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.