• Materials for Ceramics

Copper Oxide Black

Alternate Names: Copper (II) Oxide, Black Copper Oxide, BCO, Cupric Oxide

Notes

One of the oldest colorants used by potters. It is a popular source of CuO in glazes and glass. In sufficient percentages it can be a very strong flux, in a mix of 50% Ferro borax frit 3134 it will dissolve a firebrick crucible at cone 6! It is the most stable form of oxidized copper (Cuprous oxide oxidizes to cupric oxide in normal firings).

The oxide form of copper can give a speckled color in glazes whereas the carbonate form will give a more uniform effect.

Copper normally produces green colors in amounts to 5% where it moves toward black. In reduction firing, it turns to Cu₂O and gives vibrant red hues. It the glaze is fluid copper will tend to crystallize heavily. See CuO and Cu₂O in the oxides database for more information.

Above 1025C copper becomes increasingly volatile and its crystalline structure breaks down. At 1325C CuO melts. This can affect the color of other glazes pieces in the kiln. Glazes containing copper can change significantly because of loss of copper. Some potters alternate between reduction and oxidation, and even put a dish filled with copper carbonate in the center of the kiln to minimize this phenomenon.

There are many workable copper ores (i.e. tenorite, cuprite). Source: American Chemet Corp., 708-948-0800 FAX 708-948-0811

See also: "Coloring Mechanism of Peach Bloom Copper Red Glazes" written by four technicians from China published in Dec 91 Bulletin of the American Ceramic Society.

Related Information

How do metal oxides compare in their degrees of melting?

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These metal oxides have been mixed with 50% Ferro frit 3134 and fired to cone 6 oxidation. Chrome and rutile have not melted, copper and cobalt are extremely active melters, frothing and boiling. Cobalt and copper have crystallized during cooling. Manganese has formed an iridescent glass.

Copper can produce bright red glazes in correct reduction firing

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Copper red reduction glaze at cone 9 reduction

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The color red is difficult to achieve in ceramics. In reduction firing copper can turn many dazzling shades of red.

Copper is not just a pigment:

It can be a powerful flux, with consequences

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Left: The base recipe of GA6-C (80% Alberta Slip, 20% Frit 3134, 4% rutile). That recipe is stable; it does not run. Right: A 2% addition of copper carbonate transforms it into a runny, melt fluid glaze. It also produces a pleasant green even on this red-burning body (a good green is not easy in oxidation ceramics). Using a catch glaze (e.g. a frit-reduced version), this could be made safe on vertical surfaces. However, there are red flags here. Having only 2% copper would not destabilize a typical glaze (making it leachable). But phase separation is occurring here, likely because the base contains significant rutile. That means the phases are copper-bearing and possibly copper-concentrating. And, as noted, the copper has a greater-than-expected impact on the melt. Third, commercial variegated copper glazes can even leach in acids (as happened with the mug inset after a night with lemon juice). While the GA6-C recipe does have a 0.3:0.7 R2O:RO ratio, other indicators make it a course of wisdom to consider the GLLE test before use on functional surfaces.

Switching copper carbonate for copper oxide in a fluid glaze

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The top samples are 10 gram GBMF test balls melted down onto porcelain tiles at cone 6 (this is a high melt fluidity glaze). These balls demonstrate melt mobility and susceptibility to bubbling but also color (notice how washed out the color is for thin layers on the bottom two tiles). Both have the same chemistry but recipe 2 has been altered to improve slurry properties.

Left: Original recipe with high feldspar, low clay (poor suspending) using 1.75% copper carbonate.
Right: New recipe with low feldspar, higher clay (good suspending) using 1% copper oxide.

The copper oxide recipe is not bubbling any less even though copper oxide does not gas. The bubbles must be coming from the kaolin.

Copper oxide needs a fluid-melt transparent base

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Fired at cone 6. A melt fluidity comparison (behind) shows the G3808A clear base is much more melt fluid. While G2926B is a very good crystal clear by itself (and with some colorants), with 2% added copper oxide, it is unable to heal all the surface defects (caused by the escaping gases as the copper decomposes). But G3808A is melt fluid enough to do so. Because copper blue and green glazes need such base recipes, strategies are needed to avoid them running off the ware. That can involve thinner application, use on more horizontal surfaces, use away from the lower parts of verticals or a catcher glaze.

G3806D cone 6 glaze with copper carbonate, copper oxide

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This G3806D fluid melt glaze recipe demonstates the different color characteristics imparted by copper carbonate (left) and copper oxide (at 2%). The carbonate version is bluer and less intense. Copper carbonate is about 65% CuO while the oxide version is 100%. Our supply of the oxide version is not producing any specking (if yours does you may need to sieve or blender mix the slurry).

Copper blue-green cone 6 porcelain mugs

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These are among about 150+ of the best mugs I have ever made. I have them on hand, and I have cataloged, coded, photographed from 6 angles and described every detail about how I made them. Following is the kind of description each piece has. The linked words go to the Digitalfire Reference Library for endless research on particulars.

Clay: Polar Ice Cone 6 translucent porcelain
Glaze outside: G3806 base (copper-blue variation)
Glaze inside: G2926B transparent glossy
Firing: Cone 6 C6DHSC slow cool
Size: 10cm high, 9cm dia, 280, 310ml capacity
Foot: Tooled and fluted
Technique: Thrown, incised, glazes meet at the rim (pour-glazed inside, waxed, dip-glazed outside).

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