|Monthly Tech-Tip |
|Frit Softening Point||1336C (From The Oxide Handbook)|
|Dry M.O.R. (50% Silica)||620C|
-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. That means that the base clear might craze, yet the version with significant copper added will not.
-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.
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.
All common traditional ceramic base glazes are made from only a dozen elements (plus oxygen). Materials decompose when glazes melt, sourcing these elements in oxide form. The kiln builds the glaze from these, it does not care what material sources what oxide (assuming, of course, that all materials do melt or dissolve completely into the melt to release those oxides). Each of these oxides contributes specific properties to the glass. So, you can look at a formula and make a good prediction of the properties of the fired glaze. And know what specific oxide to increase or decrease to move a property in a given direction (e.g. melting behavior, hardness, durability, thermal expansion, color, gloss, crystallization). And know about how they interact (affecting each other). This is powerful. And it is simpler than looking at glazes as recipes of hundreds of different materials (each sources multiple oxides so adjusting it affects multiple properties).
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.
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.
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).
|Materials||Copper Oxide Black|
|Materials||Copper Oxide Red|
|Materials||Copper Carbonate Basic|
|Tests||Glaze Leaching Test|
|Oxides||Cu2O - Cuprous Oxide|
|Glaze Color||7% copper in glossy oxidation glazes can produce striking metallic green colors.|
|Glaze Color||Under normal oxidizing conditions CuO produces clear green colors in most glazes. The shade of green depends not only on the amount but also on other oxides present (i.e. lead in larger amounts will enhance and darken the green, the presence of alkalies or high boron will shift it toward blue). Copper in calcium/magnesium glazes gives a green very different from that produced with lead.|
|Glaze Color||Combinations of CuO with tin or zircon will give turquoise or blue-greens when the glaze is alkaline (KNaO) and low alumina. Look for a frit with this profile for best results. Glazes of this type often craze.|
|Glaze Color||K2O can turn a copper glaze yellowish. If Na2O or PbO are present, K2O should not exceed 0.15 equivalent.|
|Glaze Color||Copper in a barium/zinc/sodium glaze gives a blue. Color can also be enhanced by lithia. Tin and copper can produce turquoise to robin's egg blue.|