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[caption id="attachment_3145" align="alignright" width="228"] Johan Rudolf Glauber (1604-1670)[/caption]By Marieke HendriksenThe German apothecary and alchemist Johan Rudolf Glauber (1604-1670) spent much of his working life in Amsterdam. There, he used the facilities of a commercial glasshouse--"The Two Roses"--on the Rozengracht for some of his experiments.[1] By extracting the colours of metals by melting them into glass, Glauber hoped to come closer to the unveiling of the Philosopher's Stone.[2] Although he never achieved this, his experiments proved highly relevant for understanding a number of chemical substances and compounds. The reason Glauber's name still sounds familiar to us today is that he discovered how to artificially create hydrate sodium sulfate, a compound highly relevant for glassmaking, which Glauber called sal mirabile.[3] This white crystalline solid is still commonly known as Glauber salt, and can be bought from online chemists to intensify the colours of textile dyes.[4][caption id="attachment_3147" align="alignleft" width="180"] Frontispice to the English translation of Glauber's work, containing a recipe for sal mirabile.[/caption]Glauber salt was, and is, of great use in the making of glass, where an alkaline is necessary to lower the melting point of the ingredients for glass and to obtain good results. Before the discovery of Glauber salt, or soda ash (sodium carbonate) that had been extracted from the ashes of plants, was commonly used. The downside of using soda ash though, as Herman Boerhaave (1668-1738) put it in his Theory of Chemistry, was that 'the ashes of plants us'd herein, also vary the goodness of glass.'[5] The relative ease with which Glauber salt could be made (Glauber's own recipe!) requires nothing more than plain salt, water, a retort and a fire. Its purity ensured a more stable glass quality than soda ash. But Glauber and his contemporaries thought that his newly discovered salt was miraculous for another reason: its perceived therapeutic qualities. Glauber salt was long used as a laxative and a styptic.[6][caption id="attachment_3146" align="alignright" width="300"] Sodium sulfate or Glauber salt[/caption]These useful applications meant that the popularity of Glauber salt in the eighteenth and nineteenth century was enormous, as also shown by the numerous recipes for it in medical and chemical handbooks. In a manuscript with lecture notes, a student of the Leiden professor of chemistry Gaub (1705-1780) devoted almost ten pages to the description of three processes to create Glauber Salt.[7] Reversing these recipes, the Dutch apothecary Petrus Johannes Kasteleyn (1746-1794), who aimed to stimulate the public's interest in chemistry in the eighteenth century, wrote in his Chemical exercises for the lovers of chemistry in general, and the apothecaries, producers, and dealers in particular that through chemical analysis he had found that  '16 ounces of pure Glauber salt consist of 2 ounces of vitriolic acid, and 3.5 ounces of pure alkali.[8]From the early twentieth century onwards, the use of Glauber salt in medicine declined as safer alternatives were discovered, but it is still widely used industrially today. The story of the recipe for Glauber salt shows us the serendipity of research: while looking for something entirely different--the Philosopher's Stone--Glauber discovered a substance that would become widely used in medicine, crafts and industry for centuries to come.[1] AAR (Amsterdamse Archeologische Rapporten) 61, 2011, p. 44. The glasshouse was housed at the Rozengracht 1657 and 1679.[2] D. Von Kerssenbrock-Krosigk, Glass of the alchemists : lead crystal-gold ruby, 1650-1750 (Corning, NY: Corning Museum of glass, 2008), p. 17.[3] For Glauber's recipe for sal mirabile, see The works of the highly experienced and famous chymist, John Rudolph Glauber, trans. Christopher Packe (London: Thomas Milbourne, 1689), p. 225.[4] http://www.stoftotverven.nl/Glauberzout-500-gram; http://www.dharmatrading.com/chemicals/glaubers-salt.html[5] H. Boerhaave, Elementa chemiae, quae anniversario labore docuit in publicis, privatisque scholis. 2 vols, vo.l. I, Leiden: Isaak Severinus, 1732, p. 183.[6] Pinkhof, 'Glauberzout als Bloedstelpend Middel,' Nederlands Tijdschrift voor Geneeskunde, 1897, 41, pp. 297-8.[7] H.D. Gaub, “Chemiae praxis. Notes of lectures by an unnamed student. Produced in Leyden.”, z.d. WMS 4  MS.2479, Wellcome Library, London, pp. 67-79.[8] P.J. Kasteleyn, ‪Chemische oefeningen voor de beminnaars der scheikunst in 't algemeen, en de apothekers, fabriekanten en trafiekanten in 't bijzonder, vol. 2 (Amsterdam: ‪A. J. van Toll, 1872), p. 83. For more on Kastelyn's educational purposes, see L. Roberts, 'P. J. Kasteleyn and the "Oeconomics" of Dutch Chemistry,' Ambix 53, 3 (2006): 255-272.