To meet its many uses, plate glass is made in many thicknesses ranging from 1/8 inch to 1-1/4 inches. The principal thicknesses available are 1/8, 3/16, 1/4, 3/8, 1/2, 5/8, 3/4, and 7/8 inch. Also available, but not from all sources, are 1 inch and 1-1/4 inch. thicknesses. At the present time float glass is produced principally in 1/4 inch thickness, although some 1/8 inch and 3/16 inch float glass is made.

Plate glass containing wire netting is usually referred to as polished wire glass. This glass is comparable to rolled wire glass, except that it is transparent. Polished wire glass is available in 1/4 inch thickness. Float glass containing wire netting is not produced.

Tempered glass

Tempered glass is a type of safety glass made by specially processing flat glass to increase its strength.

When broken,

tempered glass disintegrates into small round-edged pieces, minimizing the danger of serious injury.

Tempered glass cannot be cut or drilled as it will shatter when its surface is penetrated, nor can it be bent or otherwise altered in form without losing its temper. All tempered glass products must be sized and formed before tempering.

Tempered glass is used principally for glazing motor vehicle

interior and exterior doors, shower enclosures and large fixed glass panels. Tempered glass is also used in miscellaneous industrial applications requiring glass with high thermal resistance, such as in molds used in making plexiglass (an acrylic resin product).

Production Processes

Except for some rolled glass, flat glass is made today on continuous production lines. Once production is started, it continues around the clock until interrupted by breakdown or shutdown. Flat glass production lines cannot be shifted from one type of flat glass 1/ a sheet glass line, for example, cannot be used to

to another;

produce plate, float, or rolled glass.

Essentially, a flat glass producing line consists of a melting furnace or tank, drawing, rolling, or floating equipment, and cutting equipment. In all flat glass production lines, the molten glass is made by fusing a mixture of silica sand and other materials at high temperatures; the proportions of these materials used in the "batches" or mixtures vary according to the type of glass produced. In most U.S. plants the batch is emptied into the melting end of a "continuous" or tank furnace. These furnaces are large refractory-lined tanks that are usually divided into three compartments: (1) the melting compartment, in which fusion of the raw materials occurs; (2) the refining

An exception is that rolled glass in the form of rough plate glass blanks are produced on a plate glass line.

compartment, in which at higher temperatures, the impurities in the molten glass are removed; and (3) the working compartment, in which

the molten glass is kept at the proper temperature for rolling or

drawing.

Rolled glass

In the continuous process of making rolled glass, the molten glass flowing from the furnace in the form of an endless ribbon is made to pass between two rollers. One or both sides of the glass are impressed with the desired pattern. The ribbon of glass continues on through an annealing lehr; it is then inspected and cut to the desired size.

A few U.S. companies produce small quantities of colored rolled
Molten glass is withdrawn from a

glass by an intermittent process.

furnace by means of large iron ladles and poured on a flat cast-iron table. Immediately a massive iron roller passes over the plastic

glass, rolling it into sheets or slabs of the desired thickness. The design is impressed into the glass, usually by configurations on the surface of the table rather than on the roller. The slab then passes through an annealing lehr, after which the glass is inspected and cut into stock sheets or cut sizes.

Sheet glass

Three processes, fundamentally the same, are used to draw sheet

wide is pulled upward from the working end of the tank, bent 90° over a bending roll, and then drawn horizontally over a series of rollers through a long lehr. In the lehr, the glass is gradually annealed so that the internal stresses formed when the glass was first cooled are largely removed. At the end of the lehr, the glass is cut into desired sizes.

In the Fourcault process, a ribbon of glass is drawn upward through a slot in a refractory block, or debiteuse, which floats on the molten glass in the working end of the tank. The glass is pulled upward between a series of asbestos-covered rollers placed in pairs above the drawing block. The rollers are enclosed in a boxlike structure which retains the heat and thus serves as an annealing lehr. These rollers extend upward for about 20 feet to a platform where the glass is cut.

The third method--the Pittsburgh Plate process--differs only slightly from the Fourcault process. A draw bar, instead of a debiteuse, is used for drawing the glass. The draw bar, submerged below the surface of the molten glass in the working end of the tank, forces the glass to flow evenly over its surface as the ribbon of glass is pulled upward by knurled rollers that engage both edges The series of rollers extend upward to a height somewhat greater than that in the Fourcault process, carrying the glass through an annealing lehr. In June 1969 a domestic sheet glass

of the sheet.

manufacturer (PPG Industries) announced that by modifying the glass drawing process it is able to produce sheet glass 1/8 inch and less

in thickness that is competitive in quality and cost with float glass.

Plate glass

Most plate glass, including polished wire glass, is currently

1/

produced by a continuous horizontal rolling process. Molten glass passes over a weir or through a refractory slot which gives it a preliminary shape, and then passes between a pair of water-cooled rollers which gives it the proper thickness and width. After passing through a continuous roller lehr for annealing and cooling to room temperature, the ribbon of glass is ready for grinding and polishing.

Two methods of grinding and polishing plate glass are currently in use--the single side and the twin processes. In the twin process, the uncut ribbon of glass moves via conveyor belt to a twin grinding area where both sides of the ribbon are ground simultaneously, then to the polishing area where both sides are polished simultaneously. In the single-side method, the ribbon of glass is cut into standard lengths (rough blanks) as it leaves the annealing lehr. The rough blanks are set in plaster on the grinding table, moved through the grinding machines, and on to the polishing machines. The rough blanks are cleaned, turned over, and replaced on the grinding table for a repeat process on the second side.

1/ Wire glass is made by feeding the wire netting between the rollers simultaneously with the plastic glass as it flows from the