Qualitative Study of Cennini’s Gesso for Illumination

Cennini, in his book Il Libro De’ll Arte describes how gesso was prepared. Before we examine this medieval procedure, some background information would be helpful. Understanding  where it comes from, procedures for preparing gesso, and a comparison to whiting also commonly called French chalk.

During the 14th century, the most famous mines for the production of raw Gypsum were produced from the quarries at Montmartre in Paris, France. These sedimentary rocks were called Gypsum. The common term for Gypsum in Italian is gesso.  The chemical term for Gypsum is Calcium sulphate. The coloration in its natural form is yellow. It would not be uncommon to find grey as well as brown Gypsum being mined in Italy as well as other localities in France. Gypsum becomes plaster of Paris once it goes through a burning process.

We do not have concise written information as to the laborious process used in the 14th century to roast this chemical. We do know that by burning the raw Gypsum a process known as Calcination, with or without the addition of other unknown additives a) altered and changed the color of the Gypsum from yellow to white  b) burnt off  impurities  or attachments of other materials  to the Gypsum, and c)  eliminated whatever  water moisture existed in the Gypsum from its original  state.

This Gypsum after being calcinated became commonly called Gesso grosso (thick gesso) or unslaked plaster. It was as Cennini indicated an “admirable ground for gessoing”. Unslaked plaster provided a fast set, beautiful white color, silkiness to the touch, and a variety of painting as well as non painting uses.

If the unslaked plaster (gesso grosso)  prior to any size being added, was so overloaded with water as to prevent setting;  this oversaturated gesso became known as Gesso sottile (thin gesso) or slaked plaster of Paris. Once the slaking process was complete, parchment size was added to the gesso sottile.  Now you basically had two different
uses for the same chemical, one prepared as a fast set and the other a slow set.

The above photos show the crystal like formations of unslaked plaster. These amorphous shapes (having no particular form) clearly define the structure of gesso grosso.       Photo courtesy from The Techniques of Raised Gilding, by Jerry Tresser, A michelle jordan publicaton.
Once gesso sottile was slaked it was stored as small bricks and was ready for use slaked and slaked with parchment size. It is at this point that several important facts have to be discussed in order to explain the slaking process and how it pertains to gesso and illumination.

The chapter on the preparation for slaking plaster as outlined by Cennini was confusing as to whether it was for slaking or purification or both.

Now you have to have a gesso which is called gesso sottile: and it some of this same gesso, but it is purified for a whole month by being soaked in a bucket. Stir up the water every day, so that it practically rots away, and every ray of heat goes out of it and it will come out as soft as silk. Then the water is poured off, and it is made up into loaves and allowed to dry”. CHAPTER CXVI Il Libro Dell’ Arte. The Herringham translation 1896

The procedure called for taking gesso grosso in small amounts, a pounds worth of plaster to a gallon or two of water and sprinkling it through ones fingers and stirring vigorously to avoid the plaster from setting.  .

The object was to overload this thick gesso with so much water  that it prevented  any setting of the plaster.  The process of agitating the water was continued on a daily basis for a month or even longer with short durations of stirring several times a day for weeks on end.

The key factor here was changing the water. Every time the water became murky, it was poured off and fresh water was added. A rather remarkable arbitrary method considering that Cennini had no way of actually knowing when the plaster was slaked ! It is possible that it could have been slaked on the 12th day or the 23rd day or even the 35th day!  This process ended when the water was clear, regardless of the length of time it took.  Most books seem to use 30 days as a fairly accurate measuring guide for slaking. 

The three questions that arise are simply this: why the water change? And when is the plaster actually slaked?  Or was it?

If we were to travel back in time and attempt to figure out how Cennini knew that the gesso grosso was slaked we could only arrive at one conclusion. The plaster was slaked when the water was no longer murky. The purpose of changing the water was to provide a method for cleansing the plaster. The plaster would fall to the bottom of the pail and any remaining impurities would discolor the water making it murky. The murky water would have to be poured off and fresh water was added. This was repeated as long as it was necessary for the water to be crystal clear regardless of how much additional stirring was needed. It may have taken 30 days depending on the impurities that accompanied the plaster in the initial process. 

These were the guide lines Cennini used to determine the length of time it took and to consider the plaster as slaked.

There were no identifiable markers, no chemical tests, nor any litmus papers, or pH testing kits in the 14th century. There was nothing to provide them with any kind of an intelligent barometer to know if they had completed the task of creating slaked plaster other than the clearness of the water, and the silky touch of the plaster.  They relied on their senses to judge the readiness of the plaster.                  

But was the plaster slaked?  Or was this lengthy process more for a purification process then a slaking process? Or possibly did this method provide a duel purpose of ridding the impurities of the plaster and providing the user with a gesso sottile.

The slaking process involves adding unslaked plaster (gesso grosso) to water. When these two chemicals meet, a chemical reaction occurs. In most instances heat is given off as a by product the moment the plaster makes contact with the water. Not a violent chemical reaction in the sense of an explosion, but a chemical reaction nevertheless with or without the continuous agitation of the water. If the water was not agitated enough to prevent setting, the plaster would set, even in the water.  If the agitation was vigorous enough then the plaster would not set and would remain in a powder like form in the water. Calcium sulphate is insoluble in water unless
the plaster is exposed to extreme heat.

One must remember the burnt Gypsum of the 14th century provided a greater concentration of the raw materials than what is available in today’s market as unslaked , example: dental plaster. Because of its strength as a raw product most of the burning process in the 14th century was done twice to insure that the impurities were burnt off and the unslaked plaster became a usable product.

The plaster being stirred frequently for days and weeks on end no longer changed the crystal like structures in the plaster. Whatever happened, happened at the initial contact between the water and the plaster. Heat is indicative of a chemical reaction as the water is vigorously agitated. The prevention of the setting is so overloaded by the saturation of the water that the crystal structures of the plaster undergoes a transformation to form rods or needle like filaments which very clearly defines slaked plaster.

The above photos clearly show the needle or rod like structures of slaked plaster.  Photo courtesy from  Jerry Tresser The Technique of Raised Gilding.
If the slaking process is not complete at its inception, you will have plaster slaked, plaster partially slaked, and possibly plaster unslaked in the same pail! The slaking process is immediate and does not occur over  any extended time period

When size is added to gesso sottile and allowed to dry, the evaporation of the water sets the foundation. Looking at it microscopically, the rods like structures adhere to each other to form a bond that provides an exceptional slaked plaster base.

These crystals provide the strength and ability to prevent cracking, insure flexibility and are the foundation for panel, or gesso for illumination.

The above photos show the attachment of parchment size to the rod like crystalline structures of slaked plaster of Paris. This bonding provides exceptional strength to the plaster, and sets the foundation as Part 1 of a two step process  for raised gilding. Photo courtesy from The Technique of Raised Gilding, by Jerry Tresser
Once gesso sottile with size was prepared, it was put aside for later usage. Stored in brick fashion and sold to the painters or craftsmen. This mix of slaked plaster and size became part one of a two step process in preparing the gesso for illumination on vellum or parchment. This is exactly the procedure that was outlined by Cennini in his description for using gesso in raised gilding.


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