Testing Soap/Soap as a Test Material

I admit to a longstanding wariness about this man at the left, Michel-Eugène Chevreul (1786-1889). In my real world, Chevreul is familiar as the author of a set of laws concerning the simultaneous contrast of colors.¹ In referencing this extremely influential work, writers on color occasionally suggest he invented the concept of simultaneous contrast. I tend to find in his work a regularization of ideas familiar in dyehouses and other color-production facilities, as recognized by certain taste-makers (to use an anachronism) of 18^th century society. For me, an early question about Chevreul’s work on animal fats was whether the same sleight of hand occurred here.

I won’t keep you in suspense about this concern: yes, and no.

Chevreul’s research into fatty bodies or fatty substances, which began in the mid-1810s, was published in 1823.² His work appears to be the first thorough organic analysis of a variety of animal fats, one that analyzed a variety of materials to identify constituent acids, and so characterized those fatty acids as well.³ Furthermore, Chevreul’s analysis explains and differentiates all materials, inputs and outcomes, of the saponification process,  and his use of saponification as a test method strengthened chemists’ understanding of that process: his work re-located both process and materials securely within organic chemistry. Earlier studies of soap-making focused on the alkaline substances and generally left the nature of fatty bodies to other investigations, or investigators such as apothecaries.⁴  Chevreul’s work seemed to me an interesting and perhaps useful addition to my fat collecting and soap-making efforts.

Chevreul’s Research Project

Chevreul, in the six books of Recherches sur les corps gras, defines and clarifies, or regularizes, the terms “oil” and “fat.” He describes his apparatus and methods. Having determined that saponification is a kind of acid-base reaction his identifies the acids in different animal fats, and describes their variety, relating them in some cases to their sources. As a result of this work, Chevreul was able to explain (for example) why cholesterol, a type of fat, did not saponify, thus separating it from spermaceti with which it was sometimes grouped.⁵ He also tested methods to prepare fatty materials, described how to analyze saponification products and compared different kinds of fat as well as fats from different sources.

Book 5 concerns saponification, and Chapter 5 of Book 5 addresses the quantity of fat a given quantity of potassium hydroxide (KOH) can saponify.  In Book 3,”Préparation des espèces de corps gras,” Chevreul described how much free fatty acid was available to combine with other substances in different fatty materials; in the later chapter he connects this to the quantity of caustic base needed to react fully with the acid. This is Proust’s law of definite proportions, but it is also something more.

§1049. 20 g of lard, which would have yielded 19.115 g free fatty acids on complete saponification, were boiled in water containing 3.315 g pure potassium hydroxide for at least fifty hours. Accordingly, the ratio of the potential amount of saponified fat to the amount of alkali was 100 to 18. A gelatinous, semi-transparent mass resulted that was perfectly homogeneous and dissolved completely in boiling alcohol. This solution did not turn litmus red. It gelled on cooling. The soapy mass also dissolved completely in boiling water and the solution was only slightly opalescent. After having been concentrated, it set as a gel on cooling and on dilution with water, it deposited a large amount of pearly material.⁶

I was puzzled by the mechanics of this description, and struck by its tiny quantities and extended boiling time: Twenty grams of lard is what you might expect to isolate from 2 or 3 pork chops. The quantity of KOH is less than a teaspoon. Furthermore, Chevreul specifies boiling water, not simmering or simply heating, for more than two days. This challenged my own experiences in both the chem lab and in the kitchen.

I digressed for a while, trying to determine how Chevreul used the equipment he describes in Chapter 2 of the first book; this was interesting and very detailed but the apparatus used in Book 5 is not mentioned. I was especially concerned to know if he replenished the evaporating liquid with water during the experiments, and whether these experiments relied on anonymous assistants’ carefully observations of the process. My other interest was why, when Chevreul was so specific about all other aspects, he noted only “more than 50 hours” as the timing of the experiments.  I planned to use these experiments to learn more about the materiality of the substances within them, and perhaps conjecture about what he saw and felt versus what other investigators (Berthollet, Darcet, or Hatchett, for example) saw, and what I did. We can’t expect Chevreul to have expressed such ideas in his time, but I was curious to consider what his presentation suggests about the materiality of soap and animal fat. Does it exist only within Chevreul’s theories of their chemical composition?

My immediate questions were practical, related to trying Chevreul’s tests:

• What kind of apparatus would support this kind of lengthy microtest?
• Is it likely that Chevreul is using a very large quantity of water? If so, doesn’t this affect his initial description of the product as soap, unlike lard, dissolves in water?
• Is it likely that he is replenishing the water as it boils off?
• Would he have used a closed system to limit evaporation?
• Would it have been so typical that it is unworthy of mention to use a sand bath or other indirect heat method in an early 19th century lab?

Shadowing Chevreul

Part 1. Saponification of Lard with KOH

The thought of fat + base boiling for more than 2 days worried me. I have no lab assistants to keep an eye on the burners overnight. I found a resolution in my first pandemic purchase: a small electric hotpot.

I took 20 g of lard from my earlier fat experiments. To this, I added a “heavy” 3g of potassium hydroxide (KOH) to 150ml boiling water, combined the two in the electric hotpot. I set it to low, and walked away.⁷

You should know that it didn’t occur to me to measure temperature or pH of any materials I used before I began.

Over the next two days, I checked the contents regularly, weighing the pot with its contents and adding enough boiling water to make up the contents to the original quantity when it seemed to run dangerously low.

The experiment smelled like lard that was not perfectly cleaned, a good description of the fat I used. It was a distinctive but not unpleasant odor. The liquid quickly turned yellow, darkening as it was reduced. The scum or foam on top (saponified fat) was always white. Or white-ish.

My trial did not last as long as 50 hours. Checking the hotpot after a long overnight of neglect, I found the liquid almost completely evaporated, and stopped heating before 46 hours.

The residue was white or white-ish, felt slightly greasy and looked like Parmesan crisps. From a food-and-cooking point of view, I’m not surprised.

Part 1. Numbers

Lost (total)	After (time from beginning)
24 g	2’ 20”
15 g	4’ 45”
18 g	7’ 00”
69 g	15’ 45”
84 g	27’ 30”

Total heating time:                                                                      45’ 45”
Original weight of materials (fat + alkali + water) in the pot: 176 g
Total water added during the run:                                             145 g
Weight of residue collected:                                                          14 g
Liquid collected at the end of the effort:                                       0
Lost solids:                                                                                          9 g (fat + alkali)

Part 2. Saponification with Sodium Hydroxide, as Described in Chevreul §1013

For this part, the procedure was the same as the one I described above, but the materials I used were lard and sodium hydroxide (NaOH) instead of potassium hydroxide (KOH). Hint: the result doesn’t look very different.

Chevreul mentioned that he used 100 g lard with 60 g caustic soda dissolved in 100 g water, a considerably higher ratio of alkali to fat than in the later trial. I used one-fifth of each quantity (20 g lard, 12 g NaOH) so the quantity of lard would be consistent in both parts. I doubled the amount of water recommended, to see if I could solve my evaporation issue.

I added alkali to the water in the hotpot, then the fat and set the temperature to low. After about 9 hours of simmering, the mixture had developed an odor of lard, the liquid was yellow and a small amount of yellow-whitish foam floated at the top. As the contents had lost about 70 g in mass, I added water to replace it.  Over the next day, I continued to weigh and top up the water. The liquid darkened and more of the foamy-looking substance developed. After 47 hours (again, not the “more than 50 hours” Chevreul stated), left the substances to cool in the apparatus.

The dried solids broke apart when I removed them from the container. As before, they had a greasy or waxy feeling.

This time, I tried to dissolve the uncollected residue in the pot by washing with about 30g ethyl alcohol.⁸

Part 2. Numbers

Lost (total)	After (time from beginning)
76 g	9’10”
72 g	20’10”
35 g	24’30”
84 g	35’20”

Total heating time:                                                                    45’ 40”
Original weight of materials (fat + alkali + water) in the pot: 232 g
Total water added during the run:                                              421 g
Residue collected:                                                                            26 g
Liquid collected at the end of the effort :                                       0
Unaccounted for solids:                                                                    6 g (fat + alkali)

Part 3. The Retry

Many months after these original efforts, I dedicated a portion of the fat harvested from my kitchenstuff, to another run of Chevreul’s saponification experiment.

I repeated the first trial, using again 20 g fat, 3 g lye, and 150 ml water. I put all into the hotpot without making much of an effort to mix it, and set the heating mechanism to “keep warm.” This would not be Chevreul’s boiling, but something more like the murmur of bouillon-making. I had learned my lesson.

Nevertheless, I still couldn’t keep all the materials at this low simmer for longer than 49 hours. Again, the process generated foamy materials and a transparent and viscous amber-colored liquid. This time, I managed to look in on the process just before all liquid disappeared at which point I shut down the heating. For once, it was fairly easy to remove the residues. The solid material was the same pearlescent and slick-feeling off-white substance as before, with some moist looking amber-colored areas on the bottom and dried amber something clinging to the pot.

I rinsed the pot in 100ml water, letting it sit to dissolve the particles as much as possible; some white “stuff” floated on the surface. I also added 100ml of water to the solids, to wash them, and left both for an arbitrary 6 hours. On my return to the trial, I found the amber-colored substance had completely dissolved into the water and turned slightly viscous again, like an egg white. I filtered the reconstituted liquid and the liquid from the washed solids together.

Part 3. Numbers

Residue collected at the end of the effort:   81 g
Liquid collected at the end of heating:       157 g
pH                                                                      above 13 (strongly basic)
Weight of solids collected:                             20 g
Liquid total:                                                    117 g
Solids total:                                                      20 g after 48 hours’ drying
pH of solids:                                                     about 8

I later boiled the amber liquid rather than wait for it to evaporate. When it had reduced by half, I noticed something began to precipitate out. That is, the clear amber liquid began to cloud and small particles “fell out.” You probably should know the latter is a legitimate early modern technical term.

The boiled off liquid left 13 g of materials in the bottom of the beaker. To this I then added 40 ml alcohol, swirling to dissolve the materials.⁹ The solution left a white residue about 3g, some of it as a lump. I harvested as much as I could, which was only about 1 g in the end.

Observations

I am still uncertain how Chevreul conducted the process he described. Book 1 Chapter 2, which describes his apparatus, details the tools he used to isolate the different fats but says nothing about his saponification apparatus. Nor does Chevreul describe the state of the substances within the apparatus when he stopped his experiments. ¹⁰ As I was not attempting to reproduce Chevreul’s experiments exactly, I don’t believe the differences between my findings and his are a significant concern.

Chevreul and Materiality of Animal Fat, Alkalis and Soap

Chevreul’s notions of materiality exist on several planes. He talks about the traditional criteria—odor, smell, color and taste. His focus is on isolation of specific acids to quantify reactions and use that to describe the components of the materials he engages. He does in other sections describe material characteristics of some of the fatty acids he identifies explaining, for example, why cholesterin (cholesterol) is not good for soap-making, and the differences between that substance and ethal (cetyl alcohol, initially isolated in spermaceti). His interest is in establishing species within these organic materials. Each may consist principally of hydrogen, oxygen, carbon and nitrogen but Chevreul seeks to define the quantity of each element as they establish the material differences of each fatty acid. Combinations of fatty acids are specific to each type of animal fat.¹¹ Chevreul’s underlying argument leans toward the discovery that fats and oils are made from combinations of a few fatty acids, that those materials are not limited by the source from which they are harvested.

Chevreul regards his study of the quantity of fat that a given weight of potash can saponify, as filling-in information about the salt-forming action of bases on fats. The quantities he chose were based on his estimates of the materials needed for a complete reaction.  His products were:

• A semi-transparent and homogeneous gelatinous mass soluble in boiling alcohol which was not alkaline and which becomes gelatinous again on cooling.
• A soapy mass that was completely soluble in boiling water and also became gelatinous on cooling.

Chevreul’s conclusion addressed the neutralization of the stearic, margaric and oleic acids in the given fat. And that a slight excess of alkali is needed to make the hardest soap possible.