if you were constantly stirring your plastic you will get them. Do not use any type of wood to stir with try not to stir is often and if you pour big batches. I suggest that you get a Prestel pot with a stir molder it keeps the plastic stir perfect you do not get any bubbles. But if you are only pouring of couple molds at a time I would stick to your Pyrex in the microwave just cut down on those stirring if you can.