White Gold casting alloy

I am very keen to find a good alloy for 18crt white gold. My name is
Alan Goldberg, and I am based in Melbourne, Australia. I have a
casting problem with which I am desperately seeking some help. We are
currently casting using a Mario Di Maio alloy that is nickle based (
but meets the Euro regulations Re: nickle). We are having moderate
success and would like to improve the quality. we use the alloy for
all objects, But mainly rings and settings. Rings can range from 2
to 12 gr ams. The main objective is to have an alloy that is cost
friendly, soft enough for stone setting and ring sizing and also is
as white as possible. ( Palladium based alloys are too expensive in
this case.) The problem so far is that at times it casts absolutely
porous-free and at other times (without changing anything), it looks
like craters on the moon!! Even when it is absolutely blemish free,
it is as hard as rock and very very very brittle. One wack with a
mallet and it can crack into 2, and all this using a Nuetec Jzp???
And also with a normal vaccuum assist, torch melted system,
performing fantastically with all other metals. We have also tried
using the manganese alloy from Precious Metal West, But this does
not perform well with the Nuetec Machine. HELP. If anybody out there
has anything they think could be a possible alternative for me,
please let me thank you all in advance

Alan Goldberg