Paper varied considerably in price, and the quality of paper that was used for a particular document is suggestive of how the writer valued the document.
A watermark will indicate the likely place of production of the paper. But since paper was traded internationally its place of manufacture does not give any indication of the place of the document's production.
The amount of information that can be inferred from similitude/watermark type is basically limited to region of origin and very approximate date. It is particularly important to bear in mind the limited extent of our knowledge of the early modern paper industry, especially outside the printing trade. This means that, at present, the grounds for dating by watermark type are decidedly shaky. Even if it was generally true that neighbouring mills used similar watermarks at the same time, how do we know this was true in particular cases?
The lifespan of a mould varied considerably. Watermarks are usually dated by the year of production of a printed book in which they appear, but the reliability of this date for writing paper is unclear. The details of the import trade remain unclear, the time lag between paper production and retail sale presumably varied considerably, and the time between retail purchase and use by an individual must have varied even more.
Paper is a particularly powerful heuristic if the specific paper stock can be identified, usually through its watermark. A common watermark may provide significant support for an argument proposing a common origin for a group of manuscripts. Paper, especially if it is unusual, may be associated with a specific coterie or even an individual writer. Conversely, the appearance of the same paper in different manuscripts may mean that the writers had a common source for their paper, and thus invites investigation of whether they were linked in other ways.
A good way of understanding the evidential value of watermarks is to think of watermarks as papers' fingerprints. The crucial characteristic fingerprints and watermarks have in common is that each is unique - when measured with the appropriate degree of precision.
A fingerprint, or a watermark, may provide evidence in two ways:
- A match may be found by comparing samples chosen by the investigator (e.g. from the murder weapon and the butler's fingers, or from two documents in the same handwriting).
- A given sample may be collated against a database.
The first of these is possible with watermarks but there are serious difficulties surrounding the second, for which see the LIMA page on watermark databases.
Given the current state of knowledge, watermarks can be extremely useful in resolving problems at a local or archival level, but do not form a coherent and systematically organised body of knowledge. In other words, if you want to date a Ben Jonson autograph manuscript from its watermark, you should look for identity among other watermarks in Jonson autographs, rather than turning to a database in an attempt to establish the watermark's date.
Once the object of study has been restricted, paper can be a powerful heuristic. Key to successful work with watermarks is therefore to work within a relatively small term of reference. Research is conducted on the level of the archive, in other words the written material produced by (not only retained by) a given body. Studying the paper allows for the reconstruction of the "blank archive", the material from which the archive has been produced, and thus of a particular document's place in that archive. This is discussed more fully in Paul Needham, ‘Concepts of Paper Study’, in Puzzles in Paper, pp. 1-36.
The body in question can be anything - it is most often an individual, but it could be, say, a printing house, or a government. The period of time over which a particular paper stock was used by an individual or larger body can be dated relatively precisely, and then documents which are otherwise undateable can be dated from how their paper fits into the chronology of the subject's paper use.