Lye Part 4

Lye Part 4: Saponification 

To read the first post about the history of lye, see: Lye Part 1. To read about potash lye, see Lye Part 2, and for soda ash lye, see Lye Part 3.

Saponification

Saponification is the chemical reaction between an alkaline substance and a fatty acid, making soap. Alkaline detergents can be very harsh and corrosive. Saponification lessens their cleansing ability to some extent, but it also makes them safer to use. A strong lye can burn the skin from brief contact, and even a weak lye will dry and irritate the hands in the time it takes to wash a bunch of laundry or dishes. Soap preserves enough cleansing power to aid in washing, but it is mild and unlikely to bother any but the most sensitive skins. In some cases, extra fat is added to soap so that is soothing or moisturizing—a far cry from the lye that made it.

Chemistry of Saponification

Saponification is a form of hydrolysis (1). Fatty acids consist of long chains of hydrocarbons (CH2), sometimes very long (like 16 in a row). When a lye solution is added to the liquid fat, it reacts by breaking the 'tail' off of the fatty acid chain, and then bonds itself in its place. This reaction produces a fatty acid salt (called soap) and glycerin. Glycerin can be harvested from this process to aid in making other soaps and lotions.

Like any chemical reaction, the amounts of the chemicals involved need to be proportional for the equation to be balanced. Too much of one chemical will leave an excess at the end, when the other chemical is exhausted. In order to know how much fat to mix with a particular alkali, it helps to look up the saponification value (SAP value) of that fat.

NOTE that SAP values represent a ratio of how much a particular alkali it takes to fully saponify a set weight of a particular fat. THIS RATIO CHANGES FOR EACH ALKALI. Most tables that do not specifically name the alkali probably have assumed the use of sodium hydroxide lye.

For example:

• The SAP value for pork lard for sodium hydroxide lye is 0.138; so it takes 0.138 ounces of sodium hydroxide (NaOH) to fully saponify 1 ounce of lard.
• The SAP value for pork lard for potassium hydroxide lye is 0.193, so it takes 0.193 ounces of potassium hydroxide (KOH) to fully saponify 1 ounce of lard. 

Thus, it takes more potassium hydroxide than sodium hydroxide to fully saponify pork lard. For a more in-depth explanation, one has to look at the relative molecular weights of these different chemicals. “Since the molecular weight of caustic potash (56) is greater than that of caustic soda (40), more potash is required to saponify a pound of fat. The resulting potash soap is correspondingly heavier than a soda ash soap” (1). In other words, the various SAP values are not interchangeable and care should be taken to find out for which alkali the value has been calculated.

To further complicate matters, not all sources of fat are simple to understand or predict. For example, beef tallow is a mixture of oleic acid: CH3(CH2)7(CH2)(CH2)7COOH, palmitic acid: CH3(CH2)14COOH, and stearic acid: CH3(CH2)16COOH, not all in equal or predictable measure (2). Organic sources of fats and oils vary according to growing conditions. Like the variable quantity of potassium hydroxide and potassium carbonate in potash lye, the fats have variable quantities of their constituent acids, too.

Luckily, saponification values have been pretty well studied in modern soap-literature, and, although they represent an average, they provide a pretty good target estimate, even if the fat is rendered at home from non-standard sources (heritage livestock raised on pasture, for instance). When in doubt, err on the side of excess fat.

Soap

“Soap,” the fatty acid salt produced by the reaction of alkaline salt and fatty acids, is a neutral substance (pH 7). This bears repeating; true soap is neutral, always.

However, the contents in a batch of soap may not be exclusively soap. There may be some alkali or some fat left after the other is exhausted. The soap-maker might have put too much lye in the batch or too much fat in the batch, or not cooked the soap correctly. Soap-makers also often intentionally leave an unbalanced equation to create a lye-heavy or fat-heavy product (usually fat-heavy).

An alkaline soap (a batch of soap containing excess lye) may be useful for stronger cleansing projects. Laundry, floors, dishes, etc can all be washed with plain lye, but lye may be too harsh for the person to want to use (especially if they plan to scrub by hand). Adding a small amount of fat to a large quantity of potash lye will make a detergent that is slightly less basic and gentler on the skin, but still functions primarily as a detergent, not a soap, and will not leave a greasy residue.

This works better with some alkali than others. Adding an insufficient amount of fat to a large quantity of soda ash lye will make a bar soap that excretes excess lye, resulting in a powdery metallic coating on the surface of the soap—or worse, a bar of soap that leaks a metallic ooze of unused lye. Excess lye is best kept as a solution, and sodium hydroxide is most useful in creating a hard soap, not a liquid. When working with sodium hydroxide, it is best to aim for full saponification (a balanced equation) or else slight super-fatting.

A super-fatted soap (a batch with excess fat) is gentler, safe for use on skin; hand soaps, body wash, conditioning shampoos, and various wax/polishing soaps can all be made this way. Leaving extra fats after all of the lye is exhausted results in a moisturizing, conditioning, but potentially greasy fat residue being left on the washed surface after the cleansing action is completed. Coconut and olive oil, for example, are very popular fats for moisturizing the skin; a soap-maker may wish to make a soap that leaves some of these oils behind on the hands after washing. Super-fatting is also the only way to leave a fragrance in the soap. The chemical reaction with lye will destroy the perfume oil, so fragrance is only added to the batch after the reaction is complete and the lye is exhausted. Scented soaps are therefore expected to be super-fatted by around 5%.

Super-fatting does have a point of diminishing returns, if the amount of fat left unused in the chemical reaction is sufficient to cause rot or rancidity. Most fats run this risk if the batch exceeds 10% super-fatting. Some that are more shelf-stable on their own, like tallow, may still be perfectly usable, however. Rancidity can create an unpleasant odor, and sometimes discoloration, but it does not make the soap ineffective.

Modern soap-bar enthusiasts wail and moan about “Dreaded Orange Spots,” as did commercial soap factories a century earlier (1); they generally forbid super-fatting beyond 5% to prevent the DOS. In truth, however, these objections to rancid soap are purely cosmetic. If the functionality of the soap, rather than the aesthetics, is considered more important, then a rancid soap is not a problem. This matters for certain specialized types of soaps where high fat residue may be beneficial; saddle soap and wood counter-cleaner soap, for instance, or soap used as lotion and skin conditioner (as may be helpful when a nurse, who must scrub at their hands with powerful soap many times every day, wishes to wash their hands at home with something soothing).

One way to create a creamier, more moisturizing soap without risking DOS, however, is to mix alkali in saponification. Not all alkali saponify fats in the same ways or to the same degrees. Using potassium carbonate as the main alkali, and then finishing the equation with potassium hydroxide, for example, will make a soft paste that feels like a super-fatted soap, but which has not left any fats untouched in the reaction. Since leached potash lye contains both potassium carbonate and potassium hydroxide (see Lye Part 2), this is a relatively easy combination to make at home and use to create a particularly moisturizing soap that does not go rancid.

Solubility

Since different alkali behave differently, the soaps they create behave differently. Generally, the traits of the alkali will be apparent in the soap it makes. Calcium is not very water-soluble; therefore calcium soaps are not water soluble. Sodium hydroxide is more soluble than calcium hydroxide, but less so than potassium hydroxide; therefore, sodium soaps are dryer and harder than potassium soaps. This is why sodium hydroxide can form bar soaps, while potassium hydroxide makes liquid or soft soaps; “potassium soaps are so soluble that they will even absorb water from the air and so they exist primarily as solutions. Thus the choice of alkali—either caustic potash or caustic soda—is responsible for Pliny's observation that soap comes in two varieties, solid and liquid. This distinction has been central to soap-making ever since” (2).

This has an important impact on how quickly a soap will “spend.” The more soluble the soap, the quicker it bonds with water and rinses away. A bar of sodium soap will typically last longer than a jar of potassium soap of equal size. A mixed soap will have properties in between the two.

This applies to the fats involved as well. Different fats have different properties: how solid they are at room temperature, how water-soluble they are, how pungent they are, how colored they are, etc. The soaps made from different fats will be different even if the same lye went into each. Changing the fat alters the hardness, the lather, the smell and the texture of the soap.

Simple pioneer soap

The most elementary soap (and likely the oldest form—certainly the one most associated with poor areas and rural lifestyles in European and American history) is a simple potash and tallow soap. Strong lye is leached from hardwood ashes (ideally beech tree ashes) and simmered until it will float a fresh chicken egg. Then tallow is added and melted into it while the mixture cooks, usually for 6 hours or more. This creates a soft paste soap, which is stored in jugs, jars, tubs or troughs. A scoop can be kept with the tub for scooping out soft dollops of soap at need.

This soap is typically greyish or darker in color (brown and black soaps simply indicate a higher presence of carbon/charcoal particles in the batch). It requires very little measuring and can be learned through experience (trial and error). A rough measure is a 1:1 ratio of potash lye solution to tallow. If this consumes all of the fat and turns clear, add more tallow and keep cooking the batch. If the result hardens when it cools, and its texture feels like solid fat, not yogurt, then it needs more lye. Adding the fat gradually (and holding some in reserve) may help in the learning process.

Make certain to use a crockpot or tub that is much larger than the batch of soap. Saponification can be quite expansive, and tends to boil over if the kettle is too small. Remember not to use any reactive metal utensils or containers.

Potassium hydroxide and lard soap made with homemade lye; this batch is superfatted and moisturizing, but still has not gone rancid after a year.

Going forward

Soap-making as a hobby has a robust following online, and recipes abound for virtually all types of modern craft soap. Instructions, soap calculators, and tables of SAP values for innumerable fats are easy to find. Most assume the use of commercial lye, sodium hydroxide, sold either as powder or flakes.

There are fewer websites and resources for making potassium hydroxide soaps, but they do exist and commercial potassium hydroxide is likewise sold as powder or flakes. These recipes invariably aim for clear liquid soaps, which can be colored, scented, and/or thickened (with glycerin or xantham gum or starches, etc). Occasionally, cross-over recipes will include both sodium and potassium hydroxide to make soft soaps. Typically, they all warn against super-fatting liquid or soft-soap recipes, claiming that leaving fat in a liquid will make the soap go rancid. Take this as true for a pure potassium hydroxide soap, and remember that rancid soap will smell bad or be cloudy/oddly colored, but do not let fear of rancidity make potassium soaps seem too challenging. Rebatching liquid soap is easy.

Recipes for traditional potash lye soaps are few, and mostly exist on dubiously researched forums dedicated to survivalist or homesteading skills. This stems from the variability of the materials (homemade potash lye and home-rendered fat both carry inherent inconsistency by their natures) and the lack of interest from the crafting world. Traditional potash lye soap is rarely pretty or eye-catching, and the method for making it is less about measuring precisely and more about adjusting the batch until it looks, tastes and feels right. As such, this soap mostly appeals only to those with an interest in the history of material culture, and those who idealize self-sufficiency or primitive skills. However, making potash soap is a wonderful beginner-level skill that is actually quite forgiving (the soap can be adjusted repeatedly if needed) and is a great way to explore the science that our ancestors used every day.

Remember if using commercial, pure lye concentrate in any form, that it is FAR more dangerous to use than the traditional, leached potash lye, or even leached soda ash lye. Homemade potash lye can be gentle enough to briefly handle it barehanded, and can be used as a foot wash or a shampoo, if it is rinsed away thoroughly. The danger increases with the concentration of hydroxide. Commercial lye, which is pure hydroxide, is NEVER safe to handle with bare skin. Furthermore, diluting the dry concentrate creates an exothermic reaction that has to be approached carefully. Take all precautions recommended on other soap-making websites: gloves, eye protection, ventilation, etc. Read detailed instructions on how to work with it before starting.

Sources:

1) Thomssen, E. G. Soap-Making Manual: A Practical Handbook on the Raw Materials, Their Manipulation, Analysis and Control in the Modern Soap Plant. D. Van Nostrand Company, 1922. <gutenberg.org/files/34114/34114-h/34114-h.htm>

2) Dunn, Kevin. Caveman Chemistry. Universal Publishers, 2003.