Reading about cuprous oxide, I saw a couple of plausible synthesis routes: one via Trommer’s test, which requires cupric ions from, say, blue vitriol, and reduces them at high pH with a reducing sugar, and one via electrolytically produced cupric chloride, which etches copper to make cuprous chloride at low pH, at least if chlorine is available, and which can then be precipitated as cuprous oxide with a base.
Unfortunately, both of these routes require a base, and the main bases I have handy are household ammonia, baking soda, and sodium percarbonate, which decays to washing soda and oxygen when wet. The trouble with the sodium compounds is that they both contain carbonate, which will bind any cupric ions quite firmly indeed, and maybe cuprous ones too; the trouble with the ammonia is that it will complex with the cupric ions and may make it harder to reduce them to cuprous.
I also have household bleach handy, which includes some soda lye in order to stabilize the sodium hypochlorite. One possibility might be to neutralize the bleach with hydrogen peroxide, producing oxygen, water, and sodium chloride, which I think will leave the lye intact. But I’d rather avoid lyes if I can. (I could probably produce it more easily by electrolysis from sodium chloride, and maybe leaching hydrate of potassa from wood ash would also be an option. I’d thought I could also calcine the sodas, but apparently that doesn’t produce lye until past 2000°.)
Another possibility that occurs to me is to produce a metal hydroxide through electrolysis. But the water-insoluble hydroxides like that of aluminum are of limited usefulness for reactions like those above — though they might be able to raise the pH to 7, which might be enough to convert solvated cuprous chloride to insoluble cuprous oxide. And the water-soluble hydroxides of the alkali and alkaline-earth metals wouldn’t need electrolysis to produce them if I had the metals, but the metals are a huge pain.
Slaked lime is, of course, another possibility, but it probably needs to be freshly calcined, which is also not easy to do in this kitchen.
A somewhat less mild base, with its own toxicity problems, is sulfide of soda. It is made by carbothermic reduction of sal mirabilis and can also be made from sulfide of lime with enough washing soda, though markgollum says the yield is poor. I am not sure if sulfide of lime also acts as a base, but it can be produced by carbothermic reduction of alabaster or, along with washing soda, from chalk and sulfide of soda, as practiced by that great physician; or, historically, by heating oyster shells with sulfur to red heat, though less than 1084°, again according to markgollum. Slaked lime with sulfur is also reported to work at only around 100°. Sulphuretted hydrogen over muriate of lime has also been suggested, though carbonic acid gas liberates sulphuretted hydrogen from the sulfide of lime.