch are a source to one or more forming machines.
of increased emissions and added cost for flue gas Refractories commonly used in regenerative
cleanup. Delivery, mixing, and sizing processes are glass melt furnaces are shown in Fig. 6.3. Alumina-
highly abrasive, and equipment used commonly zirconia-silica (AZS) is the material of choice for
contains ceramic-coated wear surfaces or ceramic the sidewall, with the zirconia content ranging
liners manufactured from alumina, silicon carbide, from 33 to 41%. Various chrome-containing refrac-
or tungsten carbide. While generally cost-effective tories are commonly used in the fiberglass indus-
and with adequate performance, lower cost solu- try because of their superior erosion resistance but
tions offering lower risk of contamination continue are gradually being replaced because of environ-
to be sought. mental concerns about chrome. Even though su-
perior in performance, chrome-containing refrac-
tories are not widely used in the flat, container,
and specialty glass industries because chrome im-
6.2 MELTING
purities result in glass discoloration. At the front
of the furnace where the charge is introduced, ero-
There are about 600 glass melt furnaces in
sion and thermally induced damage are the most
North America. The distribution of these furnaces
severe and limit furnace life. Materials that are
is as follows: container, 210; fiberglass, 110; flat
more corrosion resistant are sought for the
glass, 45; and specialty, 235; The life of a glass-
sidewalls. Cladding refractories with platinum or
melting furnace between rebuilds varies, but 7–8
molybdenum is considered too expensive. Not
years is not uncommon. The cost of rebuilding a
shown are the AZS electrode and burner blocks
furnace can easily exceed $1 million.
located on the sidewall below the melt line. These
Glass furnaces can be divided into those heated
blocks support the electric boost heat electrodes
electrically and those heated primarily by
and burner nozzles, both of which experience high
combustion. Often electrical heating of the melt is
local temperatures and thermal stresses that re-
used in combination with fuel firing (electric boost)
sult in reduced life.
to improve heating uniformity, provide
The main source of a problem is above the glass
intermittent increases in melt capacity at minimum
melt where temperatures are the hottest and
cost, increase melt efficiency, reduce gas
Page 6-2

3. Glass Industry
Fig. 6.2. Typical side-fired flat glass melt furnace. Source: “Development of Improved Refractories for
Glass Industries,” U.S. Department of Energy fact sheet.
Fig. 6.3. Refractories commonly used in a soda-lime glass melt furnace.
products of combustion reside. Superstructure ($10,764/m2) and is much heavier. Because of a
refractories can experience temperatures chemical incompatibility between the silica crown
approaching 1600°C when air-fuel fired, and much and the AZS sidewall, a buffer layer of zircon
hotter when oxy-fuel fired. Silica is preferred for (ZrSiO4) is used.
the crown because of its low cost ($646m 2) and its Water-cooled, stainless steel burners are located
ability to be dissolved into the melt if dislodged. on the sidewall of the furnace above the melt. In a
However, silica does not perform well under the regenerative furnace, firing alternates from side
increasingly harsh environment of oxy-fuel firing. to side, and intermittent firing is used to heat the
Mullite has been used for the crown in processing regenerator brickwork, which then preheats the
higher temperature glasses but can create cold combustion air when firing is reversed. The
inclusions in the glass. AZS, which has been stainless steel burner orifice can be either water-
considered for the crown, costs much more or air-cooled and frequently overheats. When
Page 6-3

4. Chapter 6
firing, the burner orifice is cooled by the flow of
fuel and air. During the off cycle, the burner orifice
is cooled by air alone with temperatures
approaching 1040°C. The size and geometry of the
nozzle varies, but a typical orifice is 3.17 cm in
diameter and 10.16 cm long. A longer life ceramic
orifice is sought, but it cannot cost more than $200
to remain competitive with a metal orifice cost of
$150. A typical furnace has 20ioveter a61se
of a gas-fired burner with a rectangular shaped
nozzle is shown in Fig. 6.4.
Stirring and bubbling are used in the melt zone
to improve homogeneity. Stirring is typically done
with a water-cooled stainless steel or uncooled
platinum paddle operating at glass melt tempera-
tures of 1425°C. The lifetime of water-cooled
paddles can be indefinite. While no cooling is re-
quired, platinum paddles are costly and life lim-
ited. Bubbling is accomplished with water-cooled,
high-carbon-steel nozzles located on the floor. Bub-
bler nozzle life is several years; however, when
replacement is necessary, it is easily accomplished
by drilling a sidewall, plugging the slow glass leak,
and then redrilling the sidewall for insertion of
the new bubbler tube that contains the nozzles.
Molybdenum disilicide bubblers have recently
Page 6-4

5. Glass Industry
A cost of $400 for a 6.4-cm-diam, 15.2-cm-long One approach to overcoming these limitations is to
tube with a 1.3-cm-thick wall is desired. preheat the batch/cullet mixture. As shown in
In addition to those for furnace temperature, Fig.6.5, the hot flue gas passes on one side of the
sensors are being developed to measure refractory heat exchanger while the batch powders fall counter
thickness during furnace operation. These sensors to the flue gas stream. Preheating can be provided
are based on acoustic, eddy current, or electrical by using a separate burner or using the heat
capacitance measurements that change over time. available in the exhaust stream of the furnace. In
Sensors imbedded directly into the refractory are both cases, a fan is used to pull the hot gases through
also being considered. Ceramic shields are desired the heat exchanger. Operating conditions of the
to protect the sensors from the harsh environment. preheater include inlet gas temperatures of up to
Particulate emissions contained in the flue gas 982°C and exposure to the by-products of
are removed by either cloth filters or an electro- combustion, which with the flue gas can contain
static precipitator. A conventionally fired regen- high levels of moisture and corrosive vapors.
erative furnace can generate particulate emissions Metallic designs currently used require frequent
approaching 9kg/h at the particle separator . Fans maintenance and are temperature limited. By using
are commonly used to maintain the desired level a ceramic heat exchanger, life could be extended,
of gas velocity and thus ensure the particles re- weight reduced, and temperature capability
main in the gas stream and avoid fallout. While increased. Similar benefits exist with a ceramic fan.
this system is adequate for current furnace designs One benefit of this approach is the ease of retrofitting
and regulatory guidelines, future furnace designs existing furnaces.
and increasingly stringent guidelines provide an A second approach to increasing furnace effi-
opportunity for advanced ceramics with the ciency is to introduce oxygen for firing. The
higher temperature capability and improved cor- improvement in furnace efficiency is a result of
rosion resistance sought for the filter media, fans, the higher flame temperature and diminished ex-
and ducts leading to the particle separator. haust losses caused by the reduction of nitrogen
The maximum production output of a glass flow through the combustion system. The oxygen
furnace is limited by the amount of energy that can for firing can be introduced locally through lances
be supplied inside the furnace to the melt. The limit (tubes) immersed in the melt or by substituting
is reached when the burners are at maximum heat air at the burners for oxygen. Lances are con-
output and electrical boost is at maximum power. structed from water-cooled metals, precious met-
Further increases in heat input at the melt furnace als, or, more recently, molybdenum disilicide.
are limited by the temperature capability of the While the least expensive and simplest to setup,
materials used to construct the furnace and the oxygen injection can result in local hot spots and
increased emissions (NOx) that would be generated. damage to the furnace walls, burners, etc.
Fig. 6.5. Batch and cullet preheating
system. Source: “Integrated Batch and Cullet
Preheater System,” U.S.Department of
Energy fact sheet.
Page 6-5

6. Chapter 6
Pure oxygen–based combustion introduced at Electrically Heated Furnaces
the burners is gaining growing acceptance by the
glass industry, with 87 furnaces, mostly in the Electrical heating is used exclusively in many
specialty glass and fiberglass industry, currently smaller specialty and fiberglass melting furnaces
in operation. Benefits include greater flame because of its lower initial cost and low emissions,
stability, greater glass throughput, reduced fuel even though energy costs remain high. Key
consumption, reduced furnace cost, reduced NOx disadvantages of all electric furnaces are the high
and particulate emissions, and high efficiencies energy cost and short furnace life. Electric furnaces
without the use of costly regenerators, are rebuilt as often as every six months, but
recuperators, or electric boosts. An important because of their small size, down time is limited
disadvantage is the change in characteristics of the to only about two days. An example of a bottom-
exhaust gases. A typical regenerative furnace has entry electrode furnace with a refractory-lined,
flue gas temperatures of 315–482°C at the exhaust water-cooled wall is shown in Fig.6.6. T op-entry
takeoff, with moisture content of 8–10%. electrodes are commonly used to simplify
Recuperated furnaces are slightly higher in both maintenance because of the short life of materials
exhaust temperature and moisture content. Oxy- currently used. Where bottom or sidewall
fuel-fired furnaces exhaust gas at 1204–1426°C and penetration is not desired, boost elements are
have a moisture content of 55–67%. Because of the simply placed over the side of the tank, when
high flame temperatures, corrosive volatiles (e.g., accessible. In the furnace example shown in
boron) are present in the exhaust gases. The high Fig.6.6, the melt exits thr ough a submerged throat
firing temperatures and harsh exhaust gases into a separate temperature-conditioning chamber.
present with oxy-fuel furnaces provide a number Bubblers and stirrers are frequently used to
of opportunities for ceramic materials. improve homogeneity of the melt and eliminate
A number of alternate combustion cycles are bubbles. Materials used in the construction of
being developed in unison with oxy-fuel firing. small specialty glass and fiberglass furnaces
The goal is to arrive at alternatives that are less include precious metals (for bubblers, stirrers,
difficult and costly to integrate into existing fur- plungers) and molybdenum (for electrodes and
naces while still achieving reduced emissions and flow-control hardware). Replacement materials are
increased efficiency. Many of these concepts re- sought that are lower in cost and provide longer
quire advanced ceramics for burners capable of life. Replacement materials are also sought for
higher temperatures, heat exchangers for preheat- electrode holders that do not require water cooling.
ing combustion gases, particle separators for emis- Natural-gas-fired immersion heaters, which could
sion control, and fans to move flue gases.
Fig. 6.6. Electric melt furnace for producing chopped fiber. Source:
“Advanced High-Temperature Material for Glass Applications,” U.S.
Department of Energy fact sheet.
Page 6-6

7. Glass Industry
ultimately replace electrical immersion heaters, in the melt section. Plungers and nozzles used to
will require advanced ceramic outer liners. distribute the molten glass are refractory ceramics
or molybdenum, but they experience high wear
and erosion resulting from glass flow and
6.3 REFINING corrosion. For lower temperature glasses, Inconel
600 is commonly used, with similar problems
The glass-conditioning operation (refining) reported. At higher temperatures, water cooling
occurs in the forehearth and provides molten glass can also be considered.
that is uniform in temperature. The forehearth is A number of opportunities for advanced ce-
generally natural gas fired. Electric boosting can ramics were identified within the melting and re-
be used to increase efficiency and improve fining operations of glass manufacturing and are
temperature uniformity. Water-cooled metal heat summarized in Table 6.1. Many of these opportu-
exchangers are used to help regulate temperature nities directly relate to oxy-fuel-fired furnaces. Al-
and would benefit from advanced ceramics. ternative combustion cycles are also being devel-
Temperature sensing is critical in the forehearth oped in which advanced ceramics are considered
and shares problems similar to those encountered enabling.
Table 6.1. Opportunities for advanced ceramics in glass melt furnaces
Application Industry need Opportunities for ceramics
Refractories More corrosion resistance materials Incremental improvements
potentially through graded
designs
Flue gas ducts High-temperature-capable, Fiber-reinforced ceramic composite
corrosion-resistant materials to accommodate thin wall designs
Particle separators High-temperature-capable bag filters Ceramic hot-gas filter
Greater corrosion resistance
Burners Higher-temperature-capable burner SiC or MoSi2 ceramic matrix
nozzle with longer life composite, thermal barrier
coatings
Sensor shields Uncooled thermocouple and IR Engineered material to
camera shields with longer life accommodate unique
requirements; SiC (above the
melt) or MoSi2 (below the melt)
ceramic matrix composite
potential candidates
Immersion heaters Change from electric boost to Engineered material with a Mo outer
natural gas fired liner below the melt
Electrode supports Longer life blocks, uncooled holders Incremental improvements
potentially through graded
designs
Stirrers Lower cost, uncooled materials MoSi2, ceramic matrix composite or
coating
Plungers/melt flow control Lower cost, longer life materials Engineering ceramic matrix
composite. MoSi2 and mullite
potential candidates
Bubblers/lances Lower cost, uncooled materials Engineering ceramic matrix
composite; MoSi2 and mullite
potential candidates
Glass stop insulation Longer life Incremental improvements
potentially through graded
designs
Heat exchangers Higher temperature capable, longer SiC or SiC ceramic matrix
life materials, uncooled materials composite tubular structures
Fans for air flow control High-temperature-capable fan SiC ceramic matrix composite with
materials engineered attachment
Page 6-7

8. Chapter 6
6.4 FLAT GLASS use the float process to form molten soda-lime-silica
glass into a flat sheet. No applications for advanced
ceramics were identified within the flat glass coating
The U.S. flat glass industry comprises six ma-
operation, and that operation will not be discussed
jor flat glass producers operating 28 furnaces in
further. Unlike the melting furnaces used in other
16states. These plants, which employ about 12,000
glass manufacturing processes, those used for
skilled workers, produce 3.9 million tons of glass
producing flat glass are generally very large. The
per year and achieve sales of $2.1 billion. Plants
refiner is equally large because of the need to
are mainly located near sources of low-cost energy.
eliminate seeds resulting from gas bubbles and other
In 1991 the industry used 55.2 trillion Btu of en-
defects that degrade optical clarity.
ergy derived primarily from natural gas and elec-
Two float glass forming processes are in use, one
tricity. Partly because of competitive pressures
developed by Pilkington Brothers (PB) and one
from developing countries, increases in produc-
developed by PPG Industries, Inc. Major differences
tion efficiency and energy efficiency are continu-
between the two processes involve how the glass is
ally being sought. Over the past 25 years, energy
introduced into the furnace. Essential features of the
efficiency has improved by over 50%, with much
PPG process are shown in Fig.6.7. A typical float
of the energy savings resulting from the use of im-
zone furnace is 49 m long and 9 m wide and can
proved refractories. The typical flat glass plant
hold 909 metric tons of glass. In the PPG process,
costs $100 million to build and has a campaign
the refined glass flows in a continuous, full-width
life of 12 years.
ribbon onto a molten bath of tin contained in an air-
The typical flat glass manufacturing plant
cooled, refractory-lined steel shell. In the PB process,
consists of upstream operations for batching,
the molten glass enters a much narrower region onto
melting, refining, forming, and annealing.
a molten bath of tin and takes a very complicated
Downstream operations include reheating,
flow path before it reaches its full width. In both
reforming, coating, tempering, and laminating.
processes, glass enters at a temperature of 1040°C
Downstream operations can be performed on-site
and exits at a temperature of 600°C. The tin bath
or off-site. Upstream operations are virtually
is maintained at a temperature of 815°C, whereas
identical for all manufacturing plants because all
Fig. 6.7. PPG float glass process. Source: Adapted from F. V. Tooley, Handbook of Glass Manufacture, 3rd
ed., Vol.2, Books of the Glass Industry Div ., Ashlee Publishing Co., New York, 1984, pp. 714–15.
Page 6-8

9. Glass Industry
the steel shell air is cooled to 100°C. An atmosphere densates of tin sulfide vapor can dislodge and fall
of N2plus 5–8% H 2 is used to prevent tin oxidation. on the glass surface. Current procedures call for
Alternate materials considered for replacement of intermittent cleaning of the furnace roof and dis-
the refractory-lined steel/tin bath shell (e.g., posal of the contaminated glass. Globars create a
tungsten and graphite) are either costly, difficult similar problem, but a potential solution is to re-
to fabricate into the desired shape, or lack place them with radiant burners.
oxidation resistance. Few material candidates exist From the float zone furnace, the glass sheet
because of the highly corrosive nature of tin. enters the annealing furnace, more commonly re-
The flow of glass into the float zone furnace ferred to as the lehr, to remove internal stresses
from the refiner is regulated by a fused-silica gate resulting from thermal gradients generated dur-
called a tweel. The tweel meters the glass flowing ing the plate-forming process. The lehr operates
into the float zone furnace and thereby helps con- at temperatures of 200°C, with heating provided
trol the finished product thickness. The finished by gas combustion or electricity. In an electrically
product thickness is also varied by the glass vis- heated lehr, the atmosphere is air; in a gas-fired
cosity, surface tension, and, most important, the lehr, the atmosphere contains by-products of com-
traction forces applied to the edge of the glass rib- bustion. The glass is transported through the lehr
bon by water-cooled stainless steel gears (stretch- on water-cooled stainless steel, fused-silica, or as-
ing drives). Thicker glass can be formed by flow- bestos-containing rollers with dimensions of up
ing the glass against nonwetting, water-cooled to 5 cm in diameter by 2.5 m long (as illustrated in
graphite barriers at the edges. Fig. 6.8). Problems associated with supporting the
Because the life of the tweel is only about ten glass sheet in the lehr include roller surface dam-
months, alternate materials are being sought to age, roller marking, sag between rollers, roller
increase life. In additional to molten glass com- wave distortion, and roller contact heat transfer.
patibility, requirements for the tweel include high- Uniform size and speed of the rollers is critical to
thermal-shock resistance and load-carrying capa- avoid scratching the glass surface. Radiant burn-
bility at molten glass temperatures. Replacements ers are used in many gas-fired lehrs and can be a
for water-cooled materials are also sought because source of high maintenance. Upon cooling to room
of the high level of maintenance and added cost temperature, the glass is cut and packaged for
from corrosion. More corrosion-resistant metal al- downstream processing.
loys (e.g., stainless steel) have been used but are Downstream operations can include a number
undesirable because nickel gets into the recycled of reheating, reforming, tempering, and coating
glass and forms nickel sulfide during annealing. steps. Reheating and tempering occur in furnaces
Conditioned water is commonly used throughout similar to the lehr and only vary in temperature.
a glass manufacturing plant to minimize corrosion. Large volumes of heated air are frequently circu-
The glass temperature and tin melt are main- lated in the lehr to help reduce thermal gradients.
tained by several thousand silicon carbide globars The fans are produced from high-temperature steel
located along the length of the furnace. As the glass with water-cooled drives. Fan distortion reduces
moves down the tin bath, it cools and increases in life at the higher operating temperatures. After the
viscosity, thus allowing lifting from the tin bath glass has been reheated it can be reformed by
by transport rolls. The transport rolls that carry bending on cast fused-silica molds with integral
the glass to the annealing furnace are engineered heating elements. Because of their poor structural
to avoid deforming the glass under its own weight. properties, molds are easily damaged. While they
The rolls, about 30cm in diameter and 4.3 m long, can be repaired, repairs are time consuming and
are constructed of asbestos or water-cooled metal. can be costly, both in down time and the actual
Instrumentation used to monitor the process maintenance. Release coatings or barriers of ce-
includes optical pyrometers for measuring surface ramic or metal films are used to inhibit sticking of
temperature of the glass and tin bath and shielded the glass to the mold. Because the glass relaxes
thermocouples for measuring the subsurface tem- after molding, die design is an iterative process
perature of the tin bath. Measurements of local ve- that can occur over an extended period and be
locity in the tin and glass and of glass thickness, costly.
as well as visualization of surface defects and A summary of opportunities for advanced ce-
stresses, are desirable but have met with limited ramics in flat glass manufacturing is shown in
success. Cooled instrumentation used in the roof Table 6.2. Opportunities for the batch, melt, and
is commonly a source of glass defects because con- refinement operations were previously described.
Page 6-9

10. Chapter 6
Fig. 6.8. Fused silica rollers used in a lehr. Source: Marketing brochure from Air
Liquide America Corp., Walnut Creek, Calif.
Table 6.2. Opportunities for advanced ceramics in flat glass manufacturing
Operation Industry need Opportunities for ceramics
Float zone furnace 1. Longer life tweel 1. Incremental improvements
2. Replacement of asbestos transport 2. Ceramic matrix composite or a
rolls metal/ceramic hybrid
3. Improved materials for stretching 3. SiC or Si3N4 monolithic ceramic
drives 4. Engineered material with
4. Replacement of refractory lined tin anticipated bath redesign
bath 5. Ceramic matrix composite or Si3N4
5. Uncooled sensors 6. SiC or MoSi2 ceramic matrix
6. More efficient gas-fired radiant composite. Thermal barrier
burners coatings.
Annealing, reheating, 1. Longer life air circulation fans 1. SiC ceramic matrix composite with
tempering 2. Improved hot handling materials engineered attachment
3. High-temperature-capable 2. Ceramic matrix composite or
noncontact sensors cermet
4. Low-maintenance radiant burners 3. Si3N4 or ceramic matrix composite
4. SiC or SiC ceramic matrix
composite
Reforming 1. Improved mold materials and 1. Engineered materials manufactured
manufacturing methods using 3-D rapid prototyping
2. Improved mold release coatings or technology
film barriers 2. Ceramic-coated metal or ceramic
3. High-temperature-capable cloth
noncontact sensors 3. Si3N4 or ceramic matrix composite
Page 6-10

11. Glass Industry
6.5 CONTAINER GLASS Downstream operations can be performed on-site
or off-site. The batching, melting, conditioning,
The container glass industry, which is made up and annealing operations are similar to those dis-
of 64 plants in 25 states, employs 30,000 skilled cussed previously. No applications for advanced
workers, produces 12.3 million metric tons of glass ceramics were identified within the coating opera-
per year, and achieves sales amounting to $5 bil- tion, and thus that operation will not be discussed
lion annually. Plants are located mainly near prod- further.
uct markets. In 1992 the industry used 119 trillion
Btu of energy, 79% of which was derived from Forming
natural gas. Competitive pressures occur from
market preferences for lighter weight container After conditioning to the desired temperature
materials (e.g., plastic and aluminum), the high and uniformity, the molten glass is transferred to
cost of end product distribution resulting from the the forming operation through the gob feeder. The
weight of glass containers, and the lower cost of gob feeder is an integral part of the forehearth and
labor in developing countries. The market share includes a mixer, plunger, orifice, and shears, as
lost to alternate materials is being reduced by regu- shown in Fig 6.9. The gob feeder controls the
latory concerns and the higher cost of “product weight, temperature, and shape of the gob—all of
stewardship.” The distribution cost of glass con- which are critical to container quality. Gob-form-
tainers is benefiting from the development of ing rates can easily approach 300 gobs per minute.
higher strength glass and improved manufactur- The materials of construction for the gob feeder
ing methods developed to produce thin-wall, include water-cooled metals (cut-off blades), pre-
light-weight containers. Increased production and cious metals (stirrer), and refractories (plunger) to
energy efficiencies are continually sought to effec- accommodate glass gob temperatures approach-
tively compete against developing countries. The ing 1100°C.
typical container glass plant costs $70 million to The gob next enters the forming operation
build. The last container plant was constructed in where the heat is extracted from the glass in a con-
1981. Because of their high level of maturity, manu- trolled manner during shaping. Several processes
facturing processes are essentially the same in all are available to form the gob; the process selected
plants. is based on composition, container shape, size, and
The typical glass container manufacturing production rate. The press-and-blow process com-
plant consists of upstream operations for batching, monly used to produce large-mouth containers is
melting, conditioning, and forming and down- illustrated in Fig.6.10.
stream operations for coating and annealing.
Fig. 6.9. Single gob feed cycle. Source: Reprinted from F. V. Tooley, Handbook of Glass Manufacture, 3rd
ed., Vol.2, Books of the Glass Industry Div ., Ashlee Publishing Co., New York, 1984, p. 601.
Page 6-11

12. Chapter 6
Fig. 6.10. Container press-and-blow forming operation. Source: Reprinted from F. V. Tooley, Handbook of
Glass Manufacture, 3rd ed., Vol.2, Books of the Glass Industry Div ., Ashlee Publishing Co., New York, 1984,
pp.616–17.
In the press-and-blow forming operation, the generates a high level of noise, liquid cooling may
gob enters the mold and is first pressed into a be employed as an alternative. A disadvantage of
blank. During the pressing operation, plunger liquid cooling is the increased maintenance needed
temperature approaches 550°C. Because the for leakage and corrosion. To increase cycle times,
plunger material is not as hard at these mold temperature can be further reduced by
temperatures, the plunger surface is easily scored precooling the air or liquid. Mold materials having
by the chilled glass at the interface during higher thermal diffusivity have also been used (e.g,
extraction. The blank is removed from the press copper-based alloys in neck rings). To reduce
mold, reheated, and then placed into a second mold-to-glass adhesion, glass marking, and
mold for blowing into final shape. When removed oxidation of the metal mold, the mold is swabbed
from the blow mold, the glass cools to 780°C. with a lubricant between cycles. Because mold
Production rate is limited by the rate of heat temperatures approach 500°C, the lubricants used
extraction and can approach 100 bottles per minute generate smoke and vapor and sometimes ignite
when multiple molds are used with a single gob spontaneously. Alternate approaches available
feed system. The forming operation is the single include nonmetallic solid film lubricants or the
largest contributor to both the cost and quality of injection of acetylene into the hot mold between
the finished glass container, and molds represent cycles, which cracks and forms a thin deposit of
a large part of that contribution. carbon throughout the mold interior.
Requirements for mold materials include low Alternate materials of construction for molds
cost, capability for producing complex shapes, low currently being considered include nickel-based
distortion in use, ability to take and retain a fine superalloys, graphite, and ceramic coatings.
surface finish, low thermal expansion, adequate Asbestos-containing materials were used for a
hot structural properties, scale and oxidation period but have been replaced by graphite for
resistance, high thermal diffusivity, and high reasons of regulatory compliance. Nickel-based
resistance to thermal cycling. The primary mold superalloys are being used in areas where higher
material is cast iron. Low-pressure air-cooling is hot hardness and strength is desired (e.g.,
used to accelerate heat extraction and improve plungers, baffles, funnels, blow heads, bottom
temperature control. However, because air cooling plates, and plugs). As with flat glass, nickel
Page 6-12

13. Glass Industry
contamination can be a problem with container bide) onto existing hot handling surfaces, espe-
glass. cially the plunger, by a variety of processes. While
Graphite is a preferred material for hot han- more difficult and costly to machine, resistance to
dling because it does not wet, mark, or adhere to oxidation and scaling occurs when applied to
glass; provides inherent lubricity; does not scale; graphite and cast iron, respectively. Spallation of
retains its strength at temperature; and is low cost, the coating can also occur, thus increasing the risk
easily machined, and environmentally safe. Ap- of glass contamination.
plications include blow-head inserts, take-out After molding, the containers are placed into
tongue inserts, sweep-outs, lehr-loading bars, gob the lehr for removing stresses (see Fig. 6.11). The
deflectors, and conveyor guides. Grades of graph- lehr is similar to that previously discussed for flat
ite used are selected based on their application. glass; the primary difference is the use of a con-
Porous graphite is desirable in the lehr to avoid veyor belt to move batches of the container glass
thermal checking, which is caused by rapid heat through the hot zone in a controlled manner. The
transfer from glass containers. Higher density conveyor is fully contained within the furnace to
graphite is used in molds where increased strength reduce heat loss and increase energy efficiency.
and hardness are desired. While graphite offers With the exception of coatings, no evidence was
many advantages, a key disadvantage is its lim- found of advanced ceramic materials usage in the
ited life due to poor oxidation resistance and low forming of glass containers. Opportunities exist,
strength. A greater number of glass defects have however, because of the increasing emphasis on
been seen when ceramics have been used. lightweight glass containers, continuous produc-
As containers become thinner, higher hardness tion, and improved energy efficiency as competi-
materials are desired for mold surfaces to reduce tion increases from abroad. Specific opportunities
contamination and maintain surface finish and include replacement of graphite in areas of hot
critical dimensions. Higher hardness materials also handling, with alternate materials providing
provide reduced die wear in machines where pro- higher toughness, greater hardness, and improved
duction rate is being increased in place of added oxidative stability. Lower cost materials are sought
capacity. One solution is the application of hard to replace precious metals used in the forehearth
coatings (e.g., tungsten carbide and titanium car- and gob feeder. Materials with higher thermal
Fig. 6.11. Gas-fired glass annealing furnace with an internal belt return. Source: Re-
printed from F.V .T ooley, Handbook of Glass Manufacture, 3rd ed., Vol.2, Books of the Glass
Industry Div., Ashlee Publishing Co., New York, 1984, p. 682.
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14. Chapter 6
diffusivity and hot hardness are sought for molds wool. Total energy consumption for the fiber glass
to extend life and reduce cycle time without ex- industry in 1991 was 59.5 trillion Btu, a value
ternal cooling. Higher-temperature-capable mate- exceeded only by the container industry. Unlike
rials are desired throughout the mold machine for container or flat glass, a large amount of energy
reduced maintenance and improved quality. New consumed is used to fabricate fiber from molten
sensors and sensor shields are sought to measure glass. Whereas both markets are capital intensive,
gob temperature, temperature uniformity, and the textile fibers industry is more so because of its
heat extraction during shaping. In all cases, ad- greater dependence on precious metals. Glass
vanced ceramics are central to the solution. Be- quality requirements also vary, but textile fibers
cause they are compatible with molten glass, ad- production requires much higher quality glass and
vanced ceramics of choice are oxide-based or ce- greater use of process control. Key drivers for the
ramic composites having predominate oxide use of new technology in the fiber glass industry
phases. Leading candidates are alumina, mullite, include reduced cost, increased throughput,
and silicon carbide–alumina composites. Molyb- reduced emissions, and a competitive advantage.
denum disilicide–based composites are also good Competitive pressures primarily exist from organic-
candidates for many of these applications. Silicon- based products. Processes used to manufacture fiber
based ceramics are candidates for hot applications glass can be divided into batching, melting, and
requiring optimum structural properties that are fiberization. Each process and opportunities for
not in contact with molten glass. advanced ceramics is discussed in detail in the
A summary of opportunities for advanced ce- following sections.
ramics in container glass manufacturing is shown
in Table 6.3. Batching
The batching operation is similar to that used
6.6 FIBER GLASS for producing other glass products with the
exception of different compositions and finer raw
The fiber glass industry is made up of two material particle size. Because particle size is finer,
primary markets: wool insulation and textile emissions control and contamination by ceramics,
fibers. Wool insulation is a short-fiber product used metals, organics, and alloys are of greater concern.
by the construction industry. Textile fibers are a A key ingredient for manufacturing fiber glass is
continuous-fiber product principally used as a boron, which provides reduced melt viscosity and
polymer matrix composite reinforcement. improved glass durability but is volatile and
Together, the four major producers of wool and condenses at undesirable locations. The content
the six major producers of textile fibers employ of boron in glass fiber ranges from 5 to 10 w %
about 16,000 workers. A typical producer can and represents 50–75% of the total cost. A high
operate as many as 27 glass melt furnaces. In 1991, percentage of recycled container and flat glass is
fiber glass made up 9% of all glass produced and used to produce wool fiber glass, for which quality
accounted for 21% of the total market value. In requirements are less stringent. Recycled fiber
1981, 71% by weight of all glass fiber shipped was glass is not currently used because of the boron
present.
Table 6.3. Opportunities for advanced ceramics in container glass manufacturing
Application Industry need Opportunities for ceramics
Radiant burners Longer life burners SiC or SiC ceramic matrix
composite
Hot handling Higher toughness materials with Ceramic matrix composite, cermet,
improved oxidation resistance or carbide coatings
Gob feeder Lower cost stirrer materials, reduced Engineered ceramic matrix
erosion plunger materials, and composite plunger, cutter, and
uncooled cutter materials stirrer; mullite ceramic composite
plunger
Molds Higher hardness materials, higher Engineered material or coating
thermal diffusivity
Sensors Longer life thermocouple shields Si3N4 or ceramic matrix composite
Page 6-14

15. Glass Industry
Melting homogenization section followed by a hearth for
distribution to the different fiber-forming
In addition to batching, the other major process operations. Conversion to oxy-fuel firing is also
that fiber glass shares in common with other glass occurring as a result of demonstrated improvements
industries is the melting operation. The differences in energy efficiency and emissions.
are limited to processing challenges resulting from A number of opportunities exist for advanced
glass compositions and an overall smaller furnace ceramics within the melter. Higher temperatures
size. The low alkali content of textile fibers requires of the melt in combination with the low alkali
higher melting temperatures and greater fiber content force the use of environmentally sensitive
particle size to ensure homogeneity without the chrome-containing refractories. Chrome-
benefit of fluxing additives. Both textile and wool containing refractories are also electrically
products include boron, which can foul furnace conducting, drawing current away from the glass
hardware and increase particulate emissions. Glass and reducing refractory life. The by-products of
melters include regenerative, direct-fired, and combustion, and higher temperatures of
recuperative furnaces. Large recuperative gas-fired combustion with oxy-fuel firing, also limit refractory
furnaces are still the primary source of melters. The life. Improved glass stop refractories are sought for
campaign life of gas-fired furnaces is 7–10 years; reduced maintenance. Precious metals (platinum/
rebuilds are costly and time consuming. Some rhodium) are used throughout the melter (gas
conversion to electric and electric-boosted gas-fired bubblers, plungers, thermocouple shields) with a
furnaces has occurred to reduce gas consumption, per furnace cost approaching $10 million. Platinum/
increase melt efficiency, and reduce emissions rhodium is used because of its high-temperature
(SOx, NOx, particulate). Electric furnaces use top- capability and stability when in contact with glass
or bottom-entry electrodes, the former being easier at temperatures exceeding 1371°C. Disadvantages
to maintain. The campaign life of a electric furnace include high cost, low creep strength, and uncertain
is six months; atypical r ebuild takes two days to supply. Alternatives to platinum/rhodium
complete. An example of a cooled metal wall, evaluated and deemed unsuccessful include
bottom-entry electrode melter is shown in Fig. 6.12. germanium-doped molybdenum disilicide and
The melt exits through the submerged throat into a Inconel. Ceramic-based material options include
Fig. 6.12. Example of a bottom entry electrode electric glass melter. Source: “Advanced
High-Temperature Material for Glass Applications,” U.S. Department of Energy fact sheet.
Page 6-15

16. Chapter 6
high alumina or zirconia composites with high shown in Fig. 6.13, a single stream of molten glass
purity and density. Ternary phase oxides could falls into a rapidly rotating cylinder (centrifuge)
also be considered, but phase instability is of made of a high-temperature base metal alloy. The
concern. Another option is higher creep resistant cylinder has a 12- to 36-in. diameter and contains
nonoxide composites (i.e., silicon carbide with a 10,000–30,000 laser-drilled holes having diameters
platinum/rhodium coating for molten glass of 0.1–0.015in. Operating conditions include
compatibility). steady state temperatures of 2000°F while exposed
Molybdenum is also a primary material used in to air. Typical lifetime is 40–100 h. The base metal
fiber glass melters. Applications include electrodes alloys used are generally proprietary, investment
and glass flow control devices. For lower cast, and extensively recycled to reduce cost.
temperature applications (< 1315 °C), Inconel 600 Alternate materials are sought with increased
has been used. In both cases, premature failures creep strength, resistance to corrosion (sulfidation)
have occurred due to excessive corrosion and wear. and oxidation, and reduced cost. As the molten
Advanced ceramics are sought that can operate glass is driven through the perforated cylinder,
uncooled, above or below the melt line, and exhibit downwardly directed high-velocity gas jets
high-temperature creep resistance. Opportunities attenuate the flow and form fibers of varying
for advanced ceramics also exist for oxy-fuel-fired lengths. The fibers are sprayed with organic
burners and in shields for sensors being developed binders and collected onto a moving belt where
to detect refractory wear. they are transported to a curing oven. Alternate
approaches to the rotary process for forming wool
Fiberization include attenuation of a molten stream passed
through sieve-like platinum alloy bushings and
Molten glass leaves the hearth and enters the remelting of drawn fiber by a high-velocity burner.
fiberization process directly or after an In the continuous textile forming process
intermediate marblizing process. Marblizing is shown in Fig. 6.14, glass marbles are fed through
commonly used for textile fiber production when orifices in an electrically heated platinum alloy
the forming process is not located in close bushing. A typical bushing has a cross-section of
proximity to the melter. Fiberization occurs by 25–50 cm by 18–20cm with a depth of 5 cm. About
attenuating a melt steam with air, steam, or 100 dies are used with a single melter, and a typical
combustion gas (for wool) or by drawing molten production site contains four melters. Each
glass through precious metal bushings (for production unit processes 400–3,000 filaments
24textiles). For the wool pr ocessing example simultaneously, with fiber diameters ranging from
Fig. 6.13. Example of a wool forming process. Source: Reprinted from F. V. Tooley,
Handbook of Glass Manufacture, 3rd ed., Vol.2, Books of the Glass Industry Div ., Ashlee
Publishing Co., New York, 1984, p. 721.
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17. Glass Industry
reliability. Lower cost solutions are also sought for
the platinum sieves and bushings. Oxides
dispersion strengthened with yttria have been
evaluated as a precious metal replacement with
promising results. Platinum-coated molybdenum
was also considered but failed during testing.
A summary of opportunities for advanced ce-
ramics in fiber glass manufacturing is shown in
Table6.4. Opportunities for the batch operation ar e
similar to those discussed for container and flat
glass.
Fig. 6.15. Ceramic textile fiber handling
guides. Source: Reproduced from Kyocera’s Web site
at http://www.kyocera.com.
Fig. 6.14. Example of a textile forming process.
Source: Reprinted from F.V . Tooley, Handbook of
Glass Manufacture, 3rd ed., Vol.2, Books of the Glass 6.7 SPECIALTY GLASS
Industry Div., Ashlee Publishing Co., New York,
1984, p.725.
This market segment is a catchall for glass
products not contained in the flat glass, container
3.5 to 20 µm. Bushing life is about six months, at glass, and fiber glass market segment. An impor-
which time the bushing is recycled because of the tant difference is that the specialty glass produc-
high cost of raw material. Life is limited by hole ers, unlike the other glass producers, work with
wear and sag. Operating conditions include steady higher temperature glasses, aluminosilicates, and
state temperatures of 1200°C in air. Forced- borosilicates, as opposed to soda lime glass. While
convection air cooling is used prior to the sizing compositions vary, processing methods are simi-
application to increase productivity. A winding lar to those previously discussed. The markets
drum collects the filaments and applies a traction served are very broad, with as many as 60,000 dif-
force to stretch the fibers as they are pulled through ferent products, but volumes are small. A large
a mechanical device that applies an organic sizing number of specialty glass producers are small,
and twists multiple filaments into a single strand. owner-managed, and labor intensive. Because of
Depending on the application, additional the nature of the business, the technology being
processing is performed on the multiple strands. used is frequently developed in-house and re-
Materials commonly used to handle and guide hot mains highly proprietary.
filaments are carbon based. Alumina and silicon Specialty glasses have very specific chemical
nitride guides are used for sizing application and compositions with clearly defined physical and
twisting where high wear can occur. Typical fiber chemical properties. Furnaces are generally special
handling guides are shown in Fig. 6.15. Alternate designs and principally oxy-fuel fired. To further
materials are sought for fiber handling that exhibit improve energy efficiency, cullet preheaters are
many of the characteristics of monolithic ceramics being evaluated. Melter capacities of 45 metric tons
but have improved toughness for increased are considered large. Because of the high melting
Page 6-17

18. Chapter 6
temperatures, refractories and thermocouple product. During the forming operation, molds are
sheaths are a problem. Infrared cameras are generally superalloys. Ceramic molds were
frequently used in place of short-lived considered but shown to increase the presence of
thermocouples for temperature control, and fiber stones in the finished glass. Natural-gas-fired
optic systems are being developed. Because of metal radiant burners, which are commonly used
their small size, convection currents are used in during forming and annealing, continually require
place of bubblers to homogenize the glass. A range a high degree of maintenance.
of high-temperature base metal alloys and A number of niche opportunities exist within
precious metals are used throughout the melt and the specialty glass industry. Opportunities
forming steps (e.g., platinum paddles used in the common among glass market segments also exist,
melter for homogenization). After the melting but operating conditions both above and below
process, the glass goes in several different the melt are much harsher in specialty glass.
directions for forming, depending on the final
Table 6.4. Opportunities for advanced ceramics in fiber glass manufacturing
Operation Industry need Opportunities for ceramics
Melting 1. Improve refractories 1. Incremental improvements
2. Precious metal replacement 2. Mo based engineered material
3. Longer life electrodes 3. Mo based engineered material
4. Longer life flow control materials 4. Mo based engineered material
5. Sensor shields 5. Mullite or Mo based engineered
6. Oxy-fuel-fired burner nozzles material
6. SiC or MoSi2 ceramic matrix
composite; thermal barrier coating
Wool forming 1. Low-cost centrifuge material 1. Mo based engineered material
2. Precious metal replacement for 2. Mo based engineered material
sieves and bushings
Textile forming 1. Precious metal replacement for 1. Mo based engineered material
bushings
2. Improved fiber-handling materals 2. Ceramic matrix composite
Page 6-18

19. Glass Industry
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Emissions Concerns,” Ceramic Bull. 74(4),
Publications 95–99.
Tooley, F. V. 1984. The Handbook of Glass Manufac-
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Proceedings, documenting a workshop York.
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February 27, 1997.
U.S. Department of Energy, Office of Industrial
Energetics, Incorporated September 8, 1994. Technologies January 1996. Glass: A Clear
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Glass Industry: Schuller International, Inc.”
Energetics, Incorporated October 27, 1994.
Information Supplied by Academia and the
“Materials Needs and Opportunities in the
Glass Industry
Glass Industry: Libby Owens Ford Com-
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Technol. 31(3), 85–88. Corporation
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Geiger, G. 1993. “Recent Advances in Glass Theodore Johnson, U. S. Department of Energy
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Pat Moore, Combustion-Tec, Inc.
1997. Proceedings of the Glass Technology
Dr. Robert E. Moore, University of Missouri-
Roadmap Workshop held in Alexandria, Va.,
Rolla
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Frederic Quan, Corning, Inc.
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Phil Ross, Consultant
Gridley, M. May 1997. “Philosophy, Design and Dr. Robert Speyer, Georgia Institute of
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