Is Natural Clay Earthenware?

Table of Contents

What is Clay?

Clay is a fine-grained natural material made up of hard crystalline minerals known as clay minerals (Britannica). The most common clay minerals include kaolinite, montmorillonite-smectite, illite, and chlorite (Britannica). Clays form when the minerals are weathered from rocks over time and transported through erosive forces such as water, wind or ice (Wikipedia).

The composition of clay consists primarily of silica, alumina, and water, but also contains varying amounts of metallic elements like iron, magnesium, sodium, potassium and calcium which provide color and other properties (Britannica). The small size and platy or layered structure of clay particles give clay unique properties like plasticity, shrinkage, and hardness when fired at high temperatures (Wikipedia).

Types of Clay

Clay is a type of fine-grained natural soil material that develops plasticity when mixed with water. There are many different types of clays that have unique properties and mineral compositions.

Some of the most common types of clay used in pottery and ceramics include:

Kaolin – Also known as china clay, this very pure clay is white after firing. It is composed mainly of the clay mineral kaolinite. Kaolin clays are less plastic than other clays and have a lower shrinkage rate when drying. They are often used for porcelain and whiteware. (Source)
Bentonite – This type of clay expands when water is added and contracts when dried. It is composed mostly of the mineral montmorillonite and has high plasticity. Bentonite is used for its binding, thickening, and absorbent properties. (Source)
Illite – An abundant group of mica-like clay minerals containing iron, magnesium, potassium, silicon, and aluminum. Illite clays have intermediate plasticity and firing color changes. They are important raw materials for manufacture of bricks and ceramics. (Source)

Other common clay types include ball clays, fire clays, common earthenware clays, and stoneware clays. The different clay materials each have their own characteristics that lend themselves to certain types of pottery and ceramic production.

What is Earthenware?

Earthenware is a type of pottery made from clay that has been fired at low temperatures, typically between 1,000°F to 1,200°F (538°C to 649°C). It is porous and opaque unlike porcelain which is translucent. According to Emerson Creek Pottery (https://emersoncreekpottery.com/what-is-earthenware/), earthenware has a porosity above 5% when fired. This allows gas and liquid to pass through the fired clay.

The key properties of earthenware are:

It is opaque, porous and rough in texture.
It is fired at lower temperatures between 1000°F – 1200°F.
It can be glazed or left unglazed.
Common types include terracotta, faience and majolica.
It absorbs water easily so it is not watertight.

Earthenware has a wide variety of uses including pottery, tableware, sculpture and construction materials. Its absorbent quality makes it useful for flower pots, plates and other kitchenware. Historically, it was one of the earliest types of pottery made by ancient cultures around the world.

Making Earthenware

Earthenware is made from natural clay that has been shaped and then fired at low temperatures, typically between cone 022-04 which is 1753-1945°F. The clay body for earthenware contains 25-45% quartz particles, along with varying amounts of feldspar, mica, iron oxides, calcium carbonate and other minerals.

To make earthenware from clay, the natural clay is first wedged or kneaded to remove air bubbles and create an even consistency. Next, the clay can be handbuilt into shapes or thrown on a potter’s wheel. Simple earthenware forms are often made using the pinch pot, coil and slab techniques. More complex forms are thrown on the wheel.

Once the clay has been shaped, it is set aside to dry slowly and evenly to prevent cracking. The dried clay piece is then fired in a kiln at low temperatures under cone 04. During firing, water evaporates from the clay and it becomes ceramic as bonds form between clay particles. Unglazed earthenware has a porous, earthen color surface. Glazes can be applied before firing to create various colors and effects. After glaze firing, the earthenware product is ready for use.

Earthenware is simpler to produce compared to stoneware and porcelain, as it does not require extremely high firing temperatures. It can be shaped by hand without specialized tools, making earthenware one of the most accessible clays for artists and beginners to work with.

Clay as Raw Material

Clay is the primary raw material used in the production of earthenware. Clay deposits are found across the world, but the specific composition of the clay determines its suitability for earthenware production. Kaolin clay is one of the most common types used for earthenware due to its white color and high plasticity which allows it to be easily shaped into ceramics. Other clay materials like ball clay, fire clay, and stoneware clay may also be used in varying proportions depending on the characteristics desired in the finished earthenware product.

According to Raw Materials & Chemicals, kaolin clay is one of the most frequently used raw clays for earthenware production. The kaolin clay provides plasticity and whiteness, while other clays can be added to provide additional properties. Clays can be purchased as dry bagged materials from ceramic suppliers for use in earthenware production.

Clay Composition in Earthenware

The key ingredient that distinguishes earthenware from other types of pottery is the clay used. Earthenware clay contains high amounts of kaolinite, a clay mineral that provides plasticity and strength when fired at lower temperatures (https://potterycrafters.com/what-is-potters-clay-made-of-pottery-clay-ingredients-explained/). Kaolinite is a hydrous aluminum silicate mineral with the chemical composition Al2Si2O5(OH)4. It is formed by the chemical weathering of aluminosilicate minerals like feldspar.

In addition to kaolinite, earthenware clays also contain other clay minerals like illite and smectite. The exact composition can vary, but a typical earthenware clay may contain 50-70% kaolinite, 10-20% illite, 5-15% smectite, and 5-10% quartz. Feldspar and calcium carbonate may be added as fluxes. The composition provides plasticity for shaping while allowing vitrification at lower temperatures around 1000-1200°C compared to stoneware (1200-1300°C) and porcelain (1200-1400°C).

Firing Earthenware

Earthenware clay is typically fired at low temperatures, between 1,700°F to 2,100°F (900°C and 1,150°C) or Cone 04-06 according to the Your Guide to Kiln Temperature Ranges for Pottery. At these low temperatures, the clay does not vitrify and the body remains porous when fired. For earthenware clay, the firing temperature usually ranges between 1700-2100°F (900-1150°C), which classifies it as a low-fire clay body.

According to the Guide to Kiln Temperature Ranges for Pottery, the typical firing range for earthenware is 1800-2100°F (982-1149°C). Earthenware contains a high percentage of fluxes which lower its maturity temperature. The porosity allows earthenware to be decorative and functional but requires glazes to make it water resistant.

Examples of Earthenware

Some common examples of earthenware include:

Terracotta – One of the most common types of earthenware. Terracotta is made from clay containing iron oxide which gives it a distinct reddish-orange color when fired. Terracotta has been used for pottery, sculptures, and architectural decoration for thousands of years. Famous examples include Greek and Roman amphoras and other vessels, as well as terracotta roof tiles and decorations on buildings.https://en.wikipedia.org/wiki/Earthenware

Stoneware – A vitreous type of earthenware made from finer clay and fired at higher temperatures than typical earthenware. The higher firing temperatures make stoneware less porous and more durable while still retaining an earthy quality. Stoneware glazes also melt at higher temperatures resulting in a smooth, glassy finish. Stoneware has been used for pottery and storage vessels for centuries across many cultures.https://thepotterywheel.com/types-of-clay-for-pottery/

Porcelain – Porcelain is actually a type of stoneware made from the finest white burning clays and firing temperatures between 1200-1400°C resulting in a translucent, glass-like quality lacking pores. The high firing point makes porcelain stronger and less porous than typical earthenware. Porcelain has been produced for centuries in China and Japan and used to make delicate tableware and decorative objects.

Properties of Clay Earthenware

Earthenware clay has some unique properties that make it suitable for pottery and ceramics. Some of the key properties include:

Porosity

Earthenware clay is quite porous. This means it readily absorbs water due to the many tiny holes and air pockets in its surface. According to Types of Clay for Pottery, earthenware’s open body allows water to penetrate into the clay. The high porosity makes the clay permeable.

Strength

The raw earthenware clay has low plasticity and is quite weak in its unfired state. It gains strength during firing as the clay particles bond together through sintering. However, earthenware retains some porosity even after firing, so it is not as strong as stoneware or porcelain clays. Earthenware’s strength allows it to be functional for dishes, mugs and other household items.

Thermal Properties

Earthenware matures at relatively low firing temperatures, typically between 1,800°F to 2,100°F (982°C to 1,148°C) according to The Basics of Pottery Clay. It does not vitrify or become glassy, so it remains somewhat porous after firing. The low firing temperature allows the natural red and brown colors from iron oxide in the clay to show through in the final product.

Conclusion

In summary, clay is an essential raw material in the creation of earthenware pottery and ceramics. The plasticity and workability of clay allows it to be shaped into vessels and objects which can then be fired at high temperatures to produce hardened earthenware. Clay’s composition, with fine particle size and varied mineral content, provides the malleability when wet and strength when fired that makes it ideal for pottery and ceramics. The different types of clay have properties that lend themselves to different kinds of earthenware production. But across all varieties, it is clay that enables artisans and crafters to form earthenware objects that have served purposes both practical and artistic for thousands of years.