Pour into a 500ml-glass graduate Pyrex vase (Beaker), 250ml of nitre spirit at least to 30 degrees Baume. Join to it, little by little, native martial pyrite grinded roughly, as it will dissolve in the nitre spirit. This operation should be made outdoors or in a well ventilated place, due to the great amount of glittering vapours (brownish) that the reaction emanates.
When it reaches the saturation, that is, when the pyrite doesn’t dissolve anymore in the nitre spirit, then stop. Filter the martial solution through a glass funnel, with a cotton lid, in a glass vase.
In a 1litre-glass flask with a large mouth, pour 500ml of water of the source and heat it up at 40º C. Dissolve in it, tartar salt (potassium carbonate) or sodium carbonate to the saturation.
Now, pour slowly, by successive fractions, your martial solution, as the effervescence provoked by the reaction, will diminish and rest.
It will turn in to a red brownish colour precipitate that will thicken as you pour the acid martial solution in the alkaline.
When the martial solution being poured in the carbonate solution does not create anymore effervescence, then, stop.
Stir well, with a glass rod and, then, pour the precipitate in a big porcelain or glass porringer, fill it with cold tap water. Let to rest. The martial precipitate will seat slowly at the bottom. Now, by decantation, flow the excess liquid, and tend to be cautious of not letting the martial precipitate flow together with the water.
Pour in some tap water again, and stir very well with a glass rod, to wash the precipitate well. Repeated the operation as many times as it is necessary so that the water is without acrimony and becomes limpid and by decantation, empty the water excess with caution, so not to spill the martial precipitate.
At to this point, place the porringer on an electric or gas oven, in a sand bath, with a moderate temperature, to evaporate the excess water.
As the water evaporates, the drying precipitate, will have a stain colour that, when reduced to powder, will be much clearer.
Pour it in a glass or porcelain mortar, grind it in very fine powder and pass it by a sieve of 60 lines per centimetre or 120 lines per inch.
Keep it in a large mouth glass flask. This calx of Mars is very subtle; it is a wet oxide that will be good for your spagyrical operations with regards to the extraction of the martial tincture by means of the spirit of vinegar and other means.
Rubellus Petrinus