Thanks everyone - i do appreciate all your brainstorming on this. As with any session like this - let me act as the foil to your suggestions and give feedback to add as we go through this.
Now in technicolor...
Ok - lets review - I'm a jerk
(quality is never a popularity contest) when it comes to metal design and i have been disliked by nearly every metal finisher that has had to have their work approved before i let it in the porcelain room - and the number of units now after 35+ years number 100,000+ ... literally not figuratively.
That said - i know what makes for proper framework design. So lets get that off the table... besides, I personally cast and finish these units. So I know its done correctly. Add to this my noble alloy accounts for 50% of the work we do and the high noble alloy accounts for about 40% of the rest of it. No such problems exist with the high noble alloy - though the frame design is essentially the same and the finishing is done the same as well. If it were affecting one alloy, it is logical to think it would affect the other as well - all things being equal in every other variable.
Alloy in question is Argelite 75+6 with CTE between 14.3 and 14.5.
(high noble alloy is Argedent 45 - with CTE between 14.1 & 14.3)
The Noritake EX-3 CTE is compatible between 13. 4 - 14.5.
Using a hotplate to dry opaque is a nice idea that is fundamentally flawed. The idea is that the heat from a furnace from the outside creates a shell of dry or vitrified opaque or porcelain and then when the coping heats up inside the furnace the moisture boils out causing heat tears and bubbles. Problem is - a hot plate does essentially the same thing from the bottom up. (If you've ever worked with a hot plate - like i did for over two years - you know its 6 of one and half dozen of the other and makes no significant difference in the long run. For a while we even used a Howmedica device that you put the tray in at one end and a metal conveyor belt moved it ever closer to a low side heat source till it got to the other end when a beeper went off and you could move the tray to a real furnace - a lovely piece of junk that some uninformed salesperson sold to an equally uninformed denture set-up-man-lab-owner who opened a C&B dept.)
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Now new wrinkles since last we talked that change the developing picture.
A second dentist has had one crack that was sent back late last week. Same alloy and this one still with the last batch of Ceramco opaque. His preps are totally different than Doc A, and I've worked with him for over 25 years and never had this problem with his preps or my finishing/design.
Not only that, a second crown of his cracked today before it ever left the lab - this one using Noritake opaque.
Back story - the returned crown was fired to see if the crack went away. It was a small under 2mm crack running from the margin up into the incisal from the distal lingual margin (indicating a CTE issue). After firing, the crack was gone... until I steamed it after polishing it. Then there were then three cracks.
The second crown did not have any cracks after polish or after sandblasting the inside - but one showed up after steaming it.
Its our practice to steam the waxy polish material out of or off of the crowns prior to final check out. None of the other crowns from today's batch were affected.
So - logically, 1) the steam is introducing the cracks - but only as a means of uncovering an underlying problem elsewhere. Should be able to steam off a crown after all since its not getting hot enough to expand the metal. 2) its not the brand of opaque, as it happens with both Ceramco and Noritake, 3) the alloy is the only consistent piece of the puzzle and it seems there is a CTE issue going on.
Now what possible factors can contribute to a CTE issue?
I disagree that the button doesn't tell you when the alloy is overcooked. It does 99% of the time. Safely within any margin of error.
The burnout temp is accurate at 1500 for both alloys.
The torch is set the same and alloy melted in the reducing zone for both alloys.
After finishing is complete, copings are put into ultrasonic distilled water for about 10 minutes, then placed on a paper towel and dried.
Placed on a firing tray they are oxidized at 1200 to 1850, held for 5 minutes in air. They are then sandblasted to remove excess oxide and again ultrasonic in dist. water, dried and opaqued.
Opaque is dried below a Whip Mix Pro 100 furnace at 900ºƒ and entry time is 8 minutes.
Then allowed to cool and porcelain is built, cut in and glazed in standard fashion.
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Is it possible the torch tip is getting old and shooting bronze oxidation into the alloy that would affect one alloy and not the other?
Is it possible the crucible is affecting the alloy? (though both alloys crucibles were changed - new - recently)
Is it possible the air compressor and air dryer are introducing contaminates during sandblast prior to oxidation that cannot be seen after oxidation?
question here
