Composition of Wood Ash:
1) The percentage of carbonate versus hydroxide depends on the temperature that the wood ash is fired at. At 1000 o
C, the initial composition is mostly hydroxide. At 600 o
C, the initial composition is mostly carbonate. Intermediate temperatures produce intermediate results. The temperature of wood charcoal is about 1100 o
C. Sodium carbonate is converted to sodium oxide at 851 o
2) Finely powdered freshly prepared wood ash containing potassium and/or sodium hydroxide will quickly take up CO2
resulting in conversion of the hydroxides to carbonates.
3) Making soap with carbonates is difficult (pKa of bicarbonate ≈ 10; pKa of water = 14), as hydroxide is 10,000 times as strong a base. People have been making soap for quite some time (e.g.
, the 12th century Florentine soap making guild). People in centuries past were clever, and soap makers likely determined that wood ash must be heated to high temperature followed by leaching with water and storage in a container that excludes contact with atmospheric CO2
. Alchemists in the middle ages were familiar with the construction of furnaces that could easily be maintained at 1000 o
4) Several recent papers have reported analyses of the alkaline content of burned plant ashes. These papers report hydroxide at between 1% - 20% of the total with the balance being carbonate. These results are typical when the combustion is carried out “carelessly” and the fine ash produced is allowed to take up CO2
5) Let’s assume that medieval soap makers knew that a stronger base than carbonate could be produced and that this stronger base was necessary for efficient soap making.
6) As an alternative to high T firing, sodium carbonate and/or potassium carbonate can be converted to the corresponding hydroxides by treatment with calcium hydroxide (aka, hydrated quicklime, slacked lime) in a process referred to as causticizing: Na2CO3(aq) + Ca(OH)2(aq)
CaCO3(s) + 2 NaOH(aq)
7) Processes to produce quick lime (CaO) were known before 100 BC. I am not sure when the first person realized that water could be added to quicklime to produce slacked lime, but I assume it was a long time ago.
However, it seems that medieval alchemists (i.e., chemists) would use thermal methods in preference to the process of causticizing.
9) High carbonate potash is fine for use as a pH adjusting agent, fertilizer, etc., but hydroxide is necessary for efficient soap making.