Operations
Time, speed and spacing of the various phases of the process are critical factors in the Fourcault process. Fourcault process machine operators require experience in order to judge placement of the die, location of various parts of the process, and rates of draw. These must be balanced against glass quality and the age of the draw.
As the draw continues the glass in the pit grows cooler and cooler, eventually leading to failures or diminished quality. The draw must be stopped, the pit must be "heated back" and then the process can continue anew.
Glass chemistry has a huge impact on the process since it controls the melting, forming and annealing temperatures, liquidus temperature (point where various chemicals that make up the glass start to crystallize out of the glass) and rates of change of characteristics of the glass itself.
Occasionally the ribbon will break or crack, leading to failure of the drawing process. Such breaks, known as "checks" can be alleviated by using proper operating parameters. Sometimes an expedient measure, using a portable source of heat, can be used to make the checks migrate to the edge of the ribbon where they disappear. The author has even seen crude torches made of wood which can migrate the checks.
The resultant product is a form of flat glass which is suitable for lesser quality uses. Due to process instabilities Fourcault process glass can have waves, seeds (small gas bubbles) or stones (undissolved materials).
In terms of economics and product quality the Fourcault process has been supplanted in many countries by the Pilkington developed "Float" process. The Float process lets the molten glass settle on top of a pool of liquid tin, so that gravity creates a flat sheet. Due to various chemistry and physical aspects of window glass the Pilkington Float process produces a vastly superior product.
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