Yellow spots appearing on soap

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Jun 17, 2021
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United Kingdom
Seriously? Come on. It's like death and taxes. Left to exposure, it's gonna happen, be it a living organism, or by-products of formerly living organic matter such as oils. Do you really believe that a scientific reference needs to accompany a statement that reiterates that decomposition is inevitable? Some things may take longer to decompose, like steel for example, but given time and the right conditions even steel will decompose.

I would be more concerned about how fresh your oils were to start with, than the use of aluminium foil on the top of the jar in which you made your infusion. Oils have a limited shelf life before they start to go bad (oxidation, rancidity, etc), although it varies with different types of oils. If you start out with oil past its shelf date, it's more likely to contribute to DOS (dreaded orange spots) or rancidity than fresh oils. Adding other factors to older oils will just speed up the process.

Some minerals in water are more likely than others to contribute to DOS. But so do various other factors, like contaminants from handling (our bare hands, for example, work surfaces not thoroughly cleaned could also be a factor, impure additives is another possibility.)

And another factor can be super-fat. Your recipe is set at 5% super-fat, which means the goal with that recipe is that 5% of the oils do NOT get saponified. The excess fat that does not saponify remains free to go rancid when the right conditions exist. Even while using a soap calculator, there is potential for a higher super-fat than we plan. That can happen when we accidentally put a bit more oil into the mix than called for, either by human error or equipment malfunction (the weighing scale). It can also happen if our lye has absorbed water from the air (particularly when leaving a bottle of lye open to a humid room for any length of time); then when weighing the lye, we are actually weighing lye+water (absorbed from the air.) Also the Saponification values of oils are generally a range (xxx-xxx) rather than an exact number for that specific batch of oil, so variations can occur. All these things can contribute to a higher super-fat, and if the oil is already old, that can contribute to DOS.

This does not mean that rancidity is inevitable in all soap, but excess fat will very likely go rancid eventually if the soap remains on the shelf for years and years and sometimes sooner.

Thank-you :)

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