Loading the Firing Container

It is recommended that the clay is placed on a 1" thick bed of activated carbon, and then covered over with additional carbon, filling the container to the top. We don't find it necessary to fill the container to the top with carbon, and believe packing the container with too much carbon may lead to firing failures. We recommend leaving at least 1/2" of air space between the level of the carbon and the top of the pan. Pieces can be located at the bottom of the pan, but there must be at least 1/2 of carbon between the bottom of the container and the BRONZclay. Pieces fired at the bottom often come out with a solid pale yellow, gold or green patina. This may have to do with either the temperature or the carbon dioxide that pools at the bottom of the container.

Situate the pieces in the carbon to work with gravity. The carbon makes a good firing support for many items, but some pieces can deform or slump from the downward force of gravity during firing. Counteract gravity by placing domed items face down in the carbon. Position hollow shapes vertically in the pan. Look at the piece and imagine gravity pressing down on it. Situate the piece in the pan in such a way that it has the most strength against gravity.

If you have just a few pieces, push the pieces into the carbon bed. For delicate pieces, Dig a little hole and place the piece in gently, making sure it is fully supported from below before covering it over with carbon.

Multiple pieces can be fired at once. For larger loads, it's easiest to pack the container in layers and pour carbon over each layer so you know exactly where each piece is and you don't accidentally allow pieces to touch and fuse together.

Pieces that are touching during firing may fuse together. If you do not want pieces to fuse, do not allow them to touch during firing. In firing pieces that are interlocking, such as chain links, small pieces of fiber paper can be used to separate the parts during firing.

The position of your clay pieces in the firing pan can make a difference in the outcome of your work. If you have a front-loading kiln, there are no heating elements in the door. That means the front of the kiln is going to be cooler than the sides and the back. In firing experiments, identical pieces were placed inside a firing pan and their positions noted. A piece positioned at the cooler front of the kiln did not shrink as much as one located in the hotter back of the kiln. Both pieces were very strong and could not be bent by hand, but they did not sinter equally due to the cooler temperature in the front of the kiln. Keep this in mind if you need all your pieces to sinter equally and locate them strategically. In a front loading kiln, there are no elements in the door, the floor or the roof. A top-loading kiln has heating elements all the way around the chamber. If you are using a top-loading kiln, you can distribute your pieces evenly in the firing pan, noting that your cool spots will be at the bottom of the pan.

Activated carbon particles can get into nooks and crannies and inhibit shrinkage or leave marks. A small bit of fiber blanket can be used to plug an area that you do not want filled with carbon, such as a bail.

Witness Strips

Make and keep handy small strips of dried bronze clay about 1/4" wide by 3" long and in typical thicknesses used in your work. Include a witness strip in the same thickness as the thickest piece in your BRONZclay firing. The witness strip will serve as your "done-ness indicator".

After firing, try to break the fired witness strip with using two pairs of pliers for pieces 5mm thick and less, or for thicker pieces, hammer it and see if you can break it. If the BRONZclay is fully sintered, it will distort but not break. If it breaks easily, something went wrong. The problem can be determined by what the broken piece looks like. Be sure to view our troubleshooting information noted below to determine the problem and find the solution.

Also note that you can write on BRONZclay and the writing actually survives the firing. Pencil, ink pen, india ink, and felt pen all survive the firing. The writing can be polished off, but for making notes for firing, it's good to know that you can write directly on the unfired clay and it will not burn off. This is also a great technique for identifying who's is who's when firing for a classroom.

Placement in the Kiln

The firing pan should be centered in the kiln as much as possible. Do not allow the thermocouple to touch the firing pan as it can cause incorrect temperature readings and result in over or under-firing. It's best to elevate the firing pan on kiln posts, even if they are only 1/2" tall. This allows the heated air to circulate around the container during firing and provides a more even temperature to the container.

In a brick kiln, there are single elements in a groove several inches apart. Boost your firing pan up on kiln posts to have the center of it aligned with one of these elements if you have a brick kiln.

In a ceramic fiber (muffle) kiln, the elements are embedded in the wall and run back and forth on the sides. In an SC2, for example we set the firing pan, whether it's tall or short, on 1" kiln posts, and center it left and right, pulled as much to the front as possible.

Gemstones & Inclusions

Many natural, and lab created gemstones and most cubic zirconia can be embedded directly in BRONZclay and fired in place. Check out our Gemstones in Metal Clay firing guide to see which stones can be fired in BRONZclay. Testing has shown that any stone that can be fired in silver clay can survive a BRONZclay firing for longer times and at higher temperatures than in silver clay.

Tanzanite cubic zirconia is a heat sensitive stone. It's so sensitive in fact that it can only be heated to 1110°F for a maximum of 10 minutes before it starts to darken. We fired this stone for 9 hours in 2 separate firings for a total of 18 hours in the kiln at 1550°F in activated carbon. After each firing the color was as brilliant as an unfired control sample. So, it's oxygen that causes the problem in heating this type of stone, not the actual heat itself.

We have also tested several other heat sensitive stones and they have also survived better in the carbon than in an open air firing. Diamonds are a stone that do not survive in an open shelf silver clay firing, but can survive when buried in activated carbon.

Natural gemstones must be louped to be sure they are safe to fire. If you can see little cracks or spots in a stone with your naked eye, the stone probably is not safe to fire. The pressure imposed on an embedded gemstone stone by sintering metal (which shrinks about 25%), can be enough to shatter an already fractured stone. Examine each gem with a 10-power jewelers' loupe. If you see cracks or little particles inside the stone, firing it may be a gamble. At the very least, leave some space around the stone for the clay to shrink so there is not so much pressure on it.

Most glass cannot be co-fired with BRONZclay. The firings are too long and too hot, however according to one glass artist borosilicate glass can be fired in place. I have not tested this claim.

Sterling silver and fine silver cannot be co-fired with the BRONZclay. The metals will attempt to alloy (mix) with each other in an unattractive way.

Precious metal clay cannot be co-fired with BRONZclay. These two clays can be combined, but not during the sintering phase of BRONZclay.

Copper, Brass and Bronze can be embedded in the clay and co-fired.

Shrinkage

Overall shrinkage is probably about 25%, but that's not 25% in every direction necessarily. Strangely enough, closed circles don't seem to shrink much. I formed a ring at size 9-1/2. After firing the ring still measured 9-1/2. Another ring shrank from an 8 to a 7-1/2. The same thing happened with 2 cuff bracelets which did not shrink in overall width, but substantially in length and thickness. Other items shrank from 16% to 20% in various dimensions. In general, thinner pieces shrink more in length and width than thick pieces.

Where an item is located in the firing container can have an impact on the total shrinkage of the item. Items located near heating elements shrink more than those in other parts because they get hotter sooner than those in the center of the container. Activated carbon is a poor conductor of heat, so pieces in the inside of the container will reach temperature about 30 minutes later than those near the outside edges.

Refiring BRONZclay

BRONZclay can be re-fired. I have fired some pieces several times by accident because I lost them in the carbon and found them only when I sifted the carbon to clean out the dust. Once an item has been fired, additional firings do not harm the metal.

Attaching Fired BRONZclay

If you have fired pieces that you'd like to combine, they can be pasted together and fired. First clean the items to be attached very well. Give them a rough satin finish in the areas to be attached. Make lavender paste to "glue" the sintered bronze pieces together. Make a very thick paste, but instead of mixing it with lavender water use straight lavender essential oil. Mix the paste to the consistency of soft peanut butter (a tiny bit softer than normal paste). Use a dropper as a dipper to pick up the oil a drop at at time. Cut cocktails straws work or a pipette work well for picking up a drop of oil or water. Add a drop at a time to the clay and mix with a palette knife. Attach parts with the paste and dry at 90°F for 24 hours. Fire at the 3-Hour schedule. If you add unfired components to pre-fire bronze, scratch brush the bronze and attach the component with lavender oil paste. Allow to dry at 90°F for 24 hours. Use the firing schedule for the unfired components thickness.

Kiln Maintenance

Whether you have a brick or muffle kiln, be prepared to step up maintenance when you fire BRONZclay. Oxides will build up on the outside of your firing pan. These oxides will flake off and accumulate at the bottom of the kiln. Vacuum the inside of your kiln regularly to keep it clean. Wipe the thermocouple with a damp rag or sponge whenever you vacuum to keep your kiln in top shape. A thermocouple will not read as accurately as it ages (an older thermocouple will fire hotter than it reads), so test fire your kiln every 75 firings using cones to verify the accuracy of the firings.

BRONZclay Firing - Troubleshooting

BRONZclay brand clay is fired within a narrow range of temperatures. Going too far below or over the correct temperature can result in a firing failure. The photos below show how to tell if you need to raise or lower your target temperature to correct the problem.

Signs of Overheating & Underfiring

This sample shows the surface of what appears to be a successful firing. The surface is smooth. There are no pits, cracks, dents or bubbles.



This is the back of the same piece. The back shows dents where it laid on the carbon. We were able to eliminate the dents by lowering the target temperature by 15 degrees.



Here is a piece showing bubbles that are caused by overheating. You can see a color difference in the area that is overheated. It's a crescent moon shape on the right. You can see where the metal began to bubble.



Here is the back of the same piece. You can better see how the metal bubbled up, then cooled. When you see this type of bubbling, it is an indication that the kiln is too hot. To alleviate this problem, lower your target temperature by 50°F.



Here is another example of over-heating. This one shows signs of melting. The pieces has lost a lot of it's detail. Look at about the 3 o'clock position. You'll see where a bubble erupted, broke through and then collapsed.



The is the back of the sample above. The back of this piece should have been smooth. Instead, it shows signs of melting. We suggest lowering the target temperature by 100°F if you see this type of over-heating.



This is a close up of the surface of a test ring. The surface has a very grainy appearance. It's attractive, but still technically over-heated. To eliminate this type of surface texture, lowering the target temperature by 15°F.



This piece is under-fired. It rings like metal when tapped lightly with a hammer...but it didn't pass the hammer test. When broken open, the insides are metallic, but very brittle. This piece was not fired long enough. Since thickness determines the rate of heat for firing BRONZclay, be sure to measure your piece at it's thickest point to determine the correct heat rate.



This sample not fired long enough for it's thickness. It had no ring to it when tapped with a metal object. The piece felt solid, but with light hammering broke the piece to reveal the un-sintered center. The insides are powder, not sintered like the sample above. Again, this is an example of a piece that was not fired long enough. It's very important to measure BRONZclay items at their thickest point to determine the rate of heat for your particular kiln model.

This sample was dug from the red hot carbon and quenched in water. It was then hammered to test it's strength. The thermal shock of quenching has made this piece brittle, but it's a different brittle than a piece that has been under-fired. Thermal shock brittleness gives a"sticky" quality to the metal. You can definitely feel it when hammering. It has more of a dull thudding sound when it should be ringing.



Brittleness from cooling too slowly or from cooling too quickly is revealed by a grainy break.



This sample was fired inside a fiber blanket and then buried in activated carbon. The fiber blanket was sucked tight against the piece as the oxygen was sucked away by the carbon. The binder in the clay has carbonized. Pieces can also be carbonized if they are heated at too low a temperature or if not enough oxygen is present during heating. They can simply be re-fired and will sinter beautifully.

This piece was fired to 2000°F by accident. This closeup photo shows the dendrite formations that occurred on the surface. The sample started out as a rod.



What is this? A golden pearl? How strange. All around the ball, the metal was molten and even tried to alloy the carbon into it. The surface of the piece glitters with gold, silver and green, which does not show up in the photos.