Not sure why I had not tried this method of making pigment before. The volcanic rock is mainly red but has a lot of other minerals in it, and as you can see the ground pigment varies in red-brown colours. The ground pigment fell through the file onto the paper. I made paint with the pigment you see here.
The lower left swatch of paint wet in wet shows how the different pigments can separate out. This is on 100gsm paper in my test book, so the cockling of the wet paper has enhanced the separation of the different pigments.
I had been reading up on middle stone age pigment manufacture which included tests of possible way of extracting the powder. The article discussed direct grinding on the grindstone and I wondered why I had not tried this before (even though I knew about this method). I don’t have a grindstone, so the file was substituted. This method will work well on this type of rock which is relatively soft compared with some of the water-shaped rocks of rivers I see. So on rocks similar to the one here, I will definitely use this in preference to my hammer!
‘Direct grinding is the most efficient method to extract fine powder from softer shales and siltstones.’Rifkin, Riaan F. “Processing Ochre in the Middle Stone Age: Testing the Inference of Prehistoric Behaviours from Actualistically Derived Experimental Data.” Journal of Anthropological Archaeology 31.2 (2012): 174–195. Web.