I would be remiss if I didn't say that great variations in speed from batch to batch are totally avoidable. Consistent time and temperature will eliminate all but a fraction of the variability. The variability you see is probably, mostly, from variations in the UV intensity at any given exposure situation. A basic dry plate emulsion should be between ISO 3 and 12, in full, midday light -- the lower number in the winter, the higher in the summer, with a range in between. The old timers used the charts that came with each brand plate, and/or developed their own. Here's one from Hammer plates:
http://thelightfarm.com/cgi-bin/hamm...gesize=Smaller
The trick of testing speed with the dark slide (just as you would test paper under an enlarger in the darkroom) is all you really need, and you shouldn't have to do it more than a couple of times in order to calibrate your recipe. Because the plates can be developed by inspection, you've got all the bases covered. When in doubt, bracket. It's always good insurance with handmade materials. 2 cents, d
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