Some pigments may need some help in mulling a workable paint. A higher acid value oil such as Pale Grinder’s Oil is a good start, but if that still doesn’t do the job, a little stearate paste can be used. This is often used in commercial paint to gel the oil which prevents pigment settling, increasing shelf life. In low quality paint, its overuse as an adulterant leads to a greasy feeling paint.
Heat 250 ml linseed oil to 150° C (300°F)
Gradually stir in 50 grams of aluminum stearate powder
This heated solution can be poured into an empty tube or caulking gun cartridge. It will gel on cooling so it can be squeezed from the tube much like paint.
1 part of this solution may be added to 4 parts linseed oil, by weight, for pigment grinding.
Reference:
https://cool.culturalheritage.org/waac/wn/wn23/wn23-3/wn23-304.html