Sintering Issues

This is in response to many questions that I’ve been asked about failures in sintering. I’d like to remind you first of the checklist, which is available on my blog and at the end of the instruction manual. It may help you make sure that you have done everything right. I would like to mention a few more variables here:

1. If you still have sintering problems after going over the checklist, I suggest that you try carbon from a different source than you have been using. Not all activated carbons are the same. With some there will be good sintering, with some there will be partial sintering or no sintering at all, and others are only good for a limited number of firings. Unfortunately, this information is not available from suppliers and you will need to ask for samples or purchase small amounts and test them.

This is a major issue. If you have encountered this problem and do not mind sharing your reliable source please do so in the comments to this posting. I am in the process of testing a few sources but it has not been long enough to determine their reliability.

2. Your kiln may not be getting enough power. Make sure that it is connected to a circuit of its own, with no extension cords. Make sure nothing else that requires a lot of power is running at the same time.

If you are a teacher teaching a workshop and need to use a few kilns, don’t use them all at the same time. For example, if you use two kilns, run the first phase on one of them, and when it’s done run the first phase on the other. Then run the second phase with the first kiln, and when it’s over run the the second phase with the second kiln. From my experience this should not take more than 12 hours. Another option is to ask students who live close to the venue to volunteer to fire for some of the other students, as well as for themselves.

3. No shortcuts. Let the pieces cool to room temperature before you go on to the next phase. Even if you cool the box in cold water, the carbon may be still hot. Wait until you can touch it with your fingers.

4. If you use a 4″ tall box, place your pieces horizontally, as high as possible in the box. If you place them vertically, the bottom of the pieces may not sinter. For more about the box, see here.

5. Make sure that there is 1″ space between the top of the box and the top of the kiln chamber. With no space, the top of the chamber is like a tight lid, not allowing fumes and contaminants to escape.

6. For thick pieces make sure to hold one hour in the first phase to give a chance for the binder to burn.

7. Fill hollow forms halfway with carbon. Trapped air in hollow forms may interfere with sintering. Also, avoid firing more than 4 small hollow forms in one firing session.

8. If you are not sure what your full-speed temperature is, make sure that the first phase does not last less than one whole hour. If it is shorter, lower the ramp to 1750°F (972°C). Shorter firing time may cause pieces to break.

I have just fired a big hollow form (5.2 oz.) all by itself. It is safe and sound. Hope to post photos soon.

6 Responses to “Sintering Issues”

  • LeAnn Weih Says:

    Hi there, I’ve been firing with activated carbon from New Mexico Clay ( While, I’m only just getting started, I’ve fired with the same coconut derived carbon for about 9 firings with no problems in a Paragon SC kiln.

  • Jonna Faulkner Says:

    Hi, Hadar, I just completed my first firing with Whole Lotta Whimsy’s Rainbow carbon. It went great. I fired 7 bronze pieces, medium sizes, including 2 hollow forms, using your methods. I did pre-fire the carbon at full ramp to 1516F for 15 minutes. Thanks so much for all the research you’re doing for us. Hope to see you soon! Jonna

  • Lin Altman Says:

    I’ve finally been successful with my firing. I used the Rainbow carbon with both copper and bronze and it worked great. WholelottaWhimsy is now my source for carbon. I use a front loading brick kiln. I follow Hadar’s schedule for a top loading brick kiln.

    Thanks, Hadar, for all your help!


  • Susan Dilger Says:

    Hadar, I’m using Rio coconut carbon purchased when BronzClay was first introduced 7/08. I have fired BronzClay and CopprClay about 4 – 5 times in my Rio Blue front load PMC kiln. (Is this a Paragon SC kiln?) No problems with the carbon degrading, no sintering problems either. I’ve fired both on a full ramp and hold as well as your schedule (fire in the box, cool, then fire again). I do NOT have anything else running on the circuit when I fire.

  • kim Says:

    How do you fill a closed hollow form or one with small stringing holes with carbon? And for cork clay based forms, do you do it after the first firing fires off the cork?

  • Hadar Jacobson Says:

    Hi Kim,

    I don’t put carbon in hollow forms with small holes or cork clay. I just don’t fire too many of them in one batch, and if I have the slightest suspicion that they have not fully sintered, I repeat the second phase. With cork clay, I hold up to one hour at the first phase, letting it all burn.

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