This spacer can be one, two or even more thickness of foam board. Glazing of pastels presents additional challenges. Plexiglas or other acrylic sheeting should never be used over pastel works. These surfaces are charged with static electricity and can attract the powder from the artwork surface. This condition may lessen with time, but the particles pulled from the artwork could easily slip between the mat and the glazing and soil an otherwise perfect presentation.
Clear glass is the glazing material of choice by most professionals.
Non-glare glass can be used, but with much more than the thickness of single matting, there is serious loss of detail. During the closure of the art, backing, mat and glass, one should avoid the temptation to turn the package facedown. When the pastel is inverted, it will drop much of its loose powder onto the inside of the glass and cause a redo of the project or a soiled mat. The only way to close a pastel work is to work from the back side. Finish the closing process with a brad setter or hand-placed glazing points.