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A Guide To Transparent Soaps
Author: Kevin Dunn
Monday, December 11, 2017

In 1807, Andrew Pears introduced a new kind of soap onto the market. Traditionally, soap was made by boiling fats and oil with dilute lye. The boiling mixture separated into two layers, the soap floating to the top, and the spent lye settling to the bottom. The soap was skimmed off the top and pressed into bars, while the lye was run to waste. When Pears introduced alcohol into the process, the boiling mixture no longer separated into layers. The soap, spent lye and alcohol remained mixed, and when the mixture cooled, it solidified into translucent soap. The new soap was an instant hit.

The process was a successful innovation, but its chemistry was not understood for another 20 years. Michel Eugène Chevreul discovered that fats react with lye to produce soap and glycerin. Since glycerin is soluble in water, it was run to waste along with the spent lye in the traditional process. The Pears process unwittingly left the glycerin in the finished soap. Translucent and transparent soaps came to be called “glycerin” soaps, and the general public refers to them as such to this very day.

Handcrafted soapmakers are familiar with the term, “Melt and Pour,” but not all transparent soaps can be melted, and not all meltable soaps are transparent. Water is a problem for meltable soap because the boiling point of water is lower than the melting point of most soaps. Instead of melting, a typical cold process soap turns into a steaming froth when heated. The ethyl alcohol used by Pears has a boiling point even lower than that of water, so we would expect it, also, to be unsuitable for melting. The key to a meltable soap is to replace the water with a high-boiling solvent like glycerin. A simple, meltable soap can be made, for example, with 67% soap and 33% glycerin.

Transparency in soap comes from preventing the formation of soap crystals. In opaque soaps, the soap molecules align in interlocking crystals, which scatter incoming light. Light that enters the soap from one direction, bounces off the first crystal and takes off in a different direction. It hits a second crystal and again, bounces off in a different direction. The end result is that light cannot pass straight through a typical opaque soap.

Honey gives us a familiar example of this phenomenon. Normally, we think of honey as a transparent, viscous solution of various sugars in water. But when honey gets cold, or simply sits for a long time, it can crystallize. As with soap, light entering from one direction bounces from one crystal to the next and exits in a completely different direction, and so crystallized honey is no longer transparent. To return honey to its original condition, all that is necessary is to heat it to the point that the sugars redissolve. The same principle can be used to make transparent soap.

A simple, meltable, transparent soap can be made by increasing the percentage of glycerin to 50%. But so much glycerin may be too much of a good thing. In lower concentrations, glycerin is a humectant, that is, it draws moisture to the skin. But in concentrations this high, it is deliquescent, drawing moisture from the air. A bar of this high-glycerin transparent soap “sweats” in the humid environment typical of bathrooms. Eventually, it will draw so much moisture from the air that it sits in a puddle of its own making. Sweat is the enemy of glycerin soaps, and modern formulations seek to replace glycerin with solvents that stay drier under humid conditions.

They key to transparent melt and pour soap is to find a high-boiling solvent, or a combination of solvents, that can keep the soap molecules from crystallizing without attracting too much moisture from the air. And the solvents that fit this bill best, as Pears found more than two hundred years ago, are alcohols.

Most people will immediately think of grain alcohol (ethanol) or rubbing alcohol (isopropanol), but chemically, an alcohol is any carbon-containing compound with an OH group. Ethanol is C2H5OH. Isopropanol is  C3H7OH. And glycerin is  C3H5(OH)3. Yep, chemically speaking, glycerin belongs to the alcohol family, and chemists call it glycerol. Many familiar compounds, including all sugars, belong to this family. Thus in formulating transparent soap, we have quite a few choices that can take the place of glycerin.

Crafter’s Choice Premium Extra Clear Melt & Pour soap base contains, in order of predominance, sorbitol, propylene glycol, sodium lauryeth sulfate, sodium stearate, sodium myristate, and sodium cocoyl isethionate. Let us investigate each of these ingredients in turn with an eye to understanding why they are there and what they do for the soap.

First on the list is sorbitol, and the -ol suffix in its name gives you a clue as to its function. That’s right, it’s an alcohol, C6H8(OH)6. Chiefly manufactured from corn syrup, it is used as a sweetener in diet foods and drinks. In this transparent soap, it is the principle solvent.

Next on the list is propylene glycol,  C3H6(OH)2, another alcohol. While it can be manufactured from glycerin, the purer grade used in food is more often made from petroleum or natural gas. It is structurally and chemically similar to glycerin, but with one fewer OH group, it is less prone to sweating.

Sodium laureth sulfate is a synthetic detergent. While it is often denigrated by people who value natural products, it has some advantages over soap, particularly for transparent soap. It is more soluble in alcohol solvents than soap is, and so a transparent soap can be made with less solvent than would be required for soap alone. This makes it possible to formulate transparent soaps that are less susceptible to sweating. And unlike soap, sodium laureth sulfate performs well in hard water.

Sodium laurate and sodium myristate are both soap molecules that are present in coconut oil soap. Sodium cocoyl isethionate is a mild, non-drying synthetic detergent that adds desirable qualities to the lather.

Thus all of the ingredients in Crafter’s Choice Premium Extra Clear Melt & Pour soap base fall into two categories: solvents and detergents. The solvents prevent the detergents from crystallizing, and the blend of soaps and detergents have been chosen to remain transparent with as little solvent as possible, thus reducing the tendency to sweat. Clearly an advantage for melting and
pouring soap.



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