Calculating lye amount for unknown fat mix

[OrangeCrush]

When you've collected various fats over time, mostly lard, tallow, and olive oil, and do not know the exact percentages of each, what is the best method to calculate how much lye to use, or an old time process to fully saponify it?



Thanks

 

[ChrissyB]

You know, someone else might be able to give you some sound advice here, but personally I wouldn't even go there.
I presume you are meaning used fats that you have saved in a pot or whatever. For starters the fat won't be clean, it will have food particles in it, and because it's already been used it possibly has been heated to higher temps than what we would use to soap it.
If I were you, I would start from scratch with fresh clean oils, and then you will know the percentages of each in your recipe and you will get the correct amount of lye. IMO anything else would just be guesswork.
Someone else might give you the answer you want.

 

[OrangeCrush]

It's been rendered and is clean, I just forgot what percentages of each were in it.

I do have a hydrometer, perhaps it would be possible to get an amount based on the specific gravity?

There weren't scales in the old days and multiple fats were combined, so there has to be a way to convert this into cold process preferably somehow...

 

[surf girl]

I'm interested - how did you end up with a bunch of mixed up oils? Why are they not separate?

There are some people on this forum who are very good at science and math, so hopefully they will comment on what I propose. Don't take my word for it: I am only theorizing - and I have had a fair few glasses of wine this evening. What seems to make perfect sense to me at this time of night could be a load of badly thought-out bull.

The SAP for tallow is 0.143.
The SAP for lard is 0.141.
The margin between these two is 1.4%.
If you do a full substitution of lard for tallow, you will have 1.4% excess lye. Since you will likely superfat by a minimum of 5%, the lye excess will not ultimately be harmful. If you substitue tallow for lard, you will have less lye than required and will have a higher superfat than intended.

The SAP for olive oil is 0.135.
This is a more problematic difference in SAP, in that the margin between OO and tallow is 5.6%. You would have to superfat at perhaps 9 or 10% to be safe. If you do that, and you are out by your quantities in the other direction, you will have a 15% superfat, which be quite a bit more than you would want, but would not be a terrible problem.

Here are some examples for 4lb batches using soapcalc:
33% lard
33% tallow
34% olive
at no superfat, takes 8.94 oz lye; at 5%, takes 8.49 oz; at 10% takes 8.05 oz

10% lard
15% tallow
75% olive
at no superfat, takes 8.77 oz; at 5% takes 8.33 oz; at 10% takes 7.89 oz

40% lard
50% tallow
10% olive
at no superfat, takes 9.04; at 5% takes 8.59 oz; at 10% takes 8.14 oz

So say you use the safest superfat for your mixes, 10%. Say you think you have a lot of lard and tallow, and little olive. Say you're way wrong, and instead of having 10% olive, you have 75%. You use 8.14 oz of lye, when really you only need 7.89 oz. You're still OK. Because you played it safe with that high superfat, you used even less lye than you would have needed to give you a typical 5% superfatted bar.

You can fool around with various proportions and superfat percentages on soapcalc, and see what your margin for error is.

Now, if you have a significant amount of other oils with BIG differences in SAP (like coconut, say, which has a SAP of 0.183, or palm kernal at 0.176), you may be screwed. I personally wouldn't play around with that kind of mix.

Maybe others can also comment on checking the final soap with respect to not just zap, but pH?

 

[OrangeCrush]

I used to have fairly consistent amounts of each oil and could guess the percentage pretty well off the top of my head, but had one batch of rendered fat that I can't recall the percentages. I moved onto buying store bought oils and experimenting instead of just making lard soap, and pretty much forgot about it until I found it in my freezer. And I now have a couple pounds of "whatever I used to cook with" saved up, but forgot to label the layers as I had more than enough soap around. The old was more likely high lard, and the 2nd perhaps 50% lard, 40% olive, and 10% tallow...

I really don't like the smell of unsaponified lard though is the problem. If too much lye is used will separation of the soap/water occur? I believe that has happened before in the past when I made lye heavy batches, but never experimented to find out if that was the cause. I do use a hand mixer, and it has still occured...