128 - A Proven Way to Make Rosichiero
Simmer crystal frit, which has a Iot of salt, as directed in the first recipe for rosichiero [chapter 124]. For example, put 6 lbs of this frit into a glazed crucible. As soon as it clarifies, add fine calx of lead and tin, which is to say the finer part, as was described in the chapter  for the enamel base material.
Of this calx, as an example, add 4 oz, incorporating it thoroughly in four doses. When it is well-clarified and incorporated, throw it into water, reheat, and then leave it to melt and clarify thoroughly. When it clarifies, add red copper to the glass, which we also use to make opaque red [chapters 24, 58]. For example, add 1 ½ oz of this. Add it in three portions, stir the glass well, and leave it to incorporate the powder, and to clarify the glass.
At the end of 2 hours, add crocus martis made with sulfur, reverberating it as discussed in its place [chapter 16]. Add 1½ oz, and stir the glass well. Add it in three portions then leave it to clarify, and to incorporate with the glass for 3 hours.
Now add 6 oz of singed tartar, with 1 oz of well-vitrified chimney soot. Singe the tartar as was done for the chalcedony [chapter 41]. With this powder mix VI oz of crocus martis made with sulfur [chapter 16], as above. Grind these powders well, and give them to the glass in four portions. Stir it, and wait a little while from one [dose] to the next, because it will swell and make the glass boil unbelievably. When you finish adding the powder, leave the glass to clarify for 3 hours, then return to stir the glass, and make a proof in the form of a vial.
Reheat it well. If it strikes transparent red, like blood, then it is good.
If it does not strike, add more singed tartar to [the glass] with soot, and crocus martis, as above. Add this material little by little until it becomes the desired colour. Always Iet the glass rest for an hour after adding the powder, then make another vial, and reheat [strike] it. If it becomes red, like blood, and transparent then it is good, and will be very nice for enameling, as proven in Pisa many times.