I heard a claim in the context of soapmaking that a stray grain of sodium hydroxide will decay into soda ash, i.e. sodium carbonate, on contact with air. This reaction happens in an aqueous solution, but I'm wondering how fast it would happen with only the air moisture absorbed by the hydroxide grain.
More to the point, some of the sodium hydroxide grains I have for soapmaking seems to have stuck together into a solid lump by absorbing air moisture inside the closed container. Should I assume a nontrivial fraction of that has turned into sodium carbonate by now?