Pottery and Ceramics ClayListed below are articles letting you know about all the different characteristics and kinds of clay and how the different chemicals effect the clay when they are added and the different temperatures that they are fired at.
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Clay TypesClay 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 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.
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.
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.
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