underlay
A piece of paper pasted under woodcuts, stereotype plates, etc. in a form, to bring them up to the necessary level for printing.
underlay: paper built up to make plates print level
In letterpress printing, an underlay is a laminated backing of paper or cardboard glued beneath a printing plate, block, or cut to raise its surface to the correct height for impression. Woodcuts, stereotype plates, and engraved metal blocks rarely sit at exactly the same height as surrounding type, and even small variations cause uneven ink transfer. The underlay brings them up to the strike height required for uniform pressure across the entire form.
Building an underlay is a craft task performed by the pressman or makeready specialist. Layers of thin paper, usually newsprint or bond, are pasted down one at a time using starch paste or glue, with each layer slightly smaller than the one beneath it, creating a stepped buildup. The pressman tests the height with a footstick, a specialized brass gauge that measures the distance from the bed of the press to the surface of the printing element. When the plate reaches the required height, typically 0.918 inches in British practice or 0.934 inches in American usage, the underlay is complete. In some cases, selective underlay is applied only to areas that print too lightly, rather than building up the entire back surface.
Variants and materials
Underlays range from simple single sheets of newspaper for minor corrections to carefully constructed multi-layer assemblies for fine-art reproduction. Some printers used papier-mâché or thin metal shims, though paper remained standard. Stereoplate underlays were often reinforced with strips of linen tape to prevent warping. The choice of paper thickness and the number of layers depended on how much height had to be gained and how precisely the plate needed to be positioned.
Proper underlay was essential for clean printing in commercial work. Poor makeready, including improper underlay, resulted in ink marks where plates printed too hard and blank areas where they printed too light. The technique became less critical with the shift to offset lithography and photographic plate methods, which offered better registration and more forgiving impression control, but letterpress shops still relied on careful underlay work for quality output throughout the twentieth century.