: I have been given 3 - 1 7/8 elliptical secondaries : that are gold coated via vacuum deposition. I : think they may have a chromium base and may be SiO2 : overcoated. Anyway - I want to use them for visual : astronomy and not IR - I need to strip the gold : and the other stuff. Ferric Chloride removes most : everything but it seems like there is a sort of mottled : residual of chromium. How can I get this off without : abrasives?? (Nitric, hydrochloric acid or some mix of : the two??) Thanks - Russ

Russ, The acid method works, but it's necessary to use good ventilation. I removed the gold coating from a mirror last year by polishing it off with soft polishing pads and Linde A in water. Linde A is .3 micron aluminum oxide and I think a bit expensive. Oh yeah, the mirror was 72" diameter and it took three days to strip with a three inch tool. Before starting, I ran tests on a 5 inch flat and by using a Wyco interferometer for testing, was able to determine that it would take 30 minutes of vigorous rubbing to make a 1/20th wave change to the surface. The chrome base came off the mirror after about two to three minutes of moderate pressure. On your small flat, just be sure to put a surround of glass around the elipse so you prevent a turned edge.

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