I've tried something different lately and will continue to see if it's consistent, but I seem to get cleaner castings when I hold the oven at 300 degrees for a couple of hours to dry out the investment and then ramp as fast as I can up to 1400 degrees and hold for two hours then down to casting temp. I'm bypassing the 700 degree hold.
I've observed that the models at around 700 degrees or so turn dark, hard and crack but if you heat them up really fast, they seem to liquefy better. If they liquefy and absorb some into the porous investment rather than turn hard, crack and possibly expand, that may eliminate the point at which some mold breakdown occurs. (Not scientific but I've watched models in the oven not invested to see how they react at different temps).
Two weeks ago I cast a 17 dwt. ladies ring with intricate design, followed the above burnout and got absolutely zero mold breakdown. I was blown away by the result. with that burnout schedule, I get no pink discoloration at the sprue opening and no bits of ash there either. I still use an air compressor to blow into the sprue opening to clear any lose debris, but my new burnout cycle seems to really work.
invest with plasticast, 36% water ratio to weight of investment (very thick)
vacuum and let sit for minimum of 2 hours (large flasks for 3 hours)
sit in oven at 300 degrees for minimum of 2 hours (depending on size of flask)
then ramp up as fast as possible to 1,400 degrees and sit for 2 hours
(consistencies that I feel are a must)
Completely cured models
Completely dried investment before ramping up past 300 degrees.
(Sometimes I'll even sit my flasks with bases still on into a 150 degree oven for a few hours to begin the water evaporation process. It always depends on how much time I have.)
Remember, I UV cure the model in water for a couple of hours and then in a 300 degree oven for a couple of hours after I have removed supports and sanded any surfaces. (It seems to be very important to run another two hour oven cure cycle after doing any support removal or sanding of model surface for some reason)
And in case anyone is wondering...I do not introduce any extra air into my oven by way of air pump. I have three holes drilled into the door of my Neycraft oven and that's all.
So, try it and see what you think. I'd like to know if any of you get the same results.