The Copper will normally boil out of the glaze and not be present at the end of the firing cycle when heated to temperatures above Cone 9 . Indeed the best Oxblood Glaze that I have produced are Cone 12 to Cone 14 glazes and I will discuss how to keep the copper Ions from boiling out of the Glaze. By adding Iron and Boron to the glaze, we can keep the Copper Ions from boiling out of the glaze. So we make sure that our glaze has a supply of Iron and Boron to cling to and not boil out. I use a combination of Gerstely Borate and Borax as a substitute for Colemanite.
Borax sends to dissolve into the water and form a soapy film on the pots.
The next problem we face with the Oxblood Glaze is that the Copper and the Boron acts as flux to the Glaze and may run off the pot. We must stiffen the glaze with a sufficient proportional amount of Silica in the glaze to make it a stiff Glaze that will not run off the pot. In addition, I use this Oxblood over another stiffer glaze that will catch the running Oxblood. There are two methods to achieve the desired affect. The first is to starve the kiln for Oxygen when the kiln has reached a temperature where the glazes are molten.