Not all chemicals have an odor that human noses can detect, but of the ones that do, they must evaporate into the air to reach your nose. If you slow the rate of evaporation, the scent will smell weaker. If you prevent all evaporation, there will be no scent at all.
Here are some strategies for stronger smelling and longer lasting fragrance in soap.
(1) Use scents that are known to smell strong and to "stick" (last) in soap. The two properties -- strength of the scent and its "stick" (longevity)-- are not the same. There are many essential oils (EOs) and fragrance oils (FOs) that start out smelling nice and strong, but they do not stick -- in other words, they fade to near nothing within a month or two. The only way to learn about these qualities is to read users' reviews of each fragrance, do your own testing, and ask other soapers for their advice.
(2) Use a sufficiently high dose of fragrance while staying within skin-safe limits. Even the strongest and longest lasting scent will fade as the months go by, so the soap must start with enough fragrance to withstand the test of time.
Fragrance oils and essential oils from reputable companies are 100% made of chemicals that provide scent. We also typically add a LOT of those EOs or FOs to get a definite, clear aroma. Many people dose their soap with at least 3% by weight of fats. A 3% dose generally only lightly scents the soap.
Infusions, however fragrant they might be, cannot adequately scent soap. The fragrance chemicals in an oil-based or water-based infusion are only a tiny part of the total infusion. When added to soap, the fragance chemicals from the infusion are diluted even more. My optimistic estimate is an infusion might provide about 0.3% fragrance in soap by weight -- that is 1/10th the usual 3% dosage.
Not every fragrance is safe for use in high amounts, however, so be sure to check the IFRA guidelines or the supplier's recommendations. Know what percentage of a fragrance is safe for use on the skin and stay within those guidelines.
(3) Blend essential oil fragrances to provide base, middle, and top "notes." Many essential oils are short lived in soap. If you make fragrance blends that include EOs with different evaporation rates, however, you will get longer lasting results.
Scents are often classified into top, middle (heart), and base notes. Top notes are the most volatile and evaporate quickly. Citrus and floral scents are usually top notes. Base notes are the least volatile and last longest. They are usually wood, resin, and spice scents. Middle notes evaporate moderately fast. Typical middle notes are the herbs and grasses.
For example, lemon essential oil (EO) is a top note. Lemongrass EO is a lemony-scented middle note. If you blend the two, the overall scent will last longer, mainly because the lemongrass aroma will still be hanging around long after the lemon EO aroma is gone. Add a base note of patchouli EO, and some scent will still linger months after the lemon and lemongrass scents have faded.
(4) Choose melt-and-pour soap or hot process soap rather than cold process for longer lasting results. Fragrance is usually added near the end of making melt-and-pour (M&P) soap or hot process (HP) soap. That means the scent is not exposed to active lye and it will not evaporate as much because the soap is normally cooling down at that point. Scent typically smells stronger and lasts longer in these types of soap.
Fragrance must be added to cold process (CP) soap near the beginning of the soap making process, so the fragrance is exposed to active lye and warm conditions for some hours. More scent is normally added to CP soap compared with M&P or HP soap to compensate.
(5) Store scented soap in closed containers to slow evaporation during storage. Air has to circulate around curing soap to allow water to evaporate, but once the initial curing time is done (4-8 weeks typically), you can put the soap into closed containers for long-term storage.
(6) Put a cotton ball dosed with a few drops of scent into storage containers. For personal use, this can be a pleasant-smelling method of storing soap. Be aware, however, some soap makers view this is a controversial and borderline unethical practice if the soap is to be sold. Storing the soap with a separate source of fragrance will cause the aroma on the outside of the bar to be stronger than it would otherwise be. This will mislead consumers into thinking the soap they are buying has a stronger scent overall than is really the case.
(7) Add an "anchor" to soap to store scent and release it slower. Using an anchor (aka fixative) sounds like a plausible and useful technique, but it is also another controversial practice. Many experienced soap makers say a fixative does not make a detectable difference in how long a scent lasts, and they recommend other strategies as described above. (1,2)
The materials typically used as anchors are usually powders with a lot of surface area -- arrowroot powder, powedered clays, cornstarch, etc. They are added to the soap batter in reasonable amounts. Some soap makers mix the fragrance directly with the fixative material, let it sit for awhile so the fragrance is absorbed, and add this mixture to the soap batter.
There is a key misunderstanding about the use of an anchor as a scent fixative. An anchor is a strategy for making a scent last longer. It is not a strategy for making a weak scent smell stronger.
(8) Use or sell soap promptly before the scent obviously fades. If you want the fragrance and appearance of your soap to be the most appealing to users, keep your inventory moving. Coordinate how much soap you make and when you make it with the amount of soap you use and/or sell.
(9) Have realistic expectations about fragrance. New soap makers are often surprised when their precious citrus essential oils fade within weeks or when a new fragrance oil that smelled yummy in the bottle does not smell at all in soap. Even fragrances that start strong and have good lasting power in soap will gradually fade over time. Very few fragrances will still be going really strong after a year.
More discussion on SMF
https://www.soapmakingforum.com/threads/anchoring-fos.60724/
https://www.soapmakingforum.com/threads/natural-fixtators-for-lye-soap.79579/
https://www.soapmakingforum.com/threads/best-anchor-fixative-for-an-orange-essential-oil.76262/
https://www.soapmakingforum.com/threads/why-you-no-smell.62705/
https://www.soapmakingforum.com/threads/f-o-e-o-and-fixes.34405/
References
(1) Jennifer Kleffner. Clay, Essential Oils, and Soap – An Experiment. Miles Away Farm. Version dated 23 February 2018. https://milesawayfarmww.com/2018/02/23/clay-essential-oils-and-soap-an-experiment/
(2) Kerri Mixon. Give Your Essential Oils More Staying Power. Pallas Athene Soap & Natural Skin Care.
Version viewed 3 January 2021. https://www.pallasathenesoap.com/articles/EssenOil.pdf
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