I still have a warming tray from the late sixties. It works fine for melting my chocolate wafers or almond bark. I haven't had a problem with the chocolate or bark thickening. But I've been told that adding a little cooking oil or parrafin chips made especially for coating will thin the chocolate. When using a double boiler, be cautious of not getting a splash of water droplets into the chocolate. That will seize it up immediately.
When I'm done dipping, I drop in rice cereal or cheerios into the leftover melted chocolate, mix it up well, and drop it my spoonfuls on waxed paper. The kids love the little chocolate bites.