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Clay Types

Clay is divided into three basic categories based on the fired density of the finished pottery.  The terms earthenware, stoneware and porcelain are used with other types of the clay such as color or workability and each of these terms refer to the amount of density or porosity of the fired works.  Earthenware can be applied to any type of clay which has a ten to fifteen percent absorption rate after it has been fired to maturity. 

To determine the absorption rate of a piece of pottery, you can use the following test:  Weigh the fired piece, boil it in water for two hours, dry with a towel, reweigh it and figure the percentage of absorption by using the formula below:

Saturated weight  minus  Dry weight
divided by Dry weight

Earthenware is the most commonly found clay in nature and is the raw material usually used to make tiles, bricks, and most of the pottery in the world.  Of the three types of clay it is the most porous and soft because it has the lowest firing temperature.  It has a percentage of iron and mineral impurities high enough for it to mature from 1300 degrees F up to 2120 F, which is a Cone 018 to Cone 3.  The presence of iron oxide makes this clay look brown, red, gray or green.  When it is fired it can be from red or tan to brown or black.

Because it has low shrinkage and it is so porous, it weeps liquids and stains very easy, but is ideal for making planters and oven steamers.  You certainly wouldn't want to make a pitcher or vase out of it though, unless you glazed it inside and out.

The picture above is of terra cotta pots, which are the best examples of earthen ware.

Stoneware clays are named this because when fired they have the characteristics of stone, which is a hard, dense surface sometimes with a variegated grayish brown color.  Because they are fired to temperatures from 2100 degrees F to 2327 degrees F which is Cone 3 to Cone 11, they have a absorption rate of only two to five percent.  It usually bonds well with glazes and is completely leak proof after firing to maturity.  It is tough and forgiving during throwing and firing stages, and is chiefly used for industrial ceramics.

The higher the firing temperature of the stoneware, the more durable the product is.  The fired pottery ranges in colors from tan or light gray to brown or dark gray.  Because of it's durability it is an excellent choice for everyday wares, especially dinnerware.

This is an example of a piece of stoneware.

Porcelain is the product of different techniques of many early Chinese pottery workers.  Its main ingredient is kaolin, also know as china clay.  Kaolin has a melting point of 3275 degrees F and is difficult to form because of low plasticity, but with the addition of other clays it makes it a dense, hard, white and translucent clay known as porcelain.  With the addition of other clays it can be fired to maturity at a lower firing temperature.  Usually porcelain is fired at 2300 degrees F which is Cone 9, but with the addition of the other ingredients it can be fired as low as 1900 degrees F which is Cone 04.
 

Porcelain is composed of such tiny particles it offers the most favorable qualities of smoothness and can be worked even when really thin to get translucent delicate pieces.  Put a piece up toward light and put a finger across the backside and if you can see the shadow of your finger, it is fine china.  Some china has a silhouette imbedded into it while it is being made and when held to light can be clearly seen.  It is very hard to throw because of it's low workability.  It cracks and deforms easily during firing because its highest density isn't achieved until almost the melting point of the clay.

   

These are examples of porcelain.

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There are signs that pottery and ceramics are losing appeal for a younger generation. There are pottery and ceramic departments closing in teaching institutions throughout the country. The common understanding is that younger students are reluctant to commit themselves to the intensive study required to master pottery and ceramics. Why spend three years to learn just one art form when you can pick up PhotoShop in less than a week? Pottery wheels lie idle as students flock to the computer labs. With all the stressful actions in the world today, releasing that stress through pottery and ceramics would be very beneficial to ones well being.