Arcanum Duplicatum is known by many names: sulphate of potash, arcanite, potash of sulfur, potassium sulfate. An inorganic compound, it manifests as a white water-soluble solid prized by alchemists. Arcanum dupilcatum (K2SO4) hits the alchemical shelves c. 14th century.
It's called arcanuni or sal duplicatum (double secret) as it's composed of an acid salt and alkaline salt. Other names include vitriolic tartar, Glaser's salt or sal polychrestum Glaseri. The latter are for chemist Christopher Glaser, who prepares and purveys arcanum duplicatum.
Born in Basel, Glaser becomes apothecary to Louis XIV and Duke of Orléans in Paris, France of the 17th century. He's implicated in a poisoning scheme of 1676. His death date is unclear (1670 - 1678). He may have left the earthly realm by then. If not, he does soon after.
Also known as panacea duplicata, this compound gets its ravenous power from production of aqua fortis (nitric acid, HNO3) from niter (potassium nitrate, KNO3, saltpeter) and oil of vitriol (sulfuric acid, H2SO4). It forms as residue of production processes of these compounds.
The residual salts are dissolved in hot water, filtered, then evaporated to form a cuticle or membrane. Finally, the solution is left to crystallize. The salts are then used as a diuretic and sudorific (to induce sweating). In pyrotechnics, potassium sulfate produces a purple flame.
Purple flame gives rise to the alchemical term "Angel of Violet Light". Potassium sulfate is studied by such luminaries as Johann Glauber and Robert Boyle. Already known since the 14th century, it receives its names arcanuni or sal duplicatum in the 17th century.
The recipe for sal duplicatum is purchased by Charles Frederick, Duke of Holstein-Gottorp. Schroder, the duke's physician, expounds on the wonders of its great uses in hypochondriacal cases, continued and intermitting fevers, stone, scurvy, and more.
It's hard to know if the doctor's rave reviews are true, or exaggerated to sell the products. Physician testimonials to purported miracle remedies have a bottom line in mind - money, status, or even fear of punishment by the patron who recommends it. But sure, maybe it's true.
Natural resources of potassium sulfate are minerals abundant in the Stassfurt salt, a potash salt of Germany. The minerals are cocrystallizations of potassium sulfate and sulfates of magnesium, calcium, and sodium.
Relevant minerals include:
Kainite, KMg(SO4)·Cl·3H2O
Schönite (now known as picromerite), K2SO4·MgSO4·6H2O
Leonite, K2SO4·MgSO4·4H2O
Langbeinite, K2Mg2(SO4)3
Aphthitalite (previously known as glaserite), K3Na(SO4)2
Polyhalite, K2SO4·MgSO4·2CaSO4·2H2O
The potassium sulfate can be separated from some minerals, such as kainite, as the corresponding salt is less soluble in water. Today potassium sulfate is created by the reaction of sulfuric acid with potassium chloride.
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