The same applies to the historically unpredictable metals market. Market prices for precious metals used in dental alloys (e.g., gold, platinum, palladium, and silver) have experienced unsteady fluctuations over recent short-term reviews and promise to continue in a shaky economy. A long term scan over pricing history shows a steady climb upward.

While this indicates that it may take an iron stomach for those who invest in precious metals, dental lab owners purchasing alloys to produce metal-based restorations on a regular basis need to steel themselves to metal market fluctuations that will undoubtedly affect what they ultimately pay to their alloy suppliers.

Lee states that the biggest influence over the selling price of gold is not determined by retail or industrial demand (as it would be with other commodities), but by trading and speculation. “It really ought to be treated as money versus a commodity because commodities are meant to be consumed, like oil.” He points that an estimated 160,000 tons of gold are in existence “above ground” with an added annual production from mining of approximately 2,500 tons. Of that, annual jewelry usage accounts for about 3,500 tons and the combined dental/medical and electronic industries and various other applications altogether account for about 300 tons. “The dental component doesn’t factor into gold prices.”

However, the reciprocal is not equal; gold prices definitely factor into dentistry, and will continue to play a role in to creation of metal-based restorations.

“Globally, if you look the world over, the metal-ceramic restoration is going to continue to be used by a large number of technicians and clinicians,” said Dr. W. Patrick Naylor, Associate Dean for Advanced Education and Professor of Restorative Dentistry at Loma Linda University School of Dentistry. “It’s still the major restoration of choice. And there are people worldwide who will be consumers of the metal-ceramic restoration for years to come.”

The results from the latest DLP Alloy Survey¹ of U.S. dental lab owners bear out Dr. Naylor’s world view. The survey shows nearly all lab owners (95.2%) use alloys in their laboratories, which actually is up slightly from the 92.5% indicated in the corresponding 2005 survey.² In addition, according to the April 2008 Dental Products Report Indirect Restorations Survey of U.S. general practitioners,³ PFM crowns remain the most commonly placed indirect restoration, at 92.3%, followed closely by PFM bridges in the number-two spot, at 88.5%. Coming in at number four, all-gold crowns (65.4%) were topped only slightly by metal-free porcelain veneers (67.3%).

Noble (precious) and non-noble (non-precious) metal alloys are and will continue to be a staple in laboratory-fabricated restorations for everything from single-unit all-gold anatomical crowns to porcelain-fused-to-metal or press-to-metal crowns and bridges, with non-precious base alloys also in demand for fixed and removable restorations. As a result, it’s wise to diligently look at ways to economize use of metals through improved handling techniques, while searching for alternatives to expensive high-precious-metal-content alloys.

Among the basic techniques he instills on his enlightened students is the importance of weighing sprue bases, runner bars, and all wax patterns on a digital scale to 0.1-gram accuracy prior to investing to determine the amount of wax involved (See "Casting coach"). That amount is multiplied by the density, or specific gravity, of the alloy to determine the amount of metal needed to cast the restoration and the sprues. Using only the minimal amount of alloy needed for casting minimizes material wasteage.

“What you want to do is use just enough metal to cast what you want to reproduce,” said Dr. Naylor. “You’re employing a more economical strategy here because you’re using only the amount of alloy your calculation indicates that you actually need.”

In the second edition of his textbook, Introduction to Metal-Ceramic Technology scheduled for publication later this year from Quintessence Publishing (www.quintpub.com), Dr. Naylor updates the chapter on the fundamentals of spruing, investing, and casting. His book also includes an appendix with a wax-alloy conversion chart to help translate the volume of wax to be cast with the number of ingots needed for a frugal casting.

“There’s not just an art to this, there’s a science to it as well. The idea is to achieve success on a regular basis and minimize remakes due to poor-quality casting,” he said.

Part of that science, said Dr. Naylor, involves focusing on the thermodynamics of the casting process. “Every casting has shrinkage, or what is referred to as ‘casting shrinkage.’ And where that shrinkage occurs, you’re going to have porosity,” he explained. “The real challenge for the technician is to control the location of the porosity in the casting itself. Such control can be achieved if the largest mass of metal solidifies last because that’s where the casting shrinkage (porosity) will reside. So the technician can control these thermodynamics by making certain the largest mass of metal is someplace other than in the actual restoration.”

Dr. Naylor advises using what he calls a “buttonless” casting technique, where “the sprue former network is designed so a connector bar serves as a reservoir of molten metal, permitting the restoration to solidify first, followed by the runner bar.” This is called indirect spruing. When the buttonless casting technique is used correctly, no button is cast, and you can actually see the porosity in the underside of the runner bar, the side opposite the patterns. A large ball can be used in direct spruing as long as the ball also solidifies last.

“Casting a button with either indirect or direct spruing can be counterproductive for several reasons,” added Dr. Naylor. “First, you wind up expending more metal than you actually need; that’s not cost-effective. Second, a button typically will compete to be the largest center of mass and the heat center of the entire casting. In many instances, the button supports the reservoir or connector bar, and you cast a very dense bar, but you might not get the best-quality reproduction of your patterns.”

Henson, who often refers to Dr. Naylor’s original textbook in his lectures, adds that when faced with valuable casting buttons, technicians should use them for subsequent casting as long as at least 50% fresh alloy is added to the mix. “The more fresh gold you can add to a casting, the higher the quality of the casting,” he said. “Which, in turn, could result in fewer mistakes.” He adds the idea behind the fresh alloy is to replenish any trace elements of the alloy that may be burned out in the melting process.

Dr. Naylor says one of the benefits of indirect spruing and the buttonless casting technique is the cast sprue network can be easily sectioned and reused. Labs shouldn’t accumulate a large collection of buttons. He adds that the chapter of his book devoted to spruing, investing, and casting includes a description of the “Laws of Casting,” where he covers many of these topics.

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