@Jill B Blasius -- I agree with Shari -- it's not rancidity/DOS. It looks like the soap went through a partial gel -- the oval ring on the cut face is caused by that. The edges might be darker due to the surface of the soap reacting with something on the soap mold. Or the surface of the soap is darkening upon exposure to the open air (if so, the cut face will follow suit shortly.) Or there's some other reason that may well remain a mystery. I see this from time to time, and I'm not always certain what causes it.
Folks, keep in mind there are a lot of reasons why soap becomes rancid. It's good to use reasonably fresh oils, but even so, your soap can still become rancid for other reasons. Major triggers include contamination by trace metals from your fats, water, other additives, and equipment; exposure to sunlight or bright artificial light (UV, in other words); air pollution; and contamination from your hands, soap making equipment, and soap storage area. You can do your best to minimize these problems, but it's fairly likely you can't entirely eliminate them.
That's why antioxidants and chelators are helpful additives to consider to increase the shelf life of your soap. Research I've read recently stated a chelator is more effective at preventing rancidity/DOS in soap than an antioxidant, so if you're going to use one in your soap, choose a chelator. An antioxidant is also helpful if you add it to your fats before they go into storage.
Folks, keep in mind there are a lot of reasons why soap becomes rancid. It's good to use reasonably fresh oils, but even so, your soap can still become rancid for other reasons. Major triggers include contamination by trace metals from your fats, water, other additives, and equipment; exposure to sunlight or bright artificial light (UV, in other words); air pollution; and contamination from your hands, soap making equipment, and soap storage area. You can do your best to minimize these problems, but it's fairly likely you can't entirely eliminate them.
That's why antioxidants and chelators are helpful additives to consider to increase the shelf life of your soap. Research I've read recently stated a chelator is more effective at preventing rancidity/DOS in soap than an antioxidant, so if you're going to use one in your soap, choose a chelator. An antioxidant is also helpful if you add it to your fats before they go into storage.
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