Paper should be flat, right? Well, sometimes. We think of paper as being two-dimensional, but actually it is a complicated 3-D web of cellulose fibres whose structure and moisture content determine its shape.
To understand why paper moves and changes shape, we must, in the immortal words of Maria von Trapp, start at the very beginning.
Paper is made from cellulose fibres processed by chemical and mechanical methods to a fibrillated pulp that is suspended in water and drained onto forms such as screens or racks. While draining, the forms are shaken so that the fibres distribute more or less evenly. Machine-made papers and many handmade Asian papers have a strong grain direction because most of the fibres are oriented parallel to the length of the sheet. Traditional handmade Western papers have a more random fibre distribution.
Paper grain has a great effect on the strength and dimensional behaviour of the finished product. Grained papers are strong in the direction of the grain and relatively weak in the cross-grain direction and, like a plank of wood, they have great potential for expansion across the grain. If there is a change in fibre orientation throughout the thickness of the paper (e.g., parallel distribution at the top of the sheet and random distribution at the bottom), the paper will curl.
Once free water (held between and inside the fibres) drains from the paper pulp, imbibed water (water saturating the fibres cell walls) is removed by pressure and evaporation. Evaporation may be accelerated by increasing heat (sun or artificial). As imbibed water is removed, the fibres are pressed together and become tightly joined by mechanical fibre-to-fibre bonds and chemical molecule-to-molecule hydrogen bonds. At the same time, the fibres shrink and stretch, creating semi-permanent tensions within the sheet. Ideally, these tensions are even and uniform, pulling the paper flat. Adhesives and sizes are usually added to the pulp or applied to the formed sheet to reinforce these fibre-to-fibre bonds and tensions.
Even drying creates flat paper. Uneven drying can result in curl (when one side of the paper dries before the other) and cockling. The most common example of cockling are the edge ripples that are formed if the edge of the sheet or roll of paper dries before the centre.
Provided all goes well at the papermakers, artists start with flat paper onto which they apply their media. How, then, does the flat paper end up becoming usually not by design 3-D?
Read Part 2 of this article in the next issue of Preview.