Why red is the hardest colour to get right in a stained-glass window
Red glass coloured strongly enough to hold its hue looks almost black at the thickness a window needs to survive the wind. Glassmakers solved it by fusing a thin skin of red onto clear glass, a trick called flashing that also let later artists scrape away colour to draw fine detail.
Stained glass gets its colour from metals added while the glass is molten. Copper oxides give greens and bluish greens, cobalt a deep blue, and gold a wine red or violet, though much modern red uses cheaper copper for a brighter vermilion. Glass tinted right in the furnace pot is called pot metal; the pieces are then cut, arranged into pictures and held together by lead strips known as cames, with painted details and yellow silver stain added on top.
A window has to be at least 3 millimetres, or an eighth of an inch, thick to cope with typical wind loads. Red needs such a concentration of colouring that at that thickness it blocks most light, so makers dipped a pale gather into molten red and blew the two layers into one sheet. Because the red skin can be abraded or etched to reveal the paler glass underneath, artists gained rich patterns without extra lead lines; 16th-century heraldic windows used this heavily. Abrasion came first, then dangerous hydrofluoric acid, and sandblasting arrived in the 19th century.
Traditional sheets were blown by mouth. Cylinder, or muff, glass began as a blown bubble stretched into a tube, cut open and flattened in a second oven. Crown glass was spun until it opened into a disc, leaving a thick central bull's-eye bearing the pontil mark. Rolled glass, poured onto a table and flattened like pastry, appeared commercially around the mid-1830s; it is often called cathedral glass, although medieval cathedrals actually used hand-blown sheets.
The purpose of a church window was never mainly to show the outside world but to control light, which is why these works have been called illuminated wall decorations. Many large windows have survived largely intact since the Late Middle Ages and rank, alongside illuminated manuscripts, among Western Europe's major surviving medieval art. Islamic civilization also shaped the craft from the 8th century, turning glassmaking into an industry to supply thousands of mosques.
Source: Stained glass
