Masterbatching and Small Soap Batches

[BrewerGeorge]

Help me think this idea through, Folks.

It is often recommended NOT to make batches smaller than 500 grams unless you have a scale more accurate than the typical 1g kitchen versions. But what if I've masterbatched my lye and have a good graduated cylinder?

If I assume that the goal is to get near or above 3 significant figures of accuracy, because 500 grams of typical oils will require (very roughly) 75 grams of NaOH which I'll squishily call 2-3/4 sig figs and round up to 3.

With that in mind, if I create my masterbatch solution - using my 1g accurate scale - of at least 100g each of NaOH and water, that gets me a solution accurate to three significant figures. For volume, even a class B graduated cylinder gets me 3 significant figures, and a 50 ml cylinder would be enough for the job. Density of that 50% solution is a known quantity, pretty much 1.52 g/ml at reasonable "room temps" but easy to look up. From there, milliliters of solution are easy enough to find.

What do you think?


Hmmm, the more I play with these numbers, the less this sounds like a good idea. For a 100g batch of 33% lard/coconut/olive the difference between 5% superfat and 0% is just half a milliliter. A class B 50ml cylinder will measure that - just barely - but it seems risky. And I just realized that this whole idea doesn't consider the weights of the oils. I've typed it all out, so I'll leave it to see what others think.

 

[DeeAnna]

Have you also considered what you would do to remove the residue left in the grad cylinder after you pour the bulk of the NaOH solution out? In a chem lab, the cylinder would normally be rinsed several times with small amounts of distilled water after the initial pour. Would you be adding additional water to the batch that could be used for this rinsing? To pour without rinsing could easily introduce that 0.5 mL error that you mention.

If a 0.5 mL error changes the superfat by 5%, that's a little too much variability for my tastes. I think you'd be better off using a pipette than a graduated cylinder if you want to measure by volume, rather than by weight. And pipettes are a whole 'nother can o' worms.

 

[BrewerGeorge]

Yeah, typing it out pretty much turned into me talking myself out of the idea. :-?

 

[CaraBou]

How low do you really want to go? I made quite a few 400 g batches without problems using a scale that measured only to full grams. I now have a more precise scale so worry even less. Which leads me to say, scales are pretty cheap, maybe that's an option.