Saturday, January 21, 2012

Peninsula School of Art

This summer I will be teaching a 3 day wheel throwing course at the Peninsula School of Art in Fish Creek, Wisconsin. I'm really looking forward to it, especially since I have never been up to Door County in the 12 years I've lived out here. It should be a lot of fun to work in another studio, and meet some new potters. Come take my class if you'd like, or one of the other great courses they offer.

Peninsula School of Art Home Page          PSA 3-D Courses          PSA 2012 Summer Catalog

Thursday, January 12, 2012

Kiln Controller Safety

Today I replaced a controller on a customer's Skutt kiln. It was the wall mounted type, but it uses the same parts as the kiln mounted systems. The controller had fried out, and there's no good way to fix them. By the time you send it off to have it repaired and pay for shipping and labor and everything else, it's faster and just as cost effective to buy a new one. Currently, a new Skutt KM circuit board with touch pad is $260. Replacing it is fairly simple, but you have to make sure to get the wires hooked up in the right order.

When controllers fail, they do all sorts of goofy things. The most common symptom that I have seen is when some of the buttons stop working, like on the controller I replaced today. The Cone Fire section of the board wouldn't work, and some of the number bottons were dead, but the Ramp/Hold section worked fine. Another common symptom is when the bottons still work, but the controller simply won't go when Start is pushed. The most difficult diagnosis I've had was on a very old controller that seemed to be working fine, but every few minutes the temperature readout would jump several hundred degrees for a few seconds, which would confuse the system and shut it down. Replacing the controller fixed the problem.

I worked on a kiln this last summer that was only two years old and was having trouble getting to temperature. It would just stall out at 2000 degrees, then shut down. Due to its age I didn't think that it was a controller problem, and the elements weren't necessarily worn enough to blame them. After changing the thermocouples and thermocouple wires the problem persisted. So I changed all the elements, still with no success. So I switched out the controller with a controller from one of my kilns and we still had the same problem. After talking with the tech at L&L Kilns, and the tech at Bartlett Instruments (they build the controllers for L&L and Skutt), we decided it was in electrical interference problem. We have no idea why this suddenly started happening to a two year old kiln, but it was fixed by adding a grounding wire to the center tap of the control board. It took a couple of weeks to get through all of it, but it's now working fine.

What makes a controller go bad? Age, moisture or an electrical surge. How do you keep your controller healthy? Keep it in a dry environment, and shut down the breaker when you're not using it. I've had several schools fry out controllers by leaving them on during the summer months, when we get a lot of electrical storms.

With minimal care and fuss, a kiln controller will last many years. But like any computer, they eventually wear out and need to be replaced.

Tuesday, January 3, 2012

Clay Selection

Today is the first day of a very busy pottery class session. I'll be teaching 8 classes a week during January, with lots of beginning students. I always start my new students with a short explanation of how we choose the clay bodies that we use, so I thought I'd go ahead and post it here as well:

The clays that we use to make pots are not a single clay dug from the ground. Rather, they are are a mixture of clays, binders and texturizers that are mixed to a specific formula or recipe. We call these mixtures 'clay bodies'. Every company that mixes clay bodies has their own proprietary formulas. For better or for worse, a cone 6 Buff body from one company will not be the same as a cone 6 Buff body from another company.

Clay bodies are formulated and purchased according to 3 main criteria: color, texture and firing temperature. Color is determined by the amount of iron oxide present in the clay body. Porcelain has no iron, white stonewares may have trace amounts, and stoneware bodies can have a small amount to make the body a light tan color, or a large amount to make it dark brown or brick red. The color of the clay body is important because it will affect the color of your glazes. The darker the clay, the more it will darken the glaze.

The texture of a clay body can range from very smooth to very gritty. Grit is obtained by using fire clays in the formula, or by adding grog (ground fire bricks) or silica sand. Grog and sand are available in a variety of sizes, but usually range from 20 mesh to 100 mesh when added to clay bodies. The smoother the clay body, the less forgiving it will be, meaning it will shrink, warp and crack more, therefore requiring good throwing and assembly skills. Large, thick or uneven pieces like sculptures generally require a much grittier body than wheel thrown pots to keep them from cracking during drying and firing.

Last, and probably most important, a clay body must be formulated to fire at the right temperature. If you are using cone 6 glazes, you must use a cone 6 clay body. If you are using cone 06 glazes, you must use a cone 04 clay body. If your clay and glazes are formulated to mature at a different temperature, they generally won't 'fit' together very well, and a multitude of problems can arise, from crazing to shivering. And if you over-fire a clay body, it can melt into a puddle all over the inside of your kiln. See HERE.

So there you go. If you need help selecting a clay body, the tech at your local ceramic supply store can help you out. They know their clay bodies very well, and can help you pick the right one for your project.