Drops of essential oils are the most known measurement to make an essential oil blend, but they are far from accurate or reliable across brands. It is not an issue if you are just doing experimentation where safety and repeatability are not necessary.
According to Robert Tisserand, in his blog about drop sizes (https://tisserandinstitute.org/learn-more/drop-size/), he cites three "empirical investigations" which give the conclusion that there are, on average, 30 drops per gram (or per ml since the density of most essential oils is close to 1 g/ml).
But that is an average, which may or may not be the case for your blend. He concludes that there are anywhere between 20 and 40 drops per gram. Some of the factors are viscosity, orificer size, purity, actual density, temperature, and other physical properties.
Let's break this down a bit and see what differences there could be by formulating a typical 30ml bottle of a fictional blend with Almond oil as a carrier oil and five drops each of Lavender, Peppermint, and West Indian Bay.
Two key bits of information are very evident from the chart below:
The consequences of this appear through the lens of safety and sharing.
Let's say July uses only Brand A, which delivers 20 drops/ml through its orificers, and she shares a recipe with her friend Joanna who uses Brand B, which gives 40 drops/ml. The result could be dramatically different. Even worse is that Joanna uses a mix of brands, which could result in a noticeably different blend.
If your goal is to create a diffuser ambiance, it probably is not going to matter too much. It might be noticeable to some, but for most people, it would not be noticeable, especially if they had never tried the original.
If, however, you are creating a professional clinical or therapeutic blend, the odds are that you will want to know what was effective. You most likely want to be able to repeat your blend creation, especially as an Aromatherapist and as a cosmetic formulator.
You probably also care if you have a client who needs a topical blend or possibly one to ingest and are factoring in the "Topical Max." The safety limit for a topical application is called the "Suggested Topical Max," which is specified for all essential oils in Tisserands book "https://roberttisserand.com/essential-oil-safety-2nd-edition/".
Notice also that the Dilution Ratio for a 30ml bottle, one of the most common container sizes, creeps up close to the 3% line for a 20 drops/ml type of bottle, which is getting out of the perfectly safe arena. It may be okay or maybe not depending on whether you are working with babies, pregnant women, elderlies, or other sensitive populations.
Being aware of the safety of a blend is one of the primary tasks of an Aromatherapist and is not part of what this blog post is about. Still, it is essential to know about the variability that using drops introduces into the calculation.
So what is a better measure than drops?
The answer is almost anything. Even teaspoons are better if you can accurately add exact teaspoons. Milliliters are a universal SI unit that is recognized all over the world, which makes it great for sharing.
Using volume as a unit is also a little tricky when you are using your essential oils with ingredients like beeswax or bee pollen, which are measured by weight, not volume.
How do you describe a blend with a mix of volume and weight? Easy, convert the volumes to weight by using their density. Many essential oils have a density somewhere in the vicinity of 0.91 g/ml. To find out how many grams that is you just multiply the volume you have by the density: 0.25ml X 0.91 g/ml = .2275 grams. Once you know the weight of all of your ingredients, you can compute the Dilution Ratio and the Blend Percentage.
All of this done automatically for you in the BlendPrecisely software tool.
Boo math! Yay BlendPrecisely!
A further refinement in making blends, which is the ultimate in shareability, is to simply describe your mixture in percentages.
97 % Almond Oil, 1% Lavender, 1% Peppermint, and 1% Bay.
Boom! You are done. Using percentage can be shared with anyone, and they can use whatever they want to create their own. Teaspoons, shovels, buckets, tweezers, pinches. You get the point. It is also infinitely scalable for batches. Hello, formulators :-)
The end. Thanks for reading until the end :-) You are one tough cookie!
Update 10. Oct. 2020
I just found this article from the online cosmetic school Formula Botanica and I truly find a good article to share as well:
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