As a note, I'm Canadian, so I work in grams instead of ounces. I will use this throughout this blog and this post because it gets really confusing with the weighted ounces and liquid ounces. If you aren't familiar with the metric system, 1 weighted ounce is about 30 grams and 1 volume ounce is about 30 ml. So a cup is 250 ml (give or take) - and I think this just shows the difference in using weights and volumes!
Pure water at 4 Celsius is our baseline for specific gravity and everything else is compared to it. Water weighs 1000 grams per litre - 1 kg per litre - or 1 gram per millilitre. If something is listed as being less than 1, it weighs less than water per gram. If something is more than 1, it weighs more than water per gram.
I have been mentioning cc or cubic centimetre in the mineral make-up posts. This represents 1 cubic centimetre, or 1/1000 of a litre, which is 1 millilitre. So 1 cc or 1 ml of water is equal to 1 gram. So 5 cc of water - 5 ml or 1 tsp - would weigh 5 grams.
If something has a specific gravity of 1.03, it means it weighs 1.03 grams for every 1 millilitre or 1030 grams per litre.
So if we see cetrimonium chloride listed as having a specific gravity of 0.93, we know this means it weighs 0.93 grams per 1 cc or 1 ml. Let's say we want 5% cetrimonium chloride in our conditioner. If we're making a 100 ml batch and add 5 ml, we'd only have 4.65% cetrimonium chloride. Not the biggest deal in the world. If you wanted to make 1 litre of conditioner and added 50 ml, you'd only have 46.5 grams of cetac or 4.65%.
Liquid Germall Plus has a specific gravity of 1.15 to 1.25. If you want 0.5% in a lotion and add it in volume at 0.5 ml to a 100 ml batch of lotion, you might have 0.575 to 0.625 ml preservative, which is above the 0.5% recommended!
A lot of oils have a specific gravity of 0.91 to about 0.95 (see this chart for more information!) So adding 1 cc or 1 ml safflower oil (specific gravity 0.90) would only add 0.9 grams of oil to your lotion. If you're making a 100 ml batch and you're wanting 10% oil will leave you with 9 grams of oil, not 10. Take this even higher to a 1000 ml or 1 litre batch (multiplying your recipe by 10) and you'll have 90 grams of oil instead of 100! This can throw your emulsification out of whack and will deprive you of 10 grams of lovely oil!
And please weigh your fragrance oils! I made this fatal error when I was a newbie, somehow measuring 1 gram of fragrance oil as being 2.5 ml! If my fragrance oil had a specific gravity of 0.90, then I was using 2.25 grams or 2.25% in a 100 gram batch! Now THAT'S overscented! Fragrances oil can vary - oatmeal, milk & honey from one supplier could be 1.12 and another could be 0.91 - so weigh them to make your life easier.
This goes for essential oils as well! They can start at 0.78 and increase from there. Considering how many essential oils have suggested limits, you don't want to mess up here!
This is one of the reasons larger batches of mineral make-up don't end up with the same colour as the smaller batches. If you are using scoops and cc spoons for smaller batches, then move up to teaspoons and tablespoons, it doesn't take much to add too much black or too little filler to a larger batch of eye shadow. My suggestion - use weighted measures when you can. I know, I've posted most of my stuff in volume measurements because it's easier with smaller batches, but if you want to make larger amounts - base, for instance, is a great place to start weighing things - make your smaller batch in weighted amounts, then you can convert it easier!