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What was a grozing iron used for in medieval stained-glass work?
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stained glass
The stained-glass rose window in the north transept of Chartres Cathedral, France.
In medieval stained-glass work, a grozing iron was a specialized notched tool used to refine the shape of glass pieces after they had been initially cut or cracked from a larger sheet.1
While glaziers initially used a red-hot iron to guide cracks and break glass into approximately the correct size, this method did not produce precise edges. The artist would then use the grozing iron to "groze," or systematically crumble away small bits of glass from the edges of the piece. This process allowed the glazier to accurately shape each piece to fit the design, ensuring the correct allowance for the lead strips, or cames, that would eventually join the pieces together.1