Verdet or Calx of Venus
Verdet is a green or bluish substance, almost insoluble in water, but very soluble in vinegar and other acid spirits. The verdigris is formed on the surface of the copper or brass parts that are exposed to the action of humid air, and is nothing else than a copper carbonate. The copper carbonate, in its natural state is the malachite.

Frequently, the verdigris is confused with the copper acetate.

Seek 1 kg of native Venus vitriol; place it inside of a glass or porcelain recipient and pour in source water heated up to 40º C, enough to dissolve it completely. Then filter the solution.

In an enamelled iron basin, dissolve separately, in source water heated to 40º C, 1 kg of salt of tartar (potassium carbonate) or, if you do not have it, use sodium carbonate. Filter the solution.

Now, plunge a porcelain or enamel mug in the vitriol solution and fill it. Pour it, little by little, in the carbonate solution as there is effervescence, move well with a wood spoon, and a precipitate of clear green-blue will be formed at the same time.

Proceed in the same way, always moving, until the effervescence stops. If there is no more effervescence when you pour the vitriol solution, it is because the solution is saturated. Then, stop.

Leave it to rest some time, then flow by decantation the solution. If it drains a bluish liquid, it is because there is still some vitriol in the solution that didn't react with the carbonate.

Pour abundance of tap water on your precipitate to wash it properly, always moving very well, until the water comes out clear.

By decantation, flow all the water and let to evaporate in the sun or on a gas stove, with moderate heat, in a sand bath.

When the matter is dry, remove it with a stainless steel spoon and keep it in a large mouth glass flask well closed.

This canonical Venus verdet, although prepared artificially, was made of native Venus vitriol, which is the main element of the chemical reaction.

Calx of Venus. If you want to prepare the calx of Venus or Aes Ustum (oxide), put in a mud or stainless steel porringer, 250g of verdet and place it on a gas stove at a very high heat. Move or stir well again with a stainless steel spoon, as if you where to calcinate it, until it turns completely black. When it reaches this point, stop and let it cool. Proceed in the same way, until you have calcinate all of your verdet. When cool, pour it in a large mouth glass flask well closed.

This calx will serve you to you prepare the verdigris and the tincture of Venus.

Rubellus Petrinus