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How did glaziers use a full-sized cartoon to make a stained-glass window?

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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 the traditional practice of stained glass, the full-sized cartoon served as the essential blueprint for the entire window. Glaziers would paint this line drawing directly onto the surface of a whitewashed table, carefully mapping out the design and indicating how the image should be divided into individual pieces of colored glass.1

Once the cartoon was prepared, the glazier used it as a guide to select and cut the glass. Pieces were shaped by cracking the glass with a red-hot iron and further refined through a process called "grozing," where edges were crumbled away with a specialized tool to ensure an accurate fit. The cartoon also accounted for the necessary space between pieces to accommodate the H-shaped lead strips used for assembly.1

By the later Middle Ages, the process became highly organized; master glaziers often maintained a stock of finished cartoons on parchment or paper, which could be adapted or reused for different projects to meet the specific requirements of their patrons.2