Tuesday, February 19, 2013

February 17th Hint of the Week

Warping by Dave Finkelnburg (From Ceramics Arts Daily)

Beginning clay students are frequently advised to compress the clay they are working on. If they don’t, there is a risk the work will crack or warp. This is because, they are sagely advised, “Clay has a memory!” This seems silly; clay is not alive. However, memory is a great analogy for the shrinkage that occurs when clay dries and the warping that can result.

Defining the Terms

Clay Shrinkage: The amount of change of length of a wet clay sample caused by drying or firing, expressed as a percent of the original dimensions of the wet sample.
Forming: Referring to how a mixed, wet clay body is shaped – pinching, casting, throwing, extruding, coiling, etc. – rather than the mixing of the wet clay body from raw materials.
Particle Compression: Ribbing or pressing clays and clay bodies in order to align the platelets, giving strength to the clay form and preventing warping.

Forming and Compressing Warping caused by so-called clay memory is a forming problem. Some part of a clay piece is either stretched or compressed more than the rest during the shaping process. During drying and firing, the stretched clay shrinks more than the clay that has not been stretched. The compressed clay does not shrink as much. How can there be two different shrinkage rates on different parts of the same piece? Clay’s memory, of being stretched or compressed, occurs because particles in the clay body are different sizes.

Example: Imagine standing beside a house. You are as small, relative to the house, as a clay particle is relative to an average particle of quartz or feldspar. Wet clay particles slide past each other as the clay body is formed, but the feldspar and quartz particles do not rearrange as easily and cause tiny tears or hollow voids in the clay body. Since air cannot get in through the clay body, the voids can be closed up by compressing the clay and they can be reduced by simply pressing on the clay. This compressing to close up micro-tears must be as diligent and vigorous as the forming of the clay was in creating those same micro-tears. The further and faster clay is stretched, the more tears will exist and the more compression will be helpful to eliminate warping during drying and firing.

Variables: Because there are so many variables involved in any process of forming a clay object, not to mention the variables in the clay body itself, it is not possible to predict how much compression is sufficient to prevent warping, if it can be prevented at all. If burnishing is done before a piece dries to leather hard, the effect of the burnishing will be useful compression.Compression: Forming by pinching, a type of compression, produces less tendency to warp than forming by throwing. Pinching simply opens up fewer voids because compression occurs simultaneously with shaping. Some of the same occurs with throwing but throwing shapes clay so much faster that there is a tendency to reduce density in the throwing process if one moves the clay too fast. Making slabs by pounding a clay body with a mallet or throwing it onto a firm surface may produce a slab less likely to warp than one made by rolling. Anyone who has rolled clay with a slab roller and then seen it dry in an arc because it was not flipped over and turned 90 degrees between passes understands how a well-designed machine can introduce clay memory. Smoothing: Smoothing the clay with a rib is useful, but what the rib does, while called aligning the particles, is still simple compression. The alignment extends just a few microns below the surface but the compression can be much deeper. This depends on how firm the clay has become, of course

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