All essential oils are concentrated hydrophobic liquids that contain volatile aroma compounds from plants. Volatility can be described as the tendency of a substance to vaporize. Volatile oils contain different carbon- and hydrogen-based compounds called terpenes or hydrocarbons.
When we state that an essential oil is highly volatile, it means that it has a very high tendency to quickly vaporize or evaporate. This is actually a desirable effect of essential oils and is one of the many reasons why it is so popular in aromatherapy. Unfortunately, when it comes to soap making, an increased rate of vaporization means an increase in scent fading and we aren’t left with a long-lasting fragrance. Although many people prefer essential oils because they are considered a highly regarded natural option for adding fragrance, essential oils aren’t really the best option when it comes to producing soaps with long-lasting and aromatic fragrances. Essential oils have a higher volatility than fragrance oils, which is just one of many reasons why fragrance oils are so popular.
“But I only want to use essential oils, isn’t there something that I can do?”. Yes, there are multiple different options that can help, but keep in mind, they may only be temporary solutions.
1. Hot Process is great for both EOs and FOs because they are added after the saponification process. This means that these oils will not be saponified, compared to cold process. The next question people usually ask is "But what about the heat, won't they flash off?". When adding EOs to hot process soap, there is no “flashing” or “flash off” of the oils. This is in reference to what is called a flashpoint and implies that they will ignite at a certain temperature with an open flame, but when making soap, there is no open flame (most of the time anyways). The high boiling points will also not be of concern, as the temperature of your hot process soap will not reach such high temperatures, even if using HTHP. When adding essential oils to hot process soaps, the temperature will often be much lower than the temperatures exhibited in many cold process soaps, many of which will reach an internal temperature over 160F.
2. Combining notes- Perfume engineers ascribe different “notes” to fragrances, which is often in direct correlation with the rate of evaporation (or perceived rate of evaporation). There are three different “classes” of fragrance notes which are: top, middle and base. Top notes seem to have the shortest duration of aroma, base notes have the longest and middles notes are, well, in the middle. Essential oils like many citrus EOs are classified as top notes and essential oils like clove and patchouli are considered base notes. For a longer lasting fragrance, perfume masters will engineer fragrances with a combination of the three notes in order to create an aroma that withstands time. This is also a recommend approach when selecting which essential oils you to use when fragrancing your soap. A blend of essential oils with different notes will be longer lasting than a single essential oil, especially if it has top note. If you want more information about essential oil notes or blending fragrances, you can find plenty of online and book resources.
3. Fixative- There are number of different additives that can be used that may act as a sort of “fragrance anchor”. These include arrow root powder, corn starch, charcoal, clays and other additives. All of these ingredients have either absorptive or adsorptive properties which attract the hydrophilic heads found in essential oils (which we talked about at the beginning). Clays may be one of the more popular and efficient options because they are an assemblage of crystalline minerals that are known for their adsorption properties, one of the many reasons why they are often used in facial masks. You can use a more neutral fixative like arrow root or kaolin clay or include additives like Moroccan red clay or charoal which may act as a fragrance fixative and add additional desirable properties to the soap like color, texture and other benefits. I will often add my clays first and then my fragrance/EO after for extra adsorption/absorption.
4. Recipes- If you are using essential oils as a fragrance, it is also important to consider the recipe that you are using. This means that if you are wanting a bright and airy zested lemon fragrance, using a large amount of warm and sweet cocoa butter is not a wise decision. This same principle applies to other highly fragranced oils like pumpkin seed, mango butter, unrefined shea, and hemp seed oil. This also needs to be taken into consideration when using other additives because many additives have fragrances as well (think: flowers, herbs, etc). You can also use your recipe oil selections or additives to your advantage when it comes to EO and FO fragrances because many natural ingredients have wonderful natural aromas. Sometimes I specifically add cocoa butter as my PCSF so that it adds a nice chocolate-y fragrance and combine it with a deep base note EO for a delicious, sweet, warm and long-lasting fragrance. Want a long lasting Rosemary EO fragrance? Top your soap or add fresh Rosemary. Enjoy lemon fragrances? Add zested lemon peel. Some other soap makers have reported that using palm oil might inhibit essential oil fragrances, but I have not personally experienced that myself.
5. Usage- With the knowledge and understanding that essential oils are highly volatile and may not last as long as other fragrance options, you should take into consideration the estimated usage dates and EO usage amounts. Although there is no set guideline for the amount of EO to add to your recipe, there are recommendations and suggestions as to how much to use and you should plan your recipe accordingly. Additionally, if you are planning on creating large batches of soap for an event that is six months away, you should consider creating the EO fragranced soap closer to the event date or consider using a FO instead. Soaps made with essential oils will decrease in fragrance strength as they age; however, they will also become harder, longer-lasting and more gentle.
Hopefully learning a little bit more about essential oils and some of the methods and means that might assist in preventing scent fading might help in your decision making. I use the hot process soap making method for all of my soaps, create an informed recipe with an awareness of oil and additive aromas and I often use clays or other ingredients that may help anchor the scents.