Re: clearing dichromate stain
On Jan 5, 2007, at 11:37 AM, Katharine Thayer wrote: my own comparison of last evening shows a clear superiority of bisulfite over alum for clearing. (I'm assuming that the "alum" that's meant when no specific alum is named, is the "common alum" containing potassium and aluminum; that's what I used. I don't have any chrome alum on hand at the moment to see if that works any better.) It seems to me that alum, at least this common alum, works differently from the bisulfites, in that it fades the stain to lighter tan over time (just as does plain water) rather than clearing the stain to pale blue-grey, as do bisulfites (and sulfites, given enough time). I'll post this comparison soon.Here's that comparison I promised. I usually produce dichromate stain, when I want it for some demonstration purpose, by coating paper or mylar with a layer of unpigmented dichromated gum and putting it in direct sun for 5 minutes or less to get a very dark brown stain, but there's no sun in my part of the world just now, so I set the coated paper by a window for half an hour, where it absorbed enough UV radiation from outside to produce a medium tan-brown dichromate stain. http://www.pacifier.com/~kthayer/html/Alum.html Katharine
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