I found an article titled "Sculpture Bodies" discussing some of the materials someone might consider if working with large sculptures. It is written by a member of The Quartz Inverters. This is the University of Manitoba Ceramics Club also known as the Quartz Inverters. In all honesty I do not have a technical background. I am trying to learn more. The article was easy to understand and seemed to offer good suggestions. One of the things they discuss is the importance of grog. Grog reduces shrinkage and helps protect against cracking. They suggest that the best way to help a sculpture body dry evenly and not explode during the fire process is to add lots of grog. This opens up the body and allows moisture to escape from deep within. When walls are over 1" thick this very important. They recommend a formula for adding grog. It is as follows:
"Industrial research has found that there is an ideal way to grade grog, no matter what percentage is added to the body. Of the total amount added, use:
So, if your clay body calls for 30% added grog, you would use 15% coarse, 3% medium, and 12% fine grog. This makes for ideal packing of the grog sizes in the clay."
- 50% Coarse grog, (4-16 mesh)
- 10% Medium grog (20-36 mesh)
- 40% Fine grog (40-60 mesh)
Here is the address if you are interested in the article. http://ltc.umanitoba...ulpture-bodies/
When I first wrote my article asking about fillers for clay I was wondering what percentages others were using and why they were using the percentage they chose. I was hoping to compare that ot the article and then decide what I want to do. I don't think I was very clear. Sorry about that. Hope the article is good information for others.
Tree, I'm not sure if you went to NCECA, or if you did, whether or not you caught the presentation on clay bodies by Dave Finkelnburg (CM tech editor) and Matt Katz (moderator if this very forum), both of whom are the Matt and Dave of Matt and Dave's Clays, but they discussed particle packing in clay bodies that functions in a similar way. Particle packing for plasticity means you get the most possible surface area contact between particles, and therefore maximum plasticity. It sounds as if the same principles are being applied here for green strength, rather than plasticity---since grog is not plastic.
It would seem to make sense that, while the grog-as-structural-aggregate is stronger because of the varying sizes, at the same time the grog-as-drying-facilitator is also more efficient at moving moisture through the clay wall because the varying sizes set up a system much like the vascular system in the human body, with smaller particles (capilaries) moving water (blood) to the larger particles (veins).
Ah, the intersection of science and art...