My new firing method seems to be improving as I continue to test it. Here are a few results from further testing, including thick pieces of copper that fired successfully.
Just to clarify, all testing has been done with Quick-fire clay.
The most important thing is: do not cover the carbon with the blanket! If you do, you’ll have to take the pieces out of the kiln while still hot and cool them in water to prevent oxidation. If you prefer to wait until the kiln is cool (as in overnight firing), don’t use the blanket or any other lid.
Here is an updated description of the process. This description replaces my previous posting. Eventually I will incorporate it in the instructon manuals.
1. Fold a fiber blanket into a “box” (detailed instructions in a future posting). The walls of the box should be away from the walls of the kiln chamber. If the walls refuse to stand upright, support them on the sides with posts.
2. Line the inside of the blanket “box” with a ½” layer of carbon.
Arrange your pieces on the carbon bed. As usual, avoid the center. Cover the pieces with another ½” layer of carbon.
Firing Schedule for bronze (even with very thick pieces), mixed pieces, and average sized pieces of copper
You can start the firing in either a cold or a hot kiln.
Top loader brick kiln: ramp at full speed to 1450°F/788°C
Front loader muffle kiln: ramp at full speed to 1530°F/832°C
(You may find that this temperature is too high for bronze. If it is, flat bronze pieces will warp somewhat. You can either hammer them down after firing or lower the temperature.)
Hold for 1:00 hour.
You can take the pieces out of the kiln while still hot or wait until they cool down. Overnight firing works fine. If you take them out hot, use heat-protective gloves.
After firing, most of the carbon will have turned into ash. Pick up the fiber “box” from both ends and lift it out of the kiln. All the ash stays contained in the box; not a grain of it is left in the kiln.
To retrieve your pieces you can pour the ash through a sieve while still hot, into a metal container. Discard the ash.
Here is how the pieces look when removed from the cool kiln:
The black color is not fire scale. Here is how they look after light buffing or sanding:
The pieces shown include bronze, 16 cards (2 craft sticks) thick; bronze, 6 cards thick; copper, 4 cards thick; and mixed, 6 cards thick. All are unbreakable.
Firing schedule for thick pieces of copper (works foo bronze as well)
If you fire only copper, you can always use the “Hot Firing” method:
No carbon is used. Ramp at full speed to 1690°F/920°C (top loader) or 1770°F/965°C (front loader). Hold for 30 minutes. Remove from the kiln while hot, and cool in water.
Instead of a ½” layer of carbon, cover the pieces with a 1” layer.
Top loader brick kiln: ramp at full speed to 1470°F/800°C
Front loader muffle kiln: ramp at full speed to 1550°F/843°C
Hold for 2:00 hours.
Important note: think of the pieces in the carbon in terms of food in the microwave. There is a fixed amount of energy divided among the pieces. The more pieces there are, or the bigger they are, the less energy each of them gets.
Therefore, there is no point in pushing the envelope. Since the firing is now so much shorter, if you are in a hurry (as in a classroom situation), you can fire a few pieces at a time, one firing after another, without cooling the kiln. (Not having to wait for the kiln to cool reduces the overall firing time even more.)
When firing bronze, increasing the amount of energy by increasing the temperature is not an option since the bronze will blister. When firing just copper, thin or thick, small or large, raising the temperature is an option, and it’s still being tested.
Firing Thick or Large Pieces
Again, the “Hot Firing” schedule can be used.
Cover the pieces with more carbon, as much as the box allows. Again, do not use a lid.
Ramp at full speed to
1000°F/538°C (top loader) or 1100°F/593°C (front loader)
Hold for 30 minutes to 1:00 hour (depending on the size)
Ramp at full speed to:
1470°F/800°C (top loader)
1550°/843°C (front loader)
Hold for 3:00 hours. (Time may be shorter, still testing.)
In this case, not all the carbon will have turned into ash and you may want to reuse what is left after vaccuuming.