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Float Process Gains in Glassmaking L-O-F is second U.S. glassmaker to license costcutting method for manufacture of flat glass Wide-ranging changes in the flat glass industry could well be touched off by the float glass process. To date, the nation's two largest flat glass producers have licensed the British process for making high quality flat glass by floating a continuous glass sheet on molten tin. Others are certain to follow. Float glass promises to reduce costs compared with plate glass manufacture—largely through labor savings. Furthermore, glassmen say the day may not be too far off when the float process will be competitive with sheet glass manufacturing costs. Libbey-Owens-Ford, Toledo, Ohio, has just revealed that it has licensed the float glass process from Pilkington Brothers, Ltd., St. Helens, England (C&EN, May 13, page 17). Previously, Pittsburgh Plate Glass Co. started construction of a float glass plant at Cumberland, Md., and the firm says it may build a second plant at Crystal City, Mo. Plate and sheet are the two major types of flat glass made today in the U.S. In the production of plate glass, molten glass is formed into a con66
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tinuous ribbon, annealed, ground, polished, cleaned, and cut. Grinding removes ridges left by pulling rolls. In making sheet or so-called window glass by PPG's Pennvernon process, the glass is drawn vertically from the melt tank into a continuous sheet, which is annealed and cut automatically as it rises through a multifloor building. It is less expensive than plate glass. Details of the float glass process are a closely guarded secret. However, the basic principle is simple. The flat and parallel surfaces of float glass are formed by molten tin on the under side, and by hot gases (probably nitrogen) on the other. The steps in making float glass correspond to the manufacture of plate. The same basic ingredients are fed to a regenerative furnace tank. From the tank, the glass flows onto the surface of molten tin in the float bath. The molten glass ribbon remains at high temperatures on both sides for a sufficient length of time to eliminate irregularities. While still on the molten tin, the glass is cooled until its surfaces are hard enough to run
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through the annealing oven without marring the bottom surface. Tin melts at 231° C , while glass is amorphous. Operating temperature in the melting tank varies from 1300° to 1500° C. Tin is used, instead of some other metal, because of its properties of melting range, surface tension, density, vibrational dampening capacity, and low vapor pressure. Savings in the float glass process result from elimination of grinding and polishing steps. Total plant employment for PPG's plate glass plant at Cumberland, with an annual capacity of 50 million sq. ft., is 500. PPG says it will employ 100 additional men to produce the 50 million sq. ft. per year of float glass at the plant. While exact labor savings are difficult to pinpoint, since some of the existing plate glass facilities will be used by the float plant—shipping, office, warehouse, and maintenance, for example—savings in manpower are nonetheless there. Labor costs are about half of the manufacturing cost of plate glass. According to R. F. Barker, vice president of PPG's glass and fiber group, initial reaction to samples of float glass in the U.S. market has been highly favorable. It has already captured a large section of the British market. The day is probably not far off when many existing plate glass plants must convert to the float process or be abandoned, he adds.
