Understanding Paper: Structures, Watermarks, and a Conservator’s Passion

By Leonie Müller
May 7, 2021
Index Magazine

Understanding Paper: Structures, Watermarks, and a Conservator’s Passion

A woman with long hair and glasses is leaning over a light table in a laboratory to examine a sheet of paper.
Paper conservation fellow Leonie Müller looks at an artwork on paper in transmitted light.

There is more to paper than meets the eye. Paper conservation fellow Leonie Müller shares with Index readers the material qualities of paper and how its structures reveal the process of how it’s made.

As a paper conservator, I am fascinated by the structure of paper and how different it can look, feel, and even sound—it can be rough or smooth, glossy or matte; it can have varying shades of white or be colorful; and it can sometimes rustle when carefully handled. These are properties that paper conservators try to describe when looking closely at a work of art on paper, and I invite you to take a closer look with me.

Structures and Process

In Penley Knipe’s article about blue paper making, we learned how paper has traditionally been made. But we can also learn about production processes from paper itself. The properties I described above are the ones we see under normal light, which conservators call visible light. In the Straus Center, we like to use light bulbs that resemble daylight, so the color we perceive is the most natural. But when we tip the lamps at our workbenches a bit or use a handheld flashlight, we create what’s called raking light. In this light, color is not what we are looking for. Suddenly, tiny shadows appear, cast by the uneven surface of the web of cellulose fibers that form the sheet. Also detected are slight indentations from the manufacturing process. For example, raking light can reveal the texture created when felts are pressed against a wet sheet of paper. Raking light lets us perceive paper in three dimensions.

In Europe, paper was originally made on moulds, which are wooden frames with a tight web of wires supported by wooden struts. A “vatman” would dunk the moulds in a vat filled with pulp and lift them up again, slightly agitating the pulp collecting on the mould while doing so to form an even sheet. He then would pass the mould on to the so-called coucher, who would press the now-formed paper sheet onto a piece of felt.

Asian paper is made using a flexible mat of bamboo splints sewn together and positioned on a wooden frame for support, which is also dunked in a vat. The mould is submerged several times and shaken in a rigorous back and forth movement in between dunks. The paper sheets are then pressed on top of each other by the vatman, who removes the bamboo mat from the frame and rolls it over a surface to release the paper from the mat.

In both traditions, the laid structure in the paper forms because the fibers from the pulp that collect on the mould sink into the small spaces between the wire or bamboo, leaving fewer fibers on the protruding parts of the mould. This makes the paper a bit thinner in these elevated areas. Sometimes the pattern is more pronounced, but other times you need to know what to look for: a grid of strong vertical lines, called chain lines, divided by more delicate and plentiful horizontal lines, called laid lines. We call this kind of paper laid. When we change the angle of light and let the light shine through the paper, we create transmitted light—that’s when the chain and laid lines become clearly visible.

In 18th-century Europe, the idea of covering the mould with a tight metal mesh, which keeps the pulp from sinking between the wires, resulted in the invention of a paper with a smooth surface, free of chain or laid lines. Because of its even structure, this paper is called wove paper.

Of course, not all papers exhibiting a laid structure are handmade. Industrial paper making began in the late 18th century, and with the invention of the cylinder mould machine—a cylinder covered with a mould that picks up pulp in a continuous movement—it became possible to produce laid papers on a larger scale. These machines are still used today to produce high-quality papers.


Like chain and laid lines, watermarks become clearly visible in transmitted light because they are thinner areas that show as bright lines. They are made by sewing a bent wire to the top of the mould, causing fewer fibers to form the sheet in the protruding areas. This explains why watermarks are not found on older Asian papers, as the flexible bamboo mould does not allow for wire or other attachments.

The oldest known watermarks date to 13th-century Europe and were considered marks of origin, though sometimes they indicated quality levels. They often featured a symbol or, later, initials, representing the paper mill or paper maker—an early form of a trademark or brand logo. They could also be customized to bear a royal symbol or the coat of arms of a particular city. Watermarks evolved continually, from reflecting the personal preferences of a papermaker to conforming to official guidelines on how a paper mill would need to mark its papers under penalty of law. Because watermarks and their use changed over the centuries, there is much room for interpretation. They have cultural, historical, and of course, artistic value and continue to inspire historiographic research.

By researching watermarks and closely inspecting irregularities in chain and laid lines in paper, we can make assumptions about where and when paper might have originated. This in turn can help authenticate historical documents and works of art. Because industrialization of paper making is a relatively new technology, the paper used for printed objects, both books and prints, normally shows consistency. Paper was bought in reams and was most likely used right away rather than stored, since the material was considered valuable. The prestige (and price) of high-quality paper even led some producers to forge watermarks and claim the fame of other paper mills!

Besides the mould-made structures in the paper, there are other traces of manufacture we can see. The evenness of the fiber distribution indicates the quality of both the paper and the pulp used. A cloudy paper with irregular thickness, for example, tells us that the pulp was not beaten thoroughly enough and knots of fibers floated in the vat, or the fibers could not be distributed evenly. This kind of lower quality paper may have been cheaper to buy and was possibly used for packaging and other everyday applications, such as drafting paper in offices.

Some papers clearly show traces of the production process, both deliberate and accidental. Long, strongly colored fibers that are visible without magnification can often be identified as wool. Wool was used to give paper a specific tonality and texture, making it more voluminous and rougher on the surface. Artists working in dry media, such as pastels, appreciate this surface quality. Or consider the little circles in sheets of paper called “vatman’s tears.” These are the result of water dripping on the newly formed sheet while it is still wet. At this stage of the process, paper is very vulnerable. Full sheets can have a pulled edge, which is an effect that occurs when the papermaker’s assistant stretched the still-malleable paper while removing it from the felt.

In the past, moist sheets were dried on ropes or thin wooden rods, which shows up as wrinkles, usually along the center of a full sheet. The uncut edges of a full sheet give a good indication of how the paper was produced. Compare, in the images below, the wavy, deckled edge on all four sides of handmade paper with the watercut edge found in paper made by a cylinder mould machine. To make watercut paper, the pulp of the formed sheet on the conveyor belt is incised with water, which creates a weak spot where the paper can easily be torn apart in the production process. These clearly visible differences help us guess the age of the paper, since they align with developments in paper making history.

All these features capture my attention when I look at a sheet of paper, and even though we try to take photos to document memorable paper structures, most of them are well hidden to the casual observer. But they tell us a lot about the origins of this important artist’s material that’s often taken for granted. When you see a work on paper in the future, I hope that you will not only appreciate the artistic image that sits on top, but also value the information embedded in the paper itself. You might enjoy deciphering it as much as I do!


Leonie Müller is the Craigen W. Bowen Paper Conservation Fellow in the Straus Center for Conservation and Technical Studies at the Harvard Art Museums.