How Do You Restore Something With No Instructions? Read the Scratches

Anything fixed to a surface leaves evidence of having been there. Screw holes record the size, spacing and orientation of what was fastened. A component rubbing against another wears a mark whose shape and position record the geometry of the movement. Unexposed areas beneath a part retain original finish and colour while everything around them has faded, so a removed part leaves a clean silhouette. Tool marks record how something was made and with what. Corrosion, polish, staining and dirt all accumulate differently where a part was and was not, and each difference records something about what has gone.

The evidence answers specific questions rather than general ones:

• •The exact size and position of a missing component
• •Which of several candidate parts actually fitted there
• •Whether a surviving part is original or a later replacement
• •The direction and extent of movement in a mechanism
• •The order in which repairs and alterations were made
• •Whether a claimed history is consistent with the object

Mechanical clocks and watches are where the technique is most developed, for reasons specific to the objects. They are complex, they were rarely documented, they were repaired and altered repeatedly over centuries, and every moving part leaves a record of its motion on the parts it touched. A wheel leaves a polished arc where a pivot ran, a lever leaves a mark at each limit of its travel, and a removed component leaves clean holes and unworn metal. From those a restorer can establish where a missing part sat, how far it moved and what it engaged with. The test of the reading is simple and unforgiving, since a reconstructed mechanism either runs or does not.

Reading such evidence destroys it in many cases, since dismantling, cleaning or refinishing removes exactly the traces being read, which imposes a discipline on how the work is done. Photography before anything is touched is the minimum, with raking light across a surface at a shallow angle to throw shallow marks into relief, which reveals traces invisible under ordinary lighting. Rubbings, casts and measured drawings record what a photograph cannot. Three-dimensional scanning captures surface geometry to a fraction of a millimetre. And a conservator records the sequence of what was removed and in what order, because the object's own history of repair is itself part of what the traces show.

The same reasoning runs through several unrelated fields. Accident investigators read scuff marks, paint transfer and deformation to reconstruct how vehicles met. Forensic examiners match tool marks to the tool that made them and striations on a bullet to a barrel. Building archaeologists read the scars of removed roofs, floors and staircases in the fabric of a wall to reconstruct vanished storeys. Furniture conservators distinguish original construction from later repair by the marks of the tools used. In every case the principle is identical, which is that removing something never removes the evidence that it was there.

A removed part leaves screw holes, wear patterns, unfaded silhouettes and tool marks that record its size, position and movement, which is frequently enough to reconstruct it without any drawing. Rebuilt mechanisms that then run correctly are the strongest confirmation such a reading can get. Accident investigation, forensic tool mark comparison and building archaeology all apply the same principle.