This must be compatible with the body so that during the firing they shrink together and the slip-body bond is not weakened or compromised. Slips have fired adherence properties that make them either loosely or tightly bonded to the underlying body. Slips have their own coefficient of thermal expansion and act independently of the body. If sufficient incompatibility exists, sudden heating or cooling will overcome the slip-body bond and the slip will flake or shrink off. If a glaze is applied over a slip, the slip acts as a buffer layer and it must be expansion compatible with both the underlying body and the overlying glaze.
Individual slips have a characteristic dry hardness.
Slips develop characteristic fired surface qualities. They can be smooth, silky, glassy, textured, dry, stippled, etc. The above properties can be divided between physical and fired considerations. Achieving the desired physical properties is the easiest. If you need a slip for a particular body, the first step is to determine its purpose, then propose a simple recipe.