This is a weighty topic! Weight, is that the correct answer! YUP! Faithy is dead on again in her advice. Different oils have different specific gravity and 1 cup of one oil may be by weight 7/8 of a cup or maybe 1-1/8 cup or another oil. Always weigh out all ingredients. Even FO's and EO's have different specific gravity weights. Weighing down to the 1/10 of a ounce is needed to be safe. I use grams sometimes even, which is 1/28th of an ounce.
I have read in other places that you can go by the liquid ounces. I have made all three of my batches by using measuring cups, as I don't have a scale, and don't at the moment have the money or ability to buy one.
I also don't like being so absolute about it, if I can successfully create the same thing more easily.
I do plan to get a digital scale at some point, but there are plenty of recipes listed on the internet for using liquid measurements.
Back when people started making soap, they didn't use such precise calculations. They figured it out by trial and error.
So I think this whole weighing everything out to the exact ounce is a little too fussy.
Of course, I'm not making soap to sell. If that's a person's aim, then I understand the exactness of it. I just enjoy being able to create my own personal soap.
Pine Tar has a specific gravity weight of .99.
Castor Oil has a specific gravity weight of .965.
Olive Oil has a specific gravity weight of .915.
One cup by volume of Pine Tar weighs more than one cup of Castor Oil, and Castor Oil weighs more than 1 cup of Olive Oil.
All three have different saponification values too.
All important factors when making a safe soap for personal use or for sale. Thankfully, digital scales are available for under $20 today for soap makers. SAP values and calculators are widely available on the internet.
I considered a good digital scale to be a more important purchase than my first pound of sodium hydroxide.
Wal-Mart sells a digital Postal scale for $20 that weighs up to 5 pounds in 10ths of ounces as well as even safer grams.
yup thats the one I use! although I think mine is reated at 7 pounds??? and I got it from target...but its all the same. If your going to make soaps with measuring cups...just be sure to keep some vinegar and water around (kind of like your soapmaking fire extinguisher) because once you feel the burn theres no way to stop it without neutralizing it!!...
The trouble with U.S. measurements is that there are many different meanings of the word "ounce" -- it could be a fluid ounce (volume), a troy ounce (weight), or an avoirdupois (also weight), all of which mean different things.
Most of the time, when we talk about an "ounce" we refer to the unit that is exactly 1/16 of a pound, or 28.349523 grams.
It's so confusing. I bet a lot of people, even in the U.S., would want to switch to measuring with grams, kilograms, and liters for this very reason.