Plate glass

Plate glass is a thick, transparent glass that is stronger than window glass and has little distortion. Until the 1960s, plate glass was manufactured using casting and rolling large sheets that were ground and polished. Sizes are between ¼ and 1 ¼ inches thick and can span up to 14 feet wide and 20 feet long. The float process, introduced in 1959, eliminated the need to grind and polish plate and glass. Strength, clarity, and availability of large sheets made plate glass ideal for store front windows.

History
In the United States, until the mid 19th century, plate glass had to be imported, mostly from France. Even with sufficient raw materials, plate glass was not highly produced in America. In the 1850s, experimentation led to the development of what is known as rough plate glass. The Cheshire Glass Company and the Lenox Plate Glass Company in Massachusetts used French and English methods for producing polished plate glass. John B. Ford opened a glasswork company in Indiana, where he imported grinding and polishing machinery from England. Polished plate glass was first successfully and continuously manufactured under the supervision of Ford in his Indiana factory. In 1992, Ford’s firm became known as the Pittsburgh Plate Glass Company when he constructed a factory near Pittsburgh, Pennsylvania. At the end of the 19th century, manufacturing of plate glass grew quickly. At the beginning of the 20th century, only 15% of plate glass was being imported.

Manufacturing Process
Sand, limestone, and soda ash are the traditional materials used in producing plate glass, though carbon, arsenic, and manganese are often added. Because these materials were readily available in so many locations across the country, a rise in plate glass manufacturing attempts grew rapidly. Once a system of casting, cooling, grinding, and polishing had been established, little changes were made for almost 40 years, except for ongoing efforts to increase the size of plates cast. The manufacturing of the plate glass pieces depended heavily on the use of mechanical appliances. Raw materials were melted in a furnace, and then a pot of molten glass was lifted from the furnace and poured onto a casting table. A large metal roller was then drawn over the molten glass to create a sheet, and then allowed to partially cool. Once partially cooled, the glass was moved to furnace where is slowly cooked for up to five days. The glass plates were placed in plaster of paris on a revolving table, ground to the desired thickness with fine abrasives, and then polished with felt-covered blocks and rouge. The final steps in this process included washing, inspection, and cutting the plates to size. New advancements in the manufacturing process led to better glass as well as easier production. The first innovation was the large power-driven crane. This crane was used to transport the pot of molten glass from the furnace to the casting table. The second advancement was a continuous annealing lehr (furnace). This furnace had multiple preliminary ovens and a long runway that reduced the time needed to heat and cool the cast glass. The introduction of the continuously lit furnace reduced the heating/cooling time from 10 days to roughly 36 hours.

After World War I, the automotive industry came up with the idea of plate glass for automobiles by continuous methods. The continuous process was introduced in 1923 and automated the entire manufacturing process of creating plate glass. Molten glass flowed from large tanks through a discharge spout onto a moveable table and under a roller, forming a flat sheet of glass with specified width and thickness. The glass was then transferred through a continuously lit furnace and was then ground, polished, cut, and inspected. This new process allowed for thinner plates, which gave window glass manufacturers strong competition as well as new opportunities. While plate glass invaded the glass windows market, the glass window market found itself in the plate glass industry, making the two slightly indistinguishable.

Advances in casting and annealing were adopted quickly, allowing the focus to shift to plate glass finishing methods. Libbey-Owens-Ford Company installed twin grinders that could grind rough glass on both sides at the same time. In 1959, Pilkington Brothers of England created a float process that would eventually make the twin grinder process obsolete. The float process had a continuous sheet of glass from the furnace that floated on a bed of molten tin, which eliminated the need for grinding or polishing. The Pittsburgh Plate Glass Company was the first American company that adopted the float process in 1962; by 1993, 90% of the world’s flat glass was using the float process to manufacture glass.

Uses and Installation
Plate glass was initially used only for shop and store windows. With an increase in the iron and steel constructed buildings, non-load-bearing walls created a new market for plate glass. With greater thickness and larger sheets, plate glass became a popular choice for glazing openings in tell buildings. The Woolworth Building and the Empire State Building were constructed using plate glass. With the growing popularity of skyscrapers, windows changed from individual slits in thick walls to whole walls of windows. The Hallidie Building in San Francisco was one of the boldest attempts by Americans to create a glass wall. In most commercial building façades, glass made up for no more than 50%. The ever-growing use of glass in curtain wall systems forced manufacturers to introduce plate glass with specific properties. Insulated plate glass could reduce heat loss, while heat-absorbing plate glass limited interior heat levels. Thermally toughened glass (tempered) allowed even larger areas to be glazed. These glasses made all-glass buildings more common after World War II.

Conservation
Plate glass is known for its transparency, resistance to weather, and its durability. Deterioration is usually noted due to a loss of transparency in the form of staining or etching. This damage can reduce its strength, though this is not a prevalent problem.

Deterioration
A.A Griffith presented a theory that any surface of glass has many invisible, small inherent defects which can lead to cracking when the defects correspond with high stress. The glass’s strength depends on the ‘’Griffith’’ flaws, the flaws’ location, and the presence of chemicals that attack the strained atomic bonds in the flaws. The tensile strength of plate glass is usually around 6,000 pounds per square inch. Failure always results from fractures occurring under tensile stress because the compressive strength exceeds the tensile strength. The amount of stress needed to cause failure in the glass’s strength varies, even for similar specimens. Glass strength must be expressed in statistical terms and is usually based on empirical data. Charts are available that have been written by manufacturers that suggest what thickness of glass is best suited for design conditions based on their probability of breakage. Glass strength can also be degraded by larger surface flaws, especially near edges. Edge flaws are most likely to affect the glass strength according to thermal conditions. High-tensile stress is more likely to occur at the edge of a glass sheet when the central part is significantly warmed than the edges (i.e. a building with overhangs that shades glass edges). Plate glass is brittle and deforms elastically when loaded until it reaches a fracture point. If loaded glass does not fracture, it will return to its original shape when the load is removed and should not result in deformation. Glass is susceptible to local overstressing and can be vulnerable to breakage. Alkaline exposure is also capable of damaging glass and has been known to affect its appearance. Mild alkaline solutions can be used to clean glass without causing damage as long as the residue is removed afterwards. Alkalis may also come from the concrete or masonry components of a building front. Alkalis can become difficult to remove if left on plate glass for a few days and can etch or stain the glass if left for extended periods. Alkaline solutions for cleaning nearby masonry can also result in damage to the plate glass.

Water can also damage plate glass. The water and glass react with one another, causing the water to become alkaline, possibly causing it to be more corrosive to the glass. In cases where water is allowed to puddle and come in contact with glass for long periods of time, extreme corrosion becomes an issue. Hazing and roughening of the glass surface is a common result which can cause contaminant removal to become more difficult. There is a distinct ratio of surface area to volume of liquid trapped on the surface; the more area being touched by more water, the greater the deterioration. Plate glass is usually resistant to acids, other than hydro-fluoric and phosphoric acids. These acids cause decomposing and dissolving the silicates that glass in composed of. These acids can be found in air pollution and chemical compounds that may be used to clean metal framing and masonry. Wind-born debris (i.e. roof gravel) can cause some pitting in glass surfaces. Welding and sandblasting can also damage glass if it has not been sufficiently protected. This damage is unsightly and can reduce the strength of the glass. When metal framing systems deteriorate, glazing compounds, sealant ands, and gaskets harden and lateral movement, also known as walking, of the glass sheet can damage the glass edge or cause local stresses to be applied to the edges. In either of these scenarios, glass is likely to become fractured. Annealed glass can experience fractures caused by a damaged edge and is usually identifiable by a single crack extending partway or all of the way across the glass sheet. High stress induced fractures usually present as multiple break lines extending across the entire sheet. A series of breaks radiating from one point in annealed glass is usually caused by surface impact. Tempered glass fractures in fragments and is likely to fall out of the window frame, making it difficult to identify the exact cause of the failure.

Conservation Techniques
Identifying and damaging agents and the extent of damage through chemical or microscopic analysis should be the first step in assessing the plate glass’s condition. If damaging agents are suspected, a sample of the residue from the glass and adjacent materials should be taken. Stain patterns are also an identifier and should be helpful in locating the source of the damaging agents. The samples should be taken to a laboratory to be analyzed for presence of harmful agents. The glass should be examined in place to determine the extent of damage. If damaging agents are identified, they should be removed to prevent further damage and or repetitious problems. Gaskets, sealants, and glazing system members should be evaluated for signs of damage and the metal framing should be investigated for corrosion. The visual appearance of materials is usually a good indicator of the piece’s condition. Gaskets and sealants can be judged slightly by their pliability; hardened gaskets may cause stress on the glass. When dealing with a historical building, it is important to consider its historical significance before restoring or replacing scratched or hazed glass. Polished plate glass is no longer available and efforts to repair it should be made before attempting to replace it. If an abrasive or chemical is the proposed method for restoration, a test piece of identical composition should be used.

Cleaning glass with little contaminants on it should be done with the use of a soft cloth and nonabrasive, ammonia-based cleaning solution. Rinsing the surface thoroughly with water to remove excess cleaner is necessary after cleaning has been completed. If the glass is covered by a significant amount of dirt, abrasive particles must be removed first by flushing with water to avoid scratching the glass. Organic solvents may prove helpful in removing grease or other non-water-soluble debris. Of course, regardless of the method, testing is strongly suggested. Light etching and stains may require more specialized techniques. Mineral acids, like dilute hydrofluoric acid, should be avoided because it will decompose the silicates in the glass and remove stains as well as a thin layer of glass. Another method for removing stains and light etching, other than replacing glass, is to polish with cerium oxide paste. Cerium oxide paste is made by mixing cerium oxide powder with clean water and applying with a felt pad. The glass can then be polished using a hand-held mechanical polisher. Applying the slurry and moderate pressure for several minutes should remove most of the etching and staining in that area. After 10 minutes, if no improvement has been made, the area may be incapable or repair. Continually wet the past and keep the past out of direct sunlight because the cerium oxide can become overly abrasive if sun dried. If deterioration is in the sealant, glazing compound, or gaskets, replacement of the sealants and gaskets is generally the only option. Replacements should have adequate durability. If the metal frame has corroded and resulted in significant section loss, the framing may be restored by cleaning the frame to remove rust and coating it with a retardant to prevent future corrosion. If the section loss is significant, replacing the frame with new material may be necessary to give the glass required support.

Replacement
If breakage has occurred or cleaning techniques have failed, replacement may be the only option. There are five companies in the United States that currently produce flat glass: Pittsburgh Plate Glass Company, Libbey-Owens-Ford, Guardian Industries, Ford Glass, and AFG Industries. Most glass is produced using the float process, which may not be consistent with the manner in which historical glass was manufactured. Just like applying glass in new construction, transparency, color, and strength to withstand wind and thermal load must be considered for proper replacement. The buildings configuration and façade components must be taken into account when determining the design load. The Glass industry guidelines should be followed closely during installation. Compatibility of the sealants and other materials should also be investigated in the design phase to ensure a long building life.