Cosimo “Pater Patriae” de’ Medici (1389-1464) must be counted among the alchemical experimenters of Renaissance Italy. Scholars have long known about his enthusiasm for the sorts of ancient hermetic texts that constituted the theoretical foundation for alchemical practice, such as the fourteen texts of the Corpus Hermeticum that he received in 1460 and shortly thereafter entrusted to Marsilio Ficino for translation into Latin. Less remarked on but equally important to the question of Cosimo’s alchemical activity is his acquisition of an ancient scientific text of a more practical nature: a thirteenth-century manuscript of Pliny’s Natural History, which he obtained through an agent in Lübeck.
Cosimo’s possession of these such texts cannot be discounted as the consequence of a merely general (and passive) interest in scientific knowledge, for intriguing evidence in the form of Cosimo’s own scientific recipes points to a hands-on engagement with alchemy. Two of the recipes attributed to Cosimo were published in 1894 by Pier Desiderio Pasolini in his transcription (based on a surviving later copy) of the lost original manuscript of the recipe collection of Caterina Sforza (1463-1509). Of the two recipes in Caterina’s collection that trace back to Cosimo, one is for a medicine: “a remedy to cure every species of fever, tried by Cosimo de’ Medici” (“un rimedio da guarire omne sorte de febre provata [la ricetta] per Cosimo de’ Medici”). The other recipe that Caterina attributed to Cosimo, “To give weight to a gold scudo or a ducat without a blemish on the conscience, according to Cosimo” (“A dare gran peso a uno scudo o ducato de oro sennza carigo de coscientia, secundum Cosimum”) promised a means of artificially increasing the weight of gold coins. 
In the case of the first recipe, assuming the remedy it rendered was safe and efficacious, its value could hardly be overstated; this was especially true since—from an etiological point of view—fever was regarded in his era not just as a symptom of an underlying ailment but as an ailment in and of itself. The second recipe represents a practical application of alchemical principles for a purpose that would appear to be highly suspect and likely unethical if not illegal, despite its claim to be without cause for a guilty conscience. The association of this recipe with one of the world’s richest plutocrats gave it particular credence; it also suggests the tantalizing notion that Cosimo might have inflated his wealth by habitually adulterating his gold currency.
The original contribution of the present essay lies in the publication of an additional, third recipe historically credited to Cosimo de’ Medici and recorded during his lifetime in a manuscript now held by the Biblioteca Nazionale Centrale in Florence. This manuscript is an anonymous, handwritten collection of alchemical recipes. The first and largest part of the text is written in a uniformly clear and sharp early- to mid-fifteenth-century script; it is followed by two pages of addenda in a later, possibly sixteenth-century, hand. The recipe in question, entitled “Experiment of Cosimo de’ Medici of Florence,” is found within the manuscript’s first section:
“Pratica di Cosmes di Medici fiorentino. Primo far cenaprum album ut sic poi to[li] oncia 3 di dicto zenapro poi fa aqua forte di vitriol et salnitro. Poi recipe oncia 2 di dita aqua e destilla per el manco quatro volte sopra lo dito cenapro sopra oncia 3. Poi toli oncia 1 lune fine & disolve in oncia 3 di dita aqua. Et quando tu averai destillato quelle prime tre onze di aqua sopra el dito zenapro per alambico quatro volte, tolge via quella aqua et non destilare più sopra el zenapro. Poi toli quella aqua disoluta dov’è la luna et distilla sopra li tre oncia di dito cenapro tanto che tuta l’aqua se consumi et e facta la medicina una p[ars] s[opra] xx.”
This recipe calls for very few ingredients: cinnabar (mercury[II] sulfide), which is written variously as ‘cenprum,’ ‘zenapro,’ and ‘cenapro,’ silver shavings, and a batch of nitric acid (into which the silver is dissolved, becoming silver nitrate, and with which the cinnabar is distilled). While clearly this is an alchemical recipe because of the distillation techniques and the metallic ingredients, the nature of the recipe is somewhat ambiguous; only the reference to “la medicina” in the final line suggests a therapeutic usage. Mercury-based medicines were typically used for skin maladies, and decades after Cosimo’s death they were employed in cases of syphilis. Here, however, the specific utility of this medicine is undefined.
This fifteenth-century testimony regarding Cosimo’s alchemical practice corroborates all the preceding indirect notices of his active engagement in this scientific practice. Moreover, it provides the logical link between the two recipes of Cosimo’s that Caterina Sforza later collected, one being instructions on the preparation of a medicinal simple, and the other being a purely alchemical operation. This new addition to Cosimo’s repertoire, as an example of alchemical medicine, demonstrates a vivid interest in this branch of pharmacology in Florence a century before the arrival of Paracelsus’s spagyric medicine.
About the author:
Sheila Barker directs the Jane Fortune Research Program on Women Artists at the Medici Archive Project, and has published several essays on medicine at the Medici court. This blog entry anticipates her forthcoming article entitled, “Cosimo de’ Medici and Chemical Medicine in Early Renissance Florence.”
 Brian P. Copenhaver, ed. and trans., Hermetica: The Greek Corpus Hermeticum and the Latin Asclepius in a New English Translation (Cambridge: Cambridge University Press, 1992), pp. xlvii-xlviii.
 Entry by Stefano Caroti, in La corte il mare i mercanti. La rinascita della scienza. Editoria e società, Astrologia, magia e alchimia, Firenze e la Toscana nell’Europa del Cinquecento, 4 (Florence: Electa, etc., 1980), p.139, cat. no. 1.19.
 See Caterina Sforza, Gli esperimenti de la ex.ma s.ra Caterina da Furlj, Matre de lo inllux.mo sr Giovanni de Medici, copiati dagli autografi da lei dal Conte Lucantonio Cuppano, ed. Pier Desiderio Pasolini (Imola: Ignazio Galeati e Figlio, 1894), pp. 23, 92. It is not at all improbable that Sforza could have come into contact with circulating written or oral remnants of Cosimo’s chemical tinkering, given that she lived in the territory of Florence in the early Cinquecento, and that her third and last marriage was to Giovanni “il Popolano” de’ Medici, a grand-nephew of Cosimo de’ Medici.
 Pier Desiderio Pasolini, Caterina Sforza, 3 vols (Rome: E. Loescher, 1893), I: 434.
 Although the reference is only to a “Cosimo,” the identification with Cosimo de’ Medici is made by Pier Desiderio Pasolini, Caterina Sforza, III: 680.
 A loose translation might be: “Recipe of Cosimo de’ Medici, a Florentine. First prepare white cinnabar [mercury (II) sulfide] as is usually done and measure out 3 ounces of said cinnabar. Then make a batch of Acqua Fortis [perhaps our modern nitric acid?[ out of vitriol [sulfuric acid] and saltpeter [potassium nitrate]. Now take 2 ounces of this Acqua Fortis and distill it, together with the above-mentioned 3 ounces of cinnabar, repeating the process at least 4 times, and once you are done, remove any remaining Acqua Fortis. Then take one ounce of fine silver scrapings and dissolve it in 3 ounces of Acqua Fortis. And once you have carried out the previously explained distillation process, take this silver solution and distill it together with the cinnabar that you have already distilled four times; distill these two components together until all the liquid is gone and then your medicine is done. One part to 20.”