Handcrafted soapmakers sometimes encounter a layer of white crystals that appears on the surface of soap, a phenomenon they call “soda ash.” One might imagine that the term “ash” describes the ashy appearance of this layer, but it actually goes back to the dawn of industrial chemistry. Long before there were giant multinational corporations or huge manufacturing complexes, alkaline materials (lyes) were produced in the service of three commodities: glass, paper, and soap.
The earliest alkali to be produced commercially was potash, manufactured by leaching wood ashes with water. The alkali-laden water was then boiled to dryness in a pot, and the white solid remaining was fittingly called pot-ash. Boiling fats and oils with potash produced a liquid soap, and adding salt converted it to a solid soap. Later soapmakers discovered that burning seaweed instead of wood produced an ash that could be used to make solid soap without the addition of salt. This ash was called soda ash.
The soapmaking process was gradually improved over the centuries. Heating limestone to high temperature converted it into another convenient alkali—lime, which could be combined with potash or soda ash to produce even stronger alkalies—caustic potash and caustic soda. And by the early nineteenth century, caustic soda had become the most important product of the nascent chemical industry, primarily in the service of soap production. Even today, soda ash is an important commodity, sold in grocery stores as “washing soda.”
The chemistry of the alkalies was worked out during this first round of chemical industrialization. Potash and soda ash were determined to be potassium carbonate (K2CO3) and sodium carbonate (Na2CO3), respectively. Lime was found to be calcium hydroxide (Ca(OH)2), and caustic potash and caustic soda were potassium hydroxide (KOH) and sodium hydroxide (NaOH). The conversion of soda ash to caustic soda is represented by the chemical equation:
Na2CO3 + Ca(OH)2 = CaCO3 + 2 NaOH
Notice that in this reaction, the compounds sodium carbonate and calcium hydroxide simply swap first and last names, becoming calcium carbonate and sodium hydroxide.
Handcrafted soapmakers rely on the reaction of sodium hydroxide with oil, but they are less familiar with the reaction of sodium hydroxide with air, and it is this chemistry that is essential to understanding the formation of “soda ash” on soap. To explore this phenomenon, we are going to make a single bar of bad soap on purpose. You need two Styrofoam coffee cups, one with a lid to fit it, some plastic wrap, palm oil, sodium hydroxide and water. The soda ash will be easier to see if you add a little of your favorite dark colorant. Weigh 100 g (3.5 oz) of melted palm oil into the first cup, and mix in a little colorant. In the second cup, dissolve 23 g (0.8 oz) of sodium hydroxide in 46 g (1.6 oz) of water. Pour the sodium hydroxide solution into the oil, cover the top of the cup with plastic wrap, and use the lid to make a good seal. Shake the cup to thoroughly mix the raw soap, holding the lid firmly to avoid leaks. You will be able to hear and feel the soap get thicker as it approaches trace. Set the cup aside overnight to allow for complete saponification. The resulting soap will have an excess of sodium hydroxide.
When you open the cup the next day, the soap should appear normal, but if you have made it correctly it will be lye heavy. To see this, dribble a few drops of phenolphthalein indicator onto the soap. It should turn pink. Be aware that phenolphthalein is most often sold as a solution in alcohol, and this is what is needed for the test. A solution in water will turn pink whether or not the soap is lye heavy.
To make genuine soda ash, simply leave the lid off of the cup for another day. During that time, carbon dioxide from the air will react with excess sodium hydroxide at the surface of the soap, as expressed in the equation:
2 NaOH + CO2 = Na2CO3 + H2O
The resulting sodium carbonate forms as white crystals on the surface, where sodium hydroxide comes into contact with atmospheric carbon dioxide. Sodium carbonate is a much weaker alkali than sodium hydroxide, and will not change the color of alcohol-based phenolphthalein indicator. Figure 1 shows soda ash on the surface of a bar of palm oil soap colored with red iron oxide. The cut on the surface shows the color of the soap beneath.
How does soda ash form on a normal soap, one that is not overly alkaline? If the raw soap is exposed to atmospheric carbon dioxide during the first four to eight hours, the unreacted sodium hydroxide at the surface will become sodium carbonate. The best protection against this is to cover the soap during this period. If your mold has a lid, that should be sufficient. If not, a layer of plastic film may be used. The cover need not make contact with the surface of the soap—it simply needs to prevent fresh air from entering. Any carbon dioxide trapped between the surface of the soap and the lid will quickly react, producing an amount of soda ash so tiny that it is not likely to be noticed.
Soda ash is not harmful, and because it is very soluble in water, it is easily removed with a spritz of water. Interestingly for our alkali-heavy soap, however, soda ash will reappear at the surface as fresh sodium hydroxide works its way to the surface. And if you cut the soap, it will look normal initially, but soda ash will appear on the fresh cut over the course of a few hours. This will happen again and again until all of the excess sodium hydroxide has reacted. Endless fun for the whole family!