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 Techniques Technique 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 Formation 1.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 TermAppearance Sun bakedAdobemud w/ straw Below 900˚Terra Cotta roof tiles 900˚-1200˚Earthenwarepottery 1200˚-1350˚StonewareAsian pottery Above 1300˚ PorcelainFully vitrified Term: vitrification – to convert into glass
Prehistoric Kilns Left: Yarim Tepe Iraq, 5500 BC Below Bonpo Village, China 4500 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.
Common Ceramic Terms Relating to Vessel Form base rim neck shoulder* *referred to as the upper body by Anna Sheppard 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 1 st 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.