Is fired clay biodegradable? (3 Stages of making fired clay)

This blog article shall answer the question of the biodegradability of fired clay.

It shall also cover other areas:

  • Definition and agents of biodegradation
  • The process and types of weathering
  • Stages of making fire clay.
  • Properties and applications of fired clay.
  • Types of clay for firing clay.

Is fired clay biodegradable?

No, fired clay is non-biodegradable because it is clay formed from natural rocks, mostly aluminum rock.

It however breaks into small rocky particles which then break into different particles of the soil. This process is a mechanical process called weathering.


Biodegradation is the process by which naturally occurring organic materials are broken down by microorganisms such as bacteria and fungi into small particles which are not harmful to the environment.

Biodegradation is carried out by different agents such as UV radiation, light, wind, and water but the most effective agents of biodegradation are bacteria and fungi.

Biodegradation occurs in three distinct stages: biodeterioration, bio-fragmentation, and assimilation.

The biodeterioration process loosens up the structure of the organic substance. For instance, the cell wall of plants is weakened by light, wind, water, and UV radiation.

Bio-fragmentation involves the breakdown of organic matter into smaller, nontoxic particles by bacteria and fungi, releasing water and carbon dioxide in the process.

Assimilation is the last stage of biodegradation and it involves the microorganisms taking up the products of bio-fragmentation into their biological machinery to be used to make energy.

Biodegradation can either involve the microorganisms using oxygen, aerobic biodegradation or it can involve the microorganisms which do not use oxygen, anaerobic biodegradation.


This is a mechanical and chemical biodeterioration and fragmentation of rocks and minerals into small particles.

It is the main soil-forming process.

Physical weathering involves physical processes using agents such as heat, water, ice, or human activities.

Chemical weathering involves chemical processes such as reactions involving water, and atmospheric gases with chemicals found in rocks and soil.

Water is the main agent in both chemical and physical weathering.

Chemical weathering is the most efficient type of weathering. It involves chemical compounds in rocks. The chemicals react with either carbon dioxide or oxygen, methane, ammonia, water, or other chemicals to form compounds, mostly salt compounds. 

The process results in the breakdown of rocks into small particles, these particles combine with mineral nutrients produced by weathering to form soil.

What is fired clay?

Fired clay is a type of clay that has been processed and burned for various uses such as making ceramics, such as fire bricks, or for pottery.

Fired clays are stable under high temperatures of up to 3800⁰C. They range from flint clays to plastic fire clays, including semi-flint and semi-plastic fire clays.

Stages of making fired clay.

Fired clay is made in three main steps, they include:

  • Drying process.
  • Bisque firing process.
  • The glaze firing process.

Drying process.

This process is very essential if a good clay is to be acquired.

After the pottery, it is left for some time to dry. When it’s thoroughly dry, it is called bone dry clay.

This process is important because if clay is not dried, it may explode in the kiln as a result of moisture reaching the boiling point (100⁰C) and turning into steam.

The steam enters the air pockets formed during pottering and explodes. So it is also important to make sure your pottery has very little to no air pockets.

The drying is usually through natural means of sun and atmospheric gases, however, the process can be enhanced by loosely covering the pottery using a bag but one should make sure the drying is uniform.

After the dry has dried, it can be put in a kiln on low heat for a while, a process called candling. This process helps in drying moisture lying in the deep layers of clay. 

The kiln lid is raised a little to allow moisture to evaporate out.

Bisque firing.

This is the second stage of firing clay. This should come after thorough drying of clay in the first stage.

This process is also called biscuit firing. A pottery clay that has not been fired is called bone dry pottery. Bone dry clay has the following properties:

  • Fragile- it breaks very easily even on a gentle knock. 
  • Solubility- if dry bone clay is submerged in water, it dissolves to return to its former plastic state that needs re-drying.

In this stage of bisque firing, pottery is turned from clay into a ceramic material with the following properties:

  • Hard and brittle. It is stronger than bone dry clay. 
  • Non-soluble. Bisque pottery does not dissolve in water even after it’s submerged. 

During the process of bisque firing, several changes occur.

The temperature in the internal chamber rises to almost 1000⁰C. The heating depends on the type of kiln one is using. 

There are different types of kilns including electric, gas kilns, and wood-fired kilns.

As the temperature in the kiln rises, the kiln undergoes chemical and physical changes that include:

  • Residual moisture in the clay; the mechanical water, evaporates.
  • Organic compounds such as carbons and sulfur burn out of clay.
  • Chemically bonded water in the clay particles starts to vaporize out at 350⁰C.

After the chemically bonded water has evaporated, the clay becomes ceramic, and is no longer soluble and can’t be converted into a lump of workable clay.

  • Other chemicals and minerals are burned out of clay.
  • The process undergoes the process of sintering- the surface of the clay particles begins to bond together, the particles move closer and the clay becomes denser. (read).

At the end of bisque firing, a piece of pottery has undergone a lot of changes.

The bisque fired clay becomes ceramic, hard, and porous. During this process, the clay is starting to mature. A fully matured clay is that which is harder and denser. 

The degree of hardness in clay depends on different types of clays. The three most used types of clay used are:

  • Earthenware.
  • Stoneware.
  • Porcelain.

Stoneware and porcelain are harder than earthenware. Stoneware and porcelain mature at higher temperatures than a bisque firing. Earthenware is usually mature after bisque firing.

Glaze firing pottery.

This is the third stage of firing clay. There are two purposes for this stage; decorative purpose and functional.

On the decorative part, according to an article, glazing can give a potter a range of colors, textures, and finishes.

On the functional part, Glaze coats the pottery in a layer of glass, making the pottery water-resistant and waterproof.

The glaze is applied to the pottery in a liquid form through painting, pouring, dipping, and airbrushing. The glaze dries up quickly because the pottery is porous.

Several layers of glaze are usually needed to be applied to a piece of pottery, although this might depend on the type of glaze used and the method of application.

After the glaze has dried up, the pottery is ready for the second firing, this is called glaze firing or glost firing. During this process, the material of the glaze changes.

Some of the materials in the glaze are known as glass-formers. When exposed to certain temperatures, they melt and form liquid glass. 

The liquid glass coats the pot, and as the kiln cools, the glass coat hardens to form a glazed surface.

What are the applications of fired clay?

Fired clay has several functions depending on the type of clay, the glazing used, and the density and toughness of the clay. 

The different applications are:

  • It is used as a firebrick.
  • It is used for lining furnaces.
  • It is used for manufacturing utensils used in the metal work industries such as crucibles, saggars, retorts, and glassware.
  • It can be used to make complex items such as pipes and sanitary ware.


This article has covered the biodegradability of fired clay.

In addition, it has also covered other areas such as:

  • Biodegradation process.
  • Weathering process.
  • The properties and uses of firing clay.
  • The three stages of firing clay.
  • Types of clay used.

Frequently Asked Questions (FAQs): is fired clay biodegradable?

How is clay biodegradable?

Clay does not particularly biodegrade because it does not contain organic matter, however, clay, just like rock, breaks down by weathering into small particles.

Is clay harmful to the environment?

Clay does not have any negative impact on the environment, but the activities of its mining have led to land derelict and therefore imbalance in the ecosystem.

Mining also triggers soil erosion which leads to river siltation.

Is clay a renewable resource?

Yes, clay is a natural mineral that is readily available in the ecosystem. It is always forming through the weathering process and there can never get depleted.


Shackelford, James F (2008). Ceramic and glass materials: structure, properties, and processing. Springer. p. 121.

Lesley. 3 Stages of Firing Clay- A Beginners Guide to Firing Pottery.

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Cara Gibbs. Fireclay Tiles Proves Good Design Can Also Do Some Good.

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