Somewhere around 280C to 290C, glycerin breaks down to yield acrolein.
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Acrolein is immediately dangerous to life at a concentration of 2ppm.
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Glycerin is in soap, unless extracted by the manufacturer.
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Can using homemade soap as part of a lube recipe result in a lethal smoke when shooting the boolits? I doubt the lead gets that hot, but the lube burns into smoke.