Porcelain and Pressing Furnaces

All of the technical representatives agree on one thing: 20% to 40% of all the calls they receive relating to problems with porcelain products can be directly traced to the operation of the porcelain furnace. A porcelain furnace performs only one significant function—processing raw dental porcelains to optimal levels of maturity while maintaining important features of the restoration, such as surface texture, translucency, value, hue, and chroma. In the hands of a master ceramist, even a two- or three-powder build-up is a thing of beauty; yet, many ceramists are never able to achieve the esthetics and vitality these ceramic materials inherently possess. Most often this is a result of furnace calibration.

Calibration can mean different things to different technicians. Now many furnaces include a highly technical auto-calibration feature (Figure 1 and Figure 2). But many technicians can remember the days when furnace calibration systems were quite primitive, utilizing various types of pellets and silver strip/rods. Calibration back then relied on the user to "read" the outcome and make judgments on how to adjust the temperature. Experienced ceramists often calibrate a furnace by evaluating the parameters of translucency and texture of the ceramic by firing a sample of the porcelain system’s clearest powder. An inexpensive way to make samples of clear porcelain to test the firing cycle is to form discs of ceramic using a lubricated metal washer from the hardware store (Figure 3 and Figure 4). Then the ceramist looks for things like sharply defined, unrounded edges, and maximum clarity (Figure 5), and tweaks the firing cycles accordingly to get the desired results. What many do not realize is that porcelain matures as a function of heat (time and temperature), not just end temperature.

"If a technician doesn’t have the experience to immediately evaluate porcelain maturity and understand the complex nature of its firing, then the automatic calibration features on most modern porcelain furnaces are amazingly useful," says Mark Nelson, the manager of professional relations and scientific affairs for 3M ESPE.

While the visual indicator approach has served technicians well for many years, it relies completely on the subjective judgment of the operator. "Although final verification using visual indicators is still important, the automatic calibration features of most state-of-the-art furnaces represent an important advancement in furnace technology that should be capitalized on," explains Jeff Smith, CDT, a technical representative for Ivoclar Vivadent Inc. "With most of the current furnace technologies available, the calibration system produces a much more accurate firing result." Most modern computerized furnaces go through a self-diagnostic process to check the quality of the electronics, muffle, and vacuum so the user can be sure the furnace is operating at optimum levels.

"Current VITA furnaces auto-calibrate every time you turn on the furnace, which gives the customer confidence that the equipment being used is of high quality," says Jim McGuire, the technical sales consulting and support director for Vident Inc. "However, when a difference in the porcelain is noticed, it is always advisable to double-check the calibration using a tab of clear or window-type porcelain to help determine the firing temperature." He says it is best to use a layered coping, but if unavailable, a simple tab can be substituted. He warns that calibration also involves using the proper vacuum, and a gradual slip in vacuum pressure can also cause issues in the final outcome of the restoration. "A low-cost method to confirm the vacuum on older furnaces is to use a vacuum gauge which can be purchased at a hardware store," McGuire suggests.

How often ceramists should calibrate their porcelain furnaces is a more complex question. According to Smith, they need to consider many factors, including the furnace/muffle age and frequency of use. "With a relatively new, well-maintained furnace that is operating at average production levels, calibrating every 6 months should be sufficient," he says. As the furnace/muffle age and usage requirements increase, more frequent calibration may be necessary. McGuire suggests calibrating any time a sudden change in appearance to the porcelain shows up.

"The calibration routine—be it with silver, or an independent thermocouple, or some automatic feature—will get you in the ballpark, and maybe help you get your oven to a ‘standard’ state, but that will not necessarily ensure that the porcelain will be fired properly when programmed to the chart," cautions John Slanski of the Research & Development Department at Jensen Dental. "The firing cycles are still likely to need fine-tuning to achieve the proper first-bake maturity."

Most furnace manufacturers use a calibration system based on only the melting point of silver as the known reference temperature, according to Smith. "While this calibration strategy is more than adequate for high-fusing ceramics, which fire in this same relative temperature range, it may not result in sufficient accuracy for lower-fusing ceramic materials," he says. The dual-stage calibration feature on Ivoclar’s Programat furnaces addresses this issue, he explains, by using a patented calibration method that verifies accuracy at both 660ºC and 960ºC, covering all current ceramic-firing ranges.

To ensure that each ceramic sample fired has the same exposure to the heating element of the muffle, it is often recommended to always place the ceramic restorations in a specific location in the firing chamber of the furnace. Some furnaces have muffles designed to achieve a more homogenous temperature profile, eliminating the hot or cold spots common in other muffle configurations. Whether the restoration is elevated on a firing pin or resting on a firing pillow, the same level of maturity can be expected with these muffle designs.

Pressing ceramics used to be difficult. The older air pressure-driven presses would commonly split rings open, and ceramics never pressed out with the same value as the shade guides indicated. To ensure the value of the pressings was not too low, technicians would continually lower the hold/press temperature of the oven until an incomplete pressing was produced.

Pressing time is often the most overlooked variable in the quality of pressed ceramics. "This factor is, in fact, probably the most important single cause of pressing problems," Smith says. Most press furnaces, he explains, require the user to define the press time by making an educated guess on the length of time it will take for the ceramic to completely press into the investment mold. If the press time is too short, short margins will result; if it is too long, then there will be changes in color and translucency or altered physical properties. In the case of lithium disilicate, press times that are too long will cause an excessive reaction layer. Some press furnaces use an automated process to stop the press program at the proper point to ensure a complete pressing without the adverse effects of excessive press time.

Cleaning and purging is critical for those who use the same furnace for processing metal-ceramic alloys that contain silver and for firing all-ceramic restorations. Most modern porcelains designed for veneering alloys are formulated with anti-greening properties. Occasionally, a catastrophic discoloration in the porcelain occurs when the furnace muffle or fire brick becomes contaminated with silver oxides. This is a significant problem that can affect all-ceramic materials, which are not typically formulated to resist greening. While it is always best to have a separate furnace and firing trays for alloy versus all-ceramic cases, this is not always economically possible. The best advice is to purge the furnace and replace the firing trays on a regular basis. One very economical way to purge the muffle is to run a firing cycle up to 1000°C and hold it for 15 minutes without vacuum, according to McGuire.

Long-term cooling is another important factor for processing metal-ceramic restorations to optimize the compatibility of the veneering porcelain with alloys that have high coefficients of thermal expansion. What is relatively new is the impact of long-term cooling on specific all-ceramic materials. For example, failure to follow a slow cooling process will have a detrimental effect on the long-term durability of a restoration made from IPS e.max CAD. It is a CAD/CAM-processed, all-ceramic material that is milled in an intermediate crystalline phase (lithium metasilicate) to optimize its milling properties. After milling, the restoration is subjected to a crystallization firing in a conventional porcelain furnace to convert it into a lithium disilicate glass-ceramic. The long-term cooling stage is an integral part of this two-stage firing process because it ensures that a tension-free stress state is achieved.

Zirconia-supported restorations also present ceramists with some unique challenges. Unlike alloy substructures that conduct heat, a zirconia substructure acts as an insulator during the firing and cooling processes. To compensate for this significant difference in thermal properties, most manufacturers of zirconia-veneering materials recommend a slow cooling protocol to ensure tension-free cooling of these types of restorations.

Finally, when using a modern ceramic furnace, ceramists should check periodically for firmware and software updates. Manufacturers recommend checking the companies’ websites to access and download all the current versions of furnace software.

A ceramist’s craft is rather simple. There are only so many ways to hold a porcelain brush and only so many different burrs to create a particular contour or surface texture on a restoration. But ceramists can run into difficulty when they forget that substructures should be designed according to the manufacturer’s recommendations.

Modern ceramics are designed to be highly compatible with these substructures and are very user-friendly in their handling. A modern porcelain furnace is a technological marvel designed to make processing these ceramics foolproof. If a laboratory is producing ceramics that do not resemble photos of master-level work, it may be time to consider calibrating the furnace before changing technique or adjusting the end temperature of the firing cycle.

Brad Patrick, BSc, currently operates Patrick Dental Studio in Bend, Oregon and lectures nationally and internationally on esthetics and ceramic techniques.