Ceramic Technology and Analysis. Clay and temper preparation.

Ceramic Technology and Analysis

Clay and temper preparation

Watch out for salt!

Temper

• As the clay in pottery dries, or is fired, the loss of water causes compaction of the clay platelets, which may cause cracking.

• Temper is a material that is added to clay that resists shrinkage and is also refractory under the conditions of firing. It can take many forms, including grog, which is ground-up sherds.

Vessel Formation TechniquesTechnique Appearance

Pinching Thick-walled lumpy bowl Paddle and Anvil Irregular surface and shape

Slab Cylindrical shape

Coil Smooth sided, asymmetrical

Mold visible seam, appearance

identical to other vessels.

Slow and Fast Wheels finger striations in interior,

symmetrical profile, thin walls in the case of fast wheel.

Cahuilla potter demonstrating the paddle and anvil technique

Steps in Vessel Formation1. Digging and preparing clay, adding

temper.

2. Forming the base (sometimes a mold is used, e.g. a basket).

3. Forming the walls, including scraping.

4. Applying impressed decoration while clay is still plastic.

5. Slipping pot while it is still damp.

6. Drying.

7. Burnishing, painting, and glazing (if desired).

Degrees of Firing

Temperature Term Appearance

Sun baked Adobe mud w/ straw

Below 900˚ Terra Cotta roof tiles

900˚-1200˚ Earthenware pottery

1200˚-1350˚ Stoneware Asian pottery

Above 1300˚ Porcelain Fully vitrified

Term: vitrification – to convert into glass

Prehistoric Kilns

Left: Yarim Tepe Iraq, 5500 BC

Below Bonpo Village, China4500 BC.

Terms Related to Firing Environment

• Oxidizing: a free flow of oxygen reaches the vessels during firing. Minerals in the paste, slip, and paint are converted to their oxide forms, causing color changes.

• Reducing: Oxygen is cut off to the fire during the latter stages of the firing process. Additional carbon may be thrown onto to fire beforehand, and the carbon is driven into the fabric of the pot, turning it black. Minerals turn into their unoxidized forms.

Jar

Ollas

Mug

Vase

Generally vases and mugs have unrestricted orifices (though the mug above has a restricted orifice), ollas and jars have restricted orifices.

Pitcher

Bowl

Amphora

Comal

Note that it is common to paint the inside of a bowl.

Pithoi

lamp

canteen

crucible

Common Ceramic Terms Relating to Vessel Form

base base

rim rim

neck neck

shoulder*

*referred to as the upper body by Anna Sheppard

orifice orifice

body

Asian Ceramic Technology: Han Dynasty 206 BCE – 220 AD

• Lead glazed pottery: low-fired pottery that uses powdered lead oxide as fluxing agent, mixed with quartz in a ratio of 3:1. Small amounts of copper were added that turned green when oxidized, or iron which turned brownish-yellow.

• Celedon: an innovation of the the 1st century AD within the Eastern Han dynasty. Celedon is to be equated with porcelain in that it is fully vitrified.

• The green color of celedon glaze was achieved by iron

oxide and wood

ash fired in a

reducing

atmosphere.

Porcelain

• Porcelain is fully vitrified pottery made from Kaolin clay. It has to be fired at between 1200 and 1400 degrees celsius.