I use a couple of slips that seem to respond to the ash pretty well. Funny thing is that the red colour surfaces where the slip isn't. That copper red migrates under the surface of the slip and shows up where I had a paper resist pattern.
The General Purpose Slip I use is:
General Purpose White Slip
500 Nep Syn
for copper red I add 2% each of copper carb, cobalt carb, manganese and RIO.
for Mother in Law blue I add 1/2 % cobalt carb and 1% iron.