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What is The process in which a material melts to form a crystalline phase and a liquid phase known as: and explain in detail?
When feldspar is heated at temperatures between 1150°C and 1530°C, it undergoes incongruent melting to form crystals of leucite in a liquid glass. Incongruent melting is the process by which one material melts to form a liquid plus a different crystalline material. 
This tendency of feldspar to form leucite during incongruent melting controls thermal expansion during the use of porcelains for metal bonding.
Reference: PHILLIPS’ SCIENCE OF DENTAL MATERIALS, 12th ed page no 430