Occasionally a drawing’s attribution can be confirmed by examining the watermark on the paper. The traditional method of forming sheets of paper was to pass liquid pulp through a frame on which brass wires were stretched rather like a sieve. A motif, known as a watermark, was often added by sewing a design of brass wire onto the screen. Different paper mills adopted different designs, and so these watermarks can often tell us where and when the paper was made.

Historically there have been numerous processes used to capture a paper’s watermark. These range from simply tracing the image to the more recent use of X-rays.

At the National Galleries of Scotland, we have found that one of the more successful methods is beta radiography. This is a technique of radiographic photography under beta radiation, the source being Carbon-14. The weak and penetrating power of the rays emitted by Carbon-14 produce an image that captures not only the individual watermarks but also small differences of fibre density.