Special thanks to Ken Harper of tendigitgifts.com for making this interview available in Spanish: Este entrevista en Español!

Kevin M Dunn has a PhD in Chemical Physics and is currently the Elliott Professor of Chemistry at Hampden-Sydney College, Virginia. His book, Caveman Chemistry, brought him to the attention of soapmakers and he subsequently directed a series of research projects on the chemistry of handcrafted soap. This culminated in a second book, Scientific Soapmaking, which was published in 2010. More information here.

Kevin Dunn

Kevin Dunn

It was a pleasure to meet Kevin Dunn in Miami at the 2011 Handcrafted Soapmaker’s Guild Conference, where we were both giving presentations.

Robert
Could you briefly explain the main processes used to make handcrafted bar soap?

Kevin
The handcrafted soap business is divided into two main segments: MP (Melt and Pour), and CP/HP (Cold Process/Hot Process). Melt and Pour soap base is already soap when the soapmaker buys it. The soapmaker melts the soap base, adds scents and colors, and pours it into a mold. In this process, scents (including essential oils) remain intact for the most part.

CP/HP soap starts out as a mixture of fats, oils, and lye. In HP soap, the raw soap is cooked until the lye has been consumed, after which, scents and colors may be added. In CP soap, however, scents and colors are generally added before the lye has been consumed, while the raw soap is highly alkaline. Scents and colors cannot be added to CP soap after the lye has been consumed because by this point the soap is solid. For this reason, CP soapmakers must either choose scents and colors that are resistant to strong alkali, or accommodate themselves to the changes that occur in non-resistant scents and colors.

Robert
Is mass-produced bar soap made using different processes?

Kevin
Commodity soap is produced by several processes, but they all share the property that the soap is made from fats and oils, shredded, mixed with scents and colors, and then pressed into bars. In this respect, it is more like MP or HP soap—scents and colors are not subjected to harsh alkaline conditions because they are not added until after the alkali has been consumed.

Making liquid soap

Making liquid soap

Robert
What about liquid soap?

Kevin
Commodity liquid “soap” is most often a detergent. Handcrafted liquid soap is a true soap produced by a Hot Process. Because the soap remains liquid, however, scents and colors may be added after the alkali has been consumed.

Robert
Which processes and which essential oils are most likely to lead to evaporation of essential oil constituents, and how can this be minimized?

Kevin
An essential oil with a low boiling point may suffer loss in a MP or HP soap, and this information may be found in the MSDS for the oil. Look to ensure that the temperature of an MP or HP soap is lower than the boiling points of your essential oils when they are added.

Robert
Discussions among soap makers often touch on whether or not EO constituents saponify. Is there a simple answer to this?

Kevin
There is an answer, but it is not simple. Essential oils are complex mixtures of dozens of chemical compounds. A given essential oil may contain some compounds that react with alkali, and others that do not. Lavender oil, for example, contains about 42% linalool (which does not react) and 22% linalyl acetate (which does). In fact, when linalyl acetate reacts with alkali, one of the products is linalool. Thus the scent of a CP soap made with lavender oil will smell less of linalyl acetate and more of linalool than the original EO.

The only way to predict which essential oils will react with alkali is to examine the list of components and note which of them are reactive. Such compounds generally consist of esters, phenols, and acids. There is a practical way, however, for a soapmaker to evaluate essential oil reactivity. Add a few drops of essential oil to 1 mL of the lye solution used for soapmaking (typically 25%-50% NaOH). Sometimes a reaction will be visible and sometimes not. In either case, wait a day or two and then compare the scent of the alkaline EO to that of the original. In some cases, there will be no difference in scent. In those cases where the scent changes, the alkaline scent might not be bad, just different from the original.

Lavender Soap iStock

Robert
Specifically, what happens to esters, phenols and acids present in EOs when they are used in soap?

Kevin
Phenols and acids react directly with alkali to produce odorless salts. Clove oil for example, contains a large proportion of eugenol (a phenol). If you add a few drops of clove oil to lye as described above, the resulting solution is bright yellow and very nearly odorless. Esters, on the other hand, are decomposed by lye into an acid salt (usually odorless) and an alcohol, which is often fragrant. In fact, the alcohol produced is often present as one of the components of the original EO. The scent of such an EO changes as the proportions of its components change, but it remains fragrant.

Robert
Are any other EO constituents affected by soap making processes?

Kevin
Not that I am aware of. The name of an ester is easy to spot: something-yl something-else-ate (e.g. linalyl acetate, methyl salicylate, methyl benzoate). Phenols are harder to spot, but the most common fragrant phenols are eugenol (clove, cinnamon leaf), carvacrol (thyme, oregano), thymol (thyme), and vanillin (vanilla). Phenols are actually weak acids. Other fragrant acids typically smell sour, e.g. acetic acid in vinegar.

Robert
Thank you Kevin, this is really useful information!

The Table below is a rough guide to the boiling points of some commonly-used oils, and whether or not they are likely to be chemically changed through contact with alaki. Boiling points were taken from online sources, and I cannot vouch for accuracy. In some cases, I could not find any information on boiling points. The constituent information is mine. It is representative of a typical essential oil, but there is always natural variation. Don’t confuse Low, Middle, High with the Top, Middle and Base classification used in perfumery! In this Table, “High” means high boiling point, which correlates with low evaporation rate. Conversely, a “Low” boiling point means that it evaporates readily – a “top” note in perfumery. “Reactive?” means “is the essential oil likely to react with alkali”. The greater the percentage of acids esters or phenols, the more of the oil will be changed. But remember, the oil may still be fragrant, and actually it may still be therapeutic too. (Note that very few essential oils contain significant amounts of carboxylic acids, and these are weak acids which are not corrosive.)

Oils for soap Table