24 - A Red Copper Scale that Makes Many Colors in Glass
Take small pieces of copper and put them inside the arches of the furnace. In that place, they will be within the walls. Leave them that way until each piece of copper is well calcined, using a simple fire. Do not use a fusing fire, or one that could melt them, in which case they would not come out well.
When they are calcined, grind and pulverize them and they will become a red powder. Store it for the many necessary uses it has in the art of glassmaking.