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.
Thanks!

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...[More]

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.

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...[More]

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,...[More]

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.

tell me what you do after metal work is finished dont listen to what kind of metal you are using or tip needs cleaning

Could you please let me know the alloy and the brand of this alloy? By the way when do you start to have this problem?