Cuprous

Cuprous sulfide, it happens, is non-stoichiometric, and corresponds to a Cu:S ratio of 1.88:1 IIRC.

HCR: maybe the public’s not so stupid after all 2010

Copper (I) chloride, CuCl -- Colouring Agent Cuprous chloride is probably the best copper compound for creating blue and turquoise flames, and it can be used with a variety of oxidizers.

Terror in Argentina Rosenblum, Mort 1976

Cuprous carbide or acetylide is the reddish brown amorphous precipitate which is the ultimate product obtained when acetylene is led into an ammoniacal solution of cuprous chloride.

Acetylene, the Principles of Its Generation and Use W. J. Atkinson Butterfield

Cuprous chloride is used as a solution in strong hydrochloric acid mixed with ferric chloride, and similarly absorbed in kieselguhr.

Acetylene, the Principles of Its Generation and Use W. J. Atkinson Butterfield

Cuprous carbide, however, is very unstable and prone to oxidation; so that, given the opportunity, it combines with oxygen or hydrogen, or both, until it produces the copper acetylide, or acetylene-copper, which is explosive -- a body to which Blochmann's formula

Acetylene, the Principles of Its Generation and Use W. J. Atkinson Butterfield

Cuprous chloride White crystalline precipitate, soluble in hydrochloric acid and reprecipitated by ammonia.

Transactions of the American Philosophical Society 1771