what is pottery clay made of

What is Pottery Clay Made Of? – Getting The Dirt on Pottery Clay

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If you are a potter, or simply own some pottery, you may have wondered what is pottery clay made of?  You need not wonder anymore, in this article we will dive into this topic and discuss the basics of pottery clay.  We will look at how it occurs naturally, how it is processed and the different types available.

Pottery clay is made up of different ingredients, the main one being clay. Depending on the type of pottery clay, there are varying compositions of metal oxides, organic material, and water. Different types of clay contain different compositions of particles and phyllosilicate minerals.

There are three main types of pottery clay- stoneware, kaolin, and earthenware – which will be discussed in detail as we proceed.

what is pottery clay made of

Overtime (hundreds of years), rocks on the earth’s surface weather to form fine particles. This weathering is due to climatic conditions and chemicals such as carbonic acid.

Silicate-bearing rocks eventually break down to form sediments that gather gradually to form clay particles.

Some of the clay stays put at the site of formation, thus maintaining its purity. The closer the clay is to its site of formation, the purer it is. This clay is heavy, dense, and is light in color.

However, at other times, clay can be transported by water or wind, from its original formation site to another area.

As a result, it picks up debris on the way and ends up in a new sedimentary deposit.  This clay is less refined due to the impurities it accumulates along the way.

Clay is thus classified into two categories based on where it forms its deposit: primary and secondary. Primary clay forms its deposit at the site of formation, while secondary clay forms deposits away from its area of formation.

what is pottery clay made of

What Pottery Clay is Made of: The Manufacturing Process

To achieve maximum utility, pottery clay has to go through a manufacturing process. First, clay soil is obtained from the earth’s surface. This is achieved by excavating topsoil and gravel up to 24 meters into the ground.

The deeper the excavation pit is, the purer the clay. On excavation, fresh clay has a moisture content of about 22%. Usually, it is clumped together but can easily be broken down into smaller particles that can be processed into usable chunks.

Clay is then stored in a building that has no walls so that it can partially air-dry. A visual inspection of color and consistency is then carried out in a bid to classify clay into its different types.

However, further analysis of the chemical content is conducted after every few loads to ensure consistent results. This chemical analysis helps to classify clay based on the purposes it can serve.

After that, the clay is transferred into a revolving tank where clumps of clay are further broken down into smaller pieces.

The clay is then heated in a furnace, through a process called flash-drain. This heat brings the moisture content down to 1-2%. The dry clay is then ground into a fine powder. A mill blends different kinds of clay to create specific products. The different clays are pulverized into a powder.

Next, a purification process takes place where physical and chemical impurities are eliminated. At this point, the clay is pure, dry, and pulverized.

Tests are taken to ensure the purity of the clay.  These include checking the color to confirm that it is the correct hue. Once it is confirmed pure, the clay is packaged and dispatched to pottery suppliers.

Pottery clay can be bought in a powdered form and rehydrated.  Or it can be bought in its moist state.

What are Different Types Of Pottery Clay Made of?

When it comes to pottery, different types of clay play different purposes depending on their physical and chemical characteristics. These include composition, workability, porosity, and firing temperature.

As mentioned before, varying types of clay contain varying amounts of water, organic matter, and metal oxides.  These differences in ingredients define their characteristics.

As such, particular pottery clays are more suitable for particular types of pottery.  So, if you make pottery, it’s helpful to ask yourself ‘what is pottery clay made of’ before you get started.

Let’s discuss the three types of pottery clay. 

what is pottery clay made of

Earthenware

Earthenware is the most common type of clay. There is even evidence to show that potters in the Neolithic era used it.

Earthenware is a secondary type of clay that contains iron and mineral impurities. As a result of the impurities, its maturity temperature is relatively low, between 1745F and 2012 F.

The general body formulation for earthenware is 15% feldspar, 35% quartz, 25% kaolin and 25% ball clay (1).

This type of pottery clay is highly plastic, meaning that it is sticky and easily workable. Earthenware often comes in ‘shouting’ colors such as red, orange, and yellow, and some are in light grey.

When fired, the composition of its minerals and the type of firing determines the resultant color. The most popular type of earthenware is terracotta, which translates to ‘baked earth.’

This type of pottery clay is the best in making roofing tiles, terra cotta pots, and more low-fire ware.

Stoneware

Stoneware was discovered as pottery clay soon after earthenware was discovered. Its name mirrors its stone-like features, as it is the strongest and most durable of all pottery clays.

Stoneware is made up of coarse grains and has a higher maturity temperature compared to earthenware.

The maturity temperature is classified into mid- and high-firing, where mid-firing temperature lies between 2150F and 2260F, and high-firing ranges between 2200F and 2336F.

It becomes incredibly durable after firing, with little or no absorption of moisture (non-porous). The formulation for contemporary stoneware is often around 15% quartz, 30% feldspar and chamotte or grog, and 15% ball clay.

Since it is also a secondary type of clay, stoneware contains different levels of iron and other impurities. These ingredients dictate its color, which normally ranges between buff, grey, and dark brown.

Due to its strength and practicability, stoneware is very functional and can be used for dinnerware and kitchenware. It is preferred for making coffee and tea mugs because of how it retains and distributes heat evenly.

Kaolin

Also known as porcelain, kaolin is the purest of the three types discussed. It is rich in silica and has a neutral pH.

Due to the absence of impurities, kaolin does not have a range of colors – they exist in light colors, primarily white. Thus, kaolin is also referred to as White Clay.

When moist, kaolin becomes a very pale grey. When fired, the color ranges from light grey or buff to almost white.

Kaolin is not as plastic as earthenware and stoneware, which makes it the least workable of the three. The absence of impurities means that it has the highest firing temperature at about 3272 F.

Therefore, kaolin can be challenging to work with. Kaolin is usually mixed with other clays to increases workability and reduce its firing temperature.

For instance, kaolin is mixed with ball clays to make porcelain. However, kaolin is frequently used for cosmetics and is a highly prized ingredient in some skincare products.

what is pottery clay made of
Kaolin Lake

How to Choose the Perfect Clay

The first step to choosing the perfect clay to work with has to be understanding what is pottery clay made of? You need to have a good idea as to what is contained in the different types of clay before you make a choice.

The difference in ingredients affects the characteristics, workability, and the firing temperature of the different types of pottery clays, as discussed in the article. Therefore, you want to use clay that is easy to work with, keeping in mind the equipment you have and the objective you wish to achieve.

For instance, if your goal is to make a coffee mug, stoneware would be a perfect choice. This is because it contains ingredients that make it durable; it maintains heat and distributes heat evenly.

If you wish to make porcelain products, then kaolin is the pottery clay for you. Remember, you have to mix kaolin with another clay so as to improve its workability and reduce its insanely high firing temperature.

If you are a beginner or a person interested in pottery as a hobby, earthenware would be a good fit. This is because earthenware is highly plastic, which makes it easily workable.

Other Factors to Consider when Choosing Clay:

  • Clay Powder or Ready Made Clay? You could decide to purchase either dry or moist clay. Mixing for yourself is economical because moist clay costs more than dry clay.

    Nonetheless, mixing clay is a bit of an art.  So, as a beginners it’s probably best to buy ready-made clay, until you garner a bit of experience.

  • Consider the Firing Temperature: Different types of pottery clay have different maturing temperatures. This means the kiln you are using needs to reach certain temperatures. 

    If you only have access to a kiln that fires in the lower range, you need to use a low fire clay like earthenware.  But if you have access to a mid or high firing kiln, then stoneware clay or porcelain becomes an option. 

Final Thoughts on What Pottery Clay is Made of

To answer the question ‘what is pottery clay made of?’, we need to draw on history, chemistry and geography.  Some potters will dig and process their own raw clay from the environment.  However, most pottery clay is mined, processed, and blended with additional ingredients before it arrives at the potter’s studio.  Broadly speaking, the different types of pottery clays are earthenware, stoneware, and kaolin. They all contain different compositions of water, metal oxides, minerals, and organic materials.

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Pottery Tips from the Pottery Wheel

Lesley

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Pottery Tips from the Pottery Wheel

I’m Lesley Milne, the creator of The Pottery Wheel.  Like many people, I used the potter’s wheel at school.  But then I began to focus on clay sculpture and I left the wheel behind.  However, more recently, I found myself being drawn back to pottery and the potters wheel.  And so, I have tried to pick up where I left off all those years ago at school. This blog is a chronicle of what I have learned as I got back into the potters saddle!

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