Silver Casting bubbles

Dear Friends,

I am facing some problem with Silver casting from last few days. When
I am heating the casted piece it is melting at very low temperature.
It is melting before my soft solder melts. Some times bubbles are
also coming on the top surface while spruing. For casting we are
using 92.5 % Pure Silver & 7.5 % SF928CH alloy ( 72 % Copper & 28 %
Zinc). Silve Casting temperature - 1000 deg C. We are casting in inert
atmoshphere (Nitrogen Gas). Quenching the casted flask in 10 mins in
water. If any one has observed such thing then feedback will be

Ketan Shah.