Understand Your Ovens

Optimizing a porcelain or sintering furnace requires education and attention to detail

Jed Archibald, CDT

Knowledge of ovens has almost become akin to a mystic art because of the declining number of classic ceramists. In the past, one of the first things a ceramist learned was how to look at restorations to verify that an oven was functioning correctly, and how to adjust temperatures if it was not. Ovens have progressed, but technicians have lapsed a bit in understanding how to use them optimally, even though it is more critical than ever because of the wide variety of materials we use. Many people think they can buy an oven, select a program, and put it on autopilot with no need for any human input or verification; that is untrue, because ovens can perform differently for a number of reasons, ranging from material selection to wear and tear over time to a building's electrical wiring.

What are some ways to optimize furnace performance?

Providing each of your ovens with its own designated circuit—not just its own outlet—can eliminate variables that often cause fluctuations. When other sources are drawing energy from the same circuit, the oven may be less effective. If designated circuits are not an option, have a veteran ceramist review the oven's performance occasionally and adjust it if necessary. Periodic calibration is also necessary for ovens that do not feature auto-calibration; for porcelain furnaces, this involves simply placing silver in the oven, raising the temperature to the melting point of silver, and adjusting if it melts before reaching that point or does not melt at all. Sintering furnaces, meanwhile, often require technicians to modify the recommended programs for thicker zirconia products than the manufacturers planned for; many manufacturers are in Europe or Japan, where complex restorations are less popular than in the US, so the programs are based on traditional copings.

What are some important new features of porcelain furnaces, and what would you like to see in the future?

One new feature that is not talked about enough is infrared technology that senses the dryness of the material for pre-dry purposes. This can make up for human error when assessing how much drying is necessary before firing, because that can vary for different restorations. Another feature is automatic pressing, which essentially utilizes an automatic sensor to identify the target temperature by having the plunger repeatedly contact the top of the ingot until it detects a certain viscosity. Once that viscosity is at the correct level, the machine will press and go. This eliminates a lot of overfiring for many pressing materials.

I would like to see a feature whereby ovens record their own performance; for example, if a certain temperature at a certain time was not quite reached, or if there was some unintended fluctuation. That would be particularly helpful for inexperienced users.

How similar are porcelain and sintering furnaces?

Sintering ovens are a bit of a different animal because they deal with such a different material and temperature range. Because of the higher temperatures, obtaining 100% accurate temperature readings is more difficult; most laboratories can only calibrate within 5° to 10°.

The other major challenge is that most sintering guidelines involve only time, which really does not tell you much. It is like asking someone how long they played golf and then guessing their score. Also, a zirconia manufacturer might say their material needs to be at a certain temperature for a certain amount of time, but that does not account for the differences between ovens. Some ovens are zero mass—with just the heating elements and the zirconia inside, and nothing else to absorb heat—and others have a large mass design, meaning the zirconia goes in ceramic coffins with beads, and all of that takes longer to heat up. A material that may require a 6-hour program in one oven could require a 10-hour program in another.

Key Takeaway

In addition to understanding what each machine can offer your laboratory, ask around for real-world reviews on service and support. Some manufacturers stop stocking parts for older machines after 5 years. We rely on ovens very heavily, so benefits such as loaner policies in the event of necessary repairs are extremely important. Ask around to learn who provides strong support.

About the Author

Jed Archibald, CDT, is the Owner of Archibald Esthetics and Archibald Digital in Provo, Utah.