Several raw materials of black pigments are very hard, for instance charred hartshorn or bone, charred fruit stones and some minerals. These materials are first crushed into smaller pieces in a brass or bronze mortar with a pestle (Figure 50a). As a next step, the rough powder is ground on a stone slate with a muller in circular movements (Figure 50b). Slates and mullers were commonly made from hard stones like porphyry.[1] Softer stones like marble rub off too quickly when processing hard materials and leave fine particles that impair the quality of the color. Glass-slates came into use much later when glass became available in the required thickness and size at affordable prices.
Grinding the pigment with water on a stone slate can be quite exhausting and can take more than two hours for certain pigments like those derived from peach pits (Figure 51a) (FILM 1)???. This first grinding step has two goals. Firstly, the pigment particle size is adjusted to the desired level, and secondly the particle morphology is homogenized to create uniform particles. Through the muller you can feel when the particles are still too large which is directly related to the sound of grinding which diminishes as the particles get finer. For quality control, the pigment-water paste is rubbed between the fingertips which reveals if the particles are fine enough and whether larger particles are still present. Watercolors require very fine particles and need careful preparation. Cutting corners at the step of grinding lowers the quality of the paint considerably.
After grinding, the water-pigment suspension is then left to dry. Historic sources refer to a very clever and effective method of drying: using a chalk stone. Chalk stones have the ability to absorb the water which speeds up the drying process considerably, while the pigment particles stay at the smooth surface and can easily be removed after drying[2] (Figures 52, 53). This step is necessary to prepare dry pigments for storage and is usually not part of the preparation of oil colors.
Historically, dry pigments are kept either in folded papers or in small boxes made from wood, bone or ivory. As Sanders puts it: “Preserve it for use in a paper, or clean Boxes.”[3]
[1] Peacham. 1634. The Gentlemans Exercise: p. 65.
[2] Anonymous. 1668. The Excellency of the Pen and Pencil: p. 70. https://archive.org/details/excellencyofpenp00ratc.
[3] Sanderson. 1658. Graphice, the Use of the Pen and Pensil: p. 55.