Disclosed is a free flowing dry back-up material which comprises:from 67 to 96% by weight of fly-ash;from 2 to 15% by weight of a heat sensitive binder such as boric acid;from 2 to 7% by weight of a non-wetting agent such as calcium fluoride;from 0 to 10% by weight of a heat expandable material, viz. a material expandable as a function of the temperature such as vermiculite or graphite; andfrom 0 to 1% by weight of a dust suppressant such as kerosene.