Grainy surface, often with black stains
Indicates that the mold and metal were overheated, causing a metal mold reaction resulting in oxides and silicates covering the mold
Causes of this defect are evident in the shell and metal portions of the process; to cure, R&R recommends taking the following actions:
Refractory choice for primary coats is not suitable for the amount of heat exposure during pouring or compatible with types of alloy cast (low grade refractories).
Use a high quality refractory for your primary coats, increase the zircon refractory percentage used for primary coats (100% zircon primes).
High metal and mold temperature at casting.
Improve temperature management during preheat and pouring, avoiding heat concentrations. Increase the rate at which the mold is cooled to reduce the likelihood of mold degradation.