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Re: [teacherartexchange] air dry clay & why it cracks

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From: Gayle Parent (gayleparent_at_TeacherArtExchange)
Date: Sun Apr 20 2008 - 09:43:10 PDT


On Apr 20, 2008, at 6:00 AM, Marvin Bartel wrote:
>> Marcia wrote:
>> We were making small African mask models and we used
>> different types of air dry clay and almost all of them
>> cracked in half. They were about 1/2 inch thick,
>> which may have been too thin? I've also just tried
>> using the clay to make animal figurines, with those
>> cracking as well.
>
> WHY DOES IT CRACK?
> A half inch should be more than thick enough. When either self-
> hardening or pottery clay cracks during drying, it is the result of
> uneven drying. When the last parts of the piece dries (shrinks) it
> cracks because the last parts are shrinking in relation to
> completely rigid clay.
>
> WAYS TO AVOID CRACKS:
> To avoid cracks, dry it slower so that it drys more uniformly.
> Drying pieces inside a cabinet or group them in some boxes.
> Experiment and share your findings. After they look totally dry,
> put them out to finish drying. Place things so that they are
> exposed to air equally from all directions. Often I turn things
> over as soon as possible so the bottoms are not drying slower. If
> is is clay to be fired, do not fire until they are totally dry.
> Damp pieces are cooler when they touch your cheek.
>
> If the design allows, it helps to make all the parts about the same
> thickness so they dry more uniformly. Sometimes I have to spray
> water on parts that are getting dry too fast, but this must be done
> before it is bone dry. If you add water to dry clay, it will often
> crack from the expansion caused as water goes into it. A better
> solution may be to wrap a bit of plastic on the parts of sculpture
> that tend to dry too soon (this too must be done before it get too
> rigid).
>
> Some shapes are tricky to dry. I have seen many bowls that crack
> across the bottom or down from the top. In both instances it is
> because the rim of the bowl dried first. The when the bottom
> dried, it caused the top edge to crack or it caused a crack in
> bottom because of the delayed drying of the bottom. Bowls dry best
> if turned over as soon as the top can support the weight.
>
> WHICH CLAY TO USE?
> As a clay artist, I was asked to study a group of self hardening
> clays and oven hardening clays to recommend the best one for a kit
> sold to parents. Of the self-hardening clay, the nicest clay to
> work with was the Amaco self-hardening clay. It is a bit Dellia
> Robia was the all round winner of the oven baked clays. You can
> check on prices (oven baked clay is more expensive), but I expect
> that the Amaco is cheaper. None of the oven baked or self-
> hardening clays are as strong as fired clay. I did not test
> anything for fumes produced when heating or for any safety issues.
>
> LEARNING INVENTION & HOW ARTISTS EXPERIMENT TO PICK MATERIALS
> Many kids assume that any shape that they can make from clay is
> just as appropriate as any other shape. Artists and inventors have
> learned otherwise. Sometimes I give them a little length of wire
> and a piece of clay and ask them to experiment a bit and decide
> which would be best for the legs of a spider or for the tail of
> mouse? What could leather strips add? What can they think of to
> add? With self-hardening clay, all kinds of foreign objects can be
> installed immediately. What if ears are made from manilla folders
> or plastic cut from plastic milk jugs? Wire or broomcorn whiskers?
> How many things can kids imagine and invent if you give them an
> assortment of paper clips? Some string? Clothing made from paper?
> What could be added with thin copper sheeting or aluminum cans cut
> with scissors. How could it hang from the wall or from the ceiling?
> What about self-hardening clay to join other materials in an
> assemblage. How about papier-mbchi to reinforce areas that need a bit
> more strength? What can they invent with articulated (moving parts)?
>
> With clay to be fired, I ask them how they would prepare the piece
> if they want to add something after firing. I may need to ask them
> how much bigger the openings need to be to account for drying and
> firing shrinkage. If they glaze, I need to ask them more questions
> about this while they are glazing.
>
> I have a few web pages on clay that may also be helpful.
>
> About drying clay:
> http://www.goshen.edu/~marvinpb/dryingboxes.html
>
> For reprocessing old clay or digging clay:
> http://www.goshen.edu/art/DeptPgs/rework.html
>
> Marvin
>
>
>
>
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