Glycerin in shave soap

[imagineflagons]

Hi everyone,

I've been lurking for a long while but this is my first post. I hope it's appropriate.

I've noticed a lot of artisan shave soap makers have a relatively high level of glycerin in their recipes, i.e., they're high in their ingredients lists compared to other ingredients, oils especially. Sometimes I'll see glycerin as the 2nd or 3rd ingredient in the list. That has me confused. I've been traditionally following the 1tbsp/lb oils as a SF idea, or between 3%-5%. Are they doing something different? Is it possible that they're listing the glycerin produced as part of sapping, as well as what they add? Are they adding upwards of 10-15% to their recipes? Are they adding it as part of their oils, before saponification? Have I just stepped off the deep end?

I'm a newcomer, so my inexperience is very possibly wrong but I thought that more than 5% added glycerin is risky business.

Thanks in advance if anyone replies! I've been glad to read all these posts and see such a welcoming community!

 

[DeeAnna]

You can't judge shave soap by "normal" soap standards. It breaks the rules in many ways. Adding extra glycerin is one of those rules.

One reason why glycerin is added -- in addition to the "natural" glycerin made by saponification -- is that many shave soap recipes use stearic acid. Stearic acid does not add natural glycerin to the soap. According to my figuring, you would want to add a minimum of 1 part glycerin to 3 parts stearic by weight to bring glycerin content equal to the amount of natural glycerin made by saponification of fats.

Another reason why glycerin is added is to help create a proper shaving lather -- make it denser, more cushiony, more glidy, and longer lasting.

 

[houseofwool]

Deanna, to satisfy my inner geek, how do you come to the ratio of one part glycerin to three parts stearic acid?

 

[The Efficacious Gentleman]

Oh oh oh! I know this! (Getting my geek on)

A normal fat that we use is a triglyceride. It is 3 fatty acid molecules connected by a glycerin molecule. When we add lye this breaks down - the lye combines with the fatty acids to make soap molecules and the glycerine is all alone but still in the mix. So a normal bar of soap contains 1 glycerin molecule for every 3 soap molecules.

When we use these fatty acids outside of a triglyceride, as is the case with stearic acid, there is no glycerin there, it is just the acid molecules on their own. To make up for it, we can add in the glycerin at the ratio of 1:3 to make up for it

To the op, I say do a search for 'my first shaving soap is a success' and look for the thread with that name, started by songwind. It is pretty much the best place to get caught up on shaving soap information

 

[boyago]

Oh oh oh! I know this! (Getting my geek on)

A normal fat that we use is a triglyceride. It is 3 fatty acid molecules connected by a glycerin molecule. When we add lye this breaks down - the lye combines with the fatty acids to make soap molecules and the glycerine is all alone but still in the mix. So a normal bar of soap contains 1 glycerin molecule for every 3 soap molecules.

When we use these fatty acids outside of a triglyceride, as is the case with stearic acid, there is no glycerin there, it is just the acid molecules on their own. To make up for it, we can add in the glycerin at the ratio of 1:3 to make up for it

I blew the dust off my periodic table (not really I used the dynamic one online) during the Songwind thread when I was wondering the same thing and compared the molar weight of stearic and glycerin. I came up with a mass of 92.09382/284.4772 glycerine mol to stearic mol. I did get a C in chem my last time around so I might be wrong here but wouldn't put the natural content from saponification (of a triglyceride) more like 1/9?

 

[The Efficacious Gentleman]

You still measure in weight and not number of molecules? I kid, I kid - for that, we will have to defer to DeeAnna

 

[imagineflagons]

Thanks so far everyone. This is great info that I haven't been able to pin down elsewhere!

do a search for 'my first shaving soap is a success' and look for the thread with that name, started by songwind. It is pretty much the best place to get caught up on shaving soap information

Oh I'm not that new to shave soap making. I thought it was a rite of passage before posting a question to read through all ~70 pages of that thread

I'm going to try this ratio soon and report back with results. Thanks!

 

[DeeAnna]

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:

 

[imagineflagons]

Wanted to report back, here.

I made a couple of batches last week with an upped glycerin content. I started making soap using the Badger and Blade recipe that called for 1 tbsp of glycerin/lb oil, and continued that method for all of my test batches... until last weekend. I didn't see Deanna's great molar mass post (it's been a while since I've had to use those words in the same sentence!) so measured the glycerin by weight in grams. For my 100g test batch that meant 16.6g of glycerin.

Oh. Man. My razor glided better than a physics problem disregarding the coefficient of friction. Seriously. It shaves better than one of the artisan soaps on the market that I've compared it to. For me, anyway.

I hope that the persistence of this thread helps anyone else who's experimenting and having difficulty. This was definitely my missing link. I'm looking forward to better shaves from now on and maybe trying to slowly rebatch some older formulas with added glycerin to see what happens.

 

[The Efficacious Gentleman]

Did you find that the soap was a little bit more thirsty with more glycerin?

 

[DeeAnna]

"...a physics problem disregarding the coefficient of friction..."

Love it!

 

[imagineflagons]

Did you find that the soap was a little bit more thirsty with more glycerin?

I did, but I find that pretty common with a lot of artisan soaps. If anything I prefer when soaps are thirsty; taking a bit longer during the loading process allows for the scent to bloom a bit and makes for enjoyable shaving experience overall, not to mention it allows the shaver to really ensure they have the amount of water they want.

 

[houseofwool]

This simple Portuguese major tried to read all of that but it sounded like the teacher from Peanuts.

http://youtu.be/ss2hULhXf04