Do you think that an additive like rice (or perhaps oatmeal) makes a soap longer lasting, or, since it seems to make them more bubbly (and therefore more soluble- like sugar) does it contribute to the soap dissolving faster?
I have made soap using the following additions: rice water, rice congee, rice powder, rejuvalac, finely ground oatmeal; not all in the same soap. Rejuvalac is a fermented liquid made from grain that I drink, and it varies based on the grain used. When I use it in soap, it is a water replacement in my lye solution. Same for rice water, in that it is water replacement in lye solution. I cannot say either contributed to a longer lasting bar of soap, bubble, yes.
As for the powdered additives, rice powder, oatmeal, both made my soap more scratchy, although some don't experience that, but my skin perceives these additives as rough. Does the soap last longer because I avoid the scratchy? Or does it last longer for some other reason? I think it's because I avoid the scratchy and use another soap more often, leaving that scratchy one alone more often than not. (Just so you know, my skin finds many things more scratchy that others do, so it/I may be a personal anomaly.)
What I find is that the rice soaps are significantly harder than the same soap made using the same formula (except not rice derivatives), but not longer lasting when in use. Other liquid substitute soaps I've made or used don't seem to be harder or longer lasting.
One caveat has to be, that to compare soaps with or without a particular additive or water substitute, the rest of the formula has to be the same, otherwise it would not be a fair comparison. So I would suggest that when you make a soap with a particular additive, you also make a control soap of the same formula without the additive and then test them against each other for a true comparison.
