I’ve been asked more than once by people who see my curved things: do you wet the paper? And occasionally: do you press the paper into a mold?
The answer to both questions is of course no. And as to wetting the paper, I don’t often practice wet folding (though a bit more so of late) but instead like to glue the paper to a stiff aluminum foil before starting out.
But it took me a while to understand what people were really asking here; and also to realize that even some of those who have heard of wet-folding may have a mistaken idea of what it entails.
People think when they see a curving surface (especially one curved in two directions), that since paper in its natural state is flat, its surface properties must have somehow been altered: that the paper has in fact been stretched. And they think that moistening a sheet of paper is what lets this happen. The question about ‘pressing into molds’ is contemplating the reverse: that the paper is being compressed.
This of course is wrong. A thin moistened sheet will sometimes blister, or its edges will ripple, because locally the paper has expanded and has no where to go but up or down. (A fine, interesting article on this subject was published last year by Eran Sharon.) But in origami that is never an effect you want, as it is entirely uncontrollable: indeed one uses thicker, more absorbent paper precisely to avoid any such blistering. If you get wrinkles or blisters from treatments that involve liquids (e.g., when you glue sheets together or prepaint using inks or watercolors), you’ll probably leave the sheet overnight between flat boards or under a sheet of glass, to salvage something of your labors. At all costs you want the sheet to be flat.
So if wet-folding does not alter a sheet’s “Gaussian curvature”, what does it do?
Let’s define things. Wet-folding in origami is the process of lightly moistening your model as you fold, with a damp cloth or spray mister; sometimes the moisture is applied only to specific regions.
It serves three main purposes. (1) It changes the way paper responds to indentations, and hence gives folded paper a softer and more rounded look. (2) It allows stiffer papers to be folded without cracking. And (3) it releases and repositions the sizing or glue that some papers are made with, so that when the model dries it sets into the 3D form you left it in with great permanence (I recently had occasion to handle some Yoshizawa works from the 1960s, they are as stable & solid as if made yesterday—and will be, I bet, in 500 years.) In short: wet-folding is a sculptural technique, a temporary softener of paper, and a great stabilization method.
I am most interested in origami’s aesthetic and sculptural properties, though these too of course have a physical substrate; so let’s focus on (1).
Wet-folding allows paper to take dimples and indentations softly & evenly. The look of this contrasts with dry folding, because dry paper prefers to take folds that are straight lines, or alternatively to crumple (= form straight-line segments with messy twists around vertexes, and mostly flat planes.) When paper is wet, it behaves a little more like cloth. Pressures dissipate not in a crumple-pattern but continuously. The result is that visually, you are dealing with surfaces --uninterrupted smooth areas where the form is indicated by roundedness, not by fold lines. (And where there are fold lines, they are read a little differently from their dry-fold counterparts: they are thicker, meatier affairs.)
This has a certain inexact analogue in painting, where you can represent forms either via draftsmanship or by what is called (in a different sense from origami) “modeling”—i.e. gradations of shade or color to indicate curving surfaces. (That the mind has these two ways of processing nature’s forms was a fact known & played with by, for example, Leonardo, who likes his painted smiles to hover between gladness and the ache of knowledge. To accomplish this he’ll sometimes use ‘modeling language’ to indicate innocent joy--you can see this better if you let your eye lose focus slightly--and a contrasting ‘linear’ language to indicate secret, perhaps painful knowledge. The smile seems to flicker in and out of existence as your concentration wavers...)
The comparison to the drawing/painting dichotomy is inexact, however, because of course in drawing, a line can be curved too, and can vary in stress; whereas in paperfolding, an edge-line is going to be straight and have identical weight the vast majority of the time. So really what we are comparing in paper-folds is linearity on the one hand, with a model being either flat or 3D but composed of flat planes--these can suggest ‘bones’ or architectural outlines or intellection and abstraction of form--and on the other hand, the thicker and more graduated curving forms, the haunting ‘emotional’ and expressive look of wet-folded origami.