Is water content the only predictor of shrinkage in CP soap?

[akseattle]

Question: Is water content the only predictor of shrinkage in CP soap? If not, what else?

In anticipation of giving some soaps away this weekend, I went back and looked at the soaps I made in February, March and April. Usually, I look at my soaping journal, decide which soaps I want to give away, and then just open that container and don't really look at anything else unless I decide to give away a different soap. I have given away a lot but still have a couple soaps left from each batch (only get 4 bars from each little batch.)

In Feb, March & April, I made two batches of Zany’s Castile and 12 batches of other recipes, most of those 12 a slight variation of one recipe.
I was struck by how BIG Zany’s Castile bars looked in comparison to all the others. Zany’s had 85% olive oil and a lye concentration of 37%. I made those the last two weeks of April, so they would be the newest of those 14 batches.

The other 12 recipes had small variations on lye concentration (I used to use % of water to oil but soapcalc gives me the lye concentration, too.) They were 30-32% lye concentration.

I used the same mold for all batches, except Zany’s two batches. I used one that was double in length but otherwise the same width& height.

Can I assume that it is the amount of WATER in the batter that determines shrinkage?

The only other big difference in the bars is that Zany’s had 85% olive oil. If oil shrinks, too, I assume it just takes olive oil longer to shrink?

My other soaps look so small, I’d really prefer to end up with bigger bars.
Help!!!

 

[AliOop]

Hmmm, I wouldn't say that 30-32% lye concentration is a "small difference" from 37% lye concentration. That's a fairly large difference to me! One reason I like using 40% lye concentration is that the bars shrink far less during curing. Consider giving that a try unless you are making complex designs.

I've never noticed any shrinkage difference based on the oils used, and would not expect that, because oils (to my understanding) don't evaporate like water does. But someone who understands chemistry better than I do can perhaps comment on that aspect.

 

[akseattle]

@AliOop , thank you for your thoughts. Actually, I meant that other than Zany's Castile that had a lye concentration of 37%, the rest had a lye concentration of 30-32%. I didn't think that the difference between 30% and 32% was that big of a difference. I did think that the difference between 30%, 31% and 32% was quite different than 37%.

I know that water evaporates during the curing process. I know that bars get harder during curing. For some reason, it didn't occur to me that the reason they were harder, is because they get more dense. And, if they were more dense, that they take less space. And if they are taking less space, they are smaller..... I guess it's a little bit of a 'duh' moment.

I'm pretty motivated to use a 38-40% lye concentration going forward- absent a good reason not to anyway.

 

[AliOop]

Well, along those same lines, I started wondering whether the soap crystals based on one mix of fatty acids may, over time, pack down tighter than the crystals based on a different mix of fatty acids. This would also make the bar shrink more in size, although perhaps not in weight. Dunno... interesting stuff to consider.