October 24th, 2012

December 2014, by PNH

Once More, with Feeling. ...Oops. Make That Twice More.

(This one is not my fault, as far as I can tell.)

I ordered and received a nice new thermocouple protection tube. (Let’s hear it for The Kiln Doctor: the tube arrived the day after I called them to order it.)

I have a box that was labelled “High Temp Fiber Blanket”; the blanket is 2 feet wide, and too thick for what I’m doing, but it divides nicely to half-thickness, so I cut a piece that was 18" wide and gently pulled it in half, to go around the walls. I also took a half-thickness, about 18" square, to serve as the bottom. I installed these...

...and sprayed them with a splendid material called ITC-100HT, which reflects heat: it is a high-emissivity coating. I have to mill it a bit in order to get it to go through my little spraygun, and although the inventor furrowed his brow and expressed doubt when I told him about that, it seems to work just fine.

I dried everything by heating the interior of the kiln to a few hundred C with a propane torch. (I was reluctant to apply a lot of heat very quickly because of the fresh patch material in the bottom and the inlet port, some of which is rather thick.) I allowed it to cool somewhat, and then I cured it by taking it up to about 1100° C. At that point I left it to cool again while I ate dinner.

In the evening I prepared some test tiles and cones, and did a cone 10 reduction firing. Here is a photo of the flame on top of the chimney, with the temperature inside the kiln at about 1203° C, shortly before the end of reduction:

[As you can see, I protected the lid from direct contact with the flame by interposing a piece of fiber blanket.]

A few seconds after I took that photo there was a loud bang as the steel band around the lid parted. It was under a certain amount of tension, and as the lid expanded with the heat the tension increased. Ordinarily this should not be a problem, but the tiny pop-rivets holding the hose-clamp pieces had rusted, and they came undone. As the band fell, it broke my lovely Type S thermocouple. (Platinum vs platinum-rhodium alloy wires, about as thick as a human hair.) Grrr. Fortunately, it did not break the new protection tube.

I guessed at a good time to take the kiln out of reduction, and ran the rest of the firing by watching the cones, which is one reason why we use them. It was a very clean cone 10 firing, but there was one problem...

This is actually mid-temp blanket, and it was unable to deal. (I have relabelled the box.)

I removed the hearthplate, which is probably ruined, and most of the remains of the blanket; but the layer on the floor of the kiln had melted rather thoroughly. Sigh. I sprayed the interior of the kiln with ITC-100, hoping that when it got hot it would react with the glop on the floor and the glop I was unable to remove from the walls, and that this would minimize the chance of further damage.

Fortunately, I was able to find a roll of blanket that is rated to handle 2700° F, which is more than good enough for what I’m doing. Moreover, the information is printed on the box, not handwritten. (I thought this box had already been taken away, else I’d have used it in the first place.) This blanket is 4 feet wide and 2" thick, so I took a piece about 17" wide and about 1/4 of the thickness, which was more than enough to line the walls. I also took a piece about 17" square, likewise about 1/4 of the thickness, for the floor. I put these into the kiln and sprayed them with ITC-100. Then I sprayed and installed a new hearthplate with some supports underneath it, and touched up the ITC where I had scraped it putting the hearthplate in. As you can see, it just barely fits:

I used the torch to bring the interior up to a little over 400° C to stabilize the coating, and left it to cool while I ate dinner. Then I put a set of cones and test tiles into the kiln, gritted my teeth, and fired again, using the Type K thermocouple that is ordinarily on my electric kiln. Once again, the temperature went up like a rocket, but it began to slow down after about 1150° C, and although it jumped a bit after I took the kiln out of reduction, which is usual, it seemed to stall around 1270. This was not a good sign; it is clear that this kiln is [literally] coming apart at the seams, and it leaks more heat than I would like. (If I can get a good photo that shows the orange glow from the seams between the bricks, I will add it.) It seemed to take a very long time before cone 10 started to bend, but once that happened the firing went to completion at a very reasonable pace.

Here’s what the kiln looked like, the following morning:

Cone 11 has just started to bend, so this is about cone 10¼. All three of the Rutile Blue tiles are badly pinholed, and I am beginning to have an idea about that; but I haven’t had a chance to test it yet. They were also significantly paler than I had expected, and I may add a small amount of iron to them for a retest. The Copper Red test, which is a slightly unusual recipe, is a little darker than it appears in this photo, and is closer to purple than I would actually expect at cone 10. (They get darker and more purply as they get hotter.) That’s fine with me, as it is pleasant and appears to be well-behaved:

I will be playing with this glaze a bit more, and with some luck I will have at least one piece at SFContario. That, however, may depend on whether I can get a few more firings out of the kiln before it actually falls apart.


Addendum, per comment:

[profile] gilraen2 raises the possibility of making things and putting glazes on them, which is A Notion of More-Than-Oriental Splendour, so I thought I should provide an example. As I write this, there are 12 pieces of greenware warming up in the bisque kiln — I like to be certain that they are entirely dry before I risk taking them above 100°C. This is one of them, returned for a brief moment to the wheel to have its portrait taken:

It’s a slightly unusual shape for me: I usually prefer to have the sides convex; but I looked at this as I was throwing it, and changed course. Mud is convenient that way. I don’t know whether the piece will be successful, but I guess I’ll find out.