Red copper is a reduced form of the normal black copper oxide (CuO). In normal oxidizing firings it will transform to the cupric oxide form (CuO) to produce the normal green coloration in glazes and glass. If fired in reduction, it will maintain its Cu2O structure to produce the typical copper red color. See CuO, Cu2O oxides for more information.
A conversion factor for red and black copper oxide: It might seem that it would simply be a division of the weights, Cu2O/CuO, 143 / 79.54 = 1.798. But not so, the conversion factor is actually 1.12 black copper for red. CuO (black copper) has a weight of 79.5 (1 copper weighing 63.5 + 1 oxygen weighing 16). Cu2O weighs 143, it has two atoms of copper (double what CuO has), thus it sources 127 Cu (copper) from that 143 total. 143 weight units of CuO would source only 113.7 Cu (63.5*143/79.5=113.7). So the conversion factor is 127/113, or 1.12.
Note: Red copper oxide is usually surface coated to prevent oxidation. For this reason, it may be very difficult to get the material to mix with a glaze slurry. If so you must soak the material in alchohol to wash the surface. Or use soapy water and then rinse with clean water (so there is no soap in the glaze slurry).
See also: "Coloring Mechanism of Peach Bloom Copper Red Glazes" written by four technicians from China published in Dec 91 Bulletin of the American Ceramic Society.
Information source: Digitalfire.com reference library