Pumice ? that will take a good hard sweeping i think im right in saying? quite interested in it tbh. exspensive for a long length tho.Have you looked at volcanic lining ,,it's a sock that drops down the flue ,sealed and then inflated ,,then they pour a sealer down around the sock ,in 24hrs it's set solid,deflate the sock and the flue is permanently fixed ,no loose bricks and you are left with a perfectly smooth flue.