The bottom line is that alum in medieval paint recipes is Potassium Aluminum Sulfate KAl3(SO4)2(OH)6.
Read on for how I learned this.
A google search on "alum" brings up many things. Alum for pickles, alum as antiperspirant, alum for dyeing, different alums for dyeing different fabrics and more!
I thought I was buying the correct alum for my paint experiments from Dharma Trading Company. Their alum, used for dyeing and as a mordant, turned out to be Aluminum Sulfate, Al2(SO4)3.
According to Randy Asplund Potassium Aluminum Sulfate KAl3(SO4)2(OH)6 is really what medieval recipes indicated. This is extracted from alunite.
The Dover edition of Mrs. Mary P. Merrifield's Original Treatises called Medieval and Renaissance Treatises on the Arts of Painting has a useful and helpful glossary by S. M. Alexander. Mr. Alexander defines Alum as:
"A variety of astringent substances was classed under the name of alum. Sulphates of iron and aluminium, or a mixture of the two, were commonly understood by the term." (Merrifield xii)I went down the wrong rabbit hole trying to figure out what it probably meant. I'm a scientist, I wanted the name! ;)
Randy Asplund identifies rock alum as Alunite.
This is confirmed in Original Treatises. I find 'aluminis', 'alumine' and 'alumen' in Jehan Le Begue's Experimenta De Coloribus (Merrifield pg 50). Merrifield translates this as 'roche alum' which Alexander's Glossary helpfully defines as:
"Pure form of alum prepared from alunite." (Alexander xxviii)This is where I learn that it is helpful to own the full copies of the newest books (even if you can mostly access the outdated version online). The Dover edition of Merrifield's Original Treatises contains the glossary which would have helped me find the correct chemicals before I used the wrong ones. C'est la vie!
According to the Pigment Compendium Alunite is KAl3(SO4)2(OH)6 (Walsh pg15).
And alunite can be used to manufacture the potash alum, potassium alum, alum flour, or alum meal.
In 2016 the experiments will be with this correct Potassium Aluminum Sulfate. We'll see how this effects the colors I got using Aluminum Sulfate.