What Turns Pottery Black?

Glazes and clay bodies used in pottery contain various minerals and compounds that can interact and change during the firing process. Over time and with use, ceramics can also absorb organic materials that lead to discoloration. This article will examine the common causes of pottery turning black, including reactions with iron, manganese, and copper compounds, issues with firing, and buildup of residues. It will also discuss ways to prevent blackening and restore discolored ceramic pieces.

Chemical Reactions

One of the most common causes of pottery turning black is chemical reactions that occur during the firing process. These reactions happen between components of the clay body and glaze ingredients, leading to the formation of dark colored compounds.

For example, iron oxide is a common impurity found in many natural clays. During firing, the iron can react with silica in the clay to produce black iron silicates. Iron can also interact with other minerals and flux materials like calcium, potassium, and sodium, forming dark iron-rich crystals.

In addition to iron, compounds of manganese, copper, and even cobalt can form as a result of firing reactions. These metals naturally occur in clays and glazes, and when exposed to high heat, will readily form black colored oxides and silicates. The composition of the clay body and glaze ingredients, along with factors like temperature and atmosphere, all play a role in determining the likelihood of these darkening reactions taking place.

Glazes are particularly prone to discoloration because they contain fluxes that facilitate chemical interactions at high temperatures. But even terracotta clay without any glaze can turn black if there are impurities present. Identifying the source of the metals that are causing the discoloration can help potters reformulate their materials to avoid the problem.

Iron Compounds

One of the most common causes of pottery turning black is the presence of iron compounds. Iron is a very reactive element and readily oxidizes when exposed to air and moisture. This results in the formation of iron oxides, commonly known as rust. Even small traces of iron in the clay can cause discoloration during firing.

Iron oxide compounds like hematite (Fe2O3) are commonly found as natural impurities in clay. During firing, these iron impurities get oxidized and turn into magnetite (Fe3O4), creating the typical black or dark brown color. The higher the iron content in the clay, the darker the fired pottery becomes.

In addition to naturally occurring iron, contamination from iron tools, glazes and stains can introduce excessive iron. Using iron-rich water or tools for mixing and preparing clay can also lead to discoloration. Iron particles left over in kilns from previous firings may deposit on pottery and create dark spots.

To avoid black discoloration from iron, potters should use low-iron clay and glazes. Proper clay preparation and avoiding cross-contamination is also important. Adding barium carbonate can help precipitate some soluble iron compounds. Firing in a reduced atmosphere also minimizes iron oxidation and darkening of the pottery.

Firing Conditions

The firing process is crucial for producing the characteristic colors and textures of finished pottery. However, improper firing conditions can also lead to a blackening of the clay body and glaze. One of the most common causes of blackening is insufficient oxygen flow and ventilation within the kiln. When fired in a reducing atmosphere without enough oxygen, the iron compounds in clay can chemically change into black ferrous oxide.

Firing temperature is also a key factor. Firing too low, below the maturation point of the clay and glaze, will prevent proper vitrification. The unfinished clay will then be more porous and prone to darkening when exposed to contaminants. On the other hand, excessive temperatures can also encourage blackening, especially in certain clay bodies and glazes that were not formulated for high temperatures. Regardless of oxygen levels, firing too high can directly decompose the crystal structure of the silicates in clay and glaze, turning them black.

Proper temperature ramps, holding times, and cooling cycles must be calibrated for each clay and glaze recipe. Following the manufacturer’s specifications closely can help avoid underfired or overfired results. Using a kiln sitter, cones, or a programmable kiln controller can also improve control and consistency in the firing cycle. With optimized firing, potters can successfully achieve the desired colors and finishes.

Organic Residues

Organic matter naturally present in clay can be a common culprit in causing black discoloration on fired ceramics. Clays contain small amounts of organic impurities like decomposed plant and animal residues. When fired at high temperatures, these organic materials do not burn off completely. Instead, they undergo pyrolysis and carbonize into black carbon deposits on the surface of the pottery.

During firing, temperatures often exceed 600°C which breaks down the organic compounds into elemental carbon. This carbon can leave a darkened, sooty residue. Organics like starches, proteins, oils, and sugars are most problematic as they readily decompose into black carbonaceous material.

The carbon deposition occurs because heating organics in an oxygen-limited environment prevents complete combustion to carbon dioxide and water. So traces of hydrocarbons undergo thermal decomposition reactions that leave carbon soot clinging to the clay pottery surfaces.

Preventing black discoloration from organic residues requires removing these impurities from the raw clay through processes like slip casting. Firing at very high temperatures (over 1000°C) in an oxidizing atmosphere can also allow more complete burning off of organic carbon.

Manganese Compounds

Another cause of pottery turning black over time is the presence of manganese compounds. Manganese is a common trace mineral found in clay. During firing, manganese oxidizes and turns into manganese dioxide, which has a black or dark brown color.

Over time, the manganese dioxide on the surface of the pottery continues to oxidize through exposure to oxygen in the air. This further darkens the pottery and creates black spotting or patches. Frequent fluctuations in moisture and humidity exacerbate the oxidation process. The blackened areas spread as more manganese oxidizes.

Lower firing temperatures result in more manganese remaining in the clay. Higher firing temperatures cause the manganese to burn off. However, small amounts still remain in most clay bodies. The manganese left behind slowly blackens over time.

There are a few ways potters try to prevent manganese blackening. Adding barium carbonate to the clay helps inhibit discoloration. Using clay with very low natural manganese levels also helps. Firing at higher temperatures reduces manganese content. However, small traces still exist, so blackening can still slowly occur over time.

Copper Compounds

Copper carbonate and copper oxide are common ingredients in ceramic glazes. When fired in an oxidizing atmosphere, they produce blue or green colors. However, in a reducing atmosphere, these copper compounds can turn pottery black.

During the firing process, the copper oxide in the glaze is reduced to metallic copper. This results in a dark gray or black color. If organic matter is present, the copper can further react to form copper sulfide, which produces a very dark black coloration.

To avoid copper glazes turning black, it is important to fire them in an oxidizing atmosphere with sufficient air flow. Slow cooling the kiln and allowing adequate ventilation prevents the glaze surface from becoming reduced. Proper glaze chemistry and application thickness also minimizes dark copper blotches.

Prevention

There are several ways potters can help prevent their pottery from turning black during firing:

Clean clay thoroughly – Make sure clay is free of organic matter like leaves, sticks or grass before using it. These organics can burn out and leave behind carbon that causes blackening.

Avoid iron contamination – Iron tools, water or clay with high iron content can introduce iron that causes black discoloration. Use non-iron tools when possible.

Control firing atmosphere – Fire in an electric kiln with good ventilation or in a well-drawing gas or wood kiln to provide sufficient oxygen. Insufficient oxygen and air flow can create an environment where blackening reactions occur.

Use cleaner fuels – For gas and wood-fired kilns, use clean-burning fuels to minimize carbon deposits and residues that lead to blackening.

Protect pieces – Place pieces on stilts and use kiln wash on shelves to minimize contact of clay with carbon sources. Separate pieces to allow airflow between them.

Test glazes – Make sure glazes are compatible with clay body and firing conditions. Incompatible glazes can cause black discoloration on the clay surface underneath.

Following proper clay preparation, kiln firing, and glazing techniques can help potters avoid the frustration of firing pieces that turn black.

Restoration

If pottery has turned black over time, there are methods that can be used to restore its original color and appearance. Cleaning and restoring pottery takes patience and care to avoid damaging the piece.

Gentle mechanical cleaning using soft brushes, toothbrushes, or bamboo skewers can help remove some surface residue without harming the pottery. Avoid abrasive scouring pads or wire brushes, as these can scratch the surface.

Chemical cleaners may also help lift stains without damaging the pottery. Diluted bleach, hydrogen peroxide, lemon juice, vinegar or ammonia can be applied with care using cotton swabs or soft cloths. Rinse thoroughly after cleaning. Repeated gentle cleaning over time often yields the best results.

For valuable antique pieces, consult a professional pottery restorer. More aggressive techniques like poultices, chemical dips or ultrasonic cleaning may be used, but can also damage the item if not done properly. Professional restorers have the proper training and experience to safely clean pottery and reveal its original vibrant colors.

Conclusion

There are several main causes that can lead to pottery turning black. Iron compounds in clay can react and darken during firing, especially in reduced oxygen environments. Too much organic material left on the clay can also carbonize and create black spotting. Manganese and copper compounds can further contribute to pottery turning black when fired under certain conditions.

To prevent blackening, it’s important to start with clean clay that is low in iron, manganese and copper impurities. Bisque firing at a lower temperature first, before glazing and higher temperature firing, can also help stop the reactions that cause blackening. For restoration, chemical solutions are sometimes used to convert the black compounds back to their original oxidized states and restore the original clay color.

In summary, blackening of pottery is mainly caused by chemical reactions with impurities in the clay during firing. Proper clay preparation, controlled firing conditions, and light surface restoration can help avoid and correct this issue. With some care taken to manage the chemistry, beautiful light colored pottery can be achieved.