Wednesday, September 23, 2009

Weight vs. volume

For the most part, most of the recipes you'll see for bath and body products are in weights because it's the most accurate way to get the results you want. And most recipes you'll see for mineral make-up tend to be in volume measurements - teaspoons, scoops, and so on - because we're dealing with such small amounts, it gets to be a right pain in the bum to work with 0.1 gram or lower. (And it can get expensive buying a tiny weight scale!) All you have to do is look at a container of Micronaspheres (2 ounces by weight fills an 8 ounce container) or Natrasorb bath (125 grams fills a really huge bag!) to know there can be a huge difference between the weight of a product and its volume.

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!


Pam said...

Hi Susan,

Another great post. I just realized that I am using 28.3495 as the gram equivalent to 1 ounce. How much do you think this would impact the outcome of my lotions? I will change the rate to 30.

Susan Barclay-Nichols said...

Hi Pam. If you're converting every single ingredient by the same amount, it shouldn't make a difference, as long as every ingredient is changed by the same amount. If you have a digital scale, you can just hit the "gram/oz" button and change it easily that way.

If you think about it, if you have 10 ounces of water - which would be 283.495 grams of water - and 5 ounces of oil - 141.7475 grams of oils - it doesn't make a huge different if you're considering that 300 grams of water and 150 grams of oil because you've multiplied every percentage by the same number, so you have the same ratios! In the future, it's easier just to think of it as 30 grams for sanity's sake, but it won't ruin your recipes!

Ann said...

I am brand new into making lotion bars. I have a simple recipe, equal parts of shea butter, coconut oil, & beeswax.

If I use 1 cup for measuring, I end up with 3 cups of liquid to pour into the molds.

What is the best vitamin E to add to this, & how much and what measuring method?
I don't have a special scale. I do have plastic 3 ml droppers.
Can I use vitamin E capsules? If yes, how many capsules for 3 cups liquid?
I have some Burt's Bees Vitamin E body & bath oil....can I use that?
How do I know what kind of vitamin E to use, and where do you get it?
IS IT EVEN NECESSARY to add vitamin E to my recipe?

What might the shelf be without adding vitamin E, and what might it be adding vitamin E to it?

I like the idea of making the bars to last longer, but I want to keep this simple, as reading about this makes my simple head spin.

Susan Barclay-Nichols said...

Hi Ann. The questions about shelf life are answered in the FAQ in the posts on shelf life and in the individual posts on the oils you mention. Adding Vitamin E is good for extending the shelf life, so you have to decide if you think the shelf life needs extending. As for how much to add? I can't help if you're in volume measurements as the suggested usage is 0.5% by weight. Why are you making such a large batch of lotion bars? And why aren't you using weighted measurements?

Deja said...

What about weight vs. density?

I've been using batch size calculator: for more than two years and never had a problem until my friend told me that it is inaccurate because not everything weighs the same.

For instance, he said if I wanted to make 5% vitamin C serum. LAA has a density of 1.65g/cm3, so for 10ml serum I would need 0.825g LAA. But, the batch size calculator said I would need 0.48g LAA and 8.15g water. Which one is right?

Susan Barclay-Nichols said...

Hi Deja. I don't work with ml ever, so I can't help in this regard. I work only in weights and percentages. For instance, your recipe would be 8.63 grams total, which could make 10 ml - I don't know that information. You have 94.4% water and 5.6% LAA (what is this ingredient?). Is this the right percentage for usage of this ingredient?

I can't stress enough how much you want to work with weights, not volume. Don't worry about density - start with weight.

Ashley said...

Hi Susan,
I'm trying to wrap my head around how much liquid I'm actually using by doing the math with the specific gravity and I want to make sure I'm doing it correctly, can you confirm? Just to give an example, if I use 20g of oil and it's specific gravity is 0.92, I multiply 0.92 x 20g to find out how man ml I actually used? Or do I need to divide 20g / 0.92? I weigh all of my ingredients but I also need to figure out how many ml I'm actually using. Thanks so much!