Often in alchemical works a necessary first stage of working with our Prima Materia, or starting material, we must first 'open' up our substance.  This enables us to get as much yield as possible in extracting our material.  TIn the plant kingdom for example, this can mean that once we harvest our herb we might chop it, grind it and/or pulp it until we can get a very fine consistency.

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This becomes more difficult once we move to the mineral and metallic realms.  If we were to break down a crystal, we could roast it on an open fire and quench it in distilled water, repeating this process several times.  After a while the crystal becomes brittle, and will more easily break if it is crushed.  After some labour it can then be ground to a dust, a powder and eventually a flour.  But what do we do with gold, a material that is incredibly malleable and can be hammered so fine that it can reduced to 0.1 microns?  (For reference, a human hair is on average 70 microns thick!)

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This is a problem that perplexed alchemists for centuries.  Some alchemists got around this issue by simply using gold leaf, itself gold in a very fine sheet.  Yet in some alchemical workings it becomes important to have a consistent amount of gold mixed in with other materials.  So again, how to turn gold into a powder? 

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From at least somewhere around the time of Jabir Ibn Hayyan, the Islamic alchemist, it was discovered that you could reduce gold to a liquid by combining two very powerful acids together: nitric acid and hydrochloric acid.  Together, these form what later became known as Aqua Regia, "royal water" or the "King's Bath".  This is the only acid known to reduce gold, although there are some other special materials that if, alchemically prepared, can reduce gold.  Though these are secrets, only handed down in oral and experiential ways, never on paper or in a public forum.

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So to create the Calx of gold, firstly gold is put into a beaker.  Hydrochloric acid is then added, then nitric acid, making the king's bath.  The traditional ratio of hydrochloric acid to nitric acid is about 3 to 1, although this does depend on the strength of the acids.  It sometimes requires a gentle heat to start the reaction.  After a short time, the water starts to turn yellow, then orange, and even red (if there is enough gold and the acid is concentrated down through evaporation).  As the gold starts to fizz it is dissolved into the royal water.  This operation is ideally done underneath a fume hood or outdoors away from people and animals, as the reaction gives off a toxic gas in the form of chlorine.

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When all the gold is into a liquid form, all that remains is to precipitate the gold out as dust.  This can be done with several materials.  One that was commonly used by alchemists was iron vitriol (an iron sulphate solution in water).  When the iron sulphate-infused water is added to the solution of gold, a reaction immediately takes place.  The iron and the gold swap places, and what precipitates out is powdered gold.  What precipitates out does not look like gold, because the gold particles are so fine that they do not reflect light like common gold, so it ends up looking brown.  Another substance that can be used in place of iron vitriol is water infused with sodium metabisulphate (commonly used in home-brewing to sterilise equipment).  Yet another traditional process can involve oil of tartar to break down the gold.

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The solution goes through a reaction with colour changes.  After it is finished, the gold precipitates down as a powder.  It can now be used by the alchemist in mixing or fusion, to great efficacy.