Leather Helm Crest: Part 12 Applying Gesso grosso

Here is what Cennini suggests you do to apply gesso to the leather structure.

1. size it two or three times
Done.

2. mix gesso grosso and lay it on in a thick paste, modelling it to the animal shape needed.
Done.

3."This done, take some gesso grosso ground with size, liquid and flowing, on a brush, and you lay it three or four times over this crest with a brush."
When I had moulded the gesso grosso paste onto the leather, I then mixed water with the remaining paste in the bowl and painted it over the damp gesso. It smoothed out the rougher lumps and filled any splits that had formed while the leather flexed. I was not going for a smooth finish, as I wanted to replicate a scruffy, hairy surface.

4. "Then, when it is quite dry, scrape it and smooth it down, just as you do when you work on panel."
I used some sculpture tools to remove any lumps and smooth the surface a little.

5. "Then, in the same way, as I showed you how to gesso with gesso sottile on panel, in that same way gesso this crest. When it is dry, scrape it and smooth it down;"
I applied a layer of gesso sottile with a brush, but made it pancake batter thickness. I think this is much thicker than was intended. Again, I did this so the plaster would 'catch' as it dried and pull into a textured surface to give more of a look like a wavy, bumpy coat of fur.

To make a smooth item, I would mix new gesso grosso immediately after adding the rough layer, making this second, smaller lot with less dry ingredients in the water. I would apply a thin coat with a brush, let it dry, mix a new batch, apply it etc., until there were three or four layers. I would use a rough cloth to rub down the damp gesso grosso where needed. I would also make my layer of gesso sottile much more aqueous and spend time 'polishing' the dry surface with a cloth on a chock.

http://www.noteaccess.com/Texts/Cennini/index.htm

Leather helm crest: Part 7 Preparing gesso sottile...still

The plaster is soaking to saturate it, making gesso sottile. You can see that the plaster and water have separated. It will be ready in a week or two.

Leather helm crest: Part 6 Preparing gesso sottile

About a week ago, I started to saturate some plaster of Paris to make gesso sottile. This is the second layer of gesso that Cennino Cennini recommends for the outside of a helm crest. In an earlier post I explored the chemical composition of gesso. Gesso sottile is a super hydrated version of plain plaster of Paris. By soaking the plaster, you are allowing it to take on more H2O (water), giving it the desirable property of a slower drying time. This means we will be able to add the final sculpting touches with more care.

Since this is the first time I have used plaster in this way, to sculpt rather than to make moulded objects, it is all a learning experience. I have no idea how well the plaster will stick to the leather, or how well the second layer of plaster will stick to the first, or how the paint will need to be applied. It will also be the first time I have made pigment based medieval paints. There will have to be some test pieces very soon...

Cennini suggests leaving your plaster to soak for 30 days, changing the water to ensure it remains clean. Other sources say this method is inexact - but it is the best evidence I have, closest to my chosen time period, so I have tried this method first. We can always try something else.

To make gesso sottile, I fill a bucket with water and poured plaster in. It is always recommended that you add plaster to water, not water to plaster. When making a gravy or sauce, this produces lumps but for plaster it works well. I mixed it gently and continued to add plaster until it was not absorbed immediately. I covered the bucket with another tub to stop rain, cats, frogs and leaves getting in. Now, we wait.

Gesso from calcium carbonate vs gesso from calcium sulphate

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.