Classification of Ceramics
The classification of ceramics is based principally on firing temperature and texture.
1. Firing Temperature
Pottery fired with differing degrees of temperature may be categorized as follows:
- Terracotta refers to pottery fired at less than 850 °C.
- Earthenware represents pottery fired at a temperature between 880 - 1,150 ºC.
- Stoneware indicates a high temperature firing, approximately 1,150 – 1,300 ºC.
- Porcelain refers to pottery produced at a firing temperature of about 1,300 – 1,450 ºC.
A. Earthen Texture. Ceramics fired at a temperature of lower than 850 ºC and tempered with a considerable amount of clay tend to be reddish and for the most part are not used to contain things. However, some normal-textured ceramics produced at a temperature of 850 – 1,150 ºC and tempered with clay, quartz and feldspar or with clay only, are likely to have a porous texture and can absorb water up to 15-20%. Examples of these ceramics include such unglazed pottery as Dvaravati and Srivijaya pottery. However, some type of normal-textured pottery are coated with a non-transparent glaze, like Burmese ceramics.
B. Stone Texture. Stone-textured ware tend to be formed from about 50% small particles of stone which can be melted together with soils. Ware of this type generally can absorb water up to 3-5%. Well-known stone-textured ceramics found in Southeast Asia are Lan Na, Sukhothai, Khmer, Vietnam and Chinese ceramics. Owing to high firing, we can sometimes find melted clay which looks like glaze on the surface of the ware.
C. Porcelain Texture. Ware of this texture are commonly tempered with a clay mineral known as kaolin. This clay mineral can help produce high quality ware as it contains less iron and does not absorb water. Generally, porcelain-textured ware are characterized by transparent, thin and resonant bodies. Chinese and Vietnamese ceramics are good examples of porcelain-textured ware.
The three major materials necessary for making ceramics are:
3. Fillers, such as sand, rice chaff and straw.
Ceramic production involves one of the following three recipes:
1. Clay plus kaolin plus fillers
2. Clay and kaolin only
3. Clay and fillers.
Each recipe requires skillful preparation. If the clay is very plastic, fillers are added to reduce plasticity. Ceramics that have high percentage of very plastic clay can be easily formed, but they will shrink and may be deformed during drying. On the other hand, low plasticity of clay can result in cracking. Thus an appropriate proportion or ratio of materials is important.
Common ores or earthy materials used as pigments to be applied on pottery are derived mainly from metal oxide such as:
1. Cobalt oxide (blue)
2. Copper oxide (green)
3. Iron oxide (dark brown or black)
4. Manganese oxide (brown)
5. Chromic oxide (yellow)