MMA/GMA?

I suspect that the problem may be the benzoyl peroxide (BPO) which is at 25% wetted. Actually you do not even need all those other chemicals that you are using. We use Aldrich MMA with no problems but we are also adding 3 grams of 70% wetted BPO (Aldrich) to every 100 ml of our embedding mixture. You could try making the embedding solution real simple with 90ml MMA/10ml GMA and the BPO. If you embed using layers for the larger containers the bubbling should be reduced to zero. I believe there is not enough BPO to complete the polymerization chain. The reaction goes from the liquid stage to a honey consistancy, then rubbery and finally to a hard block.