What Causes Red Color In Clay?

Clay is a fine-grained, naturally occurring earth material composed primarily of hydrated aluminum silicates. It is plastic when wet and hardens when fired, making it ideal for pottery and ceramics. Clays come in a wide range of colors, often determined by the mineral impurities present. Red clay, in particular, contains iron oxide compounds that give it a distinctive reddish hue. This unique coloring has made red clay a popular choice throughout history for decorative items, pottery, bricks, and architecture. When mixed with water, red clay can be molded into pots, sculptures, tiles, and more. Understanding what makes clay red requires analyzing its chemical makeup and how it formed over geological time. This article will examine the key factors that produce red color in clay deposits around the world.

Iron Oxides

The primary cause of the red coloration in clay is the presence of iron oxides. Iron oxides are chemical compounds composed of iron and oxygen. The most common iron oxide found in clay is hematite (Fe2O3). Hematite crystals have a reddish or brownish color, which imparts the red hue to the clay.

As clay forms from the weathering of rocks, small traces of iron can be incorporated into the clay structure. Over time, the iron undergoes oxidation and transforms into hematite. The concentration of hematite determines the depth of red color. Clays with only a small amount of hematite will be light pinkish red, while clays rich in hematite can be a deep, vivid red.

Other iron oxide minerals besides hematite can also contribute color. Goethite, with a yellowish hue, can create an orange-red color. Maghemite (Fe2O3) is reddish-brown and can darken the red color. The combination of these various iron oxides results in the distinctive red hues seen in some clays.

Chemical Composition

The red color in clay is primarily caused by iron oxides, which are chemical compounds composed of iron and oxygen. The most common iron oxides found in clay are hematite (Fe2O3) and magnetite (Fe3O4). Both of these minerals contain iron in the +3 oxidation state, meaning the iron atom has a 3+ charge. This oxidized form of iron appears red or reddish-brown.

Hematite is an iron oxide with the chemical formula Fe2O3, meaning it contains two iron atoms for every three oxygen atoms. In its pure form, hematite forms shiny grayish black crystals, but when found in clay deposits, it occurs as fine-grained red powder. This gives the clay its characteristic red hue.

Magnetite has the chemical formula Fe3O4 and contains both +2 and +3 iron. Like hematite, magnetite in clay is a fine-grained red powder. The combination of +2 and +3 iron ions allows magnetite to maintain magnetic properties, hence its name.

In summary, the red color in clay comes from the presence of iron in the oxidized +3 state, as found in hematite and magnetite. The chemical bonding between the iron and oxygen atoms results in the distinctive red tones seen in iron-rich clays.

Geological Origins

The red hue found in certain types of clay is primarily caused by high concentrations of iron oxides. These iron oxides occur naturally due to specific geological and environmental conditions that result in increased levels of iron in the clay deposits.

Certain types of rock, such as basalt and granite, contain higher levels of iron. As these rocks weather over time through exposure to the atmosphere, the iron is released. In areas with heavy rainfall and warm temperatures, chemical weathering of iron-rich rocks occurs rapidly. The iron oxides then bind to clay particles in the soil through a process called illuviation.

Environments prone to flooding can also lead to accumulation of iron oxides in clay deposits. As river waters overflow stream banks, they deposit fine silt and clay particles. Soluble iron in the water binds to these clay particles. When the flood waters recede, the iron-enriched clay is left behind. Over many seasonal cycles of flooding, high concentrations of iron build up in the soil.

Additionally, in low-lying areas with poor drainage, iron can accumulate in the soil through a process called gleying. Saturated soils allow iron to be mobilized and redistributed, moving it from upper layers into the subsoil. Over time, these waterlogged conditions concentrate iron oxides in the clay.

Understanding the environmental and geological conditions that produce iron-rich clay deposits provides insight into the origins of the distinctive red coloration. Trace concentrations of other minerals can also influence the final color, but high iron content remains the primary factor.

Processing Methods

The processing methods used to prepare clay can significantly impact the intensity of the red color. The most important step is firing or burning the clay at high temperatures. Firing promotes chemical changes that intensify the red hues of the iron oxide pigments.

As the temperature increases during firing, the iron oxide undergoes dehydration. This removes water molecules and allows more light to reflect off the crystalline structures of the iron, creating a deeper red appearance. Temperatures between 800-1000°C produce the most vivid red clays.

In addition to firing, the length of exposure to air can intensify the redness. Allowing clay to oxidize in open air for longer periods before firing produces a more saturated red. The oxygen helps convert more of the iron to hematite.

Finally, mineral fluxes like salt and wood ash are sometimes added during processing. These help fuse the clay particles together and can aid in freeing up iron oxides, promoting the bright red hues.

Types of Red Clay

There are several common types of naturally occurring red clay used in pottery, construction, and manufacturing applications:

Red Earthenware Clay – This is a very common red firing clay that turns a bright red/orange when fired. It has moderate plasticity and is often used for handbuilding and wheel throwing pottery. Red earthenware can be used for everyday functional ware.

Red Stoneware Clay – Denser and less porous than earthenware, red stoneware clays can be fired at higher temperatures to produce a stronger material. The clay body turns a deep red and is popular for oven-safe ceramic cookware and dinnerware.

Terracotta Clay – Terracotta is an orange/red colored earthenware clay. It is often used for pottery as well as brick and tile construction applications. Unfired terracotta has good workability for shaping and sculpting. When fired at low temperatures it becomes harder and more durable.

Redart Clay – A versatile, pre-mixed red clay popular for classroom and studio use. It has reliable firing results and can be used for handbuilding, wheel throwing, and sculpting. Redart can be fired to cone 04/05 for a bright red colored clay body.

Red Brick Clay – Red brick clay contains high amounts of iron oxide and turns a terracotta red when fired. It has been used since ancient times to create strong, durable bricks for construction. Modern red bricks are mass produced from clay in large kilns.

Grading and Classification

Clays are graded and classified based on several factors including iron oxide content, plasticity, texture, and color. The iron oxide content greatly impacts the redness of clay. Higher iron oxide concentrations produce a deeper, richer red color. The most common industry standards used for grading clays are:

• The ASTM C618 Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete
• The ASTM D422 Standard Test Method for Particle-Size Analysis of Soils
• The ASTM C151 Standard Test Method for Autoclave Expansion of Hydraulic Cement

These standards test for specific properties like particle size distribution, color value, and composition analysis. Based on the test results, clays are classified into categories or grades indicating quality and usage. For iron-rich red firing clays, common classifications are red art clay, red stoneware clay, and red earthenware clay. The highest iron oxide concentrations produce a rich crimson or brownish red color in the fired clay.

Historical Use

Red clay has been utilized by humans for thousands of years. In ancient times, red ochre pigments made from clay containing iron oxide were used by prehistoric peoples for cave paintings, body painting, and pottery decoration. The vibrant red color held symbolic and cultural significance in rituals and ceremonies.

Later civilizations continued to use red ochre pigments for cosmetics, textile dyeing, and ornamental arts. Red clay deposits helped establish major trade routes, as the material was highly valued yet localized. Famous examples appear in the terra-cotta warriors of ancient China and the red palace walls of India.

Indigenous peoples around the world developed unique practices using the red clay in their local environment. Native American tribes used clay for pottery and to construct domestic structures. In Africa, red ochre played a role in coming-of-age rituals. Australian Aboriginals incorporated red ochre into burials, rock paintings, and body decoration for ceremonies.

Even as synthetic pigments replaced natural earth colors, red clay maintained importance as a building material. Fired clay bricks spanning centuries still stand as architectural wonders. The natural iron oxide acts as a flux, creating strong and durable red construction materials seen in historic sites worldwide.

Modern Applications

Red clay has found numerous modern applications thanks to its rich color, texture, and plasticity. In industry, red clay is commonly used to create bricks, tiles, and other ceramic products. The natural iron oxide pigments produce a warm, earthy red hue that is highly desirable for decorative purposes. Manufacturers often blend the clay with other minerals to achieve specific properties and colors.

Artists also frequently utilize red clay for pottery, sculpture, and other artwork. The plasticity allows shaping into precise forms, while the iron content enables firing to high temperatures for durability. Many ceramists create one-of-a-kind pieces such as vases, bowls, mugs, and figurines out of red clay. The finished artworks highlight the beautiful deep red tones. Some artists will incorporate red clay into mixed media pieces as well.

Red clay’s long history of use in pottery continues today, blending modern sensibilities with ancient traditions. Contemporary potters work with the material for both aesthetic and functional wares. The striking color and texture make red clay a popular choice for handmade crafts. With its versatility and bold visual impact, artisans are finding innovative applications for red clay across a range of mediums and techniques.

Conclusion

The red color found in clays comes primarily from iron oxide compounds. These compounds, such as hematite and maghemite, contain iron in an oxidized state that gives them their characteristic red hues. The iron oxides arise from the weathering and breakdown of iron-bearing minerals within the parent rocks that formed the clay deposits.

Processing methods like crushing, washing, and firing can intensify or alter the shades of red by concentrating the iron oxide particles. The grading and classification of red clays is based on factors like color saturation, texture, plasticity, and intended use.

Historically, red clays have been used for pottery, bricks, and pigments around the world. Today, they still play a crucial role in construction materials, household products, and art supplies. Understanding what makes clays red allows mineralogists, ceramists, and other specialists to select the best materials for their needs.

Overall, the rich red hues of many natural clays stem from the geologic conditions that concentrated iron and oxygen during their formation. The abundance of red clay speaks to the prevalence of iron on Earth and its enduring colorants that span human history. While often seen as a common material, the subtle chemistry and complexity behind red clays’ vibrant palette reflects our planet’s intricate tapestry.