T h e technologist has had an increasing part in the glass industry since it emerged from the craft stage in the period fkom 1900 to 1920. Many operating difficulties were encountered during the transition period fkom hand to machine operation and in the subsequent development period during which present-day manufacturing methods were evolved. Although the regenerative furnace is unchanged in principle from Simens' original design, modifications have been possible as better materials of construction have rendered the use of higher operating temperatures practical. Practically all processes involved in manufacture of glass have been made continuous. Possible future improvements are combination gas and electrically heated furnaces, control of circulating currents in melted glass, batch treatments permitting use of finer grain size raw materials, and better refractories.
I
N THE early history of glassmaking, when fabrication was
done by hand, furnace capacities were low and fuel costs were moderate. Knowledge of the art was largely acquired by practical experience, and there was little incentive to spend money for improvement. The regenerative furnace had been invented by Siemens, but much of the glass made a t that time was melted in small inefficient direct-fired units. Between 1900 and 1920, the important mechanical inventions of Owens, Colburn, Fourcault, Bicheroux, Danner, and others tremendously increased the demand for glass of greater uniformity and higher quality and forced the adoption of larger, better designed furnaces of higher capacity. Duringthisperiod, the industry emerged from the craft stage when a group of glass workers might pool their resources, build a furnace with their own hands, and start a business. Glassmaking now demanded adequate financing and well-engineered units. ROLE OF ENGINEER AND CHEMIST
Despite the often open opposition of practical glass men, the services of the engineer and chemist were called upon to an increasing extent by the industry. Alexander Silverman of the University of Pittsburgh once stated that in 1902 there were eight chemists employed by the glass industry. The profession of ceramic engineering began during this period and has developed into an important branch of technology. The mechanical, electrical, and metallurgical engineer has been called into the industry as well as the physicist and electronics expert. DIFFICULTIES OF MACHINED PRODUCTION
The continuous processes with their round-the-clock, 365-dayper-year operation often produced glass of poor quality. These interruptions were sometimes unexplainable but were in the main due to increased production rates, more rapid deterioration of refractories, and the demand for higher quality glass. Even with the help of technical men, the defects originating from the need for increased production were not corrected in a day. The 20 years following the introduction of automatic
172
machines were filled with the intense efforts of everyone concerned with production to apply the necessary corrective measures. HIGHER FURNACE TEMPERATURES
To melt the increased tonnages demanded, furnace temperatures were slowly increased through the years. The average melting temperatures of 2450' to 2550' F. in 1910 to 1915 had been in many cases increased to 2850" to 2900' F. by 1925. In line with this increase in temperature came a corresponding increase in output per square foot of melting area in the furnace. The average container furnace of 30 years ago required 16 square feet of melting area per ton of glass produced per 24 hours. A corresponding output of better glass is now melted on 4 to 5 square feet. Superficially, the modern continuous glass furnace varies little in appearance from the furnaces of 30 years ago. Because of space requirements of the forming machines, they are built a few feet higher above the operating-floor level. This permits regenerators to be built larger and increases the furnace efficiency somewhat. IMPROVEMENT IN REFRACTORIES
The important difference in the present furnace lies in their superior materials of construction. I n spite of harder service on refractories, the operating life of melting furnaces has been materially increased. Remarkable strides have been made in the quality of refractories used, both in the furnace superstructure and below the glass line. The basic materials, silica, alumina, and zirconia, are still the most important constituents of refractories. They are now of a much higher degree of purity than formerly and are more scientifically compounded. The introduction of electrically fused refractories exerted a profound influence on the glass industry. When these blocks first appeared on the market, they secured rapid acceptance. Following the appearance of the electrocast block, rapid improvements were made in the quality of burned tank blocks and superrefractories for port and sidewall use. Successful efforts are being made to include the costlier mate-
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 46, No. 1
-Ceramics rials such as zirconium oxide, chrome oxide, and tin oxide in refractory bodies having stmillgreater resistance to glass attack. FORMING MACHINE REQUIREMENTS
.4utomatic forming machines working between fixed teniperature limits placed new operating requirements on the furnaces to which they were attached. Along with increased production rates, it became necessary to hold the temperatures in all parts of he furnace within very close limits. The operation of a glass furnace has been likened to the sailing of a ship, in that both are dependent on weather conditions such as temperature, humidity, and wind direction. The aim has been to make the furnace operator independent of changes in these atmospheric conditions, in order to reduce fuel costs and maintain uniform operating conditions. A11 furnace installations now include indicators and recorders for temperatures in the melting end, working end, checkers, and flues. These have been supplemented in many cases by glass temperatures taken by means of thermocouples encased in platinum tubes which extend through the side walls of the furnace or project downward from water-cooled tubes extending into the furnace above the metal line. Also used in increasing numbers are meters for fuels and combustion air. Power-driven fans provide a positive source of combustion air and increase furnace capacity. Fuel-air ratio meters which automatically control this mixture a t a predetermined figure are also found in increasing numbers. The control of furnace pressure is now considered important for successful operation, aa is the maintenance of a constant glass level in the furnace. Both these functions are performed successfully by accurate sensing instruments. Induced draft by means of powerdriven fans is also finding favor in many furnace installations. FUELS USED IN GLASS MELTING
Of the three fuels used in glass melting-natural gas, producer gas, and oil-natural gas has outstripped the others because of its widespread distribution and more moderate cost, although oil or butane (or both) is usually required to provide standby service. Air cooling of the flux blocks along the glass line is generally used, and, in a few instances, the top 3 inches of the flux blocks have been replaced with water coolers to prevent the severe undercutting a t the metal line. As the latter method entails from 5 to 10% higher fuel consumption, it has not met withgeneral acceptance. RAW MATERIALS
The raw materials of ordinary glass manufacture have changed very little with the years, although many new materials are used to produce the special glasses required for modern uses. All raw materials have been greatly improved as to purity and grain size. The old shovel and wheelbarrow methods of handling batch have passed through several designs of mixers and conveyors to perhaps the ultimate one of continuous-cycle automatic weighing and can conveying, which is the preferred method of preventing segregation a t present. With a few notable exceptions, batch is introduced into the furnace in the dry state. The methods of introduction include the time-honored one of floating batch piles into the furnace from a dog house and the later ones of screw- and pusher-type feeders. Each of these methods has its adherents, and in some cases methods that work well on furnaces of moderate capacity fall down when installed on furnaces which are pushed to the absolute limit. Briquetting, which appeared to have such a promising future 15 years ago when adopted by the Ford Motor Co., has been utilized very little. This is no doubt due to the added cost of batch preparation involved and the added complexity of the equipment necessary for large capacity installations. January 1954
and Glass-
I-MPROVEMENTS IN FABRICATING EQUIPMENT
In keeping with the refinements of the melting process, drawing, forming, rolling, annealing, grinding, and polishing equipment has also been improved. Marvelously complicated container and ware machines are in use today, perfected by the efforts of many men working in many groups to produce machines best adapted to the particular types of ware in which they were interested. Plate-glass rolling speeds, 80 to 90 inches per minute 10 years ago, are now crowding 300 inches per minute and the limit is not in sight. Grinding and polishing operations, also increased correspondingly in output, are being combined into straight-line production with rolling and annealing operations. This process involves a continuous ribbon of glass 8 feet wide and approximately 900 feet long, both surfaces of which are ground and polished simultaneously. Light bulbs formerly blown by hand are being produced a t rates between 20,000 and 30,000 per hour from a continuous ribbon of hot glass. Hundreds of tons of glass are being formed into fiber, a brand-new product developed by scientific research within the past 15 years. I n general, intermittent processes are becoming continuous; semiautomatic operations are being made automatic; and science is delving into the ultimate structure of glass to bring it more fully under control and permit its further exploitation. INVESTMENT AND PRODUCTION
Twenty-five years ago, an investment of $15,000 to $20,000 would allow a group of hand workers to get into business; now an investment of $2,000,000is required to build a one-unit plateglass plant. Investment in the container industry alone is estimated a t around $400,000,000. The production of containers in 1902 was around 10,000,000 gross; in 1950 this figure reached 150,000,000gross. I n 1950, the per capita consumption of containers stood as follows: United States 69, England 5 , Scandinavia 1.2, all other countries less. FUTURE DEVELOPMENT
As to future development in the glass industry, the adoption of combination gas and electric heating for melting furnaces is already under way. It is believed that the output of existing furnaces can be increased by the means from 30 to 50%. So far, large installations of this type have been limited to areas having low-priced electric power. A furnace of this type has been recently put into service on the East Coast. I t seems inevitable that the use of consolidated or briquetted batch will become more widespread, to extend the life of the costly high-production units now in use and to permit the use of lower quality raw materials when necessary. Precious metals, such as platinum and high-heat-resisting basemetal alloys, now used extensively, will find greater employment. Steps have already been taken to reduce the housing requirements of large furnaces from complete enclosures with costly ventilators to simple structures with low roofs and open sides. With wider gas distribution, manufacturing units will probably be installed nearer centers of consumption to reduce hauling charges on finished products. Regarding the development of new glass products, H. E . Simpson, of Alfred University, comments as follows ( 1 ) : Because the combination of glass fibers and plastics has resulted in many useful products, new plastics are constantly being invented. These products are often stronger and lighter in weight than the steel parts they are designed to replace. Continued research with photochemical glass has produced a new process of chemical machining. The method consists of forming in the glass itself a three-dimensional photographic image. The resultant crystalline areas in the glass contain lithium silicate crystals which are readily dissolved by hydrofluoric acid while adjacent vitreous regions are relatively resistant to attack. The rate of solution of the exposed sections after development is aome 50
INDUSTRIAL AND ENGINEERING CHEMISTRY
173
I
times faster than that of the adjacent unexposed glass. Pieces can be produced by this process which are so intricate that their manufacture by mechanical means would be impossible. Kew glasses are being developed that will transmit or retard radiations of specific frequencies. Glasses of improved faculties for the absorption of infrared and ultraviolet rays have been produced as well as glasses capable of absorbing slow neutrons. Glass is in use for automobile windshields covered with a transparent electric current-conducting film that will generate enough heat to deice itself. A glass that shuts out intense heat has been developed by permanently bonding a thin transparent film to the outer side of glass panels. The glasses used are the borosilicate type in which the coatings are similar to those used for electrical conductivity.
With the coated slide toward the source of heat, 60% of the infrared rays are reflected. Glass is being fabricated in fibers 2 or 3 microns in diameter and in 1600-pound windows for supersonic wind tunnels. There is apparently no limit to the horizons for the uses of glass. The industry is in strong capable hands and expansion will undoubtedly continue. LITERATURE CITED
(1) Simpson, H. E., J . Can. Cerum. SOC., 21, 52-7 (1953). RECEIVED for review March 11, 1953.
ACCEPTED
July 7, 1963
Recent Developments in Radiation-Sensitive Glasses S. D. STOOKEY Corning Glass Works, Corning, N. Y .
Photography in glass is growing in new applications, photosensitive compositions, and processes. Brief descriptions are given of the nature and present applications of photosensitive metal-colored and opal glasses, a glass for photoengraving and chemical machining, and a gamma-ray dosimeter glass. A new process, described here for the first time, converts a photosensitive glass to a high-strength crystalline material. Another new process reproduces photographic images in ordinary window glass, providing permanence and dimensional stability.
HE announcement of the first practical photosensitive glass
Tis,in 1947 has been followed by development of other types of photosensitive glass, new related products, and applications. Some of these have been logical developments from the same research, and others, such as gamma-ray dosimeters developed by Schulman, Weyl et al. ( 1 , 5 ) , have resulted in answer to the requirements of work with atomic energy. This paper reviews the present status of the previously reported glasses and describes new developments.
tion is nucleated by submicroscopic silver particles formed photographically within the glass. Commercial uses of photosensitive opal are based on its ability to control and diffuse light, the beauty and versatility of the three-dimensional photographic design, and durability of the material. It has been used in architecture (decorative lightdiffusing windows in the United Nations Assembly Building; interior and exterior wall facings), in lighting, and in appliances of various kinds in the form of nonglare lighting units, name plates, and clock dials.
PHOTOSENSITIVE METAL-COLORED CLEAR GLASS
Two glasses of this type (Corning Code 8600) are in commercial production as polished plate. One, in which the photographic image may be colored blue, purple, or ruby-red, contains gold as the photosensitive coloring agent. This glass is designated photosensitive “red-blue.” The other, designated as “sepia,” develops a red-brown image consisting of gold and palladium. The photograph is transparent, three-dimensional, and as permanent as the glass itself. The chief commercial use is in reproduction of portrait and scenic photographs, but these glasses are also being employed in decorative and commemorative windows and illuminated photomurals. PHOTOSENSITIVE OPAL
Photosensitive opal glass (Corning Code 8601) develops a threedimensional white translucent image, in an otherwise clear matrix. It is manufactured as flat rolled or as polished plate. The image consists of microscopic light-diffusing crystale whose precipita-
174
CHEMICALLY MACHINABLE PHOTOSENSITIVE GLASS
A photosensitive opal glass (Corning Code 8603) in which the three-dimensional image is highly soluble in dilute hydrofluoric acid was first described a t the XIIth International Congress of Pure and Applied Chemistry (3). This glass can be cut, drilled, photoengraved, or sculptured by forming the photographic image and immersing the glass in dilute hydrofluoric acid. This new glass appears to h a d an important future in the photoengraving field, and even to broaden the scope of photoengraving, because-in contrast to the homogeneous metals now employed-the image can be etched up to 50 times faster than the unexposed glass. This makes possible, by a relatively simple process adapted to mass production, more accurate reproduction and deeper cuts than can be obtained with metals. It now appears that this process may have broad applications in the printing industry, as halftone plates, engraving dies, and master plates for electrotypes, rubber printing, and newspaper printing.
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
Vol. 46, No. 1
