Hi folks
I cast a small tapered pyramid in Glasscast 50 with base dimension 13 X 13mm and top dimension 6 X 6mm, with a total height of approx 65mm. The mould was made from Polycraft GP-3481 condensation silicone. It was cured for 48 hours before use. The mould master pyramid was 3d printed using standard PLA resin, which was then sanded down to create a smooth surface.
I mixed resin and hardener using a precision digital scale with resolution 10mg and confirmed the scale calibration using a reference weight first. I mixed for several minutes to be quite sure of uniformity given the small size of the cast, but made approximately twice as much resin as I required to ensure that I would have a uniform mix for the cast.
The resin was left to cure at approximately 20 degrees for a week. At this point it turned out very nice and bubble-free and was left to cure in air for a further couple of days. However since that point it's never really fully hardened. It feels hard but will slowly deform under pressure. (i.e it will bend slowly if bent by hand). It also can't be properly polished up to the optical finish I require. 

The excess resin, which was mixed in a disposable plastic cup, has however hardened perfectly. This makes me wonder if I have an incompatibility between the resin and the moulding compound, which uses a tin-based catalyst. Should I use an additive silicon product instead, or should I have cast in two smaller pours?. Or should I wash out the mould e.g with isopropyl alcohol or something prior to use?.