Tuesday, October 9, 2012

October 8th Hint of the Week

from the Goshen College Art Department by Marvin Bartel

WHY DO TILES WARP AND CRACK

Those of us who make tile from moist clay know that it shrinks a lot when it dries. If tile is on the shelf to dry, this uneven shrinkage will warp tile into something like potato chips, or if you weight it down, it will often crack. You cannot "force" it to stay flat.

THE GENERAL PRINCIPLE

Dry it in a way that allows the water to leave at the same rate from both sides at once. On larger tile, faster edge drying causes cracks across the center in many cases.

HOW TO DRY FLAT CLAY WITHOUT WARPING IT
Here are several ways to accomplish uniform two sided drying (flat shrinkage). None of these methods is 100 percent fool proof, but with practice they work.

1. Flip them frequently during the day and cover with them plastic during the night. If they have started to warp, you can sometimes pat them flat with a paddle after they are too dry to bend.

2. Dry them on wire racks (like old refrigerator shelving) that have LOTS of air space under them. Four inches of air space under each layer is minimum. I have mounted some wire closet shelving in our ceramics room where it is high enough to be "out of the the way".

3. Sandwich them between dry sheets of gypsum board (drywall)(sheetrock). This is paper covered plaster and it draws moisture from both surfaces. Scrap drywall is available free from housing construction sites. Cut one side with a utility knife and bend it to make pieces to fit your storage places. If you tape the edges with packaging tape or duct tape, you avoid the problem of plaster pieces getting into the clay rework. Be sure it is unpainted so it absorbs moisture. These moist boards can be trashed or after complete drying they can be reused for another project.

PROTECTING THE EDGES WHILE DRYING
None of the above solutions deal with the faster drying of edges. I use one or more of the following methods to minimize this problem. Small pieces may not need any special edge protection while drying.

1. Coat the edges with wax resist. This is water soluble liquid wax used for decoration. It becomes waterproof as it dries.

2. Leave scraps of clay slab butted tightly against all the outside edges.

3. Place plastic sheeting tightly against the edges or plastic strips wrapped on the edges.

Double the clay thickness at the edges. This can be done temporarily by laying strips of clay on the edges and removing them after drying is complete.

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