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Wow, now it makes one wonder how many Non-Precious (car bumper) metal crowns fractured due to the oxide space under the opaque layer.....scary
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From what I have seen of non precious pfm failures is the bond of the oxide layer fails from the metal. The piece of porcelain that breaks away has the oxide layer bonded to it and the alloy has no oxides. I try to use non prec for Maryland bridges only. Other than that I use nobel and high nobel alloys that fit into the porcelains CTE recomendations.