Introduction
Clay comes in a variety of colors, ranging from grey to red and many shades in between. The specific color of a clay depends on its mineral composition and the presence of any impurities. Some clays, such as kaolin, tend to be white or grey, while others like red earthenware clay get their distinctive red hue from iron oxide. There is no simple answer to whether clay is universally grey or red – both colors are commonly seen. The color of clay can provide information about where it was formed and what materials it contains.
Common clay colors
Here is an overview of some of the most prevalent clay colors and what they indicate about the clay’s composition:
Grey clay
- Kaolin – This very pure, fine white clay gets its color from kaolinite, which is a clay mineral containing aluminum and silicon.
- Ball clay – More impurities give this clay a gray or tan look. Ball clays are often used in ceramics.
- Bentonite – This clay has a high concentration of montmorillonite and gets its grey color from impurities of calcium, magnesium, or iron.
Red clay
- Red earthenware clay – Iron oxide gives this clay its red color. Firing turns the clay an even deeper red.
- Redart clay – This clay has both illite and montmorillonite clays as well as iron oxide, creating a versatile red modeling clay.
- Terracotta – Italian for “baked earth,” terracotta begins as a brownish red clay containing iron and magnesium.
Other common clay colors
- White – Very pure clays like kaolin are white. They can be dyed other colors.
- Brown – Impurities create brown clays like Yixing clay used for Chinese teapots.
- Green – Glauconite or small amounts of iron can color a clay green.
- Orange/Yellow – Limonite, sulfur, or calcium create orange and yellow clay shades.
- Blue – Rare blue clays get their unusual color from vivianite mineral deposits.
What makes clay red or grey?
The specific minerals and impurities present in the clay determine whether it will be red, grey, or another color. Here are some of the key factors:
Iron oxide
One of the main contributors to red shades in clay is iron oxide. This mineral compound is red or yellow in color and widely distributed in soils and sediments. When clay forms in presence of iron oxide, it takes on a reddish hue. The more iron oxide, the redder the clay becomes. Firing clay rich in iron oxide turns it a deeper, vivid red as the compound becomes more concentrated.
Organic matter
Grey clays often contain higher levels of organic materials like decomposed plant matter. As the plant debris decomposes, it releases carbon that gives the clay a grey or dark appearance. Organic matter causes a reducing environment, limiting oxidation of iron that would otherwise produce red tones.
Impurities
Impurities in general will dull the appearance of clay, making it look greyish rather than pure white. Clays like kaolin and ball clay appear grey or tan based on the levels of impurities they contain. The more impure the clay, the darker it seems.
Geographic trends in clay color
While clay composition ultimately determines its color, broad trends emerge in the colors of clay from different regions based on local geology and environmental conditions:
Grey clays
- Kaolin – Southeastern USA
- Ball clays – Central and midwestern USA
- Bentonite – Western USA
Red earthenware clays
- Northeastern USA
- China
- Mediterranean regions
Other colored clays
- White clays – New Zealand, Germany
- Brown clays – Hawaii, Japan
- Green clays – Czechoslovakia, Cyprus
These trends exist because certain mineral deposits and soil conditions are more prevalent in some regions compared to others. However, clay color ultimately depends on the specific composition of an individual clay deposit, not just general location.
Uses of red and grey clays
The different properties of red and grey clays make them suitable for various applications:
Grey clays
- Paper filling and coating
- Ceramics like stoneware and porcelain
- Cat litter
- Cement
- Refractory bricks
Red clays
- Flower pots
- Roof tiles
- Face bricks
- Terracotta sculptures
- Red earthenware pottery
Grey clays tend to have lower plasticity and higher firing temperatures, making them suited for structural materials. The iron oxide in red clays lowers their firing temperature and allows vibrant colored ceramics. Ultimately, the specific use depends on the clay’s properties, not just its color.
Tests to differentiate red and grey clays
While color provides the simplest initial test, more advanced analysis can definitively identify a clay as red or grey:
Chemical composition
Testing the elemental composition of clay through X-ray fluorescence or inductively coupled plasma analysis reveals the presence and levels of iron that cause red hues along with other minerals affecting color like titanium, calcium, and magnesium.
Mineral analysis
X-ray diffraction identifies the clay minerals present, such as kaolinite, illite or montmorillonite. This helps determine types of clays associated with certain colors.
Spectrometry
Reflectance spectrometry measures light absorption at different wavelengths, identifying the specific pigments and compounds responsible for color. Iron oxides can be definitively detected.
Reducing atmosphere firing
Firing a clay sample in the absence of oxygen converts all iron oxide to non-red ferrous oxide. If the sample loses its redness, iron oxide was definitively responsible for the original color.
Conclusion
While both grey and red clays occur naturally, red clays derive their color specifically from the presence of iron oxide. Grey clays have lower levels of iron and more impurities that dilute any red tones. The concentration and types of minerals in the original clay deposit create these color differences based on local geology. Processing and applications choose clays of a certain color based on their specific properties and performance. Testing clay composition and minerals can conclusively identify red versus grey clays. So in summary, clay can be both grey and red naturally based on its unique mineral content.
