Once we got back from Calabria (which was Amazing!) We learned how to mix a gesso and apply it to a big painting that had been worked on by several restoration classes called Santa Caterina. I don't completely remember everything in this lesson and unfortunately my notes weren't very thorough for this section, so my explanation might be a little unclear.
As I understood it, the gesso is made of either rabbit bone or fish bone. It's mixed as 1:10 ratio. Since we were using organic material, using too much bone can make the paste moldy so it's important to get the right ratio. We made a 1 part organic bone mixture with 10 parts cold water in a small can, which then went into a bigger can to boil and melt the bone into a sticky glue. We then sifted a powdered material (maybe made of chalk and talcum, a kind of powdered gesso) into the rabbit glue. The mixture has to be the perfect viscosity. Not too fluid and not too thick, somewhere in-between where it drips at a steady rate as a bead. We then applied this to the painting where the painting had corroded. When applying we had to make sure it didn't touch the existing oil paint or it could warp the painting that was already there. After the gesso dried we scraped it down so it would be even with the original painting and applied watercolor to match the original colors.
If you notice the bottom two pictures, there is gesso filled in to the left of the face and on the forehead in the left picture. In the right picture, I'm slowly building up the color with watercolor to get a similar color tone to the surrounding area.
We also started painting a fresco in preparation for peeling it off the plaster and transferring it to canvas. This is one of the methods that can be used to preserve frescoes. The plaster was made with sand and non-acidic lime. The sand has to be from a lake or river because sea salt destroys the color of a painting. The lime is also boiled in water until in becomes alkaline (meaning not acidic). They use lime because it pushes the color of pigment forward and creates a transparency. The best ratio for plaster is 1 parts lime: 2 parts sand. If there is too much lime the plaster will crack easily. If there is much sand it becomes too porous.
We then painted on the dried plaster with colored pigments mixed with water. This would probably be called fresco-secco as the paint was applied to dry plaster. A lot of old master works were painted buon fresco, which means the paint was applied to wet plaster. The artist would plaster and paint in sections daily, or a days work called giornata. But before the painting could be done, a sketch is first outlined on the plaster, usually with a red pigment called sinopia.
We outlined our sketch by using a piece of paper with holes pricked in it. A bag of pigment, spolvero, was pounded over the paper, producing pigment on the plaster where the holes had been. I decided to paint a copy of Michelangelo's Sibyl from the Sistine Chapel. I wish I had a little more time to finish my painting but the since of urgency and time constraint definitely opened my eyes to how hard it would be to complete a wall fresco.
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