The knives I make are are of a lovely high-carbon steel, surpus mill pieces from Boeing's giant wing-cutting mills, and I usually temper them, which, in addition to the desired hardness, turns the blades fascinating colors. Blueish, usually - the traditional temper heat for a knife is about 570 F, which is very blue:
I really do like that color, and it looks pretty neat after the handle is put on, but I'd been curious about whether I could mark the temper color somehow, incorporate some sort of pattern or design into it. Temper color is caused by oxidation of the steel at specific heats (470 is orangey, 550 is purple, 630 is green, etc) - the oxidation isn't the hardening, just a useful side effect you can use to guess what temperature you're at. So I thought if I was able to cover the steel and prevent oxygen from reaching it while I heated it with a blowtorch, I might get silvery areas incorporated into the final finish.
Which means I have to find some sort of stencil to put on the steel that's 1) airtight, 2) capable of surviving 600 degree heat, and 3) capable of being removed afterwards.
Ground a piece roughly flat (Did not bother with the silly mirror-finish I put on the sharp pointies) and tried, from front to back, a clear heatproof lacquer, the heatproof paint we covered Geordie (the forge) with, a standard spraypaint, more spraypaint, and, just for fun, two kinds of sumi-e ink used in japanese inkpainting. Then tempered it with the blowtorch.
This is the result, after the paints were taken back off. Way over on the right, you can see the lacquer had some really strange chemical reactions going; it looks like the steel under the lacquer actually oxidized more. The others, no real change - it's just as blue under the paints as it is around them. The steel pulled oxygen out of the paints! WHich makes sense, given how volatile they are. The very pale lines around the paints are caused by extra masking-tape goo. I also learned that solvents will take off tempercolor, which was not really a surprise.
But the really, really wierd and also excessively cool thing? Look on the left, where there are two clear, sharp-edged gold lines in the blue? That's the sumi-e. Handmade ink, charcoal and water, applied with a bamboo brush, and removed with just water, not acetone or MEK. (YAY! MEK!)
Charcoal has no spare oxygens! It's just carbon!
Utterly thrilled. I can paint calligraphy on a naked blade, heat it to six hundred degrees, dip it in water briefly to wash off the ink, and the characters I scrawled, all sword-edged and curled like withered leaves will be marked indelibly in the steel itself.
Looking forward to the next blade like you would not believe. :) Whee!