Sizing:  When preparing canvas, the linen must first be washed and then coated with a size.  This is done before applying a primer. 

Sizing the linen creates a barrier between the oil and the canvas, keeping the acids emitted from the oils from eating away at and rotting the linen.  Size is a glue barrier that prohibits the ground and paint from contact with and penetration into the linen canvas.  Rabbit skin glue is a classic sizing that bonds the fibers to each other.  They expand and contract together and react to environmental changes as one unit rather than each thread acting independently.  When different areas of the painting react separately to the environment, cracking of the paint film and other similar problems are likely to occur.

Further information: Preparation of Canvas Grounds

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