Example 1: Dents on Carbon
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. I was able to eliminate the dents by lowering the target temperature by 15 degrees.
Example 2: Bubbles from Overheating
Here is a piece showing bubbles that are caused by overheating. Notice how the center of the bubbled area is breaking up a bit. Look at the 5 o'clock position in this photo to see where the metal bubbled up and erupted on the surface.
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 degrees.
Example 3: Melting from Overheating
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.
This is the back of the sample above. The back of this piece should have been smooth. Instead, it shows signs of melting. I suggest lowering the target temperature by 100F if you see this type of over-heating.
Example 4: Surface Texture due to Overheating
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 degrees.
Example 5: Under-Firing
This piece is under-fired. It rings like metal when tapped lightly with a hammer...but it didn't pass the whack 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 was 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 a light whack with a hammer 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.
Example 6: Brittleness due to Thermal Shock
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.
Brittleness from cooling too slowly or from cooling too quickly is revealed by a grainy break.
Example 7: Oxidation
This was an interesting sample. I fired it inside of a piece of fiber blanket. The fiber blanket was sucked tight against the piece after firing and you can see the sample blackened completely. It's totally brittle, but not from quenching or heating. This type of brittle is from oxidation in the kiln. If there is a pocket of air surrounding a piece, it can oxidize and the result is a very brittle piece. It's snaps like a cracker.
This type of brittle break is very clean and shows no graininess.
Example 8: Overheating
This piece was fired to 2000F by accident. This closeup photo shows the dendrite formations that occured on the surace. 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.
More of the surface and the dendrites.