Porosity in Silver Casting

haloo,

can help me

i work in silver casting,i have problem that porosity in silver, i casting some big ring,some porosity defects were`present in all siver cast pieces,the defects in form of round cavities,are visible only after the first polish.

i use indutherm vacum casting 925 silver is melted in furnace at 1050 celcius. last time i used only new silver mix alloy. the perporated flasks have 500 celcius when casting, the last time i tried to a big ring with large sprue, so i dont think the spuering is the problem

can anyone help me

thanks