Firing in Layers Can Work

Revisiting my instruction manual, especially the part about arranging pieces in the firing box, I was wondering whether replacing the first phase with stove-top firing makes any difference to how pieces can be arranged in the second phase.

The first thing I tried was to take advantage of the extra space on the bottom of the kiln.

On the Bottom

What you see are small containers made out of stainless steel and silica with one thick piece inside each, 2 in the rectangular one. There is a very thin layer of carbon underneath them, and maybe 1/2″ above. All were pre-fired on a stove-top and then carefully moved to the smaller containers.

A 6″ bowl was placed on top of them. It contained pre-fired pieces arranged in one layer.

Bowl

I fired for 2:30 hours instead of 2, thinking it might take more time for the heat to spread to all the pieces. Every single piece sintered perfectly, including those on the bottom of the kiln, and in the bottom center.

Despite the success of this experiment, I suspected that the pieces on the bottom of the kiln might have sintered because they had a box of their own with very little carbon. So next I tried firing in layers.

I made 21 pieces of Rose Bronze, each 12 cards (4 mm) thick, with a diameter of an average ring – 7 circles, 7 squares, and 7 triangles.

I placed the 7 circles on a very thin layer of carbon on the bottom of my 6″ SS firing box (it’s a pet dish). One piece was in the center. I pre-fired them on a camping stove.

I added a 1″ (maybe less) layer of carbon and arranged the squares the same way. I pre-fired these as well.

I added a third layer of carbon and pre-fired the triangles.

After adding another layer of carbon I went on to phase 2. This time I fired for only two hours. Firing temperature was 1700°F, the appropriate temperature for Rose Bronze.

This morning I sanded each one of these pieces, front and back, with a coarse sanding band, 120 grit. There was not one unsintered spot in any of them!

Sanded 1

Sanded 2

The overall weight of the pieces after firing was 150 grams. That’s one and a half jars of powder. Also, all pieces shrank by 10%.

This all sounds great, but some reservations should be made. As you can see in the photo, my kiln is 6.5″ x 6.5″ x 4.5″. The heat distribution is better than in larger kilns. In larger kilns, the results on the bottom and in the center of the kiln may be different. This requires more experimentation.

It seems to me that most sintering issues are caused by poor binder burnout. When phase 1 of the firing process takes place in the kiln, in layers, pieces in the bottom have hardly any chance of loosing their binder because of lack of oxygen. Stove-top pre-firing takes care of that.

Please note: Torch-firing instead of stove-top firing is not recommended. The reasons are explained in my posting Shortening the Firing Time.

In a previous posting I said that my electric bill was cut by 40% since I started to fire on a stove top (not to mention the time saved). I wonder how much we can save by firing in layers.

I will continue to experiment. It would be helpful if you could tell me about your own experience with larger kilns. When I have more data, I’ll post an updated version of the instruction manual.


27 Responses to “Firing in Layers Can Work”

  • Susi Pedersen Says:

    Hi Hadar-
    Thanks for the update. I have a question. Are you firing the different layered pieces on the stove and then moving them into the ss pan or are you firing a layer on the stove adding the next layer and then firing both layers on the stove and so on?
    Thanks, Susi

  • Hadar Jaobson Says:

    Susi, The second. I am adding carbon to each pre-fired layer. So the bottom layer get to be fired 3 times, and the second twice. Since they are in carbon, it doesn’t really matter.

  • Cindy pope Says:

    Dear Hadar,
    Inquiring minds love this kind of info. I love that you are continually experimenting to enable those who use your clay a more efficient and effective experience. Thank you so much. I have been so inspired by the new flex clay and the seminar I took with you that I am running out of clay.

    Warmest Regards,

    Cindy Pope

  • Thom Carter Says:

    Thanks for the update Hadar. Can’t wait to try firing in layers. Exciting news!

  • Melody Pierson Says:

    Can you pre fire with an electric stove?

  • Hadar Jaobson Says:

    Melody,
    Yes, if it gets hot enough. People tell me that it works. I was also told that you can get a cheap camping stove at Tire Canada.

  • Pat Roach Says:

    Hi – Just wondering if as you add layers to burn off the binder the time required to burn off the binder increases with the amount of carbon and pieces in the bowl? Or does the burn off time remain about the same from layer to layer?
    Thanks for this great news.

    Pat

  • Myriam Davin Says:

    at wich temperature
    Thanks

  • Virginia Says:

    Thanks Hadar,
    I second Cindy Pope’s sentiments. You have inspired many of us.
    As far as first firings, I began using a coleman burner for the binder burnout but had trouble seeing my smoke. I started on my electric stove (glass top) and realized that the stovetop is more efficient and was quicker and less guess work as I could see everything that was going on.
    Thanks again,
    Virginia
    South Louisiana

  • Hadar Jacobson Says:

    Pat, I haven’t noticed time increase. Bear in mind that the bowl and first layer of carbon are already hot. Also, please note that I haven’t tried it yet with mixed metal pieces and in different kilns.

  • Hadar Jacobson Says:

    Myriam, I fired Rose Bronze at 1700F. Haven’t tried mixed metals yet.

  • Leona Smith Says:

    Hi Hadar. Have you (or anyone else) tried the first phase in a beehive?

    I haven’t tried my inherited beehive at all and am not sure how to go about it, but am thinking I would need to somehow have a carbon layer.

    I am also thinking /wondering that if the binder is burned out (which ever way) then firing in layers may well work.

    Thoughts?

  • Hadar Jacobson Says:

    Leona, I have not tried the beehive. It may work if you can set it up to a low temperature. The layer of carbon is not necessary since you will have to move the pieces to another vessel for the second phase. I do both phases in the same vessel, so I layer it with carbon for the pre-firing.

    From what I’ve tried so far, firing in layers will work if you are firing the clays at their optimal temperature. When you fire mixed metal pieces, it may not work, since some of the metals are underfired. This requires further experimentation.

  • Leona Smith Says:

    Thank you. I will let you know once I have a chance to experiment, probably next month.

  • Amy Atkinson Says:

    I did a stovetop firing last night in layers.First I fired several copper pieces, from 1/4″ square to the size of a standard house key, about 5 cards thick. I fired them on a camp stove, in a pet dish, over a thin layer of carbon. After I fired that batch, I added more carbon and a layer of several steel pieces, fired them on the stove, filled the dish with carbon and fired it at 1700 degrees for 2 hours. Everything appears sintered. I have a Paragon Caldera kiln, that is 8″x8″x 6.75″.

    What was interesting was how clean everything was! The copper, which often comes out black, looks as clean and orange as dried, unfurled clay. It will be very easy to clean. And yrs, I did check to see that it actually was FIRED clay and not UNFOLIRED

  • Amy Atkinson Says:

    That was supposed to say FIRED not UNFIRED. I’m typing on my phone….

  • Hadar Jacobson Says:

    Amy, These are good news. I tried in in a Paragon FireFly, which is not as deep as your Caldera. All was Rose Bronze, and all sintered nicely except for 3 pieces that were in the sandwich layer (I fired 3 layers). I must say, though, that I may have gone too far; it was 200 grams clay! On another note, one piece showed a color that I’ve never seen before: bright red. But bear in mind: it may not work when some of the clays are under-fired, for example when you fire copper and bronze at the bronze temperature.

  • Amy Atkinson Says:

    Yes, I fire copper and steel together, bronze and mixed clays together, and white bronze combinations in a separate firing. I’ve been firing students work, so I’ve had a lot of practice lately!

  • Steve and Aundrea Says:

    Can one takeaway from this blog that it is safe to place pieces anywhere in the firing pot including the center for single layer firing? We uses the smaller Firefly kiln and stove top phase one.

    We use an inch of carbon below and then an inch above for single layer firing. What do you suggest for two layer firing?

  • Hadar Jacobson Says:

    Steve and Aundrea, My experiment was with a smaller kiln and with high-fire clay (Rose Bronze), so it would be wrong to conclude that it is safe to do in a different type of kiln and lower-firing clay. I have repeated the experiment with a FireFly (again with high fire clay) and all sintered except for 3 pieces in the sandwich layer. In the same kiln I fired lower firing clay (copper/bronze mix), this time in 2 layers only. The bowl was only half full of carbon. It went well, which means that is is possible, but not that it will work every time. One experiment is not enough.

  • Steve and Aundrea Says:

    Hi All – we just fired in our Firefly (8″ x 8″ x 4.5″ top load) in a 5″ diameter ss bowl, 17 small pieces spread throughout the bowl, including the center, in one layer (1″ carbon below, 1″ above) copper, rose bronze and steel (old formulation)@1700 for 2hrs. All pieces appear to have fully sintered (still some sanding to do). We will be trying the white bronze spread throughout the bowl tomorrow.

    If this all works out it is the equivalent, for us, of a 2 for 1 firing. Very ecologically friendly. I wonder if we can apply for carbon trading credits? LOL

  • Steve and Aundrea Says:

    We fired white bronze yesterday in our Firefly with the pieces spread out over the entire firing pot including the middle. All sintered perfectly. The piece were pretty small so next we will try with larger pieces.

  • Jocelyne Robertson Says:

    Thanks so much ! I’ll try some rose bronze and keep you informed. Can’t wait to have you back in Quebec this summer.

    Those informations you gave are also good with a SC2 ?

  • Hadar Jacobson Says:

    Jocelyne,

    I have tried it only at high temperature (firing temperature of steel,copper, and Rose Bronze) and not in an SC2. I used a top loader brick kiln. I’d love to hear your results.

  • Allison Says:

    I’ve been working with the white bronze and love how it finishes. Now I’ve tried firing some really thick (stone rounded shape) and keep running into problems. Any advice with temp. and hold times that I should be doing differently due to the thick shape?

  • Hadar Jacobson Says:

    Allison, Try to hold longer than two hours. One thing to keep in mind: if White Bronze did not sinter the first time, it cannot be fixed by re-firing.

  • Rebecca Wood Says:

    Greetings to all,

    Comment to Steve and Aundrea. Just curious if your firefly is a manual or digital?

    Thanks. New to Hadar’s clays.

Leave a Reply