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Why did medieval stained-glass sheets have variations in thickness and colour depth?

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The north rose window at Chartres Cathedral in France, featuring intricate stained glass with vibrant geometric and biblical designs in a large circular and five tall vertical panels. — stained glass The stained-glass rose window in the north transept of Chartres Cathedral, France.

In medieval stained-glass production, variations in thickness and colour depth were not considered defects but were essential to the aesthetic quality of the windows. These variations occurred because the glass sheets were produced by hand-blowing a bubble of glass, manipulating it into a cylinder, and then flattening it while it was still red-hot and pliable.1

This manual process resulted in sheets that were not uniform, with thicknesses ranging from less than 1/8 inch to as much as 5/16 inch. These irregularities were highly valued because they created rich, nuanced transitions in the depth of the glass's colour when light passed through them. While later technological advancements allowed for the production of larger, thinner, and more uniform sheets, these improvements often resulted in glass that was considered less visually interesting. Consequently, 19th-century Gothic Revivalists sought to replicate the earlier, more variable "antique" glass to recapture the aesthetic depth of 12th- and 13th-century windows.1