Kilns and Firing     

We  sell a programmable kiln that maintains a selected temperature for a set length of time, then shuts off automatically. Though not essential for success with Precious Metal Clay or Art Clay, this unit is designed specifically for silver clay and makes the firing process as foolproof as the modeling.

When Precious Metal Clay or Art Clay is fired, the water and binder vaporize and disappear completely. Because these together make up about 30% of the original Precious Metal Clay or Art Clay, the object after firing is reduced to about 70% of its original size. This offers exciting possibilities because the shrinkage is proportionate. All the images, textures and details you create in the clay original will be retained in the final version--0nly smaller! This is a little like using the reduction button on a photocopier and nearly as easy. PMC+ has a higher metal con-tent. Fired PMC+ pieces are 88% of the original size.

As the following chart describes, shrinkage, density and hardness all increase with time and temperature. That is to say, if you "short fire" Precious Metal Clay or Art Clay you will make it weaker. The longer the firing process the denser the metal, achieving maximum at the durations shown below. Firing at temperatures above these risks melting the material-it's real metal and will melt just like conventional metal when heated to its melting point.

Pieces may be fired any time after they are made, though pieces with widely varying cross sections should be dried slowly to prevent cracking. Flat pieces are laid directly on a kiln shelf, fire brick or clean soldering pad and set directly into the kiln. Work can be very close together as long as the pieces don't touch each other. Contoured or hollow pieces require a little more care to ensure that they don't slump or collapse during the process.

Support these forms on vermiculite, plaster or alumina hydrate powder (A12O3 .H2O), available from us. It can be reused almost indefinitely so a couple of pounds should be enough for most studios.

These diagrams show a few typical situations and

Jewelers use a variety of torches, all of which deliver more heat than is needed to fire PMC. People with experience in metalwork and a good idea for heat colors will be able to fire PMC+ with any traditional torch. If a person is buying a torch specifically to fire PMC+, a butane torch like the Blazer makes a good choice. These compact torches are relatively inexpensive ($35-60) and are available from craft suppliers and jewelry supply companies. In fact I even saw one at a kitchen supply company where it was sold to caramelize crème brûlée. The torches are sold empty and filled (and refilled as needed) from a can of butane that can be bought from the same suppliers, grocery stores or tobacconists. Note that the standard torch is rated at 2000° F so it can melt PMC. Rio Grande sells a modified unit that heats only to 1650° F. Another interesting torch uses a disposable cigarette lighter as the source of fuel. These clever torches are inexpensive but are limited to only very small items.

Torch Firing Process

1. Allow the work to dry overnight or drive off moisture with a hairdryer, coffee warmer or in a slow oven. Torch firing is not recommended for large items.
2. Place the work on a soldering block or fire brick, which is in turn set on either a fireproof surface or something that can be singed (like a piece of plywood). If you are working on the kitchen counter and the piece rolls off the block you don't want to scar the countertop.
3. Light the torch and hold it so the flame is nearly vertical with the tip of the cone about 3/4" away from the work. Within a minute the piece will be enveloped in a soft flame as the binder burns away. The flame will soon go out by itself. Within another minute the piece will start to glow red. Continue heating until this becomes a bright and luminous color. At this point, glance at a clock.
4. Hold this color as uniformly as possible for at least 5 minutes (10 minutes will produce a denser result) glancing away periodically to relieve your eyes. When the time is up, turn off the torch and allow the piece to cool at least until the red color is gone, at which point it can be quenched.

Educate Yourself
Here's a test I recommend to anyone who intends to fire with a torch. It takes a few minutes and requires the investment of a dollar or two in PMC, but it will go a long way toward understanding the process and building confidence. Pull off a peanut-sized bit of PMC+, split it in half, and roll out two small rods. Follow the instructions above with one added step. After the complete cycle, move the rods apart on the brick so you can focus on one of them. Concentrate the flame on this rod in an effort to melt it, which might be possible even with the regulated torch. You'll see a bright mercury-like skin form on the piece and the red color will become even brighter. The edges will start to curl and the PMC will be drawn up into a ball.

Make a mental note of what you saw and this way you'll know the signs of melting so you can withdraw the torch in time before damaging a piece you care about. To complete the experiment, allow the other rod to cool and test it by bending, filing, burnishing and polishing. This will confirm that, sure enough, torch firing really works!