Porcelain Furnaces in the Modern Dental Laboratory
What is the “perfect” oven for your needs?
George (Yatz) G. Gundrum, BA, CDT
In the last 60-plus years, the dental laboratory has undergone many expected changes, not least of which is the advancement of porcelain furnace technology. 2100°F air-fired porcelain gave way to 1800°F vacuum-fired porcelain; and reliable vacuum-fired porcelain ovens became highly sought after. The first generation featured furnaces that were horizontal and manual, with front-loading muffles (heating chambers). Restorations had to be inched into the muffle over several-minute intervals, gradually reducing the moisture from the crushed feldspar. With the work finally centered in the heating chamber, a high-heat rubber gasket was placed around the opening, then the hinged steel door was closed and secured with the locking bar and thumb screw, ensuring a sealed heating chamber so a vacuum could be drawn. The pyrometer was then manually set and the technician had to monitor the firing cycle, making sure the work was removed after the vacuum was broken and the hold time confirmed. The vacuum pumps had to be manually turned off and on as dictated by the firing cycle; they were noisy and belt-and-wheel driven. "Runaway" pyrometers were blamed for many ruined restorations, turning high-gold alloy into shriveled-looking, melted, and collapsed copings.
In the mid to late 1960s, laboratory equipment manufacturers started to make game-changing advancements. The advancements seemed to coincide with NASA's efforts to put a man on the moon. Vertical muffles were soon introduced. They too were high-heat clay but had a full circle of heating coils. These early furnaces were still cranked into the heat chamber by hand. Improvements of quartz muffles would prove to lengthen muffle life and provide a more even heat with fewer dead spots in the firing chamber. The circuit board was improving, and the footprint of the furnaces became smaller. Vacuum pumps became shoebox size and muffles were being made of space shuttle tile-like material. Programmable furnaces were introduced to the market. They featured only 10 programs, but the furnace was automated. Technicians did not have to attend to the firing station constantly, making the laboratory more versatile and profitable.
Soon after, there was another style of furnace to choose from—the clamshell. Instead of the work rising up into the muffle chamber, the muffle chamber lowered itself down onto the platform. The big advantage here was that the work didn't vibrate off the platform on to the desk, the technician's lap, and then the floor.
As materials, procedures, and equipment evolve, some basic principles still must be addressed. In creating esthetics, the technician's ability to monitor the optical and functional properties is critical. Today's furnaces are instrumental in helping control both of these functions. In ceramics, proper firing cycles and temperature are the most important part of creating optical properties that will blend into the intraoral environment. Overfired ceramics create too much internal glass and allow too much light transmission, while underfired ceramics are reflective and look dead esthetically. With the advancement of sintered materials, the accuracy of furnace temperatures is often the difference in creating translucency and stopping crack propagation.
Are 200 Programs Necessary?
Nowadays, every new furnace seems to have more programs. It's not uncommon for a furnace to offer 200 programs. An older, established laboratory probably needs this kind of furnace more than a laboratory just starting out. An older laboratory often needs to support many older materials. Low-firing materials need their set of programs, as do all the different higher-fusing porcelains. Every porcelain-to-metal alloy has its own degas and opaque interface parameters. Basically, each porcelain the technician might use has approximately 15 different programs that may be used to successfully complete a case. Every brand and type of alloy has its own degas temperatures and hold times, some with vacuum and some without. It is much easier and more efficient to have a furnace preprogrammed for all of those variables. The technician can just make sure s/he selects the correct program and hits the right button. Of course, the drawback is that the more programs a porcelain furnace has, the more expensive it is.
Manual vs. Digital Furnace Settings
Deciding whether to get a furnace with manual/analog settings versus digital settings really depends on who will operate the furnace. Less technologically savvy technicians will be more comfortable with setting programs manually, rather than needing to learn and adapt to digital settings and touchscreens. This comfort level is especially important when needing to change settings on the fly. However, if the technician using the furnace is comfortable with digital technology in general—such as setting up their smartphone and/or programming their HVAC system-then that is preferable.
Is One Furnace Enough?
There are a few variables to consider when deciding how many furnaces a laboratory might need. Long cooling cycles can be a factor in deciding whether to have more than one furnace per ceramist. When adhering to long cooling times for porcelain-to-zirconia restorations, a single furnace is limited to 16 firings a day. With multilayered build-ups, there is less material to apply but more frequent firing repetitions. The oven is doing the work, resulting in a better looking restoration. This gets the technician out of the seemingly endless loop of adding on and grinding off centric and interproximal contacts as well as incisal edges and corners.
There are other considerations as well. The amount of bench space a laboratory has will dictate which size of porcelain furnace to purchase. Clamshell ovens traditionally have a bigger footprint than the pedestal type. Having fewer programs usually corresponds to a lower price. The ability to flood the firing chamber with an inert gas like argon might be worth considering as well.
A pressing oven is most commonly built on a top-of-the-line furnace with a plunger mechanism assembled to the top of the heating chamber. These furnaces can also double as a regular porcelain furnace when the pressing unit isn't activated. Consideration might be given to making the pressing furnace a ceramic-only unit, so purging the unit of contaminants, such as chlorine gas residue, is not necessary. This can be a problem when firing porcelain to high-silver-based alloys as well as pressing to metal.
What About Sintering Zirconia?
Porcelain furnaces cannot be used for the sintering of zirconia at the present time. The sintering of zirconia is a completely different process needing prolonged high-temperature (1550°C) firing cycles of at least 8 hours, with a long cool-down period of 2 hours. These temperatures and hold times vary from manufacturer to manufacturer and product to product. In discussing sintering times and temperatures with other laboratories, the author has found real inconsistency, but porcelain furnaces as yet haven't marketed this dual purpose.
All Furnaces Require Good Maintenance
Calibrating the contemporary porcelain furnace is basically a thing of the past; they're seemingly self-calibrating. There are some calibration instruments available, but most ovens are close enough that a seasoned ceramist can tweak the manufacturer's suggested firing cycles to their own preferences. Personal variances in mixing liquids, application techniques, altitudes, preferred sheen, or texture are just some of the creative temperature adjustment results that no mechanical instrument can finalize.
Dedicated outlets for uninterrupted service are suggested. Like a computer, power surges and circuit overloads put the unit in an unadvised environment for peak performance.
Every dental laboratory makes heat and dust. Keeping the equipment in a clean, temperature-controlled air flow will prolong the life of the furnace. Putting it in the corner of the casting room and using the top of the furnace as a paper and magazine stand could have the opposite effect.
When to Purge
Cross-contamination in the muffle camber is the main reason for purging. If the porcelain firings are discoloring yellow or green around the margins, purging may fix the problem. Another tell sign may be cratering in the superficial overglaze; sometimes there is a small black speck in a shallow unglazed crater surrounded by this glaze. Purging with purge kits from a laboratory supplier should help the problem if it's not too severe.
A general purging formula is to heat from 1000°F to 1950°F under vacuum at a 50°F rate of climb with a 15-minute hold. Release the vacuum after the high temperature was reached and held for 5 minutes, leaving the last 10 minutes of hold time to be in air.
When Things Go Awry
Warranties and guarantees vary from company to company, but as great and consistent as these units are, they sometimes need to be sent back to the factory for repairs. Keep the box and the packing materials it was shipped in whenever possible. Any repair facility could tell you all kinds of horror stories of expensive damage done to equipment simply because it was incorrectly packaged—damage that, incidentally, is often not covered under the warranty. For best results, call the company and get a verification number so the furnace can be received and accepted on the receiving dock. Dealing with manufacturers or with repair facilities in the US is a priority. It is ideal to talk to someone in charge who is actually going to perform or oversee the servicing or repair of the equipment. When purchasing a new porcelain furnace it would be wise to verify that such a service exists. The internet is seemingly chock full of repair facilities with numbers no longer in service and home offices that are unable to connect to the repair facility.
Which One Should I Buy?
For porcelain furnaces, as with most laboratory equipment, there is no one-size-fits-all solution. There are so many considerations regarding design style, footprint/space, output demands, dependability, pre-installed programs, price, etc, it really is impossible for an outsider to know what will work for your business. There are suitable options for every kind of setup. Start with your laboratory's needs, and see where your research takes you.
About the Author
George (Yatz) G. Gundrum, BA, CDT, is the co-owner of Gundrum Dental Labs Inc. in Cincinnati, Ohio.