Horizontal sliding sashes allow the operator to reach around both sides of the sash while using the sash as a shield. Because the sash opening is smaller, they conserve energy by limiting the volume of air exhausted. Safety glass is the most common and economical choice for sash material. Polycarbonate sashes are recommended when hydrofluoric (HF) acid is used since this material does not fog or etch when exposed to HF fumes