In my last post, I briefly outlined what gesso was. I am still looking into how it came to be used and why type was chosen over the other. I used the websites listed in the previous post, particularly this e-book, and will refer to it again here.
Calcium Carbonate
Calcium carbonate was typically used in Northern Europe, most likely because this is where significant and accessible deposits were being mined during the middle ages. It is also known as chalk. Chalk is the remains of ancient sea creatures. It can be finely ground and to make chalk whiting. This whiting can be turned into gesso by adding a size.
Calcium Sulphate
Bulian and Graystone say it was typically used around the Mediterranean region, most likely because Bologna and Volterra have large deposits. Jerry Tresser also records that in the 1300s there was a large deposit being mined at Montmartre, near Paris. This most likely accounts for the common name of the refined product: Plaster of Paris.
The mined substance is known as gypsum. This is CaSO4.2H2O [Calcium sulphate dihydrate]
Gypsum was mined, ground up and burned to remove water and any impurities. Tresser says this process also changes the natural gypsum from yellow to white. When this dried product was ground again, it has the adjust chemical composition CaSO4.1/2H2O . It was, and still is, sometimes called Plaster of Paris. This powder is called gesso in Italian.
Gesso was used in 2 ways.
1. Gesso Grosso
CaSO4.1/2H2O + Size
Plaster of Paris, or gesso, was sifted and then moisten it, assumedly with water.
Add parchment size or hide size to this paste to make gesso grosso. This is the first layer Cennini recommends to apply over the leather crest shape. It is fast drying, but this means bulky sculpture protrusions have less time to sag or drip. This makes it a good material for roughing out the shape.
2. Gesso Sottile
CaSO4.2H2O + Size
Plaster of Paris, or gesso, was soaked in water for several weeks
Cennini says to soak it for 30 days. Jerry Tresser points out that this is an imprecise process, as it is not possible to tell when the gesso was been super hydrated. After reading a few other versions of this process, I think Cennini likely chose 30 days as a safe, standard soaking time to ensure the process was completed by this point.
The water is drained off and the paste is set into blocks to dry again. This powder was called gesso di bologna.
Mix it with size to make Gesso Sottile. This is used in the fine finishing layer of Cennini's crest making process. It has the benefit of being slower drying, allowing more time for fine detailing.
