You obviously have found the way to make it work. When you’re
looking at casting with another imbedded metal, think of it this
way: If I melted my casting metal in a crucible, would the metal
that I plan to cast around melt if I dropped it into the crucible?
If the metals involved are gold and silver, then the answer is both
yes and no. Sterling silver melts at around 1485F, flows around
1645F., and casts around 1800F. De-ox sterling, however, melts at
1650F, flows at 1725F., and casts between 1850 and 2100F. 18y yellow
melts around 1580F., flows at 1600F., and casts at around 1750F. 18k
Red melts higher, at 1625F. and flows around 1675. 18k palladium
white melts at 1895F. and flows at 1975F. 14k Pink melts at around
1700F., and flows at around 1775F.
You also know that the mass of the metal that you are adding to your
crucible melt will also affect the RATE of the melt. Tiny little
snips of metal will melt faster than a big ingot.
So, casting sterling imbedded in an 18k yellow casting is not a
problem, especially if it is a de-ox silver alloy. Casting tiny 14k
yellow wires imbedded in a heavy de-ox silver cast may not work as
well, but you might get away with a heavy 14y yellow object imbedded
in a light de-ox silver cast. Or maybe yellow puddles on an oxidized
silver surface are what you are hoping to achieve (nitric will etch
silver, but not the gold…;).
Of course, all these temperatures are not “absolute.” These are the
temperatures that Hoover and Strong lists for their alloys. Your
alloys may be different, and you may also prefer different casting
temperatures, based on your equipment, the size of the melt, etc.
Truth is … there are probably fewer metals that can’t be combined
this way than vice versa.
PS: You can also combine more that two metals … think about it.