On the German soapmaking forum, there are several posts noting that the presence of ash and the oxidation of oils(rancidity) often go together. When the ash appears in small amounts, I have not noticed any oxidation. When the ash develops over much of the surface of the soap bars, the soap has gone rancid, in two instances in my own soapmaking. If it is indeed the same ash, is the rancidity due to a lack of strong alkali in the soap, to finish the saponification process, because the sodium hydroxide has been turning to sodium carbonate? Are there other questions to ask to discover the cause of oxidation of oils? We use distilled water so that the minerals don't get in the way of saponification. We use oils as fresh and pure and we can get. We buy lye as pure and fresh as we can. We use utensils as clean as we can. We weigh the lye, water, and oils, and check them twice and thrice. We calculate lye amounts very carefully according to the values of the oils. It is costly and very disheartening to have to throw out a batch of soap due to oxidation. Any help in pinpointing the cause of oxidation would be most appreciated. Thank you very much!