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.

47 Responses to “Firing Issues”

  • Linda Reboh Says:

    Hadar have you ever fired using a large ceramic kiln? I find if I have a large class it takes a really long time to get all there pieces fired. One of the places I teach has a large ceramic kiln that I can control the temperature on, what do you think about firing in this type of kiln?

  • Sarah Triton Says:

    Hadar, should the one phase firings be taken out immediately & taken outdoors, like on a covered porch, to cool? in FL, it’s been in the 40-50s lately at night. Is that even cool enough?

    Thanks for all your hard work!

  • Melody Pierson Says:

    I have only begun designing with metal clay for about a year. When the time came to get a kiln I remember you giving me your advice. I went with it for all the right reasons. You were a professional whose work I loved. You didn’t seem into gimmicks and you were very honest with me exchanging emails. I continue to use the same techniques with your clay and other silver clay and at least 90% of the time, the pieces come out fine. I think that is some kind of miracle given my limited practice and look forward to learning more. Thank you so very much.

  • Sherry Bleiweiss Says:

    Thanks for all of your insights. They are really helpful..I have recently been using one of the new “no-flake” foil boxes that you have to build (using an origami folding method) It worked very well for about 30-40 firings and then sprung a leak and lo and behold all of the carbon leaked out during a firing..What a surprise when I opened the kiln. I did like the box well enough to build another one and I am going to see how long it lasts but everything has fired successfully(mostly)..Looking forward to meeting you for the March intensive.
    Sherry Bleiweiss

  • Barbara Briggs Says:

    I recently bought a ceramic fiber firing pot which came without firing directions. I naively fired a 200 gram batch of bronze metal clay according to the same directions I’d used previously with my stainless steel container. Unfortunately, I lost the entire load to over-firing. I’m back to using my stainless container which fires beautifully without the hassle of removing it from the kiln while hot and manually cooling the carbon. I’ll gladly put up with a few residue flakes from the stainless container for the peace of mind of knowing my metal clay will fire perfectly and that I can leave it to cool overnight inside the kiln.

  • Cindy Pope Says:

    Thanks again for helping with the mystery of firing. I have just replaced my stainless containers with mini kilns and fiber blanket and am testing so all your blog entries are helping

  • Cindy Pope Says:

    A quick question on carbon. Do you activate your carbon before using it?

  • Debbie Says:

    I have been firring the first phase open air. All pieces
    On a fiber blanket. Full ramp to 1550 for 15 minutes. I
    can Fire with silver pmc. I quickly take the pieces out
    and do the second phase fiiring. I find the more they are
    covered In the second phase, since they are not covered in
    The fist phase, the better they look. it’s been trial and error.
    Oh I also fire in a home made firebrick box,
    not covered. Little ash when done.

  • Karyn R Says:

    Hadar – thank you for your very specific advice. There is so much misinformation roaming about on this topic. A Question: Where do you get your coconut based acid washed carbon?? All I can find is either coconut OR it’s acid washed. Not both. Thanks.

  • carol douglas Says:

    Thank you for continuing to experiment and sharing your results. It is tricky to keep on trying stuff for yourself and there are so many factors to consider when you are moving around.
    Hope to catch up with you somewhere soon

  • Hadar Jacobson Says:


    The problem with a big kiln is that the temperature is not the same in different parts of it. Personally, I’d rather teach a big class with a few small kilns than on big one. For example, in a 2-day class I manage to fire twice (two 2-phase firings) for 10 people in one kiln. Of course, I need to limit each person to one, not too big, piece. If you want to use the big ceramic kiln you will need to test it first, by firing the same amount of pieces that you would fire in a big class.

  • Hadar Jacobson Says:


    Sometimes I fire the first phase overnight and just leave it in the kiln. In the morning it’s cool enough to proceed to the next phase. If you want to speed it up, you can take it out to cool outside. With the right carbon and box, 40-50 will take less than 2 hours. Here in CA the average temperature during the day is about 60, and I just put the box in the shade..

  • Hadar Jacobson Says:


    I don’t. It’s already activated.

  • ann schneider Says:

    Hadar, thanks so much for all your information. I am about to fire for the first time using the cloth from slumpy’s in the thinner size. I hope it works and I will let you know. I am so glad that I was able to see you do this. It helps more than you can imagine!

  • Cindy Pope Says:

    Thanks for the note, I was reading Rio’s instructions online about their activated coconut carbon and they say to fire it first for 30 minutes at 1550 to prepare it for use. This is quite a pain and I am so happy I don’t have to do it.

  • Linda Reboh Says:

    Hadar, I just fired your copper clay in two phases but used my old metal box. It didn’t sinter. I don’t seem to have the same problem with the bronze clay. Do you think using the ceramic fiber box will work better?

  • Hadar Jacobson Says:


    If you used the 2-phase schedule with cooling between phases and the copper didn’t sinter, it’s probably because of the box. Since steel is a poor heat conductor the temperature should be higher. You may get better results with a fiber blanket box.

  • Lucie Cote Says:

    Haddar, you described your firing with the use of fiber blanket box. What about the mini-kilns dug in bricks, do you still recommanded them with this new update ?
    Thank you very much to share your trials and instructions with us

  • Hadar Jacobson Says:


    The mini-kilns are a good idea if we could get fire bricks that are a little bigger. The problem with the small boxes, is that we can fill them with enough carbon, ans as a result some pieces can get over-heated.

  • Virignia Says:

    Thanks Hadar,
    This is exactly what I have been needing for success. The instruction manuel doesn’t really make it clear to cool between phases. This is very detailed and clear. Thank You , Thank you. I just got in my koa wool or fiber blanket and will fire in a little bit.
    Thank you,
    Thibodaux, Louisiana

  • debbie Says:

    I had my husband cut up about 2 fire bricks into strips and I cemented them together building a box. The new fire box is large enough to fire many pieces at a time, with about 3 layers. However after about 2 fireings the cement crumbles and i have to recement it together again.

  • Virignia Says:

    OK, after using the fiber blanket box for the first time, I am experiencing what everybody else is getting. An entire box full of spent carbon ash. I followed directions and brought the hot box in the cool night air to cool. The fiber blanket must be holding the heat in because that didn’t happen in the no0flake foil box.
    More sadly, I am still not sintering. I am getting ready to fire again and I will be experimenting with the temps.
    Wish me “Good Luck”
    Thibodaux, Louisiana

  • Virignia Says:

    Oh, to add to my long comment I just submitted, I am using the new “coconut, acid washed activated carbon” from Whimsey. New fiber blanket box and the two phase fire schedule in the manuel with a complete cool down between. Getting closer to the give up part, but still plugging along……..
    Thibodaux, Louisiana

  • Mary Says:

    Hi Hadar, I have ordered your clay powder but have not made even the first piece of metal clay. I have ordered one of the ceramic fiber pots/bowls and have been studying how to use it. I would like to avoid using the stainless pan. What do you think of these pots sold by the Alchemy Group?

  • Lucie Cote Says:

    Hi Haddar,
    In your explanations you write that if you take tweezers to grab the piece it will brake. It happened to me, why does it happen, how do you explain it?

  • Hadar Jacobson Says:


    Honestly, I only fired in them once. I don’t know yet. In any case, I wouldn’t use the lids.

  • Hadar Jacobson Says:


    At the end of the first phase the binder is gone, and there is almost nothing to hold the piece together. Instead of the binder there are these big spaces between particles. At the second phase, the particle get closer and closer and finally bond to a solid mass. Does that make sense?

  • Lucie Cote Says:

    Thanks Haddar
    It makes sense to me …

  • Mary Says:

    Hello again Haddar, thanks for your reply. I have ordered one of the Square Head pots. Should I lower the firing temperature in the sintering phase as you suggest for the fiber firing box? I think Square Heads maker says to use the clay manufacturer’s instructions—- which I assume is for the stainless pan. But something is telling me to lower the temperature. Any advice?

  • Hadar Jacobson Says:


    My gut feeling says you should lower the temperature. In any case, it’s better to err on the lower side, since if there is no sintering, you can always re-fire.

  • leeann Says:

    is it ok for carbon to glow in first phase or just in second phase…new to kilns and not sure what to look for to decide of my binder is completely burning out

  • Hadar Jacobson Says:

    Leeann, It’s of for the carbon to glow after the first phase.

  • Jamie Says:

    I just tried the white bronze clay and I am very discouraged. I tried to make a notebook pendant. I created it and it dried for two days before I could fire it. I have a larger kiln and when I fire bronze I have to lower the second stage firing by 10 degrees than the instructions state so I lowered the second stage firing for the white bronze. I fired the first stage at 1000 for 30 minutes…it was not a thick piece and the second stage at 1230 for 2 hours. Well, I can not even find my pieces in the charcoal, I believe it simply burnt up…very said and I do not understand.

    Anyone else have a problem with white bronze?


  • Dana Gooch Says:

    Hello Hadar,
    Thanks so much for being available to answer so many peoples questions. Besides your awesome clays these are the reasons I will stick with you and your products. You suggest taking the fiber box out of the kiln after firing to prevent burning up your carbon. From what I have read by the manufacturer of my kiln, you shouldn’t open the kiln until the temp reaches 600 degrees F. because you have the chance of busting the interior of your kiln due to the sudden temperature change. Have you ever experienced this? I am having issues with my carbon burning up, but only after I began using the fiber box. I like the size of it but may have to go back to using my “square head” box. Just as everyone has, I spent a lot of money on my Kiln and do not want to bust the interior by opening the door before I should. Thanks so much for your time and expertise!

  • Hadar Jacobson Says:


    There are two things you can do. Change carbon to one that doesn’t burn out so quickly. Or, fire in a stainless steel salad mixing bowl. This is my favorite for now.

  • Tana Says:

    Hi! I have just begun using your clay and I am still experimenting with firing. I have fired 4 white bronze test pieces at two different temperatures and times. The only question I am having right now is, how is the white bronze supposed to look right out of the kiln? Two of my pieces came out with a pink tint which, after sanding became more if a silver color and two came out basically the same color as before I put it in and sanded to a dark silvery color. Are the pink ones over fired? I absolutely love your clay and the endless design options! I am determined to get the perfect firing time in my kiln, any help from anyone would be greatly appreciated.

  • Hadar Jacobson Says:

    The color of the pieces when they come out of the kiln is not an indication to whether they are under-fired or over-fired. In most cases they are just black. They need to be sanded or buffed to reveal the true color of the metal. You can find finishing instructions in my book The Handbook of Metal Clay: Textures and Forms.

  • Tana Says:

    Thank you so much for taking the time to answer my question and doing it so quickly, I truly appreciate it.

  • Tana Says:

    I am in the process of firing my first white bronze ring. I am using a ring sizing pellet, which I have used before, once with bronzclay and my issue then was carbon between the ring and pellet. I have wrapped this white bronze ring and pellet in a small piece of fiber blanket and covered it in carbon. How do you recommend firing rings and why do I feel as though I should hashed this question before I turned my kiln on?…

  • Tana Says:

    I am in the process of firing my first white bronze ring. I am using a ring sizing pellet, which I have used before, once with bronzclay and my issue then was carbon between the ring and pellet. I have wrapped this white bronze ring and pellet in a small piece of fiber blanket and covered it in carbon. How do you recommend firing rings and why do I feel as though I should have asked this question before I turned my kiln on?…

  • Hadar Jacobson Says:

    Tana, I fire my rings with a piece of soft fiber blanket inside. I don’t try to stop the shrinkage.

  • Tana Says:

    Ok, thanks! 🙂

  • Sylvia Tevlin Says:

    Hi Hadar,

    I really appreciate the help you have given me. I tested a few pieces over the weekend (steel, copper, bronze) and the steel can be broken apart just by bending it. I assume that that means it had not sintered. I had made a few mistakes that first time so I’m not too concerned. I do have one question…is the vent hole supposed to be open ONLY during the first phase or for the second phase too?

  • Sylvia Tevlin Says:

    Also, is there a special way I should store dried pieces if I’m not ready to fire them for a couple of days?

    Thank you so much for your time.

  • hadar jacobson Says:

    Sylvia, The vent does not have to be open at the second phase. However, I strongly recommend switching to the stove-top method for phase 1. Then you don’t need to worry about venting at all.

    Pieces can stay un-fired for a long time. They don’t need special storing.

  • Sharon M Thompson Says:

    Hello Hadar- I’ve had good luck with your bronze & copper clay powders. However I’m about to do a 2 stage fire on a rather thick Copper Clay item, and wonder: how brittle will the items be when moving from stage 1 to 2? I’m a little nervous as this is already the 2nd go round with this project, and want to take every precaution. Many thanks for all your pioneering with clay, and for sharing all you learn so freely.
    Sharon M. Thompson

  • Hadar Jacobson Says:

    Sharon, It’s best to fire the first phase on a stove top. Fire on medium flame, letting the binder burn slowly. Up to 40 minutes is ok. Then just pick the box and move it carefully to the kiln. Don’t touch the piece itself. With big pieces always ecpect cracks, so just repair and re-fire. For repair you only need to fire the second phase. Good luck!

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