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How did artists cut stained-glass pieces before diamond cutters were invented?

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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.

Before the invention of diamond cutters, artists creating stained glass windows relied on a process of heating and manual shaping to achieve the desired forms.

First, a full-sized drawing, or cartoon, was created to map out the various color areas. To cut the glass, the glazier would apply a red-hot iron to the edge of a glass sheet to initiate a crack. By moving the iron, the artist could guide this crack across the sheet to break away a piece of approximately the correct size and shape.1

Once the piece was separated, the artist refined its edges through a process called "grozing." This involved using a notched tool known as a grozing iron to crumble away small bits of glass until the piece matched the required dimensions of the design, ensuring a proper fit for the lead strips used in assembly.1