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Glaze faults

Shivering - The pot shrinks more than the glaze during cooling causing the glaze to be compressed. Small pieces of glaze will pop off the pot due to the pressure of compression.

Crazing - The pot shrinks less than the glaze during cooling causing the glaze to be stretched. Small cracks open forming a fine network.

Pinholes - Volatile materials in the glaze turn to a gas during firing creating small bubbles in the glaze. The bubbles burst but leave a small crater behind. Pinholes are also caused by air bubbles introduced into the glaze during mixing.


Crawling - Sometimes a glaze will not form a continues coat of on the surface of the clay but tend to pull away from itself to form globs ­ in the same way that water will form into droplets on a smooth surface. Crawling is a characteristic of some glazes but is usually caused by dust or oil on the surface of the clay under the glaze.

Overfiring - The glaze looks thin, runny, glossy.

Underfiring - The glaze appears dry and unmelted.

 

Special glazing and decorating techniques

Slips: The word slip refers clay that has had a sufficient amount of water added to it to make it behave as a fluid. Engobes are similar but have been altered with fluxes and other materials associated with glazes. Slip decoration can depend on the slip being "fat" and creating a raised surface, being colored and showing through a glaze or both. A porcelain ware might be decorated by applying a white slip to the surface that is thick enough to remain raised. A highly translucent celadon glaze applied over this will create a delicate pattern of light and dark where the glaze drains away from the high portions of the slip and pools in the valleys. Such a slip is typically applied with a tool called a "slip trailer". For fat slips that must be applied with some force a device with a flexible container that can be squeezed and a spout like a plastic catsup bottle are used. For a more fluid slip the container is usually rigid with the flow from the spout or spouts controlled by a thumb hole that allows or prevents air into the container. Slips are also used as a medium for color as well ­ a white slip against a dark clay body or vice versa, cobalt or some other coloring oxide. An inlay technique is used with colored slips that contrast with the body clay. In this method the leather hard or dry greenware is carved with engraving or trimming tools to create depressions that are filled with slip. After a period of drying the excess slip is scarped away restoring the original smooth surface but the carved areas filled with the contrasting color.

Terra Sigillata is a type of slip decoration that has been passed down from the Greeks and Romans. Normal slips are decanted to produce a slip of very small particles. Terra Sigillata slip is applied to a surface that has already been smoothed. The slip is usually burnished further with a smooth pebble and buffed with leather to produce a refined, almost glossy surface.

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