We Made the Kiln
Work for Us

This posting may be of interest to European users of metal clay and to people elsewhere who own very small kilns.

At the end of September I taught a workshop in the Netherlands, at Non!Soesa Creatif Studio. Before the trip, my hostess Otteline and I had corresponded extensively about the firing process, trying to make sure that the firing during the workshop would work fine. Otteline did a lot of test firings, took photos and documented her process, and I made suggestions as to how to improve the results.

Alas, there is only so much information you can convey via email. Something is always left out. When I first entered the studio and saw the kiln, I was in shock. I knew it was a kiln I hadn’t used before, but I never imagined how small it was. The dimensions of the chamber are 11.5 cm x 13 cm x 7 cm (4.5″x5″x3″).

Mini kilns 002

The chamber is made out of brick, the door is in the front, and so is the venting hole. This combination was totally unfamiliar to me.

We had 2 more identical kilns and another one of very similar dimensions. This fourth kiln has no venting hole, so we left the door open a crack.

Mini kilns 004

…and there were 17 people in the class.

Based on Otteline’s experiments, these kilns fire too hot if you use the schedule for muffle, front-door kilns. This is probably because of the bricks, which retain heat well, and because of the small space, which makes the chamber heat evenly, without wasting energy on just heating air. I was also informed that pieces near the back wall tend to crack.

I did one more experiment before the class began. I placed pieces slightly away from both the back and front wall, and did not avoid the center.

The boxes that were available to us were small stainless steel ones. I filled them with carbon as much as I could. Here is the schedule that we used. It worked just great:

Phase 1

Ramp for an hour to 538 °C (1000 °F).
(These kilns do not allow to ramp by temperature, only by time.)

Hold for 0:30 – 1:00 hour, depending on how much binder needs to burn out.

Cool to room temperature.

Ramp for an hour to 795 °C (1460 °F).

Hold for 2 hours.

Here is what Otteline made right after the class. It’s a combination of copper, bronze, and Pearl Grey steel.

I am often asked what I think the ideal kiln is for firing base metal clay. I am not sure what the answer is yet, but I am pretty certain that you can make any kiln work for you – as long as it has a digital controller – if you do just a few simple tests.

Testing

Fill your box with bronze test pieces, as many as you would fire in a regular batch.

As a starting point, use one of the firing schedules for bronze, as suggested in my instruction manual (2 firing schedules are suggested for 2 different types of kilns).

Under-firing

When the firing is over, check the pieces. Sand them with coarse sandpaper. If some of the surface does not become metallic as you keep sanding, the pieces have not sintered properly.

What to do? One thing you can do is raise the firing temperature. However, before you do that, try dividing the firing process into 2 phases. Cool the kiln and the pieces to room temperature after the first phase. This is the best way I have found to completely get rid of the binder. Moreover, when I fire this way, I find that I need to lower the temperature in the second phase.

Right after my workshop in the Netherlands I taught two workshops at Studio34 Beads in Rochester, NY. All four kilns were muffle, front-loader kilns. We fired in two phases and had to lower the temperature in the second phase to 1520°F (827°C). The results were great!

Over firing

If the bronze pieces curled, blistered or became textured, then the kiln was too hot in the second phase. You need to lower the temperature.

If pieces cracked, the binder was probably burning too fast in the first phase. Cover the pieces with more carbon, or slow the ramp.

I am currently updating my instruction manuals. Instead of a separate manual for each type of clay there will be only one manual (including a table of contents), which will cover all firing information. This includes test firing, making firing boxes, schedules, and programming instructions.


7 Responses to “We Made the Kiln
Work for Us”

  • Natalie Says:

    Hadar, You never fail to amaze me!!! All best wishes, Natalie

  • Virginia Vivier Says:

    Thank you for these valuable blogs updating your research. You just explained how to troubleshoot a problem I encountered for the first time!

  • Giorgia Says:

    Hello,
    I’ve bought some bronze and white bronze a few days ago and I’m waiting for them 🙂
    I’ve read an articles about traditional copper clay and bronze clay, that explained how this clays can be dried also by waiting 24h at simple air. Do you think this is possible also with hadar’s clay? This would be a very good thing for me.

  • Hadar Jacobson Says:

    Giorgia,

    You can certainly dry them in the air. In fact, unlike silver clay, they don’t have to be completely dry before firing.

  • Bonnie Says:

    Hadar, I truly enjoyed meeting you and participating in your Rochester, New York workshop. And thank you for all your updates. They really do help me as I continue to learn more about designing with metal clays.

  • Hadar Jacobson Says:

    Thanks, Bonnie, that’s good to hear. Maybe we will meet again.

  • Suzette Says:

    Thank you I have been reading everything I can to try to decide which kiln to buy. This post was more helpful than just about anything else I have read.
    I am feeling more confident now.

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