Potato soap update

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I'm in Southern Idaho, I may have to make a potato soap. Thanks, as was already stated, I needed something else to try, now that I finally made a salt soap....which I think is awesome...
 
I might have cheated on mine. I dissolved my lye in 1:1 plain water and just added another 1.5 of potato water to my oils. Seems to work with oatmilk so figured it would work with potato water.

12 large rounds swirled with yellow oxide .. and a few little potato men (Mr. Potato heads?) left over. There's also 3 little cocoa men left over from the Minion irises but they were not made with potato water.

Everything sat over night and looked good this morning so I popped them in the freezer for a few hours to get a nice release. More pics later.

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I was curious what the old fellas had to say (or not say) about the use of potato in soap and found this tidbit. I get the strong impression from this excerpt that the potato flour actually consumes some lye. The comments in brackets [] and the bolding are mine. Here goes:

Potato-flour is to a considerable extent used for filling soft soaps [soap made with mostly or all KOH]. It does not swell much either in cold water or in an alkaline solution, but when caustic lye is added it is at once converted into a smooth, stiff paste. It possesses also the property of fixing much moisture and making soap filled with it more compact and durable. As, however, the resulting product is more turbid [cloudy], only inferior qualities of soap are filled with potato-flour.

Filling with this material is effected as follows: A large vat of soft wood is placed near the kettle for the reception of the filling material. Pour into the vat 1 part of water or potash solution of 5° to 8° B., stir in 1 part of potato-flour and finally crutch in 1 part of silicate of soda. The latter may be omitted using in place of it 1 part of potato-flour stirred into 2 parts of potash solution of 10° to 12° B., or into potassium chloride solution.

[B. stands for Baume. It is a measure of the density (heaviness) of a solution. In this context, a higher degree Baume means the lye solution is stronger or more concentrated.]

Add to the filling material thus prepared, whilst stirring vigorously, a sufficient quantity of the finished soap from the kettle to form a dissolved creamy mass and then thoroughly crutch it into the soap in the kettle. The soap, after the filling has been added becomes soft, viscid [thick] and long [superfatted], and has again to be fitted or shortened, this being effected with caustic lye of 27° to 30° B.

[To "fit" the soap or to make it "sharp" to the tongue or "short" means to make the soap slightly lye heavy. This was a way to ensure the soap was fully saponified using the soapmaking methods of the day. If the soap was to be a toilet soap, it would be finished at the end to make a more neutral soap. Otherwise most soap was finished and sold slightly lye heavy. This helped prevent rancidity.]

Add of this lye, whilst crutching [mixing] vigorously, a sufficient quantity to impart the shortness [slight lye excess] required and a sample upon the glass appears full and round as well as firm and shows the required sharpness [zap]; for 1 part potato-flour about 1 part of caustic potash [KOH] lye is generally allowed for fitting.

The operation of filling is frequently, also carried on as follows: About the quantity of lye required for fitting is crutched into the finished soap previous to adding the filling material, the soap acquiring thereby an almost torn appearance. The filling, however is more readily and better absorbed, and the soap presents a more uniformly smooth appearance.

The soapmaker's handbook, Carl Diete, 1912.
 
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I think I actually read that before but forgot about it. Sounds like they were using a lot of potato though. I wonder what is in potato that consumes lye, there is nothing to saponify so I wonder if neutralizes like citric acid does. I just know it makes a nice lather and leaves your skin feeling nice.
 
My guess is the lye is breaking down the starches. NaOH or KOH is pretty good at reacting chemically with many things, so that wouldn't surprise me.

I made some soap the weekend before last and wanted to put sugar in the water before I added the lye. Of course I remembered that only after I'd put a small amount of lye in the water. I took a chance, stopped adding the NaOH, and quickly poured the sugar into the weak lye solution. Before I started to stir the mixture, I watched the granules of sugar drop to the bottom -- the whole process seemed unusually slow for some reason. As they fell, I could see small bubbles form on the sugar granules and rise to the surface of the liquid. That probably buoyed the granules a bit, so they fell more slowly.

I know sugars can react with NaOH to form carbon monoxide (CO) gas and other breakdown products, so I suspect the bubbles were probably carbon monoxide. There was only small amount of gas so the situation was not remotely dangerous. It was interesting to see this reaction happen right in front of me, rather than just read about it.
 
The bolded part about mixing potato + lye => smooth stiff paste was the comment that caught my eye in the book quote. Y'all have been talking about adding the potato to the lye water and getting into trouble. Sounds exactly like what Diete was talking about too. Adding the potato at trace or mixing it into the oils might be the best approach to control the rate of thickening, compared with adding it to the lye solution.
 
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