Clay Body For Large-Scale Project

[perkolator]

I'm helping figure out the logistics of a proposed large-scale ceramics project.  One of the most important things we need to figure out is the clay body, which is where I'm hoping for input.

 

Was planning on simply making our standard studio clay body that we use for our large-scale sculptures.  Usually items get up around 1" thick or slightly more, occasionally students get into the 2-3" range (like at bottom of large piece) and we usually once-fire to ^04 or to ^6 without any issues, or at least minimal problems that are usually due to construction methods.

 

Sculpture body:

6 Fire Clay (Lincoln)

2 Kaolin (Edwin)

1 Ball (OM#4)

1 Grog (20m Mulcoa)

 

The proposed project will be a very large, very thick slab-based piece, outdoor demonstration with public interaction.  Original thoughts were to NOT fire this piece when finished due to size and thickness, but personally I'd like see if there is anything we can do to the clay body to make it possible to fire because I think it's going to turn out awesome and it would be a shame to put all this effort into making several tons of material, getting public interaction, and then not fire it and slake it all down 

 

Immediate thoughts went to additions of either perlite or cellulose insulation to make the body more porous, in order to fire these solid pieces.  Costs need to be kept to a minimum, so adding crushed IFB is not a possible solution.  I know perlite will work very well for what we want to do, but question whether it will be cost effective when making 2-3 tons of clay.  Paper clay made with cellulose insulation I have no idea - I tried it on a small scale, thin items, works great - but don't know how it would work on large scale like this with thickness getting into the 4-6" range.

 

Anyone?

[neilestrick]

Increase the grog a lot, and leave out the kaolin. Have you considered using wood shavings, like animal bedding, instead of pearlite? It would have the fibrous quality to help hold it together during drying, and it quite inexpensive. Large bags of it can be purchased at farm stores.

[Marcia Selsor]

I agree with Neil that more grog and some sort of binding. I have never used the animal bedding but I may try it for some project.

you can use up to 50% grog in a sculpture body. You can vary the size mesh.

You can also add paper pulp on nylon fiber for bonding. I have used cellulose. I would not add more than 5% maybe 10%. It doesn't seems to bind like paper pulp.

 

I would think you need to add some lower firing clay like earthenware or a little feldspar (5%) to make it mature at low temperature. Is this going outside?

Marcia

[perkolator]

I'm not sure why wood shavings/bedding didn't come to mind - I will look into this. Sounds like a good choice of material to use for the intended result.

 

Nylon fibers would also be a good addition, I think I have some still.

 

The 10% grog is just the starting point quantity in the recipe, it usually varies depending on the project. I've also got some 35m and two fine grogs around 150 or finer mesh that I can mix in there too.

 

The one thing that's not solid in the plan is whether or not the piece gets fired. Should be getting more information about the project tomorrow.

[ChenowethArts]

Two quick questions:

Would you mind sharing your plan for firing? 

Getting into thicknesses above 2" raises challenges for drying...how long do you have to allow this to dry before firing?

 

Best of luck on your project...be sure to document the event well!

Paul

[Marcia Selsor]

Robert Arneson built thick pieces and fired them for five days. My ex- was his tech.

So you need to be a little more specific of what you are making. Also environment plays a big part of drying. In my studio in southern Illinois, it took four days before I could add a handle to mugs. In Montana it took about an hour. So humidity influences drying time consideration. You can slow dry a large piece by placing it on even sticks and wrapping it in dry cleaner bags.They breath.

 

 

Marcia

[perkolator]

Yes, Arneson used to build some thick, solid work. Our methods were pretty much initiated by him, and were passed to us through George Grant (he was tech for like 25yrs) and our Faculty who has also worked in thick, solid ceramics most of her career. We haven't changed a whole lot in our process and our firings still take 5-7 days before work can be unloaded due to the scale and method (also because in a 10wk quarter system makes challenges when working this big).

 

We don't have humidity issues here in CA, we're actually in a severe drought - so drying should be fine, if anything it would be too fast. With a high grog content on other porosity added to the clay body, the pieces will dry out even better at the core where it's most important.

 

The piece will be a big relief that starts with a base around 3-4" thick. It will likely get both carved and added to. Piece will get cut up and put on boards (or will already be cut into tiles beforehand) and then onto ware carts for drying. Whole cart will be under a plastic sheet to slow it down. Anything not fitting on the carts I'm going to have to rig up some sort of elevation system to get airflow under the work like Marcia mentioned with the sticks, only with multiple levels due to volume of work and because many carts will be occupied with student work. I don't like to push force drying too fast, but bringing the carts into the heated building will help, as will force drying in the kiln if need be. It'll likely take a month for the work to dry enough for firing is my guess.

 

Firing will be in downdraft gas kilns, fired over the period of perhaps a week and cooled over several days as well. With the thickness, it will likely need down-firing to keep from cracking in cooling. Individual pieces will be flat on the backside, so they will all get propped up on balls of kiln wadding (equal parts silica grog kaolin) when placed in the kiln.

 

This has all happened really fast and I dunno who's idea it was to at the last minute decide to have this huge ceramic piece/demo as part of the event. This is not really one of those things you take a last minute approach like this, but we're doing it. If anything, this will be a big, fun experiment with tons of clay! It will get lots of documentation, no worries there

[Marcia Selsor]

I have been using 1/4" could I make with an extruder for firing large pieces. I bisque them first. I have found that by raising the pice off the shelf , steam is able to escape from the bottom and the piece can move as it shrinks.

 

Marcia

[perkolator]

yes, without lifting large flat expanses of clay (say over the size of a piece of paper), there is a chance you'll get a crack in the piece - especially so when you have thick slabs.  It also allows the piece to move as it shrinks, similar to how the stone blocks got moved to make pyramids.  I think a popular alternative is silica sand or grog - but it creates more problems than it's worth IMO.  we also use lots of broken kiln shelf similarly, but the wadding/putty is preferred because not all of our shelves are perfectly flat - the wadding will level out under the weight of the piece and support it evenly.  The only challenge is getting your huge slab of greenware transferred onto the balls of clay since you cannot slide the piece off, and have to pick it up...and then due to size/weight you usually need a couple sets of hands (and some on the opposite side of kiln) to help you get it up and in the kiln.  I'm talking about slabs roughly built to 22"x36" rectangle, about 2" thick.