A Restorative Facial Wash (that will probably kill you).

 

For a tired complexion.

Pour two jugs of River Water into a glazed basin, and add sollimato [salt?]. Leave it for twenty days in the sun, then each morning for nine days add a fresh egg and beat it all very well, until on the twenty-ninth day your water is settled and clear. Keep it in a flask in the sun. 

due boccali d'acqua di fiume - Thus far exploring these things, recipes have called for river water, spring water, well water, water from a fountain (public or otherwise), rain water, the water smiths quench iron or steel in, and distilled water. All have their merits, and to some extent this undermines the popular idea that Early Modern people steered clear of water because it was lethal. They understood the value of running water over still, and that boiling water was a good policy before consuming (though I don't think they did this very often outside of cookery). Different water sources meant different things and served different purposes. 

di sollimato - You know in Jaws, when Brody tells Quint he's going to need a bigger boat? That's me, but it's a historical lexicon of Italian. 

It is the business of literary translation to be dangerous, and in situations like this I can't just insert an image of me crying. So my reasoning on the basis of the form and resonance of the word and the process it is describing is it refers to the addition of salt to the water. 

The addition of salt to Holy Water is well known; the reason is it helps prevent things blooming within it; it is preservative and antibiotic, to which you can add its magico-religious connotations. Additionally, it may serve an alchemical purpose, helping turn the one thing into another. 

I freely admit that might be wide of the mark. If you have an answer, please let us know in the comments. Apes Together Strong. 

 al Sole per vinti di - My salt hypothesis is strengthened by the business of leaving river water in the sun for weeks. Give it a go, see what happens. Ask yourself, do you want to wash your chapped face with *that*?

di might be a Latin contraction of die/diebus. More likely days than hours because they add up to twenty-nine, which makes sense as a Lunar month (split the difference between 28 and 29 - they did), whereas twenty-nine hours is just a bit random. 

un'ovo fresco sbattendolo molto bene - And I wonder about sollimato, because it's now just a recipe for the world's most lethal omelette. 

e pongasi poi in uno fiasco al Sole - I think the sense is contextually easy - decant your eggy murder-water in a flask and keep it in direct sunlight - but like the contracted di, pongasi might point out beyond Italian towards one of the Iberian languages. And that's very plausible given the degree of Spanish involvement in Italy in the sixteenth century. 

We don't know who Isabella Cortese was. Cortese could be Cortes, and efforts to discover more about her might have been looking in the wrong places. 

Translation is a process. More than one reading is possible, a lot of the time, and I hope the above provides some insight into how we go about making sense of things, and the humility we need to maintain in the face of our sources. An aspect of that is acknowledging the vagaries and enigmata, and loving them the more for them.