Okay, guys, I have to confess to eating crow this morning. My ratio of 1 part glycerin to 3 parts stearic BY WEIGHT is wrong.

Ahem.

Remedial chem class is now in session.

***

The Gent is right -- the "stoichiometric ratio" of 1 part glycerin to 3 parts stearic acid (or any other fatty acid) is correct -- if we're talking about MOLECULES. But we then have to take that to a weight basis, which is how us mere humans operate.

Key numbers required:

Assume we are saponifying 100 g of pure stearic acid, C18H36O2

Stearic acid molecular weight 284.48 g/mol

Stoichiometric ratio of 1 mole glycerin created for every 3 moles of fatty acid

Glycerin C3H8O3 molecular weight 92.09 g/mol

The Math:

100 g stearic / (284.48 g stearic/mol) / (3 mol stearic/1 mol glycerin) X (92.09 g glycerin/mol) = 10.79 grams glycerin

This number of 10.79 g is the amount of glycerin that would be produced IF we had used all fat in our hypothetical shave soap recipe. We don't. We saponified pure stearic acid -- the fat without the glycerin. So if we were making a shave soap and wanted the soap to have the same amount of glycerin that it would have if we had used all fats in the recipe, we'd want to also add 10.79 g glycerin for every 100 g of stearic acid used.

***

I suspect Lee is wondering about palmitic acid, since commercial grade "stearic" acid is half stearic and about half palmitic, speaking very roughly. Let's recalculate the missing glycerin based on using pure palmitic acid:

Key numbers required:

Assume we are saponifying 100 g of pure palmitic acid, C16H32O2

Stearic acid molecular weight 256.42 g/mol

Stoichiometric ratio of 1 mole glycerin created for every 3 moles of fatty acid

Glycerin C3H8O3 molecular weight 92.09 g/mol

The Math:

100 g palmitic / (256.42 g palmitic/mol) / (3 mol palmitic/1 mol glycerin) X (92.09 g glycerin/mol) = 11.97 grams glycerin

***

Now remember the calcs are based on commercial stearic -- a mix of roughly half stearic and half palmitic acid. So if I find the average of 10.79 and 11.97, I get an answer of 11.38 g of glycerin. So I should add that amount of glycerin for every 100 g commercial stearic to bring the glycerin content of the soap up to "natural" levels.

***

Wondering how to calculate this with a bit less fuss and bother?

Go to your favorite soap calc. Set the superfat to zero. Enter 100 g of stearic as the fat. Calculate the recipe to find the total amount of NaOH needed to saponify the stearic.

When I do this in my personal calc, I get an answer of 14.8 g NaOH on a basis of 100% pure lye to saponify 100 g of commercial stearic.

When saponifying fats, 0.77 g glycerin is produced for every 1 gram of NaOH consumed by saponification.

(Want the math? See

http://www.soapmakingforum.com/showthread.php?t=47524)

14.8 g NaOH X (0.77 g glycerin/1 g NaOH) = 11.40 g glycerin

That compares pretty closely with the average I calculated in the previous section. Serious Geekily Cool, huh? :shifty: