might be your alloy. unless you are using one designed for 18kt casting it will often cause the problem you are describing. try United Precious Metals they have a very good 18kt casting alloy. they are listed in the 800 directory
To cast a large piece it is necessary to have a LARGER amount of
gold. In EVERY casting (yes every) the piece cast should be the
smallest thing about the whole system. What I mean by “system” is
the piece, the sprue(s), and the vent(s), all together as a unit. I
cast large items frequently and medium to small pieces by the
thousands. When casting a one-of-a-kind piece, your piece should be
about a third of the weight of the total cast “system”. The problem
comes when you want that heavy thing cast well without investing
(money for gold, not plaster) in the sprue system. You can discuss
alloy, flask temps, vac vs centrifugal, spruing, and how you killed
the chicken, but the fact remains you need adequate molten metal
for the piece to re-crystalize into a quality casting. J.A.