Making Traditional Manuscript Gesso: Part 1 (Slaking the Plaster)

First steps in making manuscript gesso for traditional raised gilding according to an early 15th century recipe (mostly)

For those of you who are more unfamiliar with the terminology of illuminated manuscripts, raised gilding is gold leaf applied to an elevated base of size (in this context that means the stuff the gold will stick to). It allows the illuminator to create a three-dimensional effect with raised gilding. (See the image below). Traditional manuscript gesso can then be highly polished to produce a mirror-like effect with the gold.

Raised gilding on manuscript gesso in medieval illumination — Raised gilding on manuscript gesso

But mastering manuscript gesso isn't easy so I, like most people, started out gilding with a modern product. However, none of the modern sizes used for raised gilding give the same results. They either pucker, ripple, and sink in the middle, or, in the spots where they don't, give a shine that looks artificial. There's just nothing like the real stuff.

By the way, for those of you who know 'gesso' is widely available in art stores and are wondering why I don't just use one of those, none of them are the same as manuscript gesso. Although, one day, I might experiment and see what I can cook up.

When I first decided I would give manuscript gesso a try, there was a readily available, semi-commercial source I could buy it from pre-made. It substituted at least one of the traditional ingredients for an easier to use (less poisonous) one, which I will discuss in another post. The results were lovely, although not quite as polished as I wanted. But then the man who made the gesso--Jerry Tressor--died and his passing forced me to finally dive into the finicky world of making my own and choosing a method from the half dozen ones vaguely spelled out on the internet.

Josie Brown came to my rescue. She doesn't know she did, but she did. Josie is a British artist who has been doing calligraphy and illumination for decades (check out her website here) and she just happened to be offering a series of online classes on gilding. I took a class with her once before on Weaver Writing and enjoyed it and she did not disappoint this time either. She's patient and explains things well, unfussy and has a good sense of humor.

So, the first thing needed to make manuscript gesso is the actual gesso. Gesso is the Italian word for plaster. It's the base ingredient. But it's also not just the straightforward plaster of paris you can buy pretty much anywhere: hardware stores, art stores, amazon, etc. It has to be slaked. Plaster is a sponge for moisture. It sucks it up and holds onto it like crazy. Its thirst has to be slaked, so to speak. It has to be chemically altered. Somehow, medieval illuminators figured out how to do this and Cernino Cennini (an Italian Renaissance artist), fortunately, included the instructions in his rather famous book on painting, which you can get in translation.

So what does slaking require? Three months of time and lots of stirring.

Slaking Plaster

Mix plaster of paris with water. I used a 1 to 4 ratio, but it doesn't need to be exact because that water is just a starting point. After stirring it for 15-30 minutes (I kept to the longer time mostly), let it sit overnight. Be very careful during that first stir, however. If you haven't used enough water and stir for too long, the plaster can thicken too much and suck up all the moisture and start to harden. A further note, I chose to use distilled water. Josie doesn't, but the tap water in NYC has fluorine added to it so I decided to be safe and use pure water.
After letting it sit a day, the plaster will settle a bit and some of the water will rise to the top. Pour that off and add more water. Now stir again.
Repeat this daily for a month.
After a month, you can now slow your stirring to once a week ... for the next two months.

Once your three months of stirring are done, you can now dry your slaked plaster. To do this, strain the plaster through a cloth to squeeze out as much of the water as you can. I just use an old t-shirt for this. Then set it aside and let it dry. I keep mine on a high shelf (because of cats) and cover it with the same t-shirt (because of dust and cats).

After putting it away to dry, I periodically broke it up into smaller pieces to speed the process. Regardless, it still took my gesso approximately a month to fully dry. If left uncovered, it would taken less time. If I had done this during the summer with our higher humidity, it would have taken even longer to dry. Keep all of this in mind when setting out to make manuscript gesso. This is not something that you can cook up for a project you want to make imminantly.

Once it's dry, you can just store it in chunks or you can grind it back into its powder form. I prefer to grind it now and store it as a powder, since that will make it easier to measure it out during the next part of the process. Then I just put it away in an air-tight container and label it. The good news is, even just two cups of plaster of paris will translate into enough slaked plaster to last you for quite some time and you wont have to repeat the slaking process again for (potentially) years. Of course, that depends on how much gilding you do.