Helps prevent thermal shock and therefore cracking. Use in the bottom of clay chimeneas to prevent the hottest part of the fire from touching the clay. Suitable for any clay chimeneas. One bag is enough for one chimenea. Can be used time and time again. Does not burn. Easy to use. Just place the contents of one bag directly on the clay in the bottom of the chimenea and build your fire on top. Lava stones are pumice formed by the froth of molten rock in volcanoes. Lightweight with excellent insulation properties, it is widely used in construction and similar applications. The lighter the lava, the better the grade quality. So don't be fooled by the weight. The best quality by volume.