During the Renaissance, various painting techniques such as egg tempera, oil, fresco and secco, illumination of manuscripts and even drawing (preparation of toned papers and grounds for silverpoint) required the colors to go through a meticulous grinding process on a slab before being applied to a panel, canvas, paper or wall.
Typically, artists obtained dry and coarse powdered pigments, which they then had to prepare for painting according to their specific requirements. Depending on the type of pigment, medium, and intended use (e.g., glazing), pigments were ground to varying grain sizes.
In most instances, pigments were initially ground with water and stored in jars with excess water, ready for immediate use. For oil paints, pigments were ground directly with oil binders (such as linseed, walnut, or poppy oil) and stored under water to prevent polymerization.
While this labor-intensive process was supervised by the artist, it demanded considerable time and effort and was often delegated to apprentices and assistants in the studio.
THIS SHOWS YOU THE NATURAL COLORS, AND HOW YOU SHOULD GRIND BLACK. CHAPTER XXXVI
[…] Let us go no farther, but return to the black color. To work it up properly, take a slab of red porphyry, which is a strong and solid stone; for there are various kinds of slabs for grinding colors, such as porphyry, serpentine, and marble. Serpentine is a soft stone and is not good; marble is still worse, for it is too soft. But porphyry is best of all; and it will be better if you get one of those which are not so very much polished, and a foot or more in width, and square. Then get a stone to hold in your hand, also of porphyry, flat underneath, and rounded on top in the shape of a porringer, and smaller than a porringer, shaped so that your hand may be able to guide it readily, and to move it this way and that, at will.
Then take a portion of this black, or of any other color, the size of a nut; and put it on this stone, and with the one which you hold in your hand crush this black up thoroughly. Then take some clear river or fountain or well water, and grind this black for the space of half an hour, or an hour, or as long as you like; but know that if you were to work it up for a year it would be so much the blacker and better a color.
Then get a thin wooden slice, three fingers broad; and it should have an edge like a knife; and scrape over the slab with this edge, and gather the color up neatly; and always keep it liquid, and not too dry, so that it may run well on the stone, and so that you may be able to grind it thoroughly, and gather it up well. Then put it into the little jar, and put enough of the aforesaid clear water in with it to fill up the jar; and always keep it under water in this way, and well covered from dust and all contamination, say in a little chest arranged to hold several jars of liquors.The craftsman’s handbook by Cennini, Cennino, active 15th century; translated by Thompson, Daniel Varney, 1902- ed
The process of grinding
The grinding process is a critical aspect of working with traditional or handmade materials. It is essential to have a firm grasp on the characteristics of these materials to produce high-quality results.
While it’s true that many modern dry pigments are already ground to the desired particle size, achieving a consistent and uniform paint with superior working properties can often require additional dispersion in water or a binding medium.
Creating paint by simply mixing pigment with water, oil, or egg emulsion using a brush or a stick can result in a powdery and grainy consistency that is difficult to work with. This method also doesn’t allow for achieving the translucency and luster of the egg tempera technique.
To achieve a more consistent and workable paint, it is necessary to first disperse the pigment in water (or binding medium) by ensuring that any agglomerates of pigment particles or clumps of color are completely broken up.
For optimal results, it is recommended to procure a slab and muller made of porphyry or granite, as Cennino advised. However, using a crystal muller and a thick glass plate is also a viable option and much more accessible.
To prepare the pigment for tempera painting, it should be mixed with water to create a thin paste and ground on the slab in circular motions while slightly rocking the muller. It is essential to maintain the right consistency by adding water as needed. The pigment should be creamy or pasty for optimal grinding. The ground color can then be scraped into a color jar using a small wooden slice or a pallet knife.
It is important to clean the slab thoroughly with warm water and soap immediately after grinding.
Storing the Ground Colors
Store ground colors in wide-mouthed bottles with air-tight covers, filling them up almost to the top with water to prevent the color from drying out and becoming granular. Use small bottles and grind only a small amount of color at a time to reduce the risk of contamination. The bottles should not be more than half-filled with the ground color to leave space for water above. Small quantities of fresh color are more convenient than large jars that may collect dust and traces of other colors, so use two-ounce screw-cap bottles for ordinary use.
If you don’t have access to wide-mouthed bottles with air-tight covers, small plastic containers can be a good alternative for storing ground pigments. Look for containers that are made of a material that won’t react with the pigments, such as high-density polyethylene (HDPE) or polypropylene (PP), and that have a tight-fitting lid to keep the pigments from drying out or spilling. It’s also a good idea to label the containers with the name of the pigment and the date it was ground, so you can keep track of your supplies and their freshness. However, keep in mind that plastic containers may not be as durable or long-lasting as glass bottles, so consider replacing them periodically if they show signs of wear or damage.