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Question asked 2019-09-07 16:08:30 ...
Most recent comment 2019-09-09 16:00:11
In my search for the best oil medium for thinning oil paint to the extreme without being up overly glossy, I came across a thread that suggested using pumice powder for a matte appearance. This sounds intriguing because my aim is to paint mountainous scenes by first pouring the paint and letting things happen as they do and then working into those poured layers. Some might say to use acyrlis for this type of approach but i'd rather start to explore other ways of getting an alcohol ink type look on the canvas.
Answers and Comments
I have no direct experience with adding pumice to oil paint (I
have used it many times as an addition to animal glue bound ground) but would
worry about adding dry pumice to tubed oil paint as it could easily push the
pigment load beyond the point where it is properly bound.
Pumice also has little to no hiding power. Given this, it
would have little effect on hue, as compared to a white pigment. I would,
however suggest coarsely ground calcite or marble dust as a probably better
choice for this purpose. I would grind up some calcite (or whatever you choose)
into an oil binder to a relatively lean consistency and add this to the paint
rather than add the dry powder to pre-made paints. This will change the
consistency but avoid creating an underbound paint film. You will likely have
to experiment with proportions before coming up with the optimal material.
We have lots of experience with extender pigments or fillers (white pigments with a refractive index of 2 or less) in oil paint. Extender pigments include calcite or chalk, kaolin, marble dust (another form of calcite), mica, nypheline syenite, silica, talc, wollastonite, etc. These are white pigments of low refractive index.
Most extender pigments have a refractive index between 1.5 and 1.7, so that they do not have much effect on colored pigments in oil paint. Pumice is not typically considered to be an extender pigment and usually is not used in paint as its color (off-white) has a slight affect on white and light colored paint. It can be mixed into oil paint, but most available grades are very coarse and you will find difficult to work in paint. Simply adding chalk to oil paint can significantly reduce gloss and makes for a better filler.
Pumice can be positively brutal on brushes but it can give a beautiful paint surface if not overused. Braque used sand and sawdust as fillers/textural inclusions in oil paint, and may have also used pumice (it was present in his studio, at least for sanding grounds). I am aware of one painter who reported good results using diatomaceous earth as a bulking agent for oils, and who surmised that the irregular shape of the particles would interlock within the film.
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