Convergent Series

learning, using & teaching metal clay, and other aspects of life

Some more notes from Hadar’s workshop.

Posted by C Scheftic on 2011/04/10

I don’t want to go on forever about this, but I am using this blog to keep some notes for myself about our recent workshop with Hadar Jacobson, as well as to write some comments for others to read. So, if you’ll bear with me, this post will cover a number of “loose ends” from that, then I’ll probably add one final note, and then I’ll move on to posting about other things.

The photo at the left shows Hadar’s own design for “firing vessels” that are made using a piece of “ceramic cloth” formed into a bowl shape and held together with stainless steel T-pins. In this workshop, we started out having three kilns available to us, so here you see three vessels lined up with the results of a test-firing in each of those three kilns. Test-firing means to run some “test pieces” through the entire two-phase firing process each and every time you change any (one or more) of the variables: try a new kiln, use a different size or type of firing vessel, introduce a different carbon (type or supplier), make pieces out of a different metal or brand of clay, etc. In fact, we also tested and then used a fourth kiln later on and, at one point, tried a different material for firing vessel construction that required its own test as well. (I’ll try to remember to write a bit more about testing and test pieces eventually, whenever I get around to trying out in my own kiln some carbon from a new source.)

The vessel on the bottom in that photo (far left, if you faced the table straight on) shows a bit more ash than do the other two, but not enough to be of concern. Once the test pieces had been sifted out from the carbon, we examined each piece by looking at and feeling it. Since they all seemed ok, the next step was to begin to buff a “back” edge very lightly. (Of course, I hear that and think, “Wait! My reversible pieces don’t have a back!” There’s no problem, of course, if the piece has sintered. If it hasn’t, then I’ll just be stuck with doing any necessary patching on one of the two “front” sides of the piece but, hopefully, that will be a rare occurrence.) Once they pass that test, they can be sanded a bit more aggressively.

As I know well from previous experience, when Alice and I were experimenting with these clays on our own and together, an unsintered piece typically makes its condition known very quickly. Luckily, all of our test pieces seemed just fine. That meant we could proceed with both firing some of the pieces that had taken more work to construct and the lesson on all the steps for post-firing polishing.

I will mention that Hadar had brought an example of an unsintered piece so folks could see what one looked like. Because I’d wanted to ask her a few questions about “sintering issues,” I had also brought a couple problem-pieces. So we were both able to share our examples for everyone to learn from, in addition to having the successful pieces from our kiln-tests to use in the polishing demo.

Speaking of firing our actual artworks, to the right here is a photo of the first vessel loaded up and ready to go. Several things to note:

  • Each piece is at least half an inch from the edge of the vessel and from all of its neighbors.
  • Each piece is positioned horizontally. Some people write or teach about positioning them vertically. That does allow you to fit more pieces in at one time, but Hadar insists that there can be enough of a difference in the amount of heat reaching the top and the bottom of a piece that one end may sinter while the other does not. She says she has not found as much discrepancy edge-to-center as she has top-to-bottom, so this way produces more consistently successful results.
  • The pieces are positioned in a single layer. Once again, you can fit in more pieces if you use multiple layers, spaced at least a half inch apart, but Hadar reports more examples of unsintered pieces from that arrangement. Better to just do as many different firings as you need than to have to worry a lot more about sintering with every single piece.
  • The “front” of the vessel should be clearly indicated. Here, it is marked with a T-pin, though that doesn’t really show up in the photo. In top-loading firebrick kilns that have heating elements running around all four walls, this is not crucial. In front-loading muffle kilns, however, with elements on the back and sides but not on the door in front, this is necessary. Since we had a mix of the two types of kilns, better to be safe than sorry, and just mark all the vessels. (Remember: once the pieces are covered with carbon, you can no longer see what’s where! That’s why you mark the outside of the vessel itself. And you do it in a way that will survive the firing, in case you need to remove the vessel from the kiln between the two firing phases, especially because the pieces are extremely fragile at that point.)
  • In a front-loading kiln, in most cases, pieces should be placed around the back and sides of the vessel. Not in front, towards the door.
  • In either a front- or a top-loading kiln, in most cases, pieces should not be placed in the center of the vessel.
  • The one exception to the previous two rules is this: If you have one of the really round firing vessels, you may want to test whether pieces placed in the center, or near the front in a front-loader, will sinter. In some cases, this does seem to work…
  • It is best to fire together only pieces that are roughly similar in size. If you mix sizes, curiously, the ones most likely not to sinter are the smaller ones. (Yes, that;’s what Hadar said but it’s also something I have observed.) It seems almost like the big pieces somehow draw the sintering-heat away from the smaller ones… Experience has shown that, the more even the size of your pieces, the more even will be the sintering. (Here’s yet another situation where I wish I better understood the whole science of these processes!)
  • You can make a few exceptions to those rules, if you know what you’re doing, and take care in the process. In the load illustrated above, for example, Hadar placed pieces of similar size around three sides but, finding she had one small piece left, she put it on the fourth side with extra space around it and fired this in a top-loader.

The next photo on the left shows our first load going into a front-loading muffle kiln. Note that:

  • The vessel is raised up on kiln posts, so that air can circulate all around it.
  • There is no lid on the vessel.
  • The vent-hole plug has been removed.

The last photo, below, shows our second load as it began to fire in a top-loading firebrick kiln. While you can’t see inside this one, it also has the vessel raised to allow circulation, and it does not use a lid on the vessel here either. The purpose of this particular photo is to show that:

  • While there is no actual vent-hole on this kiln, an equivalent effect can be achieved simply by placing the lid slightly askew.

Whew! That covered a lot more than I was expecting when I started this post. What can I say: the workshop was just loaded with interesting and useful information!

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