Copper sulfate pentahydrate is the most widely available form of copper sulfate. Copper sulfate is useful as a water soluble source of copper (as opposed to insoluble forms like copper(II) oxide and copper carbonate).
When I refer to copper sulfate on this page, I am only referring to the pentahydrate version.
By weight, copper sulfate is about 32% black copper oxide (CuO), i.e, 1g copper sulfate contains the same amount as 0.3g black copper oxide and 1g black copper oxide is equivalent to 3g copper sulfate.
| Temperature (C) | g/100ml |
|---|---|
| 20 | 203 |
Copper sulfate is highly soluble in water. For the purposes of glaze chemistry, any (sane) amount of copper sulfate will be dissolved.
Chinese Jun (also spelled Chun) glazes can be partially or entirely red to purple. This is caused by spraying or splashing copper onto the pots. I've found success using a copper sulfate solution.
For my kiln and firing, I find that spraying copper on top of the glaze produces the strongest colors. Only a very small amount of copper is needed, but the firing is also important. I use a 1wt% solution (1g CuSO4 5H2O in 100g water) and a spray bottle that makes a fine mist. One or two sprays can produce a dark purple but can also result in no color at all. This is probably because of kiln placement and airflow. My jun glaze contains no tin. Probably the phase separation of the jun is responsible for controlling the oxidation states of copper, giving reds and purples.
I've done some tests where the copper sulfate is dissolved into the dipping glaze mix, so no spraying is needed. However, the results are still quite variable. Because the copper is soluble, its distribution in the glaze is highly dependent on the glaze application. If the glaze is very deflocculated, then the water in the glaze can slowly evaporate from the surface, leaving a high concentration of copper near the surface. If flocculated, the water and copper are easily absorbed into the bisque.
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