Clinkers form when you get your firebox temps up to around 2500Degrees F. What happens is the heat liquifies the metals and minerals in the wood and forms clumps of the metal. These metals are mixed with ash to make clinkers. Seems to be common when burning elm, but I have never burnt elm and I have found them in my stove, so I just guess the mineral content your wood was grown in has a big part to play in how much clinkers you end up with.