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Zinc Oxide (ZnO)

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Garden Gives Me Joy

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New to Zinc Oxide (ZnO). It appears to have hardened one batch and not another. ???

#1. Castile soap: soap was harder than otherwise or at least similar with slightly over 1 tbsp ppo.
#2. Brine soap that usually hardens acceptably well because it's a brine soap and despite having lots of soft oils: soap was softer than otherwise with 24g ZnO.

Is ZnO acidic which can break and consequently soften soap, especially if it is already a little soft? Outside of changing the oils, can working around the excessive softness be a matter of quantity or method or timing of adding the ZnO?
I used apx 1tbsp ppo ZnO. What is the best percentage of ZnO ppo added to soap?

Also happy for any other stories or observations.
 

ResolvableOwl

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ZnO is amphoteric and will be attacked by lye. In this respect you might call it “acidic”. But during saponification, lye will be used up, and ZnO recovers. A probable side product is zinc soap (heavy metal soap scum), since ZnO is also alkaline (binds to fatty acids like Ca and Mg in hard water).
 

cmzaha

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Okay, I suppose I will show my stupidy, why use Zinc Oxide in soap? If it is to harden you can use SL or Vinegar. I did not understand the above post if it acts as a chelator or not, but there are better chelators such as SG I am sure and Vinegar with SG works wonderfully as a chelator and at least a temporary hardner for de-molding faster.
 

cmzaha

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I will agree with the itchy skin, but it does not stay on the skin long enough when it comes to soap and the percentage would not be high enough to make much difference to warrant the cost of Zinc Oxide As for Calamine in soap the same issue applies, you are still dealing with "Soap" which causes itching for some folks. You can add Calamine and Zinc Oxide but you cannot make Soap from Calamine or Zinc Oxide.

The OP mentioned hardness so I guess that is what threw me, and the second post looks like it made mention of binding metals, (chelator)?
 

ResolvableOwl

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Zinc is a metal. If at all, zinc compounds will use up chelators, but won't act as one themselves. Zinc citrate/gluconate or zinc EDTA waste chelator capacity, that would be needed to keep other trace metals from their annoying (calcium, magnesium) or harmful (iron, manganese, copper…) effects.
 

Garden Gives Me Joy

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Sorry about any confusion re the purpose for using ZnO. Having never done so before, I was testing it to observe its performance in general.

Example; while I found that it smothered lather, it felt soothing in a powdery way to me.

Re hardness; I later noticed what @ResolvableOwl said re it being amphoteric. I wrote the post BEFORE the ZnO in that different batch recovered. When it finally hardened, it was rock hard ... It just took its time.

Re Chelation. Thanks for clarifying that ZnO is NOT a chelator and can in fact use up the chelator being already used. I currently use Citric acid and adjust the NaOH accordingly. However, when/ if using<ZnO, should and or by how much should one increase the citric acid (and NaOH)?
Would increasing my chelator be made all the more urgent when I make brine soap (which, in addition to rancidity concerns, to my understanding also needs a chelator more than non-salt soaps ... to reduce soap scum from salt).

The ZnO has made harder both Zany-style 3.5% salt Castile & Bastile soaps on one hand and more linoleic acid-rich brine soaps on the next. However, so far, the latter are more sponge - like and crumbly. It's too soon to tell re shelf life. I am curious about this because they are all sweating, even the Castile and Bastile soaps. Wondering if the ZnO helps or hurts re humidity in salty soap. (I am currently in a rainforest climate). Will it just absorb moisture and crumble away in a few months?! Need to read about weighing samples.
 

ResolvableOwl

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Just to clarify: I am not an expert on ZnO in soap at all. I had to do with it in a totally different context where ZnO (chemical grade) was dissolved into concentrated NaOH lye. ZnO made for cosmetics/pigment use might behave differently dependent on pre-treatment. But if the zinc should be any effective (besides being white), it must somehow find its way into solution.

I'm glad that your batch “did it” and became hard! Who knows what's really going on in zinc-bearing soap batter? Maybe the powder grains help with draining water (good for hardening, but promoting dew)? Any chance you can give it another week or two of curing in a dry climate?
 

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