Tuesday, February 26, 2013

Newbie Tuesday: Learning how to read a recipe and convert it into weighted measurements

You cannot make good bath and body products using volume measurements. It is hard to scale the ingredients to make smaller or larger batches, and, in general, people assume that volume is the same as weight. It is not, and it is vital for the success of your recipes that you get this concept!

Just about every recipe on this site is done in weighted measurements. (Mineral make-up is an exception because I might be dealing with tiny tiny amounts and my scales aren't that great! I am working on changing those recipes!)) And when you visit a supplier's or manufacturer's sites or get textbooks on cosmetic chemistry, it is all done this way. A recipe should total 100%.

As an aside, when you see something that says "water q.s", it means add enough water to make the recipe total 100%. 

When you see a recipe in percentages, switch the % sign for the word grams. Do not switch the % for ml or liquid ounces or anything to do with volume. This is all about weight.

For those of you using ounces, do you not switch the % for weighted ounces. You will make 100 ounces by weight, which is freakin' huge! 6.25 pounds of product! Massive! And the math is annoying. If your scale can do grams, use grams. Please! 

PRETEND RECIPE - do not make this as it isn't workable! 
10% water
10% aloe vera
10% chamomile hydrosol
10% cocamidopropyl betaine
10% ACI
10% olive oil
10% rice bran oil
8% shea butter
10% Polawax
10% cetyl alcohol
1% essential oil
1% preservative

Convert the % sign for each ingredient into the word grams, and you will have a 100 gram or about 3.5 ounces by weight of product. It's hard to predict the volume of the recipe - is it 1/4 cup or 3/8 cup? - but it's easy to know the final weight because it should add up to 100 grams.

If you want to make more of a product - and I don't recommend making more than a double or triple batch the first time you make something - substitute the % sign for the word grams, then multiply the ingredients by 2 or 3 or whatever amount you want.

10% aloe vera becomes 10 grams aloe vera, which becomes 30 grams aloe vera if I want to make a triple batch.
10% cetyl alcohol becomes 10 grams cetyl alcohol, which becomes 30 grams cetyl alcohol if I want a triple batch.
1% preservative becomes 1 gram preservative, which becomes 3 grams preservative if I want a triple batch.
And so on...

Because it's more accurate and easier to use when we want to get into larger batches.

Let's say we get into our workshop today and we use 2 tsp Polawax in the recipe above (2 tsp = 10 ml). We love the lotion and make the same amount again next week. But it fails! Hmm...but we love it so much and it worked once, so let's make it for Christmas. We want to make at least 15 - 4 ounce bottles. So we figure we need 2 tsp x 15 emulsifying wax in this product. (30 tsp = 150 ml). The lotion fails again. Why?

Two reasons...The first is that you might not have used 150 ml emulsifying wax. You might have added more or less because it's not that easy to use measuring cups. Is it a level cup or a packed cup or a melted cup? There are just too many ways to mess up this kind of measurement. And you definitely didn't use 10% as required by the recipe.

But wait - I used 10% by volume. But our recipes are meant to be created by using weighted measurements. You didn't use 10% by weight. Why is that? Specific gravity! Specific gravity "is the ratio of the density of a substance to the density (mass of the same unit volume) of a reference substance." (Wikipedia)

This means that when we compare our ingredient to a similar amount of water, we'll come up with a number. If we have 1 ml of water, it weights 1 gram. Everything is compared to this. Water has a specific gravity of 1. If my aloe vera liquid has a specific gravity of 1, this means that 1 ml aloe vera weighs 1 gram. If my rosemary hydrosol has a specific gravity of 1, this means that 1 ml rosemary hydrosol weighs 1 gram. And so on.

But what about the volume measurement of something with a lower specific gravity? Rice bran oil has a specific gravity of 0.916 to 0.921 grams per millilitre. So let's say we want 10% rice bran oil in our product. If we use 10 ml or 2 tsp, we would actually have 9.16 grams of rice bran oil in our product. If we use 60 ml or 1/4 cup of rice bran oil in our product, we'd have 54.96 or 55 grams in our product, not 60 grams. And when we get to using larger amounts, 500 ml of rice bran oil isn't going to be 500 grams but 458 grams. We'll be using much less than we wanted!

Polawax has a specific gravity of about 0.8, meaning 1 ml weighs 0.8 grams. If we use volume measurements we'd end up having 8 grams Polawax instead of 10 grams. Too little emulsifier means the lotion fails. As well, this product is a solid - it comes in pellets. But it comes in pellets. Do we melt it before or after the volume measurement? If we use weighted measurements, we know we're using the same amount every time.

What if we wanted to add lactic acid at 10% to a recipe? If we add 10 ml (2 tsp) to the recipe above, have we added 10%? It has a specific gravity of 1.2, which means that 1 ml will weigh 1.2 grams. 10 ml would weigh 12 grams, which means we'd have 12% lactic acid by weight in the recipe! If the safe as used amount for lactic acid is 10% in a product, we've could be putting our skin at risk.

Or look at sodium lactate with a specific gravity of 1.32. The safe usage rate is no more than 3%. Going over that amount can result in sun sensitivity. If you used 3 ml (3/5 tsp) in a product, we'd have almost 4 grams going into the product. Too much!

As you can see, volume and weight measurements are not equal. Using volume measurements means that you aren't following the recipe correctly and things might fail or the consistency isn't right or you are going over suggested safe usage rates.

Plus, you always leave something behind in the measuring cup or spoon when we're done!

A bonus question for the morning: What about using drops for essential oil? I have never understood the rationale for measuring essential oils in this way. I encourage you to get a very small scale and learn how much those drops weigh.
Why? Let's say you make a 4 ounce bottle of something with 20 drops essential oil, but you want to make 15 bottles for Christmas. Do you want to stand there and add 300 drops of something?

And besides, what happens if you use 21 drops or 19 drops? I haven't found an essential oil that is so potent that you need to use teeny tiny amounts like drops. Can our essential oils enthusiasts chime in here as I'd love to learn more! 

And another bonus question: Why did the lotion in the example above work the first time using 10 ml of emulsifying wax? We were lucky! There are three ways we create an emulsion - heat, mixing, and emulsifiers. Sometimes we can heat something and mix it so well that it will work for a short period of time. If you make a lotion today and use it all up by next weekend, it might not have failed in that period of time. Give it ten days or ten weeks and it probably will.

This is one of the reasons I get upset when I see people selling products shortly after learning to make them! You need to take the time to watch how the product changes over time. (Click here for the post Give it time!)

My suggestion: Next time you're in your workshop, choose a few ingredients and check out the weighted measurement vs. the volume. You'll be quite surprised to see what you find!


Lise M Andersen said...

Hi Susan, Some really excellent points in this post! As to measuring essential oil by drops, you are correct about this not being specific enough. They each have different weights and droppers aren't even all the same, so it's tricky work. Over the years, I have measured my EO's into ml measures to see how many drops equalled 1ml, and it ranges from 20 -26 drops depending on the EO. Obviously, it is not accurate to measure EO in ml! Even more difficult to measure by weight when your total batch is 100ml, so I do work with drops for the most part, keeping in mind how many drops to a ml each EO takes. Like I said, tricky work.

Lyn said...

I wish more suppliers gave the SGs of all of their supplies. I have a program that converts all the formulations into approximate volume equivalencies, but it only works properly if you input the SG of every ingredient, a lot of which I don't know! A lot of the time I want a certain volume of something, like 4L of shampoo, but I know making 4 kg won't always equate, so I have to guess (usually I overshoot). SG values are just so valuable!

Diane said...

Hi Susan, since the recommended level of essential oils in cosmetics is really low - like .5% or often less, I generally use the eyeball method of weight relative to water and be conservative even so - generally following the 20 drops = 1 ml = 1/5 tsp ratio AKA 600 drops per fl. oz. This is close enough; you are right that seldom do the measurements need to be more precise (a little goes a LONG way), with the exception of cinnamon and clove and lemony aldehydes (citronella etc)which need to be applied very sparingly - say 2 drops per fl oz. Some, like the citruses, seem quite light and others, like vetiver or pathouli, are thicker. Generally the flowers and leaves are about water weight in my estimation. I'm unwilling to "spend" the EOs on weight experiements. Also inclusion of EOs might well influence viscosity, so my general rule is to go very light and maybe add more the next day if you so desire. I generally go with what turns out; once made a 400 g batch of lotion and added 3 drops of neroli (expensive, exquisite) instead of my usual 1 or 2 and found it to be sickeningly sweet. Just that quick.

melian1 said...

at the beginning i held stubbornly to the teaspoons and tablespoons... until the very first time i actually weighed out stuff. it is easier, more accurate, and can be sized up and down in a snap. i used eo in drops and went by what i found on google for converting drops to ml and teaspoon measurements, tho rarely did i make so much that i needed to do more than count the drops. but, after converting everything else to weight, i also converted my eo.

probably the hard way, but i took each eo i used and put a container on my scale and put in the number of drops in my recipe/formula i was converting, and then wrote down the weight. there were differences between them, so i did all of them. then i figured what the percent of that weight was in my overall formula and had my percentage.

a long way around maybe, but it gave me my basis. it has been years since i did anything not by weight. i have a scale that weighs to .1 gram (and will do up to nearly 15 lbs) and one that weighs to .01 (that does up to 400 grams).

melian1 said...

oh, and i forgot to say that yes, a drop or two can seriously change the product. neroli is one that is so powerful that if i add 5 drops to a 12 ounce cream batch it overpowers everything. i can add 3 drops and it is ok. some don't make such a difference, but there are some that seriously change drop by drop.

amisha said...

there is a very helpful table over at ananda apothecary that converts drops into mL for figuring out percentages in formulas that use essential oils-- i have used it a number of times in recipes where measuring in drops is helpful (like the other commenters, i do this for small batches/ strong oils). using this table makes it a lot easier to scale up a drop-based formula.