Copper Reds
GLAZE OF THE WEEK 10
COPPER RED CAN BREAK THE HEAD!
What makes it turn red; composition
Best uses of Copper Reds
Firing conditions required
What happened when it does not turn red v Copper = Cu from the Latin, Cuprium
Advice: Don’t cry, best to take it as it comes
All HPG glazes can be tricky in some way, but Copper Reds take the prize. There has been so much written about this subject, most of which is contradictory, that it becomes confusing to know what is the best procedure. Every glaze expert seems to have a preferred glaze formula or technique of firing . Sometimes the best results happen at Cone 5 oxidation or mild reduction when a professional potter can adjust conditions just right in the kiln dedicated to red ware. At HPG the kiln is always fired up to Cone 10 or a bit more, and the aim is for good reduction in order to satisfy all the different glaze requirements in each kiln load.
If one sees copper rocks in the “wild”, for example, veins of it in British Columbia or in the Southwest, it is a bright turquoise color because it is in one of its oxidized forms bound to other materials. Metallic copper as seen in copper wire comes from smelting at high heat to get rid of all the oxygen and other stuff. Raku firing with reduction of copper compounds develops an irridescent golden sheen of copper. The oxidized forms are cupric oxide called Black Copper Oxide which looks black in raw form (CuO), often used in red glazes though it tends to speckle, and cuprous oxide (CuO2), which looks brownish and is responsible for blues, greens and turquoise colors.
Copper Carbonate (CuCO3) looks leaf green when raw, but when used in glazes can become blue, turquoise, green, or even red depending upon the glaze mix. HPG green glazes use this form of copper in those formulae up to 5%. Copper Carbonate is most often used in red glazes because it does not cause speckling. It has less coloring power than Copper Oxide, so the weight in a glaze formula must account for the difference in molecular weights.
Either form of copper can be used to get reds. During the firing copper forms molecular complexes under the right conditions. The complexes are aggregates of copper “colloids”. These are tiny crystals containing copper which reflect light so that a ruby red color is seen. If the crystals get too big, a muddy red surface forms, rather like the color of liver. Who wants that! Sometimes tin is added to help keep the crystals tiny. If a glaze formula has too much silica, the resulting weaker color is less even and light like “strawberry” or “flambe”. Only 0.2% of copper is needed for a good red. More turns green. A “bit” of Copper Carbonate can not be dumped into any glaze and get red. The composition of the feldspars, alumina and the presence of magnesium or iron affect the red color drastically by preventing copper crystal formation. Feldspars (fluxes) composed of sodium favor brilliant reds; those with potassium favor the darker transparent red.
HPG has two active red glazes both of which fire out to a good glossy red in reduction with a careful cool-down of the kiln: Oxblood and CR-17. They both have formulae which work well in HPG’s gas kiln. Reduction is required to get most of the oxygen out and Cone 10 is the maximum temperature point.
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The red glazes are best on white clay and porcelain. On darker clay they become too muddy. Both require a thick application. Copper is volatile and will burn out, so having enough copper in the glaze available to the heat is important. Both tend to run, so keep well away from the bottom of the piece. Copper red is considered “fugitive”. If reduction conditions are not maintained, the copper goes back into its gray form on the surface of the glaze. Sometimes this can be prevented by spraying a thin coat of a cone 06 clear over the red glaze.
The first glaze is Oxblood which forms a deep red where there is plenty of glaze, such as inside a bowl. Reduction is often greater inside a piece. On the rim the glaze may be clear gray because it is thin and the copper burns out. Gray may also appear on the outside because of some oxidation. Gray can make a handsome contrast on the raised part of a carved or fluted piece. It is not always a bad effect.
The second glaze at HPG is CR-17 introduced by Shelle Avecilla, and might be a Ceramics Monthly glaze. It is a good bright red like Oxblood except that some cobalt blue is added to the formula. Cobalt is good insurance against having the glaze gray out if firing is not quite right, but there is not enough cobalt to overpower the red. Instead of being gray where thin or oxidized, CR-17 becomes clear light blue which is itself very pretty. Sometimes both blue and red are seen on the same piece. A mixture of the two in some areas creates a purple color. This is quite rare. Shelle gets ruby red jewels.
Many things can change the reds during the firing; and potters seem to have their own special firing cycles. Too little reduction and oxidation will prevent small crystal formation, causing clear gray areas, or murky dark red areas. Too much heat or time at high temperatures causes overfiring and thus copper completely burns off the pot leaving a colorless glaze, even if the red color was initially formed in glaze that was thick enough. Refiring might not work in this case. Proper maturation of the glaze toward the end of firing is important to reduce bubbling and form a clear smooth finish with no pitting nor crackling. The presence of impurities like magnesium can make the red become matte pink like bubble gum. Iron globs make the liver color, or prevent color, therefore porcelain is the best clay to use, or B- Mix.
The HPG reds work well enough as an accent on certain other glazes: Chun, Miller White (comes out more pale if at all), Rachel’s Black, maybe MacKenzie Celadon and Tenmoku. Although some of these glazes contain iron, the two glazes may not mix during firing so iron would not interfere. The red glazes are not used for designs because they run and will not develop red color evenly.
For decorated ware using prepared red or orange stains, Concepts for example, several coats of stain must be applied to make the color appear, otherwise the color is weak. Oxidation firing works best for these reds and oranges which are designed for cone 5-6. New underglazes from Amaco,the Amaco Velvets, are good at cone 10. The clear overglaze must not contain zinc or colors will fade in firing.
In a community kiln in which conditions must be satisfied for all the glazes, there is amazing good luck with getting reds at HPG. It is not easy to control. A small gas kiln dedicated to particular kinds of firing, like only careful reduction and temperature for copper red would be ideal. Usually everyone is quite happy with the result, however there are times when red does not happen. The best solution? Take it as it comes,.
Pat Harwood 6/23/05ed January 2009 ed April 23, 2009 Nov 20
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