One of the common tests that many educators, essential oil companies and consumers will conduct is a blotter test. The tests are typically done from a layman`s point of view in an effort to judge quality. But what is it, and what does it actually reveal about an essential oil? Does it indicate quality? Is it even worth doing.
The basic process is quite simple, in that you apply a drop of essential oil to a slip of paper. The paper should be white so as to see colour, viscosity and track absorption. You let it sit in the open air for a period of time and monitor what happens.
A true essential oil is a mix of naturally occurring chemicals which are volatile and prone to evaporation. If you are testing an essential oil which is claimed to be pure and obtained via distillation, the drop will dry up at some point leaving a non- oily mark. If there is a colour associated with the essential oil, it will leave that residue. For example, bergamot will leave a faint green colour while lemongrass has an amber tone. Oils which are typically clear such as eucalyptus or lavender will be untraceable once complete evaporation has occurred. The aroma will also change as lighter molecules evaporate first.
The other important aspect to observe is how long will it take for that drop to evaporate. This again depends on the essential oil’s composition and varies accordingly. An essential oil high in Oxides or Monoterpenes such as eucalyptus or ti-tree, will evaporate relatively fast. Depending on the room temperature and air circulation this could occur in a few minutes to a few hours. Oils containing high levels of esters or aldehydes will evaporate much more slowly as in the case of immortelle or lemongrass.
If you want to inject a degree of scientific approach, do blotter tests in a controlled environment of consistent temperature and air circulation to create a baseline of approximately how long each one should take to evaporate. I use an incubator set to a specific temperature. You do need a reliable sample of an unadulterated oil sample to compare.
I conduct a test in class with my students using a sample of a fairly well balanced essential oil such as petitgrain or lavender. I apply a drop of pure oil and a sample which I have adulterated 50% with propylene glycol. This is the most common and easily adulterated trick many essential oil companies use to extend their oils without detection and increase profits. The drop contaminated with propylene glycol will evaporate, but at a slower rate than the pure Petitgrain. It may also leave a slightly blue colour around the edge of the drop.
For those out there that think they could immediately detect the glycol, think again. Even at a bold 50% I have duped many respected and self- declared experts!
This test is not effective for resinoids such as benzoin or myrrh, or absolutes such as Jasmine and Rose or expensive essential oils which are diluted with a carrier oil such as coconut or jojoba oil. Dilutions should be declared on the label to avoid confusion.
While rather unscientific, a test like this can display the bouquet for your nose to analyze, because you should never assess the smell of an essential oil from the bottle as the aroma is still mostly locked inside. Oxidized residue is normal around the top of any opened essential oil which will impair the aroma.
It will give you a view of viscosity and colour. It will reveal whether you have a pure distilled oil or one that is diluted with a carrier oil. If a dilution is not declared on a label, then that is a problem.
If you have a good baseline of evaporation for particular oils, you can compare to see if it dries much more slowly. Beyond that, it is not a true test of quality or purity rather a fun exercise to do to learn more about your essential oils.