I am trying to make my own repousse black pitch using black tar and
adding some red baked clay powder.
Adding Plaster of Paris to tar. Chip up the hard tar and place in a
pot that you have dedicated to this task, bring to a heat slowly to
melt/liquify the tar (use a well ventilated area). Then begin adding
Plaster of Paris, mixing it in completely until the 'former' tar
becomes almost plasticized in its texture. It takes a surprising
amount of Plaster of Paris, but the results were very good.
Pre-mixed pitch is much more expensive and so the effort is
definitely worth it.
K. David Woolley