I have a small Victorian open fireplace. It has been used recently and just been swept. I see that the rear 'firebrick?' is loose and behind it there is no insulation between it and the brick wall. There is some (looks like shiny gravel) to the sides and I was told this is a particular heat insulator put there for when the fire is lit. Does anyone know what this material is called and am I right in thinking this should be in the cavity behind the firebrick and the side firebricks?