This may be old hat for you, but for me it was a better idea moment. Sometimes when casting I realized after creating the RTV mold that I needed an additional vent hole. The problem is that when I tried to drill the hole the drill bit would drill through the RTV but would not create a hole. It would just split the RTV. When the bit was removed the RTV would close back together as if the hole never existed. So today I put the mold in the freezer for about three hours then took the mold out and then drilled the RTV to create a vent hole... and to my surprise and satisfaction I actually drilled a vent hole in the RTV mold. This now gives me the ability to correct vent hole mistakes and or omissions after the mold has been created!!
Below is the the hole drilled after pulling out of the freezer.