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After a silicone food mold has been used to prepare soap using very caustic materials (such as sodium hydroxide), is such a mold safe for use with food again? Is a regular dish washer cycle enough to clean away such materials? Or is some of the sodium hydroxide potentially absorbed into the silicone?

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  • 1
    Personally, I wouldn't, but I can't back that with any citations.
    – FuzzyChef
    Aug 31, 2023 at 17:18
  • @FuzzyChef: I agree with you, but I need to convince somebody who is doing just that. So I need a real source.
    – dotancohen
    Aug 31, 2023 at 17:47
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    It appears that sodium hydroxide is often an ingredient in dishwashing detergents/tablets, so that would make the brief answer to the title question a 'Yes'.
    – user85471
    Sep 1, 2023 at 1:48
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    Pretzels (and other things things with "pretzel crust") are literally dipped in lye (sodium hydroxide) solution before baking. And soapmaking neutralizes the lye used in soap-making. kingarthurbaking.com/blog/2021/05/20/… "Lye - it's hazardous, but it's not plutonium."
    – Ecnerwal
    Sep 1, 2023 at 3:23
  • It's safe but may not taste good. Taste is very sensitive and could potentially taste the soap. Sep 2, 2023 at 1:25

3 Answers 3

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I think it sounds fine.

Strong acids and bases do their thing by dissolving into water. They are highly soluble. Once diluted through rinsing, they’re not really a problem. (A dishwasher would be sufficient for this; detergent would not be necessary.)

Now, there’s still a worry that the lye could have survived the rinsing by seeping into porous cookware. But pure* silicone is not very porous. The last pretzel you ate probably had more sodium hydroxide clinging to it than those moulds do.

 * Silicone with fillers may be porous. It’s not common in cookware. If you suspect fillers, try bending the cookware hard. A color change in the bent area indicates fillers.

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    I agree with this. Chemically "silicones" are organo-siloxanes; basically a plastic with silicon as the backbone instead of carbon, and with carbon side-chains, so they are very chemically inert and unlikely to absorb anything to any significant degree.
    – bob1
    Aug 31, 2023 at 20:18
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    So there is no worries about leftover caustic materials, but what about permanent changes in the material of the mold that may result in it being less then safe? Like when heating of plastic containers makes them leach chemicals: scientificamerican.com/article/…
    – Ivana
    Sep 1, 2023 at 10:18
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    @Ivana No, there’s not. Silicone does not react with sodium hydroxide.
    – Sneftel
    Sep 1, 2023 at 11:01
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    @Ivana Silicones are quite heat resistant, up to about 200 C (390 F), but I don't know if there have been any studies looking at products produced by heat degradation and health effects of these. Most plastics melt or burn well below 200 C, so let off more damaging products that way. My feeling is that we'll know for sure on these things in about 10-15 years, given the pace of medical research. I think we'll also find that silicones are "forever" things, just like plastics...
    – bob1
    Sep 3, 2023 at 22:54
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I use my own homemade soap to clean dishes on occasion. In particular I usually let my mixer bowl, measuring cups etc. stand overnight after a batch of soap, with the residue still in them, and clean them the next day along with any other undone dishes. I also use the soapmaking lye as a cleaning agent every now and then. As one commenter pointed out, sodium hydroxide is used in dishwasher detergents.

Generally, I'd say dishwasher safe stuff is safe to use in soapmaking. As for cleaning with lye, I have only had issues when I forgot I had an aluminum pot in the sink when I dumped the solution. Oh and when I tried to dry NaOH grains in the oven it ate through the glaze on the ceramic bowl I used.

Finally I'd point out that soap batter is at maximum about 15% NaOH by mass, some of which is neutralized by the fatty acids before pouring into molds. You would expect the saponification process to neutralize the rest as well, with oil left over. Any residue that infiltrates surface pores will be more problematic as an off flavor and actual lye will be in homeopathic dilutions, and will probably just saponify some of the oils in whatever food is cooked in the mold next.

Incidentally, sodium hydroxide itself is not toxic, and only harms living tissue by corrosion. In foodstuffs like pretzels, it is neutralized by the Hydrochloric Acid, naturally produced by your stomach, into water and table salt.

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Frankly, I wouldn't risk it. My understanding is that strong alkalis are sometimes used to clean up part-set silicones (RTVs etc.), and as well as the risk of chemical combination there's the risk of something nasty being absorbed (into the structure) or adsorbed (onto the surface) and only released slowly.

Now quite frankly I tend to be pretty blase about things like use-by dates. But in the current case and considering the probable price of replacement molds: it's just not worth it.

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  • I agree with you, but I need to convince somebody who is doing just that. So I need a real source.
    – dotancohen
    Sep 1, 2023 at 13:43
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    @dotancohen well, I might be complex but I can assure you that I'm not imaginary. The other thing to consider is that the lye sold for soap making is likely to be contaminated with sufficient metal compounds etc. that it would be immediately disqualified from any culinary use. But if your friend feels like risking it, and making that decision on behalf of family and visitors... Sep 1, 2023 at 14:11
  • Yes, well, I should have properly defined the term "real" )) Something authoritative that I could point to, not just "some guy named Mark on the internet disagrees with you, dahling!".
    – dotancohen
    Sep 1, 2023 at 15:14
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    Can you put anything about metal contamination in lye into your actual answer? That's a plausible concern, unlike the NaOH itself, which as other answers have convincingly argued, is not a health hazard in low concentrations. (Neutralized by stomach acid if any does get on your food.) Sep 1, 2023 at 21:21
  • I am extremely uncomfortable expanding on it directly, because my second-hand knowledge of this derives from my customers- major chemical suppliers- who go to a lot of trouble to control what production process is used for intermediates (including lye) to be used for what ultimate purpose. Sep 2, 2023 at 6:50

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