About as All-Natural As You Can Get Soap February 27, 2013 18:45

When I described this project to me mum some weeks ago, she brightly exclaimed "Oh!  We tried that when you were a little kid and it was a MISERABLE failure!" So I guess this post chronicles my second effort at making soap like in the old days.

The Goal:

Make useable bar soap starting with suet (beef fat) and wood ash.

The Plan:

I divide this project into phases. Phase 1: render the beef fat to get pure tallow.  Ha!  I already did this.  See my blog post about it here. Phase 2: leach potassium hydroxide (aka KOH or caustic potash, a form of lye) from hardwood ash. Phase 3: use science and guesswork to combine these two into bar soap.

Phase 1: Done!

Phase 2:

 2013-02-11 15.35.15
 First I poked some holes in a bucket.  This is a 5-gallon bucket left over from coconut oil.


 2013-02-11 15.38.20
Nest the bucket inside another and line the bottom with some kind of non-reactive filtering material.  Here I've used a cotton rag.  You can't see it, but the lower bucket is larger, so there's plenty of space between the bottoms of the buckets when they are stacked.


 2013-02-11 15.41.48A couple inches of sand poured in over the filter material.  The sand is really the main filter bed; the rag is just to keep the sand from escaping.


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Hardwood ash packed in about 4 inches deep.  There was still a bit of residual charcoal in there, as well as some old burned nails.  It helped to wet the ash in a big garbage bag (which is where I'd been keeping it) before transferring it to the bucket.  Otherwise it's really dusty.


 2013-02-11 15.46.15
Any homebrewers reading this will recognize this process as similar to lautering during an all-grain brew.  Basically, you add small amounts of water at a time and allow it to trickle through the solid filter bed, dissolving solutes (in brewing, sugar) along the way.  In this case, we are trying to dissolve the potassium hydroxide (hereafter KOH) present in wood ash.


 2013-02-23 10.30.13
Leached ash water

 This is the liquid that collected in the bottom.  I collected it once and poured it back through the ashes to leach more out, but since I only measured the density once I don't know how effective this was.  Those floaters are bits of coconut oil that were leftover in the bucket I used.  At this point I'm pretty sure that liquid contains some lye, so Phase 2 complete!

Phase 3:

Now for the pièce de résistance; actually make the soap. The classic problem of old-timey soap makers is that it's difficult to know the concentration of the lye solution produced by the leaching process.  Many recipes (widespread on the internet) call for such folk remedies as floating an egg or dissolving a feather to determine whether it is strong enough.  A more quantitative solution would be to boil off all the water and weigh the remaining solids.  This, of course, would assume that KOH is the only solid present in the water. Luckily, I came upon this website that offers a neat conversion between density and mass ratio (i.e. what % of the mass is KOH) of a KOH solution.

 2013-02-23 10.32.18
ascertaining the density

 Not having a graduated cylinder or other accurate measurement for volume, I weighed equal volumes of tap water and lye solution, assumed the water had a density of 1kg/liter, and used the ratio of the two weights to estimate the density of the solution at 1.07 kg / L.  From the table on the aforementioned HandyMath website, this puts the KOH content around 8%.

 2013-02-25 14.07.32
soap in the making... hopefully!

 I decided to use 1kg solution because I figured it would fit my pot well.  At 8% KOH, this amounts to 80g KOH and 920g water.  Now I want to see how much tallow to use.  SoapCalc.net's Soap Lye Calculator only lets you input fat amounts, so I used 1kg tallow at 5% superfat and it spat out 190g KOH.  A little math and I see I need 421g tallow for my 80g KOH.  Shown above mixing and heating.

 2013-02-25 14.12.57much more watery than the cold-process recipes I'm used to!

 ... and mixing and mixing.  But it doesn't come to trace like your regular cold process soap.  You see, there is a lot of water in here, and since we're using potassium hydroxide instead of sodium hydroxide like we normally do for bar soap, we get potassium tallowate (soap) instead of sodium tallowate.  This will clean the same, but is more soluble in water and hence forms a liquid soap unless nearly all water is removed.  I have a lot of water in this batch but I want hard bar soap, we need to add some sodium. I poured half the batch into a jar as the control, then to the rest I added some sodium chloride - one tablespoon.  I added it 1/2 tsp at a time while mixing until it reached a pudding consistency resembling the trace stage of cold process soap.

 2013-02-25 15.20.00
This is the stuff without any salt added.  The whiter layer solidified enough to prevent the bottom (liquid) layer pouring out when the jar was tipped.


 2013-02-27 15.36.43
solid enough to mould, just barely

 This is the stuff with salt added, 24 hours later.  Pardon the impromptu mould.  As of this writing it attained the consistency of fudge.  I'm going to let it cure for a while and see if it hardens as more moisture evaporates. If that doesn't work, I'll consider doubling the salt.  Here's why: 80g of KOH is 1.43 "moles."  A "mole" means a gazillion molecules.  When describing chemical reactions, it's useful to talk in terms of molecules rather than weight or mass. I only added salt to half the batch, so we had 0.72 gazillion KOH molecules containing potassium.  We want to displace them, so we might reasonably add 0.72 gazillion salt molecules containing sodium, and assume that most of the soap molecules will end up with the sodium rather than the potassium.  This is probably true because the sodium-soap is less soluble than the potassium-soap and will leave solution as it forms.  Or something like that.  I think.  Any real chemists, help me out here? Anyway, this gives us a ballpark amount of sodium to add.  But I did not do this math when making the soap, I did it just now. I put in a tablespoon of salt, or 22g (again, measured just a minute ago).  A mole calculator tells us this contains .37 moles of sodium, roughly half the 0.72 moles of potassium.  So, doubling the salt would give 0.74 moles of sodium, or about one molecule of sodium for every molecule of potassium we're trying to displace.

 2013-02-27 15.37.02
Here's the non-salted stuff  after another day.  The upper part is a semi-solid foam which does appear to be soap.  I thought it was interesting because the watery layer at the bottom is reduced in size relative to the previous day.


 To be continued?

I'll post an update in a few weeks to report on whether the salted soap cured hard.  If not, I may re-melt it and add the extra salt as proposed above. Postscript on assumptions: One big (too big?) assumption made above is that potassium hydroxide is the only significant solid dissolved in the lye solution.  To see if this is reasonable, I checked the recipe found here using 18.2 oz dried homemade lye and 6lbs fat against soapcalc.net.  For 6lbs tallow soapcalc recommends 18oz pure KOH, suggesting that this homemade soap recipe (widespread on the internet) assumes that homemade lye when dried contains 99% pure KOH.  Ok, it's a little fuzzy, but at least we're going to be ballpark here. I've also assumed that the equilibrium of NaCl + Potassium Tallowate <> KCl + Sodium Tallowate will favour the right hand side.  I tried some googling to back this up but all I got was cosmetic ingredient lists.