What Makes Paper Crease?
In a posting yesterday, 22nd February, Sainatee asked: What makes paper crease?
Before I attempt to give an answer, I should like to draw attention to the two words "crease" and "fold". We tend to use the two words as the same, but when we look closer, they are really different.
To me, "creasing" means to make a crease in paper (or some other material), the mark of which remains after the paper has been opened up again. We also speak of creases in fabrics, such as linen, which are notorious for leaving a person wearing such materials looking crumpled and as though they have just got out of bed. A crease in this sense is "remembered" in the paper, fabric or other material.
Yet we do not call our activity "paper-creasing". It is always "paperfolding", whether in English or some other language such as Japanese, in which it is "origami".
"Folding" is not identical with "creasing". We fold things in half, even if we do not intend to make a permanent crease. So we fold our clothing to put it away in a drawer or a suitcase. We talk about folding a sweater, even though wool will not take a permanent crease. Some people go to great lengths to fold up their clothes, lining them with tissue paper to prevent them from acquiring permanent creases during the journey.
Clearly, you have to fold something over before you can go on to press it down and crease it. I know that there are soft folding techniques in origami, but the essence is making creases that the paper or other material will "remember". I suggest therefore that we should stop cling our activity paperfolding and instead call it "paper-creasing". (Should we rename it the "British Paper-Creasing Society"?)
Some materials we use for origami take and hold creases more readily than others, Some people use metal foil (usually backed by thin paper) when they are practising a model, because it keeps the preliminary creases better than soft paper. Others deliberately use soft paper, especially for wet folding, because they want softer creases.
But I'm getting way from the quieten. Sainatee's question is: What makes paper crease?
Paper is made up of vegetable fibres, which are felted together. They felt together because the fibres are somewhat rough, although other agents, such as size are often added to encourage the fibres to stick together. But the fibres are brittle and when you crease paper the fibres along the line of the crease they are permanently fractured. It is this permanent line of tiny fractures that forms the crease. Because it is indelible, the paper is said to "remember" the crease". Even ironing with a hot ironing will not get rid of the crease.
It is much the same with metal foil, although in this case there are no fibres to fracture. Instead, metals have a crystalline structure and the process of creasing permanently fractures the crystalline structure along the line of the crease, leaving a permanent line of fracture which is usually sharper than a crease made in paper.
Other materials either have fibres that do not easily fracture, or they do not have fibres at all. Parchment and vellum are examples. Parchment "remembers" a crease only to a limited extent. (I do not understand how this happens, but I suspect it is caused by the stretching and distortion of the elastic animal tissues, but not their fracture). Parchment can be "bent", but does not hold an indelible crease. If it is flattened again, (except where it is folded for a very long time as with old deeds) it looses the "bend".
It is even more so with some plastics, Some, like parchment, will hold a temporary "fold", but others refuse to retain any mark at all. This is because the plastic does not have fibres, is not crystalline and does not have any other structure that can be distorted. to leave the mark of a crease or fold.
I expect that it is possible to get very scientific about this, and no doubt I will be contradicted by scientists, but I do not chink I am very wide of the mark.