December 16, 2012
Charles Selser, CDT · Owner at Selser Dental Lab, Inc.
A possible contributing factor to bubbling is the firing temperature of your first opaque cycle relative to the oxidation cycle temperature. If you are oxidizing the alloy prior to opaquing, even if you then blast off the oxide layer before you apply opaque it is very important that your first opaque firing does not go to close to the oxidation temp. You want to stay at least 20 degrees Celsius below the oxidation temp. Your second opaque firing should be 10-20 degrees lower than your first opaque firing. Hope this helps.
Jeff Walker · Owner at London Bridge Dental Lab
It's most likely from contamination somewhere in the process of conditioning and opaquing, as I'm sure you realize. Here's what I do to avoid the same thing:
First, I use specific finishing/grinding tools for each metal and I don't cross contaminate. When using stones, I only use aluminum oxide.
Second, I fiinsh my metal in one direction, never crossing over perpendicular to previous grindings , or reversing direction. This can fold metal over and cause bubbling later on.
When I'm finished conditioning, I hold the coping or bridge with hemostats and never touch the coping with anything...See more again until at least after my second opaque layer. This is critical to avoid getting any contaminants, like oil from your skin on the framework.
Next, I sandblast the coping well with 50 micron aluminum oxide at 60 psi.
Next, I thoroughly clean the framework with high pressure steam. If it's a bridge framework, I place the framework in a clean, disposable cup of clean, distilled water for about 20 minutes in an ultrasonic.
After degassing, I sandblast at 50 psi for noble alloys and I don't for high noble alloys that have a light oxide layer.
I only use hemostats to hold the framework, remember!
I use paste opaque that is always kept covered to avoid getting dust particles in it, or if using powder opaque, only fresh opaque!
Then I pre-dry the framework (place on the firing peg and firing tray) on an electric hot plate. This dries the opaque from the inside out. It's most important for paste opaques.
If you follow all these steps consistently, you can eliminate your conditioning and opaquing procedure as the problem. If you continue to have these problems, you can assume that you have other problems such as dirt in your porcelain, or opaque, a bad, or contaminated muffle, or you are using incompatible metal/porcelain combination.
I hope this helps.
Trisha Schweinsberg, CDT · Owner/C&B at Custom Crowns Dental Lab
I have had this also, please let us know what type of metal and if it is a press to metal porcelain. I am just curious if it is the same I have used that I have seen this with. I am not saying it matters I am just curious.
Mike Andreas · Owner at Dominion C&B Lab
I agree with Jeff. It's likely contamination of the metal from finishing stones. We had problems when a stone used for removing oxides from implant cases before polishing was then mistakenly used to finish copings. If any of the metal from the machined gold interface gets into the stone, it can no longer be used for finishing.
Jay D. Gomer, CDT · Owner/Ceramist at Restorative Solutions, Inc.
If you are using base metal alloys frequently, you may want to look at using some type of furnace purging material. It will help to remove many contaminants that attach themselves to the muffle and thermocouple. You would want to do this weekly if using predominantly base metal alloy.
It is always best to follow the manufacturers recommendation for surface treatment of the alloys, although, all of these recommendations are great.
Michael Kudrna, CDT · Owner/President at New Vision Dental Creations, Inc.
May want to replace the Muffle even if you just had done this in the last few months. Also, we had this happen while we used rubber gloves while opaquing that were contaminated.