After much more testing with the indicator paper - it seems that anything less than extremely acidic goes a bluey-grey colour with a purpley outline. Only the very acidic substances turn the paper pink,which is a shame, since some of the geothermal samples I've been given seem to be quite acidic (according to real pH paper and sample tests).
Nevertheless - an interesting range of colours coming up in the different portraits. Need to have reproductions of 6 decent ones, a statement and an application form done by Friday, so having a break from graphite portraits this week - sorry to those waiting for theirs to appear.
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