Lapis Lazuli extraction – Fra Angelino natural ultramarine recipe

I am first very surprised that any conservation professor would ask for such specifics due to the vagueness of the original recipe. I have tried to reconstruct Cennini a number of times and it is very difficult to get satisfactory results. Many years ago I asked a similar question on the site that this forum is inspired, AMIEN and I was unable to find anyone who could give me the info that you seek.
Kremer Pigments has this down but I have asked a couple of times and they are reluctant to give it out their procedure. There are many variables and the short answer is that I have not worked out a system but have achieved satisfactory result using a simplified version.
First, you need to select the purest and deepest pieces of lapis. Remove any obvious impurities first, then crush and again remove any impurities. You can heat the stone to a high temp and quench in water where it will break into much smaller pieces. I tried this but ended up pounding and crushing in a mortar and pestle. Then grind the powder to a mid-coarse powder (something like 50 micron or so). You will need a series of sifting screens. It is still an enormous amount of work to hand grind the powder to even this moderate degree of fineness. Access to a mill would make this so much easier. I have then melted together something like 1 part beeswax, 3 parts rosin, and 2 parts mastic resin. This mixture is the 1 part by volume. To this I added about an equal volumetric amount of powdered lapis and heated and stirred all together. I did not add any oil like suggested. This was then poured out onto a piece of mylar. My previous attempts at this process ended up saponifying the oil in the alkaline water solution and complicated the process. Probably my pH was too high.
I made balls of this after it cooled. I then let them age for a couple of weeks. I them put them in little scraps of artist’s linen. I made up a number of basins of water (maybe a quart apiece) to which I added a tablespoon or so of potassium carbonate (the lye of Cenneni’s time as opposed to caustic lye which is sodium or potassium hydroxide). I did not add oil to my hands but used nitrile gloves other than that I followed the remaining extraction process mention in Cennini.
My result were visibly purer ultramarine but the yield was rather low and the extraction process took a very length period of time.
In the long run, I am not sure that anything that I provided here will be of any help. I had many frustrating attempts to carefully follow the recipe in the distant past but my failures dissuaded me from taking precise notes even though I would appreciate having them today. My attempts also uses some modern materials like mylar and omits others like the use of oil. This is the best that I can do.
Just so you know, there are later recipes for the extraction of ultramarine. You can probably references that contain them by doing a search on Art and Archeological Technical Abstracts. Some are clearly influenced by the Cennini recipe but others are a different take on the process. I am sorry that I could not be of more help.
Please let us know if you do find a modern recipe that adds precise measurements and pH to the Cennini recipe.