I'm not sure how effective any chemical or domestic product solution might be as it will depend on how coarse or fine the stone surface is. Soot particles are very fine and will lodge in the 'pores'. In the past I've stuck to a good wire brush and lots of EG as a first cut and then tried with soapy water and a fine stiff brush but cannot claim that was very effective - what soot washes out soon finds another crevice in which to take up residence. :evil:
Thank you - think hubby nicked the wire brush for cleaning his chain saw chain - will invest in a new one tomorrow and try the old EG and maybe bicarbonate of soda - maybe