Hand Made Paper
'Laid' paper was made by dipping a mould into a vat of pulp made from undyed rags that had been left to rot, then been pounded and diluted with water. A mould is essentially a wire sieve, mounted on a removeable wooden frame called a deckle. As the mould was lifted out of the vat and the liquid drained through, fibrous matter collected on the mould. The deckle was removed from the mould and the wet sheet of paper turned onto a piece of felt. Before it could be used for writing or printing the paper had to be dried and treated ('sized'). Illustrations showing paper manufacture from Diderot's Encylopedie can be found here (at the Louisiana State Museum).
An outcome of this process is that the resulting paper retains an imprint of the mould on which it was made. The pattern of the wires of the mould is visible when laid paper is held up to a light source, in the form of closely set laid lines running between the narrower sides of the sheet, and thicker, more widely spaced, chain lines running along the length of the sheet. From the thirteenth century onwards, most paper was also given an identifying mark, a watermark, formed from the impression of a wire pattern sewn onto the mould. Since every mould, and every watermark, is made individually and by hand, each one makes a different impression on the paper. It is thus possible to identify two sheets of paper made from the same mould.
It has been estimated by Bo Rudin (Making Paper, p. 223) that 500 reams (250 000 sheets) of paper could be produced before a mould needed to be replaced. Given that the average annual output per vat/mould pair was 1500 reams, a pair of moulds in continuous use would wear out in about 8 months. However moulds were seldom used continuously (given the need for different paper-sizes and stoppages in production): a common mould would usually last for about 2 years, but others could last many years longer. Identifying a particular mould, usually through its watermark, can thus help to identify an approximate date for the paper's manufature. This is one of several reasons why it can be useful to identify the paper on which a particular manuscript is written. The evidential value of paper/watermarks is discussed in a separate page.
Although many watermarks are based on the same general design, every mould had its own watermark, and no two watermarks are exactly identical. Watermarks are the easiest way to identify paper: a watermark can be thought of as the paper's fingerprint.
A huge number of devices were used in watermarks. Hold a sheet of seventeenth century paper up to a light source and you may be confronted with a mythical beast like a basilisk or a unicorn, or a pot, or a fleur-de-lis, or a pennant, or a heraldic shield, or something else entirely.
Watermarks were originally the marks of the individual papremaker or mill, but by the sixteenth century they had taken up certain other functions. Many watermarks were identified with a specific geographical area: the Basilisk with Basel, for example, or the pot with Normandy (a particularly common watermark in paper used in England).
Some forms of watermark could also become marks of quality. But well known watermarks, especially those used in high quality paper, were open to imitation.
There is a large amount of variation even within the most conventional watermarks like pots. The initials of the master of the papermill was often also included on the watermark. There are also definite trends in watermark design: over time, pots' crowns tended to gain more jewels, for example, and later crescents, then fleur-de-lis. These changes can be of assistance in estimating a very approximate date for the paper's manufacture.
Since the watermark was part of a mould of fixed size, that particular watermark was correlated to a particular size of paper. It is therefore not surprising to find that some watermarks, such as pot or foolscap (fool's cap), also became the names of paper sizes. This correlation also had an effect on the lifespan of a mould, since a mould in a common paper size would wear out more quickly.
Further Details about Watermarks
A mould, and therefore the watermark, deteriorated with use (which is why it had to be replaced), and there would be corresponding changes in the physical appearance of an individual watermark over its working life. The exposed edges and corners become increasingly battered and the watermark may have to be re-sewn onto the mould. On the whole, however, it will remain recognisable as the same watermark. The usefulness of sewing dots in identifying a deteriorated watermark is discussed in Allan Stevenson, 'Paper as Bibliographical Evidence', The Library, 5th Series, 17 (1962), 197-212.
Paper making required two moulds and at least two workers. The vatman dipped the first mould into the vat and passed it to the coucher, who removed the sheet from the mould while the vatman was making a second sheet with the second mould. The coucher then passed the first mould, now empty, back to the vatman and received the second mould in return. So a given paper stock produced by a mill, while all of the same size and from the same raw material, was made from two different moulds, and contained not one but two watermarks. Since moulds were always used in pairs they were usually designed in pairs, so most watermarks have a twin - a very similar (but not identical) watermark with which it is typically found. If a single sizeable body of paper, such as a manuscript or printed volume, contains two watermarks randomly distributed in roughly equal proportions, you are almost certainly dealing with twins rather than two paper stocks. This concept is discussed in detail in Allen Stevenson, 'Watermarks are Twins', Studies in Bibliography, 4 (1951-52), pp. 57-91.
The Location of the Watermark on the Sheet:
Most, but by no means all, sheets of paper had a watermark, and it was usually placed one quarter of the way down the length of the sheet. So when the sheet was folded in half, it would appear in the centre of one of the halves. Paper was folded in this way in large format books or manuscripts - folio. A volume in folio will therefore usually have a watermark in the centre of about half of its leaves. However, producing smaller formats involved further folding and cutting of the paper, and in such cases the leaves may not have a visible and complete watermark. To make a quarto, for example, the paper is folded in half a second time. This means the watermark is in the gutter divided in two by the spine fold, and therefore difficult to see. Format and related bibliographical terms are discussed in introductions to textual bibliography such as Philip Gaskell, A New Introduction to Bibliography (Oxford, 1972) and D. C. Greetham, Textual Scholarship: An Introduction (New York and London, 1994).
Having located the watermark in the paper, it is then necessary to describe and (perhaps) identify it.