While experimenting with firing in more efficient ways I discovered something that can shorten the firing time significantly. It seems that the first phase – in which the binder is expended – does not have to be done in a kiln and can be completed in as little as 10 minutes, with no cooling time between phases.
This can be done with the SpeedFire Cone system, on a camping stove, or on the kitchen stove.
Here is how it works:
Pour a 1″ layer of carbon into a small stainless steel bowl. Place your pieces on top of it without covering them in carbon.
Put the bowl on the burner.
Cover the bowl. You can use the cone from the cone system, or the circular fiber box that is actually a firing box, or a fiber board, or a piece of fiber blanket. If you use the cone, cover it with a fiber board or a fiber blanket.
In each of these lids drill a pencil-size hole. To do this in a fiber board or the fiber box you can use a screwdriver. Just twist it in; the fiber is very soft. With the fiber blanket just stick in a pencil to make the hole.
Here are some photos of the set-up:
This photo was taken inside my studio, but this should be done outdoors.
On the left is the fiber box; on the right is the fiber cone. The pans are flat, but I mostly use simple bowls.
If you do this in the kitchen, the fiber blanket is not recommended. You can protect your stove with aluminum foil.
Turn the flame on to full capacity. After a few minutes you will smell and see the smoke coming out of the holes in the lids. This is the binder burning. I tried to photograph it, but it turns out that smoke and fire are hard to shoot.
After about 10 minutes the smoke will stop. Remove the lid with a glove and peek inside. The pieces should look black.
Pour more carbon into the bowl to cover the pieces and put them in the kiln for the second phase of firing. The whole process takes about 2:30 from beginning to end.
I have just fired a batch of thick pieces. If I had done the first phase in the kiln, I would have had to hold for at least 2 hours. Given that the ramping takes about 30 minutes and the cooling time at least on hour, I saved myself 3:30 hours.
Why not burn the binder with a torch?
I have tried this many times. The pieces were positioned the same way, in a bowl with a 1″ layer of carbon. This works only sometimes. In most cases pieces crack from the direct heat, and the cracks do not necessarily show until after the second phase is over.
Why not do the same thing in a kiln?
I have tried this as well. A kiln takes a long time to ramp. This exposes pieces to oxygen for an extended length of time and most probably causes internal oxidation. That means that when they come out of the kiln after the second phase, they may crumble, but not necessarily because they are not sintered: it’s because they are oxidized.
Why is the fiber cone or box necessary?
Without them, it would take a long time for the pieces to heat up and they might oxidize, just as in a kiln. The fiber cone and box create a hotter chamber, partially shielded from oxygen.
A word about internal oxidation: before the advent of base metal clay, I used copper pipes, rods, and wire (solid, or bare, copper, from plumbing supply stores). I used to wrap them with silver clay (low shrinkage) and fire. Cracks would occur because of the shrinkage of the silver, and I had to repair and re-fire. Nothing in the way that the copper looked showed any sign of oxidation. However, after a few repairs, the copper would crumble in my hands.
This is one of the reasons that I avoid firing copper in open air. Even if it looks fine after firing, you never know what is going on inside. Furthermore, sometimes the copper needs to be re-fired, as in repair, enameling, soldering, or adding silver clay. With each firing the copper oxidizes further and weakens, just like what happened with the solid copper.