OptiKast is a coarse-grained kaolin used primarily in ceramic casting applications. It has a very fast cast and exceptional deflocculation characteristics. Thus OptiKast is preferred in pressure casting systems.
Velvacast kaolin is no longer available, this is the recommended substitute. One concern with Velvacast was its dark fired color (compared to other common plastic kaolins). Opticast has higher iron and TiO2 contents, this could impact translucency in porcelains, for example. Thus you may need to fire tests and adjust other materials in the recipe if needed. We have done fired shrinkage, porosity, dry performance and plasticity comparison tests between these two materials Oct 2011 and they do appear very similar, OptiKast is a little more plastic and does not appear to fire darker at cone 8 and 10.