Properties of oil paints

To make oil painting, powdered dye must be grated with a certain amount of oil.
The oil can be used raw or oxidized, or with resins or wax added.
The amount of dye used to grind the paint is a very important issue, as both too little oil and too much oil will detract from the painting.
Each dye has a certain percentage of oil needed for rubbing. This amount depends on many factors, such as:
Specific gravity - The heavier the paint, the less oil it needs. (There are exceptions here, however. For example, cinnabar, although heavier than lead white, needs twice as much oil as white.)
Paint particle structure - Paints with the same chemical composition, need different amounts of oil.
The behavior of paints when exposed to moisture - The drier the paint, the less oil it needs. Therefore, the dye should be dried before rubbing.
Ease of blending with oil - Paints that blend easily with oil need less oil. Paints that combine poorly with oil include zinc white and ultramarine.
Temperature of the room in which the paints are rubbed - In the heat, the oil is thinner and takes more dye.
Presence of free fatty acids in the oil - Oxidized oil combines more easily with the dye and needs less dye.
Pressure - Pressurized mashing reduces the amount of oil needed.
Both an insufficient amount of oil introduced and an excess of oil reflect badly on the durability of the painting.
In the first case, the painting is dull and rubs off after drying. In the second, it causes wrinkling, cracking and yellowing of the paint layer.
An appropriate amount of oil is taken for 100 parts by weight of dye.
From 15 to 55 parts by weight of oil are needed for the following dyes:

• lead white - 15,
• zinc white - 23,
• cinnabar - 23,
• ultramarine - 43,
• aurealin - 49,
• burnt green earth - 52,
• kraplak - 54,
• yellow chrome - 55.

Dyes needing from 67 to 100 parts by weight of oil:

• light yellow cadmium - 67,
• English red - 70,
• Paris blue - 78,
• earth green - 87,
• burnt umber - 87,
• cobalt blue - 90,
• kraplak brown - 93,
• natural umbra - 95.

Dyes needing more than 100 parts by weight of oil:

• ivory - 112,
• asphalt - 127,
• roasted sienna - 150,
• natural sienna - 240.

Dyes can be rubbed with oil both by hand and by machine.
Dyes that combine poorly with oil are grated in two stages (zinc white, ultramarine). After the first rubbing, they are left alone for 24 hours. They then become runny and liquid. More dye is then added to them.
The more intense the dye, the thinner it must be grated.
The following dyes belong to this group:

• yellow and red ochres,
• iron reds and other metal oxides,
• organic blacks,
• cinnabar,
• natural and burnt umber,
• ultramarine,
• lead white.

Coarser rubbed paints include:

• yellow chrome,
• manganese,
• cobalt,
• ultramarine green,
• schweinfurt green,
• smalta.

The above dyes lose their color when grated finely.
Some dyes (e.g., zinc white) are fine enough on their own that they just need a good mixing with oil.
After prolonged storage of oil paints, heavier ones (e.g., cinnabar, caput mortuum) and light ones (e.g., cobalt, ultramarine) give off oil, thicken, gelatinize or harden and become unusable.
These changes are caused by the interaction of binders and paints.
All cracklings jellify. Dyes that have metal compounds in their composition (gley, minia, zinc white) harden.
Paints grated on oxidized or untreated oil harden most often.
Poorly purified oil contains water and protein bodies in it, and as a result it becomes bitter - it breaks down into free fatty acids and glycerin.
The resulting fatty acids react with lead, copper and, in part, zinc dyes to form soaps that promote paint hardening.
In oil painting, similar changes are caused by the action of resin acids on oil. In addition, in these paints, resin can be released from dissolving oils, which also causes hardening.
To remove this defect, the paint must be grated a second time and, with excess oil, drained on blotting paper.
Cinnabar, the longer it is in the tube, the sooner it darkens later on the painting.
It is best to grind the paints by hand, because then we are sure of their quality - it is easier to adjust their fineness accordingly.