Put a chunk of compressed pigment or mica in a grinding bowl or mortar and pestle. Grind the pigment or mica into a very fine powder, with no lumps, clumps or solid pieces. To be positive you have no chunks, you can sift the powder through a fine-grate sifter.
Place a pile of pigment on a piece of clean glass or a clean palette. With the tip of the handle of a large brush, push a small hole into the center of the pigment pile.
Add a few drops of linseed oil to the hole in the pigment pile. Using a spatula or paint knife, mix the oil with the pigment until the mixture is the consistency of butter. If it is too thick for your liking, add more oil and mix again. When mixing your oil paint, keep in mind that the oil will darken the pigments. Add white pigment to lighten the color, if desired.
Store extra paint in airtight containers and keep them in a cool, dry place. Even in a sealed container, oil paints will dry up or thicken beyond use after a few weeks so they should be used as quickly as possible.