One bookbinding secret that you’d learn in a bookbinding class, but not in many online tutorials, is the importance of grain direction.
Machine-made paper, binder’s board, and book cloth all have a grain direction. The grain direction is determined by how the cellulose fibers are lined up during the manufacturing process. In bookbinding and box-making, it is very important to make sure the grain direction of all your materials is consistent so that your book or box does not warp. Additionally, the pages of your book should always be folded with the grain rather than against the grain. Folding against the grain causes the cellulose fibers to break, which in turn causes the fold of the paper to weaken and crack each time the book is opened.
Because handmade paper is created using pulp which settles in all different directions on the paper-making screen, it does not have a grain direction like machine-made paper does.
Fabric itself has a grain direction, which is a topic for another article, but here I’m focusing on paper-backed book cloth. The direction of the grain is very easy to feel thanks to its paper backing.
The easiest and most surefire way to test for grain direction is the bending test. Take your piece of paper, book cloth, or binder’s board and gently bend it first in one direction, then in the other direction.
There will be more resistance when you bend against the grain. It will be easier to bend with the grain.
I usually draw a light arrow on the material to remind myself of the grain direction after I have figured it out.
I hope this answers your questions about grain direction in paper, cloth, and board. If you have any additional questions, don’t hesitate to contact me.