Red copper oxide supplies (Cu2O).  A colourant and flux.
Melts at low‐fire temperatures and become more volatile at higher temperatures. At high‐fire, it may fume adjacent ware. 
Red in reduction. Green to green‐blue oxidized.  Blackish at high amounts, where it may leach from the glaze. In raku post‐firing reduction gives metallic copper penny surfaces which are thin and may re‐oxidize over time. Used for metallic copper luster. Copper plus alkaline flux gives turquoise, 
Copper plus lead gives transparent grass green. High amounts give gun‐metal greys.