Clotted cream used to be made like this in farmhouses. You would take the heavy cream from the milk, set it in large, shallow pans on top of asbestos (yes, this was the old days) mats on the back of the stove, where the slow heat would evaporate out the water, leaving you with this lovely orangey-gold crust with little cracks in it on top of a thickened cream the consistency of old schoolroom paste. Note - the cream does NOT sour during this slow heating process, it's yummy and incredibly thick.