Oh the frustration....

This week I have been fusing/slumping the glass elements of the piece.  First one came out fine.  Second one, using a different mould, just a slight bit not worked, so I went back to the first mould.  And it did this:

(excuse the dodgy photograph).

Fine you might think, that bubble is cool!  Well, yeah it is, I love it, but unless I can reproduce that eight times, I can't use it.  And it won't be good for engraving on.

My son said "can't you do them all like that?" to which I replied "not a chance".

Seems I was wrong.  I put another two in last night, one new mould, the other one a used twice before.

And yup, I have more bubbles.  This is not good, and I am stumped as to what's happening.  Something on the glass that I haven't cleaned? Moisture on the glass where I have tried to clean?  Or, something is happening to the mould as it's being re-used?  The previously used mould has blown a bigger bubble than the first time mould.  But that could just be coincidence.

Not sure what is causing it, but I need to resolve it, and fast!