Ceramic Shell

Once the model is set up with its gates and vents, it is surrounded with a material that will cover it smoothly when wet and withstand high temperatures when baked. In some places an ancient technique involving cow-dung and clay is still used; a very fine mixture is painted on to capture the detail, then coarser layers are added to build up a meld that can be handled, baked, and poured into. A 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 "fluidised 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 meld will hold together through the burnout and pouring.