Most pigments come ground to optimal size, but occasionally an historic or earth pigment (anything larger than about 40 microns) benefits from a bit of grinding. A glass muller (about 5.5 Mohs – yes?) isn’t necessarily hard enough for grinding (depending, of course, on the hardness of the pigment being ground), nor does a muller’s shape direct force in such a way as to grind effectively (rather, its flat bottom shape seems best suited to dispersing pigments). So a mortar and pestle seems a much better tool for grinding (and the harder the material it’s made from, the better; i.e. agate, porcelain). Yet I see mullers often discussed or even recommended for grinding pigments.
Is a muller suitable for grinding (I think of mullers as primarily suited for dispersing)? Is a mortar/pestle unquestionably preferable? Is any substance added to the base of muller to increase its hardness? Any other thoughts? Thanks.
I think that your intuition is correct. A muller is only meant for dispersing pigments in a binder. There may be some crushing but that would only be with very friable pigments.
A mortar and pestle is better for this operation but even then it can be very difficult to evenly grind the pigment. I have found that it is often necessary to pour the ground pigment through a sieve of the appropriate degree of fineness and regrind the pigment that remains larger than the sieve screen. You would continue this until all of the pigment is of the desired grind.
All of this is quite laborious. It is far easier to just purchase pigment of the desired grind. I have only relied on the above when making my own pigments from minerals like lapis or when I am performing an analytical experiment about art materials and I absolutely need to know that there are no additional additives (eg hand griding my own green earth from lumps of the mineral).
Thanks, Brian. I like the additional clairification of using a sieve, it’s a good point. Koo
It’s my understanding that impact, crush, and grinding stress are the forces that reduce stone to powder. Glass muller and roller mill mainly apply shear stress to paint, with not so much crushing force. Shear is the same physical stress as when one presses a spoon into cornstarch in water, sliding it across the bottom of a bowl to break up a cluster or lump. For an improvised studio solution for small quantities, maybe a small rock tumbler would work for this purpose. I know that some have modified electric buffers as a “machine assisted” hand muller, which I think would apply enough rotations to cause the pigment to self-abrade and reduce to a fine grain. I don’t know what the buffer disc material was in the modified electric tools, however.
Thanks, Matthew – I appreciate the imput. Koo
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