A Aluminum Investment Casting modern variation on this method is called "ceramic shell"; instead of the cow-dung and clay, a silicaceous slurry is used to cover the model by dipping and/or pouring. Special dry aggregate is then applied to the wet pattern, either by hand or by using a "fluidized bed", which blows the dry particles around in a confined space, covering the wet areas until no more will adhere. The coated pattern is then left to dry a while and another layer of wet and dry material is applied. This is repeated, using coarser aggregate on the outermost layers, until a sufficient thickness has built up so the mold will hold together through the burn-out and pouring. lost wax casting, alloy steel casting