Hand building with clay is fun. Good hand building clays make the experience even better. What are the characteristics of a good hand building clay body?
The primary requirement of a hand building clay body is strength, with plasticity running a close second. Happily for us, and unlike a throwing clay body, water absorption is not an issue.
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Because many hand-built forms need a high degree of strength in the green stage, hand building clay bodies often have 20% to 30% grog or other non-plastic filler. The grog can either be fine or coarse, depending on the surface qualities the potter wants. Such high additions of grog also results in an open clay body. This means the clay will dry quicker and with a reduction in the possibility of cracking.
When extreme levels of strength are needed from the clay as it is being worked, a few handfuls of chopped nylon can be added to each 100 pounds of dry clay as it is being mixed. Warning: Do not use chopped fiberglass as a substitute.
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A hand building clay still needs to be plastic enough to be workable. The more extreme the shapes being built or being used in the building, the more plasticity the clay will need. For example, it takes a high degree of plasticity in the clay body in order to roll out long, thin coils that do not have any cracks.
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Hand built forms can be very large. When that is the case, it is a good idea to decrease the weight a finished product will have. This is accomplished by mixing combustible substances into the clay. Some materials that have been used successfully by a number of potters are sawdust, coffee grounds, ground nut shells, and ground fruit pits.
Organic additions generally burn out of the clay body without any difficulty if a normal firing schedule is followed. Perlite (pearlite) is also sometimes used to reduce weight. For perlite and the organic fillers, as well as grog, it is best to add them when the clay body is being mixed.
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Finding the Right Hand Building Clay for You
The right-hand building clay for you will depend on your artistic needs as well as the technical requirements. Using the above as overall guidelines, you will also want to determine what temperature range you wish to work in, what surface(s) you want in your finished pieces, and what color you want the fired clay to be.