The Chinese court official Ts’ai Lun (often written Cai Lun) is credited with the invention of paper in 105 A.D., although he and his countrymen were almost certainly experimenting for some time before his announcement of their success to the emperor. The manufacturing process did not change much for hundreds of years, until the invention of suitable machinery in the nineteenth century, and is still used to some extent to produce hand-made paper today.
The method involves soaking rags in water and beating them to a pulp. (NB – if you employ children of differing heights for this task, chopping blocks and planks of wood of different thicknesses should be provided!)1 A mould consisting of wire mesh in a removable frame2 was dipped into the wet, fibrous material, lifted slightly and shaken so as to remove as much water as possible and ensure that the pulp was evenly spread. The vatman would then remove the deckle to use with the next mould and pass the mould he had just used to the coucher, whose first job was to lean the mould against the drainage horn. 
When enough water had drained off that the sheet had solidified sufficiently, the coucher turned it out onto a larger piece of woollen felt. 144 sheets (six quires) were thus piled up. This pile (known as a post) was then placed in a press and more water squeezed out. 
The layman then separated the sheets of paper from the layers of felt (which were returned to the coucher for reuse) and piled them neatly to go into the dry press. They may then have been piled up in a different order and pressed again, and this step repeated several times. Finally, “spurs” of four or five sheets were hung in the drying loft over ropes made of cow- or horse-hair.
The images above are from Diderot’s Encyclopédie ou dictionnaire raisonné des sciences, des arts et des métiers. They accompany the article on Papeterie (paper-making) by Louis Jacques Goussier, one of the regular contributors to the encyclopedia, aided by M. Prevost, whose mill appears in one of the plates. Published between 1751 and 1772 in 28 volumes, of which 17 are text and 11 plates, the Encyclopédie contains a large quantity of information on bookmaking under a number of different headings. Our copy is housed in the Long Room, at shelfmark S.aa.4-31.
Occasionally, an imperfect sheet of paper would go unnoticed until it was too late. The images below show the two sides of a page in Speech of Lord Grenville in the House of Peers on the motion of the Duke of Bedford for the dismissal of ministers, … March 22, 1798, published and printed in Dublin for J. Milliken in 1798, shelfmark OLS 184.p.38 no.12, in which the rag is still clearly visible. The previous page, which is part of the same gathering (ie folded from the same large sheet) is very thin in places and even has a hole in it (shown with a sheet of green paper behind the page).
Worse still, there was not much that could be done about the intrusion of a ‘foreign body’ if it was not spotted in time. This fly landed in the paper which became Debrett’s Parliamentary Register Vol.VIII (the third session of the eighteenth Parliament of Great Britain), printed in London in 1799, and now kept on the shelves between the windows in the EPB reading room.
- Denis Diderot et al: Encyclopédie ou dictionnaire raisonné des sciences, des arts et des métiers Vol.XI, Neufchastel, 1765
- The frame is sometimes known as a deckle. If the sheet of paper was not later cut straight, the sides would be slightly indented wherever the wires met the frame. This is the origin of the term ‘deckle-edged’.