Is a face coat necessary for using R&R GLASS-CAST mold materials?
No, each of the R&R GLASS-CAST mold materials can reproduce the finest detail without a face coat. During the mold making process, plaster rich water flows into the tiniest crevasses even though coarser refractory particles cannot. This plaster rich water will transform into gypsum and set hard. In a sense, a face coat is naturally formed during this process. Rough castings can result if this plaster rich skin is somehow disturbed. A common mistake that leads to rough castings is excessive steam de-waxing, which erodes away this rich gypsum layer. Scrapping the inside of the mold also will remove the gypsum-rich layer, resulting in in rough areas on the casting. If using a face coat, the mold material may not bond consistently with this layer. This can cause inclusions in the casting and loss of detail in reproduction. Face coats often have different expansion characteristics than the rest of the mold materials as well. This results in cracking and shifting in the mold, leading to fins
