I pour thick tissues at about 1.2 ml of glop per square inch. If I do not have a fan (no heat) on the tissue during the drying stage, I risk mold forming on the surface of the tissue. I have had mold growing on a tissue that was just out of reach of the air being pushed by the fan.
Once mixed (no preservative), my glop can sit in a water bath at 110F for up to 10 hours -- no trouble there...yet. I have tried to refrigerate the glop (again, no preservative) but ended up with the classic science experiment.
I can imagine a workflow that a safe preservative would be useful...make a lot of glop, then store it in the fridge. As needed, remelt and pour. I have made up to 2.5 liters of glop at a time -- that is all I can pour in one session and have enough room for the tissues to dry.