This one turned out to be a lot harder than imagined… it took three attempts and a month of trying to get something that worked.
Plan A:
I bought a 4 quart stainless steel mixing bowl from Blood Bath and Beyond. Very nice, sturdy, light weight, very shiny inside surface. It is made from stamped or spun molded stainless steel sheet. Whatever made it, work hardened the material out the wazoo. It took around 20 hits with an automatic center punch to dent it. Harbor Freight titanium nitride drill bits wouldn’t even scratch it (why am I not surprised). It took cobalt steel and carbide bits to get through. Finally got the emitter mounted. Epic fail! I could only drive it with about 35 watts before it went into thermal shutdown. I want around 200 watts… Stainless steel is a VERY poor conductor of heat. It could be spit sizzlin’ hot behind the emitter, but an inch away it was room temperature.
Plan B:
Well if stainless blows chunks, copper is king. Finding a copper bowl with a flat bottom is rather hard… particularly one made of solid copper. I finally found a nice one on Ebay for $35. When it arrived… phooey! The outside bottom was nice and flat. The inside was curved. Milling the bottom flat would have been a no-no for a hand-built light.
Plan C:
Well, about all that was left was aluminum (aluminium for you foreigners). I located a Mikasa pasta bowl on Ebay (sorry folks, no longer made). It is big (13” in diameter). Heavy (3.3 pounds/1500 grams). Material is over 0.20 inches thick. Nice flat bottom. Nice shiny inside (ok, scratched to hell but polished up nice). The outside has nice ridges to increase the surface area.
Smooth/shiny surfaces are rather poor for shedding heat. Flat black, textured surfaces are good. Just how much difference could it make? See http://www.molalla.net/members/leeper/coatbar.htm
So, what’s a boy to do? Why paint it, of course. Painting aluminum/aluminium is easy. Getting the paint to stick is not. First it got an acetone bath. Next, I roughed up the surface by wire-brushing the outside with a rotary brush in a drill. It had to be a thin wheel to get in the ridges of the bowl. Wet sanded it as best I could. Etched in by weighting it down in a vat of sodium hydroxide. Etched it again in a couple gallons of vinegar.
Masked off the decorative rim. It’s a compound curve and took a zillion small pieces to tape to follow the contours. Sprayed it with Rustoleum Self Etching auto primer. Etch, retch, etch, retch, etch. Finally sprayed it with a flat black Rustoleum Truck Bed paint. It (supposedly) dries to a tough, textured finish. Seems to work.
The results: