The Mystery of the Purple Fire

We've been looking at sheep with a view to stuffing one or two up chimneys.  This led to a review of which of the multitude of fireplaces still have an open chimney, which in turn led to looking again at the curious fireplace in the purple room.

My assumption is that this was part of some (water? air?) based secondary heating system.  We've not had floorboards up in this area of the house (yet) so there's not much in the way of related clues such as pipes or ducts.