How did medieval glassmakers get red and blue out of sand and beech ash?

Medieval glassmakers got their reds and blues from metals: some already present in the beech ash they melted into the glass, others such as copper and cobalt added on purpose, with the furnace itself acting as a paintbrush. In Northern Europe, most stained glass was potash-lime-silica "forest glass", and the 12th-century monk Theophilus named beech wood as the preferred source of the ash. Beech ash contains iron and manganese oxides, which are particularly important for generating colour in glass.

The trick is that the same metal gives different colours depending on the furnace air. In an oxidising atmosphere iron turns glass from pale blue to yellow or brown, and in a reducing one it goes back to pale blue. Manganese is yellow in its lower oxidation state, purple in its higher ones, and pink when mixed. Theophilus described molten glass passing through a saffron yellow and a flesh-like tawny, then a light purple, to an exquisite reddish purple, as it was heated.

For the strongest reds, blues and greens, glassmakers added copper oxides: copper gives blue in an oxidising environment and red in a strongly reducing one. The brilliant blue of Chartres came from another metal: cobalt, which gives that blue at only 0.025% to 0.1% of soda-lime glass. Yet the picture is not only added metals. At York Minster, 90% of the medieval glass was shown to be coloured by its iron and manganese content.

The ruby red was so dense it was usually "flashed", a thin coloured layer on another sheet, which was developed in the 15th century. Yellow was hard too, until silver stain arrived in the early 14th century.