CuO (Cupric Oxide)
Notes-Copper can be produced from many different raw materials, the main being black tenorite (CuO), deep red cuprite (Cu2O), bright green malachite (CuCO3.Cu(OH)2), and bright blue azurite (2CuCO3.Cu(OH)2).
-Under normal oxidizing conditions the CuO molecule remains unchanged and produces clear green colors in oxidation glazes. The shade of copper greens can vary with firing rate and soaking changes. The best colors are generally obtained with fast firing and little soaking. CaO is unlikely to affect the color of copper in a glaze.
-Copper is well-known for its ability to produce blood-red and fire-red colors in reduction atmosphere firings where it is altered to Cu2O (see Cu2O). Purple copper reduction glazes are the result of a mixture of copper in its green oxidized and red reduced forms. This effect appears most frequently in high lime glazes or where early stages of firing are oxidizing or latter stages are light or neutral.
-Copper is a strong flux and even 2% can considerably increase the melt fluidity of a glaze.
-Generally additions of copper to a glaze will reduce its thermal expansion. However there is some doubt about the truth of this, it needs to be confirmed.
-Crystalline glazes can be attractive when done with copper.
-Copper and titanium can produce beautiful blotching and specking effects.
-Pure copper metal filings can make an extremely potent specking material (and contaminant, depending on your viewpoint) in reduction firing for both bodies and glazes.
-Leaching: When added to low lead solubility glazes copper can cause the solubility of the lead to be greatly increased. Copper is well known for this effect in glazes at other temperatures also. Do a leaching test to be sure.
-In the enameling industry, copper is used in combination with small quantities of cobalt, manganese, or nickel in making black where the black is produced in the smelter.
The same copper red glaze on the inside and outside of a vase
The Copper Red effect shows the importance of correct firing to achieve a specific effect with certain glaze recipes. The inside of this vase was more heavily and consistently reduced, simply because it was isolated somewhat from the outside kiln atmosphere. The outside of the vase, by contrast, is grey (a product of periods of oxidation during the firing).
How do metal oxides compare in their degrees of melting?
Metallic oxides with 50% Ferro frit 3134 in crucibles at cone 6ox. Chrome and rutile have not melted, copper and cobalt are extremely active melters. Cobalt and copper have crystallized during cooling, manganese has formed an iridescent glass.
Copper can destabilize a glaze and make it soluble
A closeup of a glossy Cone 6 glaze having 4% added copper carbonate. The bottom section has leached in lemon juice after 24 hours. This photo has been adjusted to spread the color gamut to highlight the difference. The leached section is now matte.
Ceramic Oxide Periodic Table in SVG Format
The periodic table of common ceramic oxides in scalable vector format (SVG). Try scaling this thumbnail: It will be crystal-clear no matter how large you zoom it. All common pottery base glazes are made from only 11 elements (the grey boxes) plus oxygen. Materials decompose when glazes melt, sourcing these elements in oxide form; the kiln builds the glaze from these. The kiln does not care what material sources what oxide (unless the glaze is not melting completely). Each of these oxides contributes specific properties to the glass, so you can look at a formula and make a very good prediction of how it will fire. This is actually simpler than looking at glazes as recipes of hundreds of different materials.
Does copper cause glazes to leach?
These are four cone 6 glazes of diverse chemistry. They have varying melt fluidities. They are soaked (half way up) in lemon juice over night. None show any evidence of surface changes. All contain 2% copper carbonate. If the copper was increased, especially to the point of going metallic or crystallizing, likely the leaching test would have different results. So, if you use copper sensibly (in moderate amounts), there is a good chance you can make a glaze that resists leaching.
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