G’day again; It is here that I tell you how, not knowing
something I was too damn lazy to look it up! But now I have
found it (actually my wife did!):- Sodium sesquicarbonate is a
mixture of both carbonate and bicarbonate. (sesqui, believe ittt
or nottt means one and a half!) One way of making sodium
bicarbonate is to pump carbon dioxide gas into a solution of
sodium carbonate, and if you were to stop before the reaction was
complete you would get both salts. There are other ways of making
the stuff, and it is virtually a by-product of some industries.
All that said, I can’t really think one might go out of one’s
way to buy it. So there you are, Eureka!!! (three cheers for
wives, eh?) And one for you lot, cheers,

   / /    John Burgess, 
  / /
 / //\    @John_Burgess2
/ / \ \

/ (___)