Dental Lots: Customer Sampling Aids
The toughest material to sample is dental material, especially if the customer is mixing different dental alloys.
Why: Dental alloys vary widely in the metals contained and many of these metals are considered deleterious because of their wide range of melting points and their resistance to forming a homogeneous mix needed for properly sampling.
What metals cause problems when melted together in a dental melt?
Gallium – low melting point
Tin – low melting point
Indium – low melting point
Chromium – Extremely high melting point.
Nickel – Extremely high melting point.
Platinum and Palladium – high melting point.
Why can’t you take a pin sample of a dental melt? Because of the large difference in melting points of the metals involved (see group above), the pin will solidify with the low melting metals concentrated at the ends and the higher melting point metals solidify towards the middle. The assay for Au, Ag, Pt and Pd will vary greatly dependent on which part of the pin you sample. In fact, the most common complaint I get concerns the sampling of dental lots where the customer has taken a pin. They usually cut off the ends of the pins and take a sample from the middle and they come with very high gold and palladium assays.
When we receive the melt, we add copper to dilute the effect of the different melting points of the metals involved (average out the melting point of the entire lot) and then pour into grain and sample the grain. Our grain samples are very homogeneous. We usually agree with the customer on the silver and platinum but not on the gold and palladium.
Why can’t you take a drill sample of a dental melt? For actually the same reasons. When you pour the bar, the different metals solidify at different rates as they cool and the bars contain high and low concentrations of the precious metals. When you look at a bar and it contains a lot of different colors, you can be assured that the bar is not homogenous. A homogenous bar will be very similar in color and texture throughout the bar because you have something that is closer to an alloy. The best example of an alloy that you can readily see is your wedding or some other ring that has the same color and texture throughout.
So, where ever the bar is drilled, it will give a different assay than another drill sample on the bar. Hence, when we get a dental scrap bar, we add copper and take a grain sample.
So, what can the customer do so that he does not over or under pay his dental customers?
Here is how you advise the customer to sample his dental material:
1. Pull out all ceramics prior to melting (using a hammer to break the ceramics if needed).
2. Advise him to add 25% copper to the melt.
3. Have him pour the entire lot into cold water onto a flat board just at the point where the board and top of the water level meet. (pour quickly and steadily)
4. The material should form “grain”.
5. Take several of the cleaner (no slag) and flatter grains. Pound flat and run on xrf (several grains).
6. If the results on the xrf are close, dry the grain and ship to Metalor.
7. If the results show much variation (basis the weight of the lot), add an additional 25% copper and repeat the process until the grains shows only a slight variation on the xrf
