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Here we go :)
+1 on the art supply stores, also for fine grade chromium oxide.

Jimmy, cigar ash together with saliva forms a very mild alkaline mix.
Thats why. :)
(Works with cigarette ash as...

Sounds great!
Graphite is a mix that includes clay.
Depending on the hardness of the pencil its more or less.
Should be interesting to find out which ones work best.
(And what type of clay it...