This iron crystal glaze is Ravenscrag slip plus 10% iron oxide fired to cone 10R on a buff stoneware (Plainsman H550). Since Ravenscrag slip is a glaze-by-itself at cone 10, it is an ideal base from which to make a wide range of glazes. It has its own website at http://ravenscrag.com. It was originally formulated using Digitalfire Insight software. The project built on the merits of a specific silty clay that was noted to couple very good suspension and drying properties with a low firing temperature. The process involved calculating what minerals needed to be added to it to produce the chemistry of a middle-of-the road silky cone 10 glaze; the product was Ravenscrag Slip.
GR10-L - Ravenscrag Iron Crystal
Plainsman Cone 10R Ravenscrag Slip based glaze. It can be found among others at http://ravenscrag.com.
Ceramic glazes form crystals on cooling if the chemistry is right and the rate of cool is slow enough to permit molecular movement to the preferred orientation.