Firing Issues

First, a class update: a 2-day back-to-back workshop has been added to the workshop in Chicago, at the Local Metal Clay Guilds and Chapters. The added dates are April 11-12, 2011. Please contact: Katie Baum at to inquire about available places.

There is also still room in the May intensive at my studio (May 7 – 11).

I’d like to address a few issues that have been raised.

Carbon Turning into Ash

If you fire overnight and in the morning you find your pieces exposed and most of the carbon has turned into ash, it’s not a good thing. First, you lose a lot of carbon. Second, your pieces are exposed to air and heat (who knows for how long), and are at risk for being oxidized or over-fired. There could be a few reasons for that:

1. Some carbons stay hot for a long, long time after the firing is over, even outside the kiln. Try to switch to another type of carbon. The carbon should be coconut-shell based, acid washed, size 12 x 40.

2. Some kilns stay hot for a long time after the firing is over. In that case, don’t fire overnight. Take the box out of the kiln as soon as the firing is over. Covering the carbon loosely with a fiber blanket box helps to some extent, but does not solve the problem.

3. The new fiber box does not cool down for a long time (even outside in the snow, as has recently been reported to me). It is not recommended for overnight firing. Take it out of the kiln and empty it out as soon as the firing is over. The box also takes a long time to cool down between the first and the second phase. Try to find a way to pour out some carbon until you see the pieces. They will cool down faster when exposed, and then you can carefully pick them up with your fingers (don’t use tweezers or the pieces will break!) and lay them on a soft surface such as a fiber blanket. Empty the box and re-fill it with new, cool carbon. Put the pieces back in, cover them with carbon, and proceed to the next phase. Remember that in the second phase you need to fire 30°F lower than in a fiber blanket box.

Note: if your pieces get oxidized, there is no need to pickle them. Just put them back in the carbon and re-fire for about one hour.

Testing the Kiln

When I go to teach in an unfamiliar venue, I’ve gotten into the habit of testing all the kilns the night before. What I want to find out is whether a certain kiln is under-firing or over-firing when I use my regular firing schedule. Under-firing can be fixed by re-firing; over-firing cannot. Here is what I do:

For each kiln I prepare at least 4 test pieces: 1 bronze, 1 copper and 2 mixed piece – one mostly copper, and one mostly bronze. The surface of all the pieces (including the mixed ones) is smooth and flush.

As a starting point, I use the firing schedule from my instruction manual, using a fiber blanket box. I have tried this schedule with brands other than mine, and it works.

Depending on the kiln (front or top loader), I fire the first phase at 1000°F or 1100°F, and open the kiln as soon as the firing is over.

Here is something I have recently found out: at this point, when I open the kiln, sometimes I see that the carbon is on fire. It shouldn’t be. That means that the first phase temperature is too high. The whole idea of the first phase is burning the binder. The binder won’t burn off if the carbon is on fire, for lack of oxygen. So, if the carbon is on fire at the end of the first phase, lower the temperature. Start lowering by 50°F. In one of my top-loader kilns, I had to lower the temperature to 900°F.

Note: when you open the kiln right after the second phase, the carbon should be on fire, or at least glowing. In an overnight firing it should be cool.

I let the box cool down by taking it out of the kiln and leaving it outside (not if it rains). From many of your emails, I have learned that this step is often skipped. Until the next improvement comes, this is an essential step. When I can touch it without burning myself, I proceed to the second phase. I fire at 1470°F or 1520°F, again, depending on the type of kiln. This time it’s an overnight firing.

The first thing I check in the morning is the temperature in the kiln. If its 700°F or more, I will not fire in this kiln overnight.

Then I check the pieces. I sand the copper piece. If it does not look all metallic, and parts of it are powdery, then the temperature was too low. From my experience, with a 2-phase firing schedule, this hardly ever happens. If it does, though, repeating the second phase will sinter the copper, so it is better to err on the lower side.

If the bronze piece curved or blistered, then the temperature was too high. There is nothing you can do to fix this.

If the metals in the mixed pieces look separated somehow, if there is a relief instead of flush surface, or the copper does not show at all (it is covered with bronze), then the temperature was too high. Again, It can’t be fixed. Next time I will try firing at 1450°F or 1500°F.

Testing White Bronze

As stated in the instruction manual, White Bronze has a very narrow range between under-firing and over-firing. I can only report results from my kilns and other people’s kilns. I fire my White Bronze in 2 top-loading kilns. In one of them I get sintering at 1310°F; in the other, at 1250°F. A student with a front-loading kiln reported good sintering at 1325°F.

Changing Conditions

Bear in mind that every time something changes – carbon, box, kiln, location, or weather – the firing temperatures may change as well. If you are testing your kiln for firing a certain clay, don’t change more than one factor at a time. If you are gradually changing the temperature for better sintering, make sure all other conditions stay the same: same kiln, same box, same carbon, same size of pieces, same number of pieces.

There will always be new products on the market, some very tempting. Just remember that if you purchase something new, you may have to change your firing process. The consideration in purchasing a new piece of equipment is not “Is this product going to improve my firing results?”, but rather: “How do I make this product work for me?”. These products don’t come with their own firing schedule, and we can’t expect them to. My only advice is: stay with what works best for you.

In general, what are we looking for in our equipment?

Box: made out of a good heat conductor; takes advantage of most of the kiln’s space; deep enough for carbon below and on top; vented on the top; does not contaminate the kiln.

Kiln: small chamber for even heating; firing elements on all 4 sides; vented on the top (I fire more pieces in a 6.5″ x 6.5″ x 5″ than in an 8″ x 8″ x 6″; first, because I can take advantage of the center of the chamber, and second, because I can fire vertically. The temperature is not lower at the center or the bottom part of the chamber); I think a circular small kiln would be great!

Carbon: coconut-shell based and acid washed; cooling down fast. Except for that, we don’t know much, nor do carbon manufacturers at this point. It’s a matter of trial and error. If you are looking for nice colors, I doubt that the carbon creates them. I have used many kinds of carbon, and with each of them, sometimes I get nice colors, sometimes I don’t. Also, the colors do not indicate if pieces are properly sintered or not.

Leave a Reply