A simplified version of Jerry's hot plate setup, which has
worked well for me, is an inverted can with a tea candle
underneath it. I cut some vents around the base and the top,
place the item to be blackened on top. When the item has warmed
(pretty quickly), I swab it with the liver of sulfur solution.
When it's dry, it's done (and often too hot to pick up by hand).
Saves me from having to heat the solution.
Simple solutions from a simple mind...
Charlotte, NC (USA)