For Hot fabrication I was thinking it might be useful to be able to smelt copper into a mold with a thermite reaction. But for that you need cuprite, cuprous oxide, which I do not know where to buy.
I think you’d probably have to synthesize it.
Copper has many salts, but more cupric salts than cuprous salts, and its cupric salts are more often water-soluble.
Red cuprous oxide (cuprite) is insoluble in water, though not in acids, and “degrades to [cupric oxide] in moist air”, while black cupric oxide (tenorite) is also insoluble, though, again, not in acids — or alkalis, with which it yields cuprate salts; and there is additionally a hypothetical trivalent copper oxide which would be a strong oxidizer. White cuprous sulfate “decomposes rapidly in presence of moisture” to copper and the highly soluble skin-staining blue vitriol. “Little evidence exists for” yellow cuprous hydroxide, which is “extremely easily oxidized” to insoluble cupric hydroxide. Cuprous nitrate is almost unknown; blue-green cupric nitrate is a common water-soluble bulk chemical, easily synthesized from copper and silver nitrate. Cuprous iodide is precipitated from cupric ions and potassium iodide, releasing iodine; there is no cupric iodide, or at any rate only a very unstable one. Rare blue cupric fluoride is “highly soluble in water”; there is no cuprous. Dark green water-soluble cupric acetate, a component of verdigris and used to make Paris green, can reportedly be heated with copper to produce white cuprous acetate. Basic cupric carbonate hydroxide, another component of verdigris, is the insoluble and hard malachite and azurite, depending on the amount of hydroxylation; the neutral gray cupric carbonate readily hydroxylates given half a chance, or decays to cupric oxide at low CO₂ concentrations; there is no cuprous. Blue-green cupric phosphate and phosphate hydroxide are the insoluble rare mineral libethenite, or more commonly pseudomalachite, depending on hydration and hydroxylation — or, with aluminum, turquoise; again there is no cuprous.
So, excluding exotics, there seem to be no stable water-soluble cuprous salts except (to an almost undetectable degree) the chloride, iodide, and bromide.
(Incidentally, the above suggests that copper ions offer a way to separate iodide and, if cupric, elemental iodine from a mixture of chlorides and iodides, though I’m not sure it would work as well for sodium iodide.)
WP suggests synthesizing cuprous oxide by the following routes: directly oxidizing copper by heating it in air (but then how do you separate the two oxides?); reducing cupric solutions to cuprous with sulfur dioxide (maybe sodium thiosulfate might also work; the ammonia in ammonium thiosulfate would probably form a tetraammine-copper(II) complex, which which would be pretty but might interfere); reacting cuprous chloride with bases to precipitate the cuprous hydroxide; or reducing alkaline cupric solutions with reducing sugars in Fehling’s or Benedict’s tests, where the copper is complexed with potassium sodium tartrate or sodium citrate, respectively, to prevent precipitation of cupric carbonate. (Fehling’s test is mentioned in Cooley’s 1880 Cyclopædia, for example under “Urine”, along with Trommer’s test, under “Sugar”, which mixes blue vitriol with sugar and “an excess of hydrate of potassium [KOH]”, then heats it to precipitate what sounds like cuprous oxide. The advantage of Fehling’s test is reputedly that it was more selective toward sugars and easier to prepare. Benedict’s test wasn’t introduced until 1907.)
Benedict’s test sounds relatively easy, but it consumes reducing sugars, such as glucose, fructose, or lactose — but not sucrose! Though you can hydrolyze sucrose with HCl — or, apparently, by boiling it with citric acid.
I think you can also, crossing the streams, use a cupric solution to oxidize copper metal, reducing the cupric ions to cuprous ions, and simultaneously producing more cuprous ions from the metallic copper; this is reported to work with cupric chloride, for example. The etchant is maintained at an acidic pH with HCl so that the cupric chloride remains soluble, and the temperature is ideally maintained at 50° to accelerate the etching process, though Adam Seychell explains that this is not necessary; he prefers 30° to reduce the HCl fumes.
Electrolytically pure copper metal is readily available, if expensive, and easily oxidized electrolytically to the chlorides, probably with a little contamination from the iodide which is insignificant in this context. Somewhat less pure blue vitriol is available for US$5–$7/kg as a fungicide and alguicide for swimming pools, though it, too, is of course cupric rather than cuprous. At about 250 g/mol (pentahydrate) it’s about 25% copper (63.546 g/mol) by weight.
The thermite reaction with cupric oxide is hazardously rapid, so cupric contamination is to be avoided here; washing the nearly-insoluble white cuprous chloride thoroughly with water should suffice to purify it, but after purification the resulting cupric chloride must be kept dry to prevent disproportionation from recontaminating the mix. Similarly, cuprous oxide oxidizes to cupric oxide in moist air, which is dangerous in this context.
Insoluble tribasic copper chloride (atacamite, another component of verdigris) is manufactured in bulk as a nutritional supplement (though not for humans — a substitute for blue vitriol for livestock) and a fungicide.
The electrochemistry chapter of Simon Quellen Field’s scitoys suggests making cuprous and cupric oxide by heating copper in air to red heat for half an hour, then flaking the black cupric oxide off (by allowing it to cool) to reveal the cupric oxide underneath. I suspect this is more practical as a way of making cupric-oxide semiconductor thin films than as a way of obtaining bulk cupric oxide. (The author reported 50 μA at 0.25 V from their 0.01 m² cupric-oxide solar cell, using salt water as the other electrode.)
There’s also “red plague”: red cuprous oxide forming on the surface of copper over months in humid environments due to galvanic corrosion with silver plating.
Of all the options for producing cuprous oxide, I think the most practical is probably to generate soluble cupric chloride by electrolysis of aqueous sodium chloride by copper electrodes, followed by reduction to cuprous chloride with copper and HCl, then precipitation as cuprous oxide with ammonia, yielding also ammonium chloride — although perhaps a different base like sodium carbonate or bicarbonate, or calcium hydroxide, is required to prevent the copper from complexing with ammonia. It would be super cool if acetic acid were strong enough to allow the reduction and etching of copper to proceed; Cooley’s 1880 Cyclopædia reports that dibasic acetate of copper is “prepared on a large scale in France by exposing copper to the air in contact with fermenting wine-lees”, so I suspect that vinegar may be sufficiently strong to etch the copper. Where I am most uncertain is as to whether it is sufficiently strong to maintain cuprous chloride in solution. The Cyclopædia claims that heating cupric chloride reduces it to cuprous chloride “at a high temperature”, which Wikipedia suggests is 993°, well above its 498° melting point, and a bit too high for this kitchen; WP claims cuprous chloride is stable to 1490°, though melting at 423°.
Distillation of ammonia by absorption into pure water may be feasible over days at room temperature; some random YouTuber reported successful deposition of a great deal of verdigris by putting their vinegar-and-salt-painted copper in a sealed box with a dish of ammonia nearby. Another video experiment report by the impressive mopatin claims success at reducing copper oxide, or perhaps the sulfides, or perhaps dissolving them, with a mix of vinegar and salt. Others report success at making copper acetate from copper metal, vinegar, and low-concentration H₂O₂, which last they claim is not necessary.