IN ENGLAND IN the later Middle Ages, people from a wide range of social and vocational environments authenticated and validated their documents with seals. “By the early thirteenth century,” Harvey and McGuinness argued, the seal-using group “comprised everyone who had free land or property to convey or other business to be agreed in formal writing.” The seal impressions most people appended to documents are typically small objects—a few centimetres in diameter—and single sided. Grand seals, such as those used by kings, could be larger and include text and images on two sides. Mechanically speaking, seals were, and remain, a simple technology; to create seal impressions, a type of stamp, called a seal matrix, was used. The matrix was pressed into a plastic material such as wax, and the impression could then be attached to the object to be secured. Because seals were difficult to copy, medieval document-makers relied upon them for some protection against forgery; the best way to copy a seal was to create a new matrix, a task that required considerable skill. Despite the simple technology behind these tools, the three-dimensional nature and complex combination of text and image make seal impressions difficult to record or reproduce in other media, and therefore challenging to study.

Millions of seals survive from medieval Europe, but because they are numerous and by nature hard to record, they present scholars with particular challenges. The DIGISIG project, begun in 2013, initially aimed to address a long-standing challenge for sigillographers. Hundreds of repositories in England hold medieval seals, but in 2013 there was no single resource that could tell a researcher where any particular seal was located. The first version of DIGISIG addressed this problem. But once the prototype system was in place, it proved to have further unanticipated capacities. DIGISIG enabled researchers not only to discover seals, but to analyze, compare and combine information from multiple reference works. These capacities may transform sigillography by enabling scholars to consider long-standing problems in the history of sealing practices, based on a fuller understanding of the evidence.