hot-cure soap recipe from reclaimed oils?

Ages ago I saw a recipe for hot-cured soap made with any generic or mix of oils.  It was an old recipe, probably circa 1920-1930s.

Does anyone know where to find a recipe for this?

Unlike cold-cured soap, the measurements don't have to be as precise as the cure happens before we put the soap in the mould.  We also don't have to get the oils in a perfect ratio.  So it's really good for used cooking oils.

Yes, used cooking oils stink, but the recipe I had included cleaning instructions for the oil to remove the smells.  I did that stage before and it worked great.  But I lost the recipe before I could make the soap.  

I prefer making hot process soap, myself. I have a couple recipes I've developed, but they don't use scrap oils.  But, brambleberry.com has a lye calculator that might help you, here: https://www.brambleberry.com/calculator?calcType=lye

If that one doesn't work, this one is a bit broader, and might: http://soapcalc.net/calc/SoapCalcWP.asp

With either one, you fill in the type of oils you want to use, and how much extra fat you want in it, for moisturizing, if any, and it will tell you how much lye you'll need.

If you need more info with technique, or how to decide this stuff, I can help with that, too. 🙂

Jill Winger of The Prairie Homestead has a recipe.  What I like are her comments:

I have since learned from a few soap makers who shared their knowledge that while hot process doesn’t necessarily require a six week wait, you will find the texture and hardness of the soap improves after a 1-2 week wait time. So while you can use it right away if you want, it’ll be better if you still wait a bit.

https://www.theprairiehomestead.com/2015/05/hot-process-soap-recipe.html

Great idea. Here's a link to eHow with a recipe that I haven't tried but will now. https://www.ehow.com/how_6404573_make-soap-used-oil.html

Wow, that's neat.  I hadn't seen cleaning oils with flour before.  That would take a lot less electricity than the boiling method.

r ranson wrote:Wow, that's neat.  I hadn't seen cleaning oils with flour before.  That would take a lot less electricity than the boiling method.

Yes, you mentioned you'd cleaned oil of smells in the first post - were those oils ones which are liquid at room temperature and do you remember what you did?  I will check out the flour method also.
For a while I had a lot of soap that had been given to me as gifts, but our use has gone up and I'm getting low, so this thread is timely!

The recipe I have lost had something like this for cleaning used oils.

Heat the oils, add cold water to the oils and it precipitates the particles and odour causing stuff to attach to the water and fall to the bottom.  Let it sit a while, then pour off the oil and repeat a few times until the oil is clean.  

r ranson wrote:The recipe I have lost had something like this for cleaning used oils.

Heat the oils, add cold water to the oils and it precipitates the particles and odour causing stuff to attach to the water and fall to the bottom.  Let it sit a while, then pour off the oil and repeat a few times until the oil is clean.  

I'll try this soon and report back. I have tons of old cooking oil and I am running low on all purpose cleaning soap.

I want to say there is a way to calculate lye based on Specific Gravity of the oils, but I am not sure. I will look into it.

The issue I have is that I just pour all my used oils into one vat and there is no real way to know how much of each is present.

Hamilton Betchman wrote:

I want to say there is a way to calculate lye based on Specific Gravity of the oils, but I am not sure. I will look into it.

exactly!  With cold cured soap this is hugely important.

However, with the hot-cured soap, this wasn't needed because you can tell if there is excess lye before you pour the soap into the mould to cure.  Since our cooking oils are mixed, I want to find that hot cured soap recipe to try.  

r ranson wrote:

Hamilton Betchman wrote:

I want to say there is a way to calculate lye based on Specific Gravity of the oils, but I am not sure. I will look into it.

exactly!  With cold cured soap this is hugely important.

However, with the hot-cured soap, this wasn't needed because you can tell if there is excess lye before you pour the soap into the mould to cure.  Since our cooking oils are mixed, I want to find that hot cured soap recipe to try.  

I will be making liquid soap, hot process, and I will share my results.

One of the other things to remember is that people have been making soap since Roman times if not before, and the standard household didn't have any way of measuring ingredients to the nearest gram.  So there are ways to do this without knowing the exact ratios of the oils.  I want to rediscover what those ways were.

r ranson wrote:

One of the other things to remember is that people have been making soap since Roman times if not before, and the standard household didn't have any way of measuring ingredients to the nearest gram.  So there are ways to do this without knowing the exact ratios of the oils.  I want to rediscover what those ways were.

Trial and error?  We make something in the spring called, "Grandmother's Rhubarb Pie". My mother watched her mother, who's watched her mother! My mom came to visit and I wanted the recipe. I had her use the sort of implement she'd use at home, and before adding it, we measured it with modern measuring cups/spoons, because she'd always gone by, "what looked right".

I made soap once years ago, so I have limited experience, but it seems to me that part of what determines the ratios needed to make soap work is what temperature the oil goes from solid to liquid. What I've noticed recently using fats from my own animals for cooking/pastry, is that the time of year I harvested the animal makes a difference both in the quantity of fat (no surprise there!) but also in the temperature at which it goes from liquid to solid and back. The species in question make a big difference also, but for the majority of the time, I'm dealing with Muscovy duck.

To remove odors from used cooking oils, warm the oils to about 160 degrees and filter it through some food grade diatomaceous earth. The DE removes most of the carbon particles and "off" odors

On this filtering issue, I'd imagine using d.e., bentonite clay, activated charcoal, etc. would all be excellent options. The key would be finding the medium that's most easily acquired, most sustainably used &/or repurposed, afterward.

Beyond that, cold process and hot process recipes are pretty much interchangeable, with some exceptions based on certain, often superfluous additives, that don't react well, to heating beyond certain temps. For example, two of my favourite additives are goat milk, for its loads of benefits, and honey, for additional skin benefits, plus amazing lather - both of which are easily 'burned' by the lye & the hot processing. Altering the method of when & how those additives are incorporated makes all the difference, in the world - the primary difference being the aesthetics. Cold process soaps tend to be 'prettier', in that their appearance is lighter, creamier, smoother. Hot process soaps are typically darker, more rustic looking. I added my honey and goat milk too soon, in my last batch, and it came out an interesting, mottled, chocolate-y brown. But, it does exactly what I want it to - it lathers beautifully, moisturizes my skin and hair, helps repel ticks, fleas, mosquitoes, and even makes a great dog shampoo. It even smells better than I expected. I'll admit - I do wish it lasted longer, but some curing might help. I didn't wait. It's my own recipe, and the first 24 - 48hrs, it was very soft, and kind of oily. I'll make a couple adjustments, next time - but, overall, I'm pretty happy with it.

I promised I would take notes, so I will deliver!


For liquid soap:

I did not use any "fish grease," because the fish smells are the fish fat, amd I didn't think this method would help. I'd love it if this weren't true, so please prove me wrong! Anyhow, without knowing the exact amounts, the following oils would most likely have been present: Canola, "Vegetable," peanut, grapeseed, avocado, coconut. I also had some frozen, used, duck fat, pig lard, and bacon grease." All of this went into the pot for cleansing.

I found these instructions online: "For every cup of frying oil, whisk together ¼ cup water and 1 tablespoon cornstarch. Add mixture to warm or cooled oil. Heat oil gently over low heat (do not let it simmer), stirring constantly with heatproof spatula, until starch mixture begins to solidify, 10 to 12 minutes." To clarify, you want the oil warm and melted, but not got. I then poured this mixture through a cheesecloth lined strainer.  

I then let the mixture cool in a clear container. Since oil is lighter than water, I noticed some layering and poured off the good oil up top, into a steel pot.

I wanted to stick to 1920's era tech for this, so i simply weighed the oil. It came out to 8.4 lbs. I found an old recipe that gave a generic calculation and an average KOH value of
.2X where X is the weight of the oil.

Balances and scales were certainly readily available in the 20s, so I used an old doctor's 2 beam balance to weigh my ingredients.

So anyways, back to my calculations.
With 90% KOH and a .2 factor that meant (8.4 * .2 ) / .9 = 1.867 lb of KOH needed and for a 3% superfat .97 *1.867 = 1.81 lbs.

Now you will mix that much lye into 2.5x as much water.

Edit** A note on water quality. It is absolutely crucial to use a soft water for good soap making. You can use distilled water or rainwater if you aren't sure.
That is 4.5 lbs or a little more than a half gallon water
Be careful and always use proper PPE!

With the oil on the stove on low heat, add the KOH solution and begin whisking thoroughly. I cheated here and used a hand mixer. Increase the heat to medium and and continue whisking until you get a mayo like consistancy.( You can actually pour these into molds for a solid soap now, but it will not be really hard and will go rancid quicker.)
Remove from heat amd forget about it overnight
It will remain warm until fully saponified, and may take 24 hours.

At this time, dilute the solid soap with an equal part of water
Stir vigorously for about 10 minutes. Then let it sit. You can forget about it for a week, or stir it every few hours to get a quicker result.

edit note: this is not the time to get impatient, put the hand blender away. if you over mix, you will get a frothy mess that will ruin your soap.  This mess will eventually recover, but it will not be of the same quality.

After the mixture becomes homogeneous, you can add more water for a thinner consistancy, if that is what you like.

Edit note: if you find the soap too hot after a while, you can always add carrier oils and fragrances. Be careful though, only add a small amoint at a time. .5% of the total weight is a good target for fragrance oils. You dont want to overdo it here.

My first thought was that you wanted to render the old nasty oils rather than just use them directly. If you have food/other particles in the fat, you just need to heat it and strain it. Bacon grease makes great soap, btw. Here's a decent link on how to render wild boar fat, which is pretty stinko:   https://www.americanhunter.org/articles/2011/3/7/how-to-render-and-use-wild-game-fat/

I don't know if the directions are still on the back of a can of Red Devil Lye, but that is what my mom used for directions for making her soap. She would save all the left over cooking grease, when she had enough she would make her soap for laundry. Once made she would pour into half gallon milk cartons for it to cure. After a few days she would remove the milk carton, let it cure for about a month. My job was to grade it up to use for laundry. Floors were mopped with it, dishes were washed in it. bodies were cleaned with it.