Only glass (high quality clear) or acrylic sheeting should be used as glazing material. Acrylic sheeting should not be used on pastel or charcoal drawings. Under no circumstances should the artwork be allowed to touch the back of the glazing material. As there is little possibility of any moisture or condensation forming on the inside of acrylic sheeting, an acrylic 'sandwich' frame is reasonably safe. However, only 100% ragboard should come into contact with artwork of value, never the glazing material. There must be sufficient clearance between the framed piece and the back of the glazing material to allow for the possible expansion of the work of art without it touching the glazing material.