Crawling of a cone 10R Ravenscrag iron crystal glaze. The added iron oxide flocculates the slurry raising the water content, increasing the drying shrinkage. To solve this problem you can calcine part of the Ravenscrag Slip, that reduces the shrinkage. Ravenscrag.com has information on how to do this.
A ceramic glaze fault that occurs during firing of the ware, islands of glaze form as it crawls, leaving bare patches of body.
Calcining is simply firing a ceramic material to create a powder of new physical properties. Often it is done to kill the plasticity or burn away the hydrates, carbonates, sulfates of a clay or refractory material.
GR10-L - Ravenscrag Iron Crystal
Plainsman Cone 10R Ravenscrag Slip based glaze. It can be found among others at http://ravenscrag.com.