This is because the sheets go on a conveyer mesh belt through a series of ovens with specific temperatures (gradually progressing from hot to warm) with air blowing evenly around the entire sheet, so that the temperature of the sheet is as uniform as possible throughout. (This long long tunnel of ovens is called a "lehr", pronounced like "lair" (or "layer", if you are from the South of the United States).
In contrast to this, in a stationary kiln, there is no blower, so the glass is more likely to stay hot on the insides and shelf side of the sheet compared to the edges and the top side. This temperature differential creates stress. To counteract this tendency, in a stationary kiln, the glass needs to progress through the different temperatures at a much slower rate than in the lehr.
Mary Kay Nitchie
Bullseye Glass Co.
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