Copper oxide is an active metal that combines easily with oxygen, which means that it is very sensitive to oxidation and reduction atmospheres. It produces a very wide range of colors in glazes, from greens (delicate light greens to turquoise to deep emerald green), to red, pink, blue, black, yellow, and copper luster.
Defining the Tests
Copper Oxide—Black Copper Oxide (Cupric) CuO; melts at 2419°F (1326°C). Red Copper Oxide (Cuprous) Cu2O; melts at 2255°F (1235°C). Cupric oxide decomposes at 1847°F (1008°C) to cuprous oxide and oxygen. It is an active flux, so adding it to a glaze may cause the glaze to run. It has high coefficient of expansion/contraction, which may increase crazing in larger amounts. It is toxic, volatile (fume hazard), and can leach into food. It can migrate through a clay body, and almost any copper glaze with a matte black surface will leach copper in the presence of acidic liquids. It can also cause pinholing.
Copper Carbonate—The idealized formula for this green powder is CuCO3, but the material may come as a variety of compounds and may contain impurities. Cu2(OH)2CO3 (Malachite) may be a more accurate formula representation. Since it is reactive chemically, it disperses better in a glaze thus giving more even results than copper oxide. It off gasses and can cause pinholes or blisters in a glaze. At approximately 572°–608°F (300°–320°C) copper carbonate releases carbon dioxide and water, and then at 1922°F (1050°C) it loses more oxygen as it restructures. Copper carbonate makes greens in amounts of 5% or less, blacks above 5%, and at 0.3–0.8% it makes blues in oxidation and copper reds in reduction. Approximate conversion: 5% copper carbonate = 3.6% black copper oxide = 3.24% red copper oxide.
Copper Sulfate—This blue crystal is an agricultural fungicide. It is soluble in water, starts decomposing at 302°F (150°C), loses four water molecules by 392°F (200°C), then changes to copper oxide and sulfur trioxide by 1202°F (650°C). Often used in pit and low-temperature saggar firings. Produces grays in soluble salt firings as well as pinks and reds in heavy reduction.
Copper Chloride—Often used in water soluble metal salt firing (aka water coloring on porcelain). Produces burgundy colors in pit and saggar firings.
Copper Filings—Chips of copper metal. Copper filings are sometimes sprinkled in or on a wet glaze to give black spots with flashes of red on the perimeter.
Copper was one of the first metals worked by humans (6000–4000 BCE). It shows up in glass as early as 2000 BCE and in glazes of the Han Dynasty (200–25 BCE). Some of the most famous copper green glazes are known as Oribe, which comes from Furuta Oribe, a general and tea master during the Japanese Keicho period (1596–1615 CE). Another popular copper glaze is a copper red. It most likely came about from the accidental reduction of copper green glazes fired in a wood-burning kiln. Some of the early sources of copper in glazes are believed to be from spraying water onto hot bronze. This produces a black flaky substance composed of copper oxide and tin oxide (the metals in bronze), which was then finely ground and used to produce copper reds. Copper red colors include strawberry, oxblood, flambé, black-red, peach bloom, apple red, rose carmin, etc. There are four major sources of copper in glazes: Black copper oxide (CuO); red copper oxide (Cu2O); copper carbonate (CuCO3); and copper sulfate (CuSO4), which dissolves in water. Each has different properties that can make a significant difference in the outcome of the glaze. Copper carbonate is the most commonly used form. It disperses well in a glaze slop and melts well in glazes to give uniform color. Black copper oxide has a larger particle size that doesn’t melt well and can cause specking in glazes. Red copper oxide is the strongest form and has a hydrophobic coating (oleic acid surfactant) that keeps it from reoxidizing in the air. Because of this coating, it won’t mix with water and simply floats on the surface. This can be corrected by adding several drops of liquid soap, which breaks the surface tension and allows it to disperse.
Copper Glaze Tips
Oribe pieces are decorated on one side with an iron oxide design over a transparent glaze while the other side is decorated with a transparent copper green glaze. Then they are fired in oxidation. Oribes can get a scummed layer on top that dulls the color. The traditional method for getting a clearer copper Oribe is to soak chestnut husks in water and then soak the pots in this acidic solution. But today potters just use a weak muriatic acid (hydrochloric acid) solution. (This is toxic so use in a well-ventilated area with safety glasses and a mask.) Copper volatilizes above 1877°F (1025°C) and becomes increasingly volatile, making it a fume hazard. The volatilization can affect adjacent pots, particularly those with tin whites or celadons, resulting in a pink blush. This property can also be used to decorate a pot. Glazing the inside of a saggar with an Oribe glaze and then placing a tin white glazed tea bowl in the saggar will give a delicately blushed pink tea bowl. Copper glazes are often used in soda and salt firings because the introduction of volatile sodium during the firing turns copper glazes various shades of blue/turquoise/green. Sometimes potters use high amounts of copper (10%) in a green salt glaze, which turns black, but when the salt fumes hit that part of the pot, the area turns deep green on one side with the other side of the pot fading to black. Raku: With sufficient post-firing reduction, the copper oxide/carbonate can be reduced to metallic copper finishes. These copper lusters are only microns thick so they can reoxidize to produce green colors (much like a penny oxidizes) if the pots aren’t coated with a polyurethane sealant. Copper is also used in Islamic luster firing techniques as well as Egyptian Paste (ancient Faience), which is a self-glazing, low-fire clay body that goes back 7000 years. It was probably discovered by firing sand, clay, and salt or soda ash. Then colorants were added to make colorful beads and ornaments.
While there is no legal limit set for safe leaching of copper in glazes, potters should be aware that the legal level of allowable copper deemed safe for drinking water is 1.3 mg/L (based more on its effects on taste than toxicity). Levels of above 5% can create black metallic surfaces and should not be used in functional ware.* Excessive amounts in a glaze can be leached with prolonged or repeated contact with acidic foods or beverages. The extremely basic conditions in dishwashers can attack a glaze surface, causing erosion of the surface and resulting in increased leaching over time.
*Digitalfire Reference Database, http://digitalfire.com.
This article was written by John Britt and excerpted from the June/July/August 2012 issue of Ceramics Monthly.