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.
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.
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!
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.