Super-duper super fat!

Soapmaking Forum

Help Support Soapmaking Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.
Joined
Nov 16, 2018
Messages
6,299
Reaction score
16,708
Location
Hamilton, New Zealand
So I was right in the middle of melting oils and mixing lye, when i realised i did not have enough lye!!! I would have had enough if I was making my usual size batch, but this was a bit bigger. So I just ran with it and made it anyway. I did have a bit of KoH here, so I topped up with a bit of that rather than just leave as-is.

Soap made, and cut, and it looks great, but now I'm running it all through soap calc to see what the numbers are, and it turns out that the difference between 30% lye concentration, and 28% lye concentration is absolutely minuscule in terms of the amount of lye required. So I adjusted the superfat in the calculator to find that I have a 28% lye concentration and a 16% superfat! ( that's without counting the KoH).

Essentially I needed 204g Lye and I only had 180g. I topped up with 20g KoH. How on earth do i crunch these numbers? Mathematicians please help!

What will the addition of KoH do? Just decrease the superfat? My soap set up just fine and cut just fine but it feels like it has more 'give' when i squeeze it than usual.

So in attempting to answer my own questions here, I've entered all my numbers into the soap calc to find that the closest I can get results in the following:
NaoH - 93% (178g), KoH 7% (20.8g)
This is approx 29% lye concentration with 11% superfat.

Will my soap always be soft with that amount of KoH? And that amount of superfat?
 
I don’t believe so. I often do a 95/5 ratio of lye. I think you’ll just have a longer cure time due to the water content. While I don’t SF that high normally, I don’t believe it will keep the bars soft. I could very well be wrong though🤷‍♀️
 
Been meaning to try this myself, since a few long-time soapers on this forum regularly use a bit of KOH to increase solubility/lather in their bar soaps.

From what I understand, how much more soluble it will become, and how long it will take to full harden, will depend on the NaOH/KOH ratio, as well as the oils used.

Keep us posted as to how it turns out!
 
Essentially I needed 204g Lye and I only had 180g. I topped up with 20g KoH. How on earth do i crunch these numbers? Mathematicians please help!
Not a mathematician by profession, but I'll help out anyway 😉 (I just don't believe that people should fear numbers)

Grab your periodic table (or believe me) that KOH has a molecular weight of 56.1 g/mol (capital O, btw, it's the same Oxygen O as in NaOH or O₂ or CO₂), and NaOH has 40.0 g/mol. That means that replacing the saponification quantity of each 40 g NaOH needs 56.1 g KOH.

Or, the other way round, 20 g KOH are equivalent to 20/56.1×40 g = 14.3 g of NaOH (minus some 10% to account for a typical KOH purity). Hence, eyeballed, your 20 g KOH have replaced some 13 g NaOH. Effective NaOH is 193 g, which is 11 g (or 5.4%) less than called for = 5.4% higher superfat. Not ideal, but not the end of the world either.

To replace all of the missing 24 g of NaOH by KOH, you would have needed 24×56.1/40 g = 33.7 g KOH (again, a bit more to compensate purity).

Or: Each % of KOH that replaces its own weight in NaOH will increase superfat by (1-40/56.1)×1% = 0.29% (accordingly, the more the lower the purity is)

All that math is built into the more advanced soap calculators (like soapmakingfriend), but it's not directly accessible, since the normal use case is to have enough of either ingredient before starting 😜


Lye concentration can be misleading when using anything else but water + NaOH. It's useful for practictioners who have laminated recipes in their workshops. But it's not overly scientific. A 29% KOH solution is a weaker lye than a 29% NaOH solution (can saponify a smaller amount of fats – not that it is any less dangerous!). But at the common usage rates for dual-lye bar soap (95/5), the impact is negligible. Unfortunately, that doesn't help in ambiguous situations like yours, when you can't tell if it makes a difference or not.


I have used KOH in dual-lye bar soap a few times, but then without comparison to single-lye soap, so I can't tell the difference. There is some evidence that it helps against castile slime. Not sure about the narrative developed there. Potassium prevents crystallisation (ordering of soap molecules in neat layers/lattices – that's why liquid soap stays, well, liquid), but I have no reason to assume that a few % (and I'd call 10% KOH still a low concentration) make a whole lot of a difference. On the other hand, excess superfat also does impede crystallisation, so these effects add up, and your soap bars might be softer than usual, and use up quicker.

In any case, keep us updated how it turned out! Will people rave out because it was so super mild and emollient to the skin, or will they complain about smeary scum and clogged plumbing?



ETA: One thing I'm regularly doing is, when receiving a bottle of X, weigh it as a whole, and write the difference to the net weight onto the bottle (weight of the empty bottle). When in doubt, I can always weigh the half-empty bottle and check if I should worry about running low on X, long before starting anything with X.
 
Not a mathematician by profession, but I'll help out anyway 😉 (I just don't believe that people should fear numbers)

Grab your periodic table (or believe me) that KOH has a molecular weight of 56.1 g/mol (capital O, btw, it's the same Oxygen O as in NaOH or O₂ or CO₂), and NaOH has 40.0 g/mol. That means that replacing the saponification quantity of each 40 g NaOH needs 56.1 g KOH.

Or, the other way round, 20 g KOH are equivalent to 20/56.1×40 g = 14.3 g of NaOH (minus some 10% to account for a typical KOH purity). Hence, eyeballed, your 20 g KOH have replaced some 13 g NaOH. Effective NaOH is 193 g, which is 11 g (or 5.4%) less than called for = 5.4% higher superfat. Not ideal, but not the end of the world either.

To replace all of the missing 24 g of NaOH by KOH, you would have needed 24×56.1/40 g = 33.7 g KOH (again, a bit more to compensate purity).

Or: Each % of KOH that replaces its own weight in NaOH will increase superfat by (1-40/56.1)×1% = 0.29% (accordingly, the more the lower the purity is)

All that math is built into the more advanced soap calculators (like soapmakingfriend), but it's not directly accessible, since the normal use case is to have enough of either ingredient before starting 😜


Lye concentration can be misleading when using anything else but water + NaOH. It's useful for practictioners who have laminated recipes in their workshops. But it's not overly scientific. A 29% KOH solution is a weaker lye than a 29% NaOH solution (can saponify a smaller amount of fats – not that it is any less dangerous!). But at the common usage rates for dual-lye bar soap (95/5), the impact is negligible. Unfortunately, that doesn't help in ambiguous situations like yours, when you can't tell if it makes a difference or not.


I have used KOH in dual-lye bar soap a few times, but then without comparison to single-lye soap, so I can't tell the difference. There is some evidence that it helps against castile slime. Not sure about the narrative developed there. Potassium prevents crystallisation (ordering of soap molecules in neat layers/lattices – that's why liquid soap stays, well, liquid), but I have no reason to assume that a few % (and I'd call 10% KOH still a low concentration) make a whole lot of a difference. On the other hand, excess superfat also does impede crystallisation, so these effects add up, and your soap bars might be softer than usual, and use up quicker.

In any case, keep us updated how it turned out! Will people rave out because it was so super mild and emollient to the skin, or will they complain about smeary scum and clogged plumbing?



ETA: One thing I'm regularly doing is, when receiving a bottle of X, weigh it as a whole, and write the difference to the net weight onto the bottle (weight of the empty bottle). When in doubt, I can always weigh the half-empty bottle and check if I should worry about running low on X, long before starting anything with X.
What a wonderful reply Mr Owl - you are indeed very wise. So without all your clear calculations it seems that I happened upon a similar answer in that I seem to have about 6% additional superfat to my requirements. And yes - we shall wait and see about the soap scum issue after curing is over. Finger's crossed all will be well.
 
Finger's crossed all will be well.
Cross Fingers.jpg
 
Back
Top