Mark, I use a slab roller fro some of my work and had the same issue until I began the process of ribbing both sides of the slab going in multiple directions each time. When one side is ribbed, press thin plastic (I use a roll of drop cloth plastic from Home Depot and cut the right sized pieces for each project) and smooth it onto the surface making sure no wrinkles or air bubbles are present. Placing a 1/4" piece of plywood on top, I flip it over, rib that side and transfer either to a form for draping, a wheel for throwing (placing the bat on the top allows flipping without distorting), or leave flat for slab work. Most warpage has gone away.
Posted 30 November 2014 - 01:53 AM
Debora, I would to add to the other comments - 1- bottom thickness and 2 - speed of drying- I like to throw 14'' plates and leave the bottoms thicker than normal for a nice foot- I work the bottom from inside out then, outside in with a wood rib as my last step, cut it off the bat then cover with plastic to slow down the dry time.. if I dry them fast, about 25% will crack pretty nasty..
I like to throw red clay, it balls nicely and hurts like hell when it hits you...
Posted 01 December 2014 - 10:58 PM
For compressing, I personally love this tool:
The COWS TONGUE!
Every firing is a test. One day I will pass.
Posted 08 December 2014 - 09:56 PM
If I read correctly, you're making the slab plate and then throwing on a foot, right? Is the slab plate mostly flat or is it slightly curved before the foot is added? Or is the center of the plate slightly curved with a wide flat rim? If the plate is flat or has a wide flat rim and the diameter of your foot ring is too small for the diameter of the rim, this can cause the rim to drop and the center of the plate to pop up in the glaze firing because the foot acts as a pivot point (think see-saw) when the clay softens in the glaze firing. Gravity wins. Whether the slump is consistent around the entire rim or is worse in some places than others is usually related to how carefully or casually the clay was handled during construction of the piece.
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