With copper or other oxide-based glazes, the worst that happens is a muting of color. Things are a bit more frustrating with 'white' glazes. Some of the most beautiful effects in Raku emerge with 'crackle'- the superficial, post-fire fracturing of a glaze, with the lines blackening in the smoky atmosphere. An uneven ashy smear does little to improve the effect.
Raku 'guides' (of which there are a redundant hoard) suggest that you tackle these stains with a combination of abrasive cleaner, brillo pads, and copious amounts of elbow grease. Unfortunately, I've spent hours scrubbing to no avail (earning myself an over-developed wrist that earns me a snide comment or two, though).
This is a beloved effect in Japanese 'shino' glazes. Malcom Davis has generated a famous variant on this type of glaze (left). I fooled around with shino back when I audited a couple ceramics classes at the University of Idaho (below- another recipe called 'Moon Rocks' for obvious reasons). Someday, when I build an expansive anagama kiln in my backyard, I plan to do more.
My main concern lay with thermal shock. Raku is already a stressful process, and you lose a frustrating number of pieces simply through the reduction process. Adding a kiss of localized flame seemed like a step too far.
Still, you can see (in the image at the head of the page, or to the left) just how disfiguring these carbon stains can be. I'm all in favor of 'artful imperfections'... but only in context...
In this instance, I'm scouring a pot that was fired months ago, and that I was considering consigning to my 'red-headed stepchild' bin (things that I give away to office-mates and skeet shooters).
You can see the difference in the 'after' shot (below). I found a buyer for this piece at an Art Fair this weekend- so time well spent.
Still- it's a nice example of a learned technique that's going to improve a lot of my work (and save me a boatload of torque on my elbow).