Window design in the middle ages was dictated mainly by the limitations glass manufacture imposed. Historically there were two types of glass, Cylinder and Crown named after their method of manufacture. To manufacture cylinder glass the glassmaker blew a sphere with the molten glass and then swung it back and forth until it was shaped like a cylinder. He then cut the cylinder along its length and flattened it into a sheet.
In crown glass manufacture the blown sphere was attached to an iron rod before removing the blowing iron, which left a hole at one end. The sphere was then rapidly rotated, centrifugal force expanding the hole until the sphere formed a disc from which panes could be cut.
Cylinder glass was inferior to crown glass, which could be manufactured thinner. From both types of glass it was however only possible to produce very small window panes (max 6″ x 6″) without distortion.
Initially glass was used almost exclusively for stained glass windows in cathedrals and churches. It was then gradually adopted by the wealthy and then eventually by everyone else.
The first ecclesiastical windows used wrought iron frames manufactured by a blacksmith into which the small panes were joined together using lead. This then led to the development of wrought iron casement windows (with at least one opening light); which albeit lacking the weather ability of modern windows ensured that houses could be ventilate.
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