I used a double boiler (two pots) to melt chocolate for truffles but half way into dipping the truffles in the chocolate, the chocolate hardened and became very clumpy.
How do I keep the melted chocolate from hardening?
There are two things to keep in mind while melting chocolate:
Keep a low uniform heat
I start off the melting process with low to medium heat. Once the chocolate fully melts, I reduce the heat to low and keep gently stirring all the while.
If you allow the chocolate to cool, it separates out into non-uniform areas of heat, and the cooler pockets start crystallizing. This causes lumps or spikes to form. However, if you go too high, the chocolate may burn.
Keep the chocolate absolutely dry
This also includes steam and condensation from the boiler. The water in the boiler should not splash over or steam into the chocolate. Also make sure that the dipped materials (truffles in your case) are totally dry.
Presence of water causes rapid cooling of parts of the chocolate, which again causes lumps to form. It also causes sugar to crystallize out of the fine chocolate mix. This is called "seizing", and such chocolate is very brittle and difficult to melt.