The Microscopic Grain Hidden in a Sheet of Paper

There is more to paper than meets the eye.

Just like a piece of lumber, paper has a grain that runs in one direction along the sheet, changing the physical properties of the paper depending on how it is being used or manipulated relative to the microscopic differences in the structure. In modern paper manufacturing, the grain is introduced as sheets of paper are cut from a large roll. You can witness this for yourself by tearing a piece of paper along the grain, which will tear in a fairly straight line, and then rotating the paper and tearing it perpendicular to the grain, which makes it impossible to tear straight.

You can also perform the simple experiment that is demonstrated in a new video from YouTube channel Numberphile. By taking two squares of paper and holding them together at the edge, you can see how they will either stick together or separate depending on which direction the grain runs. Flipping the squares of paper over will change the grain direction of the bottom piece of paper, changing whether they stick or separate. To off a better illustration of what is going on here, mathematician Tadashi Tokieda of the University of Cambridge folds ridges into two squares of paper so you can see how one orientation is stiff enough to remain rigid, and the other orientation sags. The exact same thing is happening with the grain of the paper on a microscopic scale.

Why does this matter? Well, people who work closely with paper, like bookbinders, need to know which direction the grain is running and how it affects the paper they are working with to make sure the pages of finished books do not bend or fold awkwardly. Japanese calligraphers have an even more personal interest in the grain direction of paper, as these artists say they can tell when the direction of the grain in a sheet of paper influences the ink being applied to it. For this reason, Japanese calligraphers insist on using paper that was made the ancient way. By compressing pulpy wooden fibers into one sheet at a time, the grain of the resulting paper does not run in a single direction, but rather spreads randomly across the sheet, similar to plywood.

Even something as common as a sheet of printer paper has a complicated structure if you look close enough.

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