77 - The Way to Make Oriental Emerald
Take 2 oz of rock crystal ground impalpably, as I have described above [chapter 76]. Add 4 oz of ordinary minium, blend these powders together thoroughly, and unite them well so they incorporate. In all you will have 6 oz of material, to which add 8 grains per ounce of good verdigris, finely ground.
Therefore, in all give 2 pwt [pennyweight] of ground verdigris of material. Stir in 8 grains of crocus martis made with vinegar, as described in the seventeenth chapter. Mix this thoroughly, and then blend diligently with the aforementioned materials.
Put them in a good crucible that is resistant to fire, and is large enough for the entire mass of powder. Fill it, leaving a space the depth of one finger. Cover this crucible over with a terracotta lid, and lute it securely, then Iet it dry. Now put it in a pottery furnace to bake along with the pots, plates, and other vessels. Place it in an area of the furnace that will develop a good fire. In this heat and flame, the material purifies and cooks thoroughly. Leave it to stand in as strong a fire as bakes the pottery vessels, and then remove it from the furnace.
Open the crucible, and expose beautifully vitrified material of a lovely emerald colour. In grand works and in the form of jewels it will seem superior to oriental emeralds from ancient rock. If the first time this paste is not sufficiently cooked and purified for Iack of heat, re fire by looking in from the top, take care not to break the crucible but only Iift the cover. If the paste is purified and transparent through to the bottom then it is good. If it does not sufficiently clarify, and it is opaque, then re-lute the cover to the crucible, and return it [to the furnace] another time to purify.
Take care never to break up the crucible before you look at the paste, and make sure that it is cooked and purified thoroughly. If you break the crucible when the paste is not cooked, and you recover it into another crucible, then it will clean up, however you will see it filled with specs, and it will make for ugly and imperfect work. This is important to avoid.
If you do not have access to a pottery furnace, you can use a purpose-built kiln. Fire it for 24 hours, which will make the work perfect. Firing many crucibles of various colours at the same time makes for a good saving of work. Build the fire with hardwood, which is to say well-dried oak; green wood will not work well. The fire should be continuous and never allowed to slacken, because then the work will be imperfect.
In Antwerp, I built a furnace that held twenty glass pots of various colours, and when fired for 24 hours everything fused and purified. For large loads, continue the fire for 6 hours more, with the assurance that the materials will cook thoroughly. This way, you will not consume excessive firewood. Be sure that the crucibles are of good quality and tempered to the fire.
Mount these pastes like the ordinary gems, and work them the same way in all respects. They will take on the polish and Iustre of the natural ones, and you can mount them in gold, backed with foil as usual. With these doses, the pastes will be harder than ordinary.