Tuesday, March 16, 2010

clay body stuff.

I received an email asking about my claybody today so I thought I'd share it here.

Hawthorn Fireclay 35 Mesh 50
Hawthorn Fireclay 50 Mesh 50
Goldart 15
OM4 Ball Clay 25
EPK(or Tile6) 33.3
Silica 200 Mesh 5
Custer Feldspar 15
Grog/Mullite(50/48Mesh) 25

This is a personal modification of a pretty standard fireclay based clay body. I use several different clays so I have a variety of particle sizes. Here's a little blurb from Hank Murrow via clayart about particle size:

"I am pleased to introduce you to the "Room Full of Basketballs" theory of clay formulation. The theory goes like this: imagine a room full of basketballs. Now fill the spaces between with tennis balls, then the remaining spaces with shooter marbles, and peas, and buckshot, and sand grains, and fine sand. The idea is to make sure that there is a very wide range of particle size available, so that all particles are touching each other(or as nearly as may be configured) so that all particles are just separated by the water that can be held(by molecular attraction) to their surfaces. The reason folks use several clays is to insure(in our age of over-'improved' and therefore mono-sized clays) that there is such a range of particle sizes. "

In my experience basic clays don't need to be weighed out to decimals or even real precise amounts(except for spar and silica), general amounts seem to do just fine. The above formula is one batch in my blunging barrel. To save time the main ingredients are either full bag amounts or some variation, half-bag, 2/3 bag so that the next batch can be weighed out faster. I've recently had a few pots with lime popouts...probably caused by the 35 mesh fireclay. One of these days when my stash is exhausted I may switch to all 50 or 45 mesh. The next time I buy clay I'm going to try some new things, I'd like to try Kentucky Stone in place of the goldart and some of the ball clay...I'd also like to try C&C Ball Clay in place of OM4, it's supposedly cleaner and whiter than OM4.

My body used to have no free silica in it but my ash glazes didn't look nice on that body and a small addition of 2.5% made a huge difference. That's something I did in the last year. I've never tested the absorption of this body...the feldspar is technically on the low side but it's vitreous and doesn't leak.

It'd be really cool if I could find a local clay to add to my body for a little bit of local flair but this is red clay country and I don't want a dark body.

Well, wasn't that exciting stuff? Cheers!


Joe and Christy said...

Thanks for sharing your clay formula. I've never seen your pots in person, but I always figured you had more kaolin in your clay body (seems like you get good flashing). You mentioned trying something cleaner and whiter than OM4, have you tried XXsager? Seems to work well for us (we use a fairly low iron body as well). Lastly you said "EPK(or Tile6)" have you used batches of both? Are they really interchangable in your clay body? Sorry too many questions. Looks like everything is going well.

brandon phillips said...

yeah, i've slowly been creeping up the amount of kaolin, it used to be like 10# in a 200# batch. I've been thinking of swapping out a bit of the fireclay for more kaolin, maybe another 5-10%. The body is still a little dark and I think I could get better yellows with a lighter body. I've used both epk and t6. I've not noticed any difference, probably because the kaolin content doesn't account for a significant part of the body. i use epk now because here it's significantly cheaper than T6. If it were a more significant portion of the body I don't know that they would be interchangable. Tile6 is only carried by one supplier here and not in huge quantities. They only order it once a year so I have to stock up when I can.

i really want some sagger clay but my supplier doesn't carry it, there is another place that carries it sporadically but never more than a few hundred pounds at a time. so i'm somewhat limited with material options. i've tried to get both suppliers to special order stuff for me in large quantities but they won't do it. i'm trying not to have to get stuff shipped across the country(more than it already is anyways...) so I try to work with what i have available...wish it was more though.