You know, you're right. I was going off the demonstration of partial Transfiguration:

And a few minutes later, Harry was strapped into the safety chair and resting his wand against a metal ball - one that, based on his current test scores, should have been too large for him to Transfigure in less than thirty minutes.

And a few minutes after that, Minerva was leaning against the wall, feeling faint.

There was a small patch of glass on the ball where Harry's wand had rested.

Which seemed to me to indicate that the glass didn't appear until a few minutes after he started the transfiguration. And the bit where Harry cuts through Azkaban's interior wall seems to make somewhat less sense if it's gradual- the motor oil would just drip out, for instance.

But the fact that Goyle could recognize the cauldrons before the transfiguration was done pretty conclusively rules the idea out. Oops.