Page "Stained glass" Paragraph 6

Glass factories were set up where there was a ready supply of silica, the essential material for glass manufacture.

Silica requires very high heat to become molten, something furnaces of the time were unable to achieve.

So materials ( potash, soda, lead ) needed to be added to modify the silica network to allow the silica to melt at a lower temperature, and then other substances ( lime ) added to rebuild the weakened network and make the glass more stable.

Glass is colored by adding metallic oxide powders or finely divided metals while it is in a molten state.

Copper oxides produce green or bluish green, cobalt makes deep blue, and gold produces wine red and violet glass.

Much modern red glass is produced using copper, which is less expensive than gold and gives a brighter, more vermilion shade of red.

Glass colored while in the clay pot in the furnace is known as pot metal glass, as opposed to flashed glass.