Lye Part 3

Lye Part 3: Soda ash lye 

To read the first post about the history of lye, see: Lye Part 1. To read the previous post on potash lye, see Lye Part 2.

Soda ash lye

Soda ash, as opposed to potash, comes from burning very specific vegetable matter—usually plants growing in sodium-rich or salty conditions, such as barilla and seaweed/kelp. It contains sodium carbonate, Na2CO3, also called black ash, washing soda or just soda. It behaves quite noticeably differently from potash lye, and was thus distinguished early on for the production of hard soaps (instead of soft soaps).

Soda ash is related to natron (the cleansing salt favored by the Egyptians because it could be dug up locally); natron is a natural mix of sodium carbonate decahydrate (Na2CO3*10 H20) and sodium bicarbonate (NaHCO3, baking soda).

Sodium carbonate is a water soluble base that, like potassium carbonate, is capable of saponification on its own, although the process will be incomplete and leave about 10% of the fat untouched (1). It usually is leached (dissolved in water, and the ashes filtered out) to make sodium hydroxide, or lye.

Plant sources

Barilla (salsoda soda), also called saltwort, is a small annual native to the Mediterranean Basin. It is a low, succulent bush that likes salt water. Burning the dry plant produces ash containing about 30% or more sodium carbonate in a fairly pure form. Barilla can be cultivated on farms near salt water sources, and irrigated with salt water. It can also be eaten as a vegetable salad, but its primary historical value was for the ash.

Glasswort (salicornia europaea) is a salt-marsh plant that, like barilla, could be cultivated in saline conditions and harvested for burning to produce soda ash. The ash from glasswort contains less pure sodium carbonate (about 20%) than barilla, but this is easily remedied by leaching. Sodium carbonate dissolves readily in water, while most of the impurities from the ash settle to the bottom of the container, allowing extraction of a mostly pure sodium carbonate solution which can be evaporated afterwards. However, glasswort ashes do also contain potassium carbonate (potash), which likewise dissolves in water, so the lye leached from glasswort ashes would result in a mixed potassium carbonate and sodium carbonate alkali—perfectly acceptable for use as a flux, but producing a softer soap than barilla ash lye. Glasswort ash came into particularly popular use in the 1500's in England, replacing potash as the primary flux for glass-working, creating a clearer and more delicate glass than the forest glass that had been the standard for several centuries.

Seaweed (several species of fucus and other genera) or kelp is a marine algae plant that grows in salt water and reaches about 2m long. In northern Europe, where conditions are too cold for barilla to grow, seaweed was collected, dried and burned as a cheaper substitute. Seaweed ashes contain about 2-5% sodium carbonate, which was leached out. Although the concentration of soda ash is much lower in seaweed, requiring far larger quantities than other soda-ash plant sources, the seaweed was easy and cheap to harvest along the coasts, and the remaining ashes were used as garden fertilizer, so it was not a waste of effort.

Historical uses for soda ash

Glass-working: Plant-based soda ash served as a flux for lowering the melting point of glass to around 1,000ºC. Soda ash from barilla had fewer impurities than mined sodium carbonate (less iron and fewer earth carbonates), so it made clearer, more delicate glass. This higher-purity glass was called crystal.

Food: Soda ash is a mild alkali that improves nutrition, flavor and texture in grain-based foods like cakes and noodles (especially Chinese moon cakes and Japanese ramen noodles). German pretzels are traditionally made by dipping the dough in a solution of soda ash (a weak lye containing both sodium carbonate and sodium hydroxide). Uses for sodium hydroxide in food are less common than uses for potash lye or for hydrated lime, since soda ash was more expensive in the medieval period than either of the alternatives. Modern lye recipes, however, assume that lye means sodium hydroxide, and so it has far more use in food now than it did historically.

Soap: Both sodium carbonate and sodium hydroxide saponify fats to make soap. According to even the earliest records of soap-making, sodium-rich plant ashes have been used to make hard soap (see Lye Part 1). Hard soap was easier to transport and sell, and lasted longer in use than soft or liquid potash soaps.

Chemistry of soda ash lye

Water plays a vital role in extracting sodium carbonate, which is very water soluble, from ashes. Water also converts some of the sodium carbonate into sodium hydroxide:

• Water (H2O) self-ionizes naturally, producing a quantity of hydrogen (H+) and hydroxide (OH-) ions in equal portions.
• Sodium carbonate (Na2CO3) is a soluble ionic compound; when fully dissolved in water, it dissociates into sodium cations (2Na+) and carbonate anions (CO3 2-).
• Some of the carbonate anions (CO3 2-) abstract with the hydrogen (H+) in water to make bicarbonate anions (HCO3-). The remnants of the water molecules remain as more hydroxide ions (OH-).
• Some of the sodium (Na+) and hydroxide (OH-) bond to form sodium hydroxide (NaOH).
• The remaining positive sodium ions (Na+) react with carbonate anions (CO3 2-) to form sodium bicarbonate (NaHCO3), or baking soda. The sodium bicarbonate (NaHCO3), in turn, reacts with the water, forming carbonic acid (H2CO3) and more sodium hydroxide (NaOH).
• The carbonic acid (H2CO3) is volatile, so it breaks down into carbon dioxide (CO2) and water (H2O). The water remains while the carbon dioxide evaporates. 

In other words, leaching water through soda ash creates a solution of sodium hydroxide (NaOH) and sodium carbonate (Na2CO3) in water.

Just as potash lye could be treated with hydrated lime to increase the amount of potassium hydroxide in the solution, soda ash lye can be treated with hydrated lime to increase the amount of sodium hydroxide. In solution, the calcium hydroxide (Ca(OH)2) reacts with the sodium carbonate (Na2CO3) to form sodium hydroxide and calcium carbonate. Again, the calcium carbonate is not very water soluble, so it precipitates out of solution quickly, leaving just the sodium hydroxide in water.

Ca(OH)2 + Na2CO3 → 2 NaOH + CaCO3

Calcium hydroxide is more alkaline than sodium carbonate, but sodium hydroxide is more alkaline than either. It is also far more water soluble than calcium hydroxide. This makes it a much more useful detergent, and a faster, more complete saponification agent.

Soap made from sodium hydroxide is more water soluble than soap made from calcium hydroxide, but less water soluble than soap made from potassium hydroxide. This means that sodium soaps can harden into bars (which potash soaps cannot), and the bars do not “spend” too quickly.

Salting Soap

Although the extraction of soda ash from plants like barilla dates back at least to the Roman period, there is another way that medieval soap-makers found to create sodium soaps, without needing soda ash. They took potash lye, and during soap-making, added salt (2).

This process is actually relatively simple, and by the time that this method came into use, salt was cheaper in some regions than importing specialized ashes. During saponification, sodium chloride (NaCl) reacts with potassium hydroxide (KOH) to create sodium hydroxide (NaOH) and potassium chloride (KCl)—resulting in a sodium soap that dries into hard bars like regular sodium hydroxide soaps do.

Many later medieval soap recipes call for salt precisely for this reason. Salting hardened the batch of soap, which could then be formed into bars, balls or flakes. These recipes are often for luxury products, again, not the everyday soap used by poorer and rural communities, but they do illustrate the diversity of fine soaps available in the late Medieval Period (for those who could afford them).

To read about saponification, see the next post: Lye Part 4.

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) Verberg, Susan. “Of Potash and Lye.” Ithaca, NY, 2015. <academia.edu/27755101/Of_potash_and_lye>