How did artists create detailed designs by abrading flashed glass?

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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.

Artists created detailed designs on stained glass using a technique known as the abrasion of flashed glass. This process was made possible by the creation of "flashed" glass, which involved dipping clear, pliant glass into molten colored glass to coat its surface with a thin, translucent film of color.1

To achieve intricate designs without the need for complex leading, artists would grind away specific portions of this colored film. By removing the top layer of color, they could reveal the clear glass underneath, allowing for detailed patterns—such as the ruby roses with white centers found in early examples—to be rendered on a single piece of glass. This method, which gained popularity in the 15th and 16th centuries, provided artists with greater creative freedom by reducing the reliance on lead lines to define shapes and colors.1