During a raku firing, carbon often gets trapped in the superficial glaze matrix. This leaves a scour of greasy looking, ash-tone schmutz on the surface.
This is generally not a lovely effect (although carbon trapping can be beautiful in the right context). The 'raku manuals' (of which there are a plethora) generally recommend elbow grease, abrasive soap, and maybe a brilloo pad for extreme cases.
Unfortunately, there are many pots that don't seem to follow the program. Take the example in the picture above. I spent hours hacking at this thing with a brillo to no effect. In the end, I had a very unsatisfactory pot that I wasn't comfortable selling.
And then... a miracle...
I was reading an obscure monograph on raku, and the author talked about using a blow-torch for cleaning carbon trap. I tried this about a month ago with a great deal of trepidation. I can't begin to articulate my delight when the ashy sheen began to flow away from the flame. I should note that (contrary to my fears) the blow torch had no impact on the glaze colors... whether oxidized or reduced.
The only problem... heat shock sometimes makes things crack. In the case of my beautiful ling cod plate (see below) the plate cleaned up nicely, but split down the middle near the end of the process. Bleagh!!
Still, it's a great new tool for my arsenal.