First of all.....what the heck do I know? Maybe there's a simple mechanical/chemical condition that turns the gold hot stamp to silver (that seems to be the consensus direction.) Also I never knew that hot stamping was so complicated.
But....the only gold and silver hot stamp pair in my collection are the same Sands chips that Barry originally mentioned. I must have well over a thousand hot stamped chips in my collection. The vast majority are gold. Why don't I have lots of chips that have transitioned to silver? Just saying.