Tuesday, June 1, 2010

Stack furnace v1.0: The claywork

Now based on several articles, videos, and some photos found online, I've decided to build what I think should be termed a clay stack furnace. I will create a small pocket of high temperature surrounded by reflective (or at least insulative) material. Thus the heat energy will stay trapped in that area, rather than leaving via the flue/chimney.

Shopping for high-cone clay turned out to be the hardest part. I knew I was going to be pushing all sorts of temperature boundaries in the pocket, so I didn't want to mess with something that would melt, slag, or droop below that. Unfortunately, my local bigbox stores, craft stores, and even school supply stores didn't carry anything beyond air- or oven- dry clay. I went to a couple of the boutique paint-your-own pottery places, and although they sometimes sell clay, they had none. Even the specialist art supply place on the edge of town only occasionally carried pottery clay, and that was neither high-cone nor in stock. Much gas wasted driving around, so I returned home and started calling around.

In fact the nearest place that was open and selling cone-10 clay was the student store at a college about an hour's drive out of town. Happily, they were dumping their stock at the end of the semester, and had 35-40 kilos at "sub cost" prices. Whee!

OK, so now I have clay, but I'm no potter, so I have no idea what I'm doing as far as construction, drying time, firing time/temp, etc. A quick browse through the internet does not reveal much in the way of "how long do you let a 5cm thick wall of clay dry before you fire it?". Most of the instructions are for dainty little teacups or objets d'art. The five large cubes of clay made me bold, and I decided to just forge ahead. (No pun intended)

First layer
Second layer
Third Layer
Fourth and fifth layers
Inside view, after smoothing

Basically I started with a tight little horseshoe shape, built on that, then closed the tuyere at the top, and built it up a bit taller.

When I was done, I realized that I had expanded the internal volume considerably beyond what I had originally intended, so I sliced (vertically) a couple of chunks out of the sides, dampened the edges, and stuck them back together firmly. While I fully expected this to result in faults during firing, this did not happen: apparently working the edges enough resulted in a good "weld". The removed clay I placed on top again, making the stack still taller.

Finally I wet my hands a bit, and smoothed and compressed all the surfaces. Yes, smooth is a relative term, but I reasoned that reducing the surface area would increase heat reflection and decrease heat absorption. Likewise, the opportunity to compress the sides together would hopefully knead out any larger air pockets. This was by far the quickest part of the build.

So now I have this... er... thing. I know it's got a lot of water content: I didn't add water to the clay out of the box, but it was still pliable, so its water content was not zero, and I did wet my hands for the smoothing effort, and the clay drank that water in.

Looking up drying times on the web, I get a variety of answers: from 36 hours in Arizona to ten days in Minnesota. Since I'm leaving on a trip in four days (on April 28) I decide to try for three days. Moving this (base + forge) off my patio and into the relative dry of my apartment was a big job, and bringing it out again was going to be another one. Happily, family and friends were understanding enough to help.

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