Pottery is the process of making pottery and ceramic ware while clay is the raw material that is utilized for this purpose. The pottery clay is broadly categorized into major classes based on the type of clay material and the specific features of the clay. Different parts of clay and distinctive kinds of clay are essential for potters and ceramic artists to differentiate when making a choice of the material to use in any project they are carrying out. They are broadly classified into three groups which include earthenware, stoneware, and porcelain.
Earthenware Clay
Earthenware is one of the most common pottery clays in the world and is generally made from a combination of clay, water and other minerals. It comprises major amounts of iron and other minerals which makes it possible to be fired under lower temperatures of between 1700°F-2100°F. At these temperatures the clay does not melt but just burns and does not become hard and close-pored after burning as in the case with pottery. Enameled products such as vases, bowls and plates must be coated with glazes because the material used to create them is porous.
The most common types of clays include earthenware clays, which are available in various colors and the use of colored clay slip will help to achieve various effects. It is softer and more delicate than stoneware and porcelain products, and for this reason it is not as suitable for constant usage. Nevertheless, the low firing temperature is an advantage by offering a better production rate than other types of clay. Together, earthenware clay is popular and favourable material for use in items such as decorative pottery, ceramic tiles and plant pots.
Stoneware Clay
As compared to earthenware clay, stoneware clay comprises of minute mineral fragments and therefore a rigid and sturdy ceramic product. It can be fired at higher temperatures ranging from 2100°F to 2400°F and this fuses the clay particles into the kiln and come with a non-porous and vitrified body. Stoneware does not need a glaze layer after firing because its structure is very dense hence being waterproof. Nevertheless there are still many cases when the decorative glazes remain as part of varnish.
Higher firing temperature also enable stoneware clay to better handle thermal shock therefore it is suitable for more usage, often and repeatedly in everyday life. It is suitable for formation of tableware accessories such as plates, bowls, mugs and casserole dishes besides vases, pots and tiles. Stoneware clay burns to medium dark earth tone bodies but can be wedged or otherwise manipulated to produce a complete spectrum of results. It has good properties for throwing on pottery wheel.
Porcelain Clay
Porcelain is desired for its white colour and opaqueness after firing a block of clay and showing light through thin walled sections. Such qualities are achieved by using the finest clay particles and some of the ingredients such as feldspar granite. The clay has considerable plasticity and thus is very malleable and can be deformed in any way when wet.
Because porcelain is plastic and possesses a low shrinkage, elaborate patterns and designs can be carved and painted on the body of the porcelain without losing their distinctive outlines even after the porcelain has been dried and fired at the high temperatures ranging from 2200°F to 2400°F in the glost firing stage. The high temperature used in this process enables the porcelain to achieve full vitrification, leading to low porosity and high density. This makes porcelain stronger than stoneware which makes it ideal for repeated use thus making it a perfect material for the production of tableware. It also stands high temperatures and thermal shock for a very short period before it cracks. While codes do not always mandate the use of glazing, they are often used for aesthetic concerns.
As such working properties, strength and aesthetic beauty makes porcelain considered as the most premium type of pottery. Its most commonly used for fine wares such as china tableware, art wares and figurines but is technically more demanding in shaping and firing so as not to exhibit imperfections.
Key Differences
Categorized by the type of clay that is used, it is possible to differentiate among earthenware, stoneware and porcelain types, allowing the potter to decide which type is the most appropriate to use. Key points of difference include:Key points of difference include:
- Firing temperature range where the lowest is for earthenware, slightly higher for stoneware, and the highest for porcelain.
- Smoothness of surface and water absorption – earthenware is highly absorbent, stoneware is moderately absorbent while porcelain is non-absorbent when fired.
- Finally, durability – although earthenware is more porous than other ceramics, it is relatively brittle and has low mechanical strength, whereas porcelain has very high strength for its thickness.
- Sturdiness and permeability – earthenware has a red tint; stoneware has a darker tint due to iron content; porcelain is white when it is of purest variety.
- Uses suitability- decoration is done on earthenware, table setting are done on stoneware, china dinner setting is done on porcelain and delicate artform.
When working on certain properties, potters will be able to know which type of ceramic clay is suitable for a ceramic project on the basis of these unique characteristics and how they affect aesthetics and functionality of a property. Always test small samples first when experimenting with a new clay type to learn these traits yourself. The following sections will present an overview of earthenware, stoneware and porcelain pottery clay including their potential and their shortcomings.