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!!