overlay
A piece of paper pasted upon the tympan sheet to improve the impression by making it stronger at a particular place.
overlay: targeted impression reinforcement on the press
In letterpress and offset printing, an overlay is a small patch of paper or thin material applied to the tympan sheet (the resilient cushion under the platen or impression cylinder) to locally increase pressure where the image requires heavier ink transfer. The tympan itself distributes pressure evenly across the entire sheet; an overlay builds up that pressure at specific points, forcing the paper harder against the type or plate precisely where needed.
Overlays solve a real problem on the press: uneven type height, worn or shallow characters, image areas that print too light, or substrate that resists ink at certain points. Rather than adjusting overall impression pressure (which risks crushing fine detail elsewhere), the printer cuts a patch of glassine, onionskin, or similar thin stock and pastes it under the tympan directly behind the weak area. Different materials offer different thickness; multiple layers can be stacked for stronger correction. The patch is shaped to the problem area, not extending beyond, so unwanted impression build-up is confined.
Application and variations
Overlays are made and fitted for a specific job and printing setup. The printer examines a proof, marks the low-impression zones, and measures the deficit (typically 0.002 to 0.010 inches). The overlay is cut, positioned, and pasted down with rubber cement or paste. Once the tympan is reinstalled, the press is run at normal impression; the build-up from the overlay adds just enough extra pressure at that spot. They are temporary fixtures, removed or replaced between jobs or when the underlying issue is fixed.
The term 'overlay' is sometimes confused with process overlays in color separation work, but in tympan context it refers strictly to the physical pad build-up. In modern offset printing, overlays on the tympan are less common than they were in hand-fed letterpress work, but the principle persists in any press where impression adjustment is localized rather than global. Their use reflects the reality that no printing surface is perfectly uniform and no substrate perfectly responsive everywhere at once.