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Michael Sorum

Plasticity in earthen ware?

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I am wanting to make stone ware more plastic for the wheel?  Mine is cracking, alot, when I try to stretch out a bowl, after I have rolled a texture on it? The stone ware was mixed, following a recipe, and I am doing fine with simple bowls but when I faceted one of my attempts and tried to stretch it i got alot of strecth cracks?  My clay feels fairly soft and it was easy enough to center and pull. My sides were between 1/2" and 5/8", before I faceted with a cheese cutter. I cut slabs that are a little less than 1/8" thick.  When I tried to strech it into the bowl shape with 5/16" sides, it cracks and splits?  Any tricks? I will post my recipe at a later date, as it is at the campus studio, 50 miles away!  I will repost later tonight or probably tomarrow.  Thanks to the Hulk for tech advice! I am an old timer, trying to learn new tricks! 

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Plasticity is a combination of materials (particle sizes), and moisture content. In college we would mix clay from reclaimed slop, and use numerous clays to develop different bodies. The clay was mixed in big blue bird mixers. The wet slop was added first, and then dry materials until the clay was the proper consistency. Unless one was using a large amount of plasticizers (bentonite, macaloid, etc) the clay was always fairly short, even after thorough wedging on damp canvas covered tables, or aging for a couple of weeks.

The only process I found during college to make my clay more plastic, without bumping the plasticizers up to silly levels was to mix it on the wet side, and let it sit in the concrete tub of the mixer to evap excess moisture. This allowed all my particulates to be properly wetted, and increased the plasticity greatly. I came to this conclusion after noticing that the short clay, once it had been thrown, and re-wedged, and thrown again, was MUCH more plastic. My deduction was that my clay wasnt thoroughly wetted if the clay was mixed to throwing consistency in the mixer.

Of course, different clay bodies were better than others; what we called a 25/25 body was 25% each feldspar, kaolin, ball clay, silica +2%  bentonite, was a plastic clay out of the mixer.

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