To give this colour to glass, set in the furnace a pot of pure metal of frit from rosetta or barilla, but the rosetta frit does best; as soon as the metal is well purified, take for a pot of twenty pounds of metal fix ounces of bras calcined by itself; put it by degrees at two or three times into the metal, stirring and mixing it well every time, and diligently skimming the metal with a ladle: at the end of two hours the whole will be well mixed, and a proof may be taken; if the colour be found right, let the whole stand 24 hours longer in the furnace, and it will then be fit to work, and will prove of a most beautiful sky colour.