The organic chemistry laboratory curriculum is frequently more interesting to faculty than to students. Most faculty are able to see the beauty in a single, large crystal of a Diels-Alder adduct or in the elegance of a simple technique. This appreciation is sometimes not shared by students. To a student it may seem that many hours are spent producing small quantities of crystals or liquids of dubious utility or–more recently–samples so small that their presence is only confirmed by spectroscopic analysis. While the chemistry exemplified by these methods is, indeed, important their scale (and ultimate disposal of the product) is somewhat unsatisfying and the end results are often too abstract to spark student enthusiasm.
Accordingly, it is useful to provide experiences where the end product’s utility is clear (e.g. the synthesis of aspirin, preparation of soap, distillation of essential oils from spices). This paper reports a preparation of soap which not only produces a useful end product but also yields a practical quantity of the material. Furthermore, the procedure closely mimics traditional methods (an additional factor heightening student interest).
Numerous laboratory scale preparations of soap have been published (1-4). The method reported here is closely patterned after Appalachian folkways, reported in The Foxfire Book (5), and in several craft books on soapmaking (6-11). The laboratory is a cooperative project which can be easily completed in a normal lab session (with minor followup) and yields enough product that each student can keep a bar of the soap.
Soapmaking begins with fats or oils which are treated with aqueous hydroxide to convert the fatty acid esters into long chain carboxylates. This reaction, called saponification, is summarized in Figure 1 (12). The nature of a soap depends upon the fats and oils which are used in the saponification as well as on additional materials (e.g. colorants, fragrances, preservatives, exfoliants) which are added during processing. Traditional folk methods use animal fat such as tallow (rendered beef fat) or lard (rendered pork fat) and very few additives to enhance the aesthetic appeal of the soap. Most craft-oriented books favor the use of vegetable fats and oils for numerous reasons ranging from animal rights to cosmetic properties. Since the cost of vegetable fats was prohibitive at a large scale, we eschewed the conventional wisdom of craft soapmakers and chose to follow the advice of “Ma” Pearl Martin who stated, “This is breakfast bacon grease. ... I’ve always used hog grease myself.” (13) We found that the dining services department at our institution was easily able to provide a large quantity of fresh bacon grease.
Additional materials which were used (but not found in most laboratories) include a food scale with metric graduations, a galvanized 12 quart pail from the hardware store, a large spoon for stirring the soap, a roll of freezer paper, a small cardboard box (9-by-12 inch), a small package of uncooked oatmeal (not instant variety) and several bottles of essential oils.
The technique below is for use with bacon grease. If other fats are used (as a substitute or in a mixture) it is necessary to recompute the amount of sodium hydroxide as per published formulas (14). The Supplementary Section gives conversion factors which may be used to calculate the amount of sodium hydroxide needed when substituting a different fat or oil. Sodium hydroxide is available as Red Devil Lye for a more traditional ingredient. More involved procedures involve “dripping the lye” by extraction from wood ash (15).
Caution must be exercised when using the final soap since it may be rather harsh, even when properly prepared and cured. Students may wish to keep their product more for its appearance and fragrance rather than as a cleaning agent; however, as Ma Pearl remarked, “It certainly won’t hurt you. ... If I had’n’a known what lye soap was it’d scare me t’death. You needn’t be scared of that though.” (16)
Dissolve 250 g NaOH in about 700 mL of water (18,19). CAUTION: Goggles, gloves and fume hood recommended. Use caution to avoid splashing the caustic solution on skin or in eyes. Do not breath vapors from the solution. Stir constantly to prevent a solid mass of sodium hydroxide from clumping on the bottom of the container. The solution will become quite hot. Cool the sodium hydroxide solution in a water bath until the temperature is about 35°C. While waiting for the sodium hydroxide solution to cool, transfer 2.0 kg (about 4 pounds, 6 ounces) of bacon grease (refer to Supplementary Section if using other fats or oils) into a galvanized 12 quart pail, using a food scale to measure the amount of grease added. Warm on a hotplate until a temperature about 40°C is reached. Stir the fat while warming. Once both liquids are at the proper temperatures, add the sodium hydroxide solution to the fat with stirring.
Stir as vigorously as possible without splashing. While stirring it is important to occasionally scrape the liquid from the sides of the pail down into the mixture. Stirring continues until the mixture has become thick enough that one can faintly trace lines onto the surface of the liquid in the pail by drizzling a little liquid from the spoon onto the surface. In our hands this required roughly 90 minutes; actual time needed may vary–be sure to stir to this “trace stage,” but don’t continue beyond that point or it will be difficult to pour the soap into its mold. It is helpful for students to take five minute turns at stirring.
While stirring progresses, others can prepare additional ingredients. We found that two handfuls (traditional measuring units) of uncooked oatmeal give the soap an interesting appearance. Gently mill the oatmeal by hand until it has a medium grained texture. As soon as the soap has reached the trace stage, fold the oatmeal into the mixture, stirring well. Add 15 to 30 mL of essential oils (20) in a slow, steady stream with continuous stirring. Blend all of the materials well.
Pour the soap into the mold and cover the top with towels, plywood or any other insulating material. Let the soap rest in the mold for at least 24 hours. During this time the saponification will continue. Wait until the large slab of soap should reaches the consistency of soft clay (21) before continuing.
Remove the soap from the box by gently lifting the paper liner out. The soap is conveniently sliced into bars. Alternately, thin slices of the slab can be cut off and rolled into soap balls. Creative individuals might wish to mold the clay-like soap into interesting shapes at this point. Avoid contact with the skin since the raw soap is still quite alkaline at this stage. Form all of the soap bars into their final shapes at this time.
Within a few days some surfaces of the soap may possess a powdery residue of sodium carbonate or unreacted sodium hydroxide. Using a sharp knife, slice this thin layer from the soap.
Allow the soap to cure by wrapping in paper towel and air-drying for 4-6 weeks before considering use. The yield is approximately twenty-four 2-by-3 inch bars of soap, depending on thickness.
CAUTION: The final cured soap may still be relatively harsh and care should be taken in using the final product, especially in the
case of individuals with sensitive skin. The final product may contain unreacted sodium hydroxide which is a severe irritant. Take care
to avoid contact with the eyes. It is recommended that the final product be used only for decorative purposes, though when properly
prepared as described, the author has obtained a product which is suitable for use as a hand soap.
| Beef tallow | 1.012 |
| Canola oil | 0.898 |
| Castor oil | 0.927 |
| Cocoa butter | 0.996 |
| Coconut oil | 1.377 |
| Corn oil | 0.987 |
| Crisco (solid) | 0.986 |
| Lard | 1.000 |
| Olive oil | 0.975 |
| Peanut oil | 0.987 |
| Safflower oil | 0.987 |
| Sesame oil | 0.966 |
| Soybean oil | 0.979 |
| Sunflower seed oil | 0.970 |
| 1.0 kg beef tallow | (50% by weight) |
| 700 g canola oil | (35% by weight) |
| 300 g olive oil | (15% by weight) |
| beef tallow | 250 g NaOH x 1.012 | = 253.0 g NaOH |
| canola oil | 250 g NaOH x 0.898 | = 224.5 g NaOH |
| olive oil | 250 g NaOH x 0.975 | = 243.8 g NaOH |
| beef tallow | 50% x 253.0 g NaOH | = 126.5 g NaOH |
| canola oil | 35% x 224.5 g NaOH | = 78.6 g NaOH |
| olive oil | 15% x 243.8 g NaOH | = 36.6 g NaOH |