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I was making soap last weekend and I got distracted, and thence I put too much lye into my oils blend. Basically, the whole soap paste is unusable to make soap because the cure will never be enough to neutralize all the lye contained in the soap paste.

I wanted to recalculate the lye quantity and add the quantity of oil needed to salvage my soap paste but I remembered a piece of advice I read on the recipe: never pour oil into lye, always pour lye into the oils.

Why is that?

At first I thought it was because of similar reasons to why one shouldn't pour water into acid (as these two substances react strongly when put together and the hydration reaction may provoke projections). It has been a while since my high school chemistry lessons but I think that a similar thing happens with water and basis. However, oil has pretty much nothing in common with water when it comes to chemistry. Another reason may be that what a user from a soap making community calls the "splash potential":

You are more likely to splash whatever is being poured into than what is being poured. You do not want lye water splashing out onto you and your surroundings.

This sounds more plausible but it isn't a scientific explanation. So, is there a scientific reason behind the restriction: "do not pour oil into lye"?

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  • $\begingroup$ I would say that it's more or less the splash potential thing as oil is non polar. chemistry.stackexchange.com/questions/893/…. $\endgroup$
    – Avyansh Katiyar
    Commented Jan 7, 2019 at 12:38
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    $\begingroup$ Olive oil, while not as reactive as water in this setting - is still the dilute solution and you should add the concentrate to the diluant and not vice versa. It is just common practice. Without ever doing saponification I do think it is easier to keep them mixed that way as well, with oil being so oily and all. $\endgroup$
    – Stian
    Commented Jan 7, 2019 at 13:04

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Lye is denser and tends to pour through the lighter oil, improving mixing (and possibly cutting down on the splattering of lye). The same is true of concentrated mineral acids added to water.

In any case the hazards are minimized by also adding slowly and with stirring.

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