[Val]

I usually throw cone 5 stoneware without grog. I have recently moved from Southern Africa back to USA and by mistake got some clay with fine grog. I am at a loss how to smooth it, as a damp sponge just brings up the grog. B.T.W. Is grog needed in clay for throwing? The only way I could solve the problem was to make a sieved slip from the clay body, or burnish. How to others deal with this?
Is there a rule of thumb about when grog is needed? I've read it's a must for tiles, but what else?

I 'm sure my question is very basic and dumb, but I learned to throw in sort of a vacuum being in Africa and all.........
Thanks!
Val

[neilestrick]

Grog is not necessary, although it does improve the workability of the clay, plus reduces shrinkage, cracking and warping. You can make just about anything with any clay body, but grog makes it more forgiving. I just bisque fired 28 inch tall porcelain vase with no problems.....

To smooth a pots with grog, use a metal or rubber rib, not a sponge.

[OffCenter]

Grog also weakens the clay.

Jim

[JBaymore]

OffCenter, on 11 November 2012 - 09:53 AM, said:

Grog also weakens the clay.

Jim

Wise man. MOR tests bear this out.

While it can improve the plastic throwing/forming qualities of some clays, and can help with potential drying issues by letting moisture more easily migrate through the walls, and even out/decrease shrinkage helping to prevent things like warping, grog is a double-edged sword.

The shrinking clay surrounding the already fired grog particles sets up a network of micro-cracks surrounding the grog pieces that form the basis for later failure under stress. This process starts in the wet to dry shrinkage, and continues in the dry to fired stage.

best,

.................john

[neilestrick]

OffCenter, on 11 November 2012 - 08:53 AM, said:

Grog also weakens the clay.

Jim

But not to the extent that it makes your ware too fragile for daily use. If you need it, use it.