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How did medieval glassmakers turn a blown cylinder of glass into a flat sheet?

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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.

To transform a blown cylinder of glass into a flat sheet, medieval glassmakers employed a technique known as the production of broad glass.2

The process involved several distinct steps:
* Blowing: The glassmaker gathered molten glass and manipulated it through repeated blowing and swinging to form a large, hollow cylinder.12
* Trimming: Once the cylinder was formed, the ends were cut away.1
* Slitting and Flattening: The cylinder was slit lengthwise down one side. While the glass was still red-hot and in a pliable state, it was opened and flattened into a sheet.12
* Annealing: Finally, the flattened sheet was placed in a kiln to cool very slowly, a process called annealing, which ensured the glass was stable and could be cut into the specific shapes required for a stained glass window design.1

This method allowed for larger pieces of glass than the alternative crown glass process, though the resulting sheets often retained some surface waviness and variations in thickness.2