Larry,   Your query prompted me to re-read an early paper on the subject: THE PRODUCTION AND CONTROL OF COPPER REDS IN AN OXIDIZING KILN ATMOSPHERE Arthur E. Baggs, Edgar Littlefield Journal of the American Ceramic Society Volume 15, Issue 5, pages 265–272, May 1932 ... the abstract is freely available at: http://onlinelibrary...3930.x/abstract   For those without access to the contents of the paper, here are a few comments.   (1) They obtained a cone 9 copper red with 0.3% copper