What are the physics behind Vionnet's bias cut?

December 2, 2021 2:11 AM

The French fashion designer Madeleine Vionnet (1876-1975) is known for popularizing the use of the "bias cut" to create garments with soft, flowing lines that cling to the body. What physical forces, on the warp and weft level, are involved in adding elasticity to the fabric, and how do they differ from those forces affecting cross-grain or straight-grain cuts?

Also, if you were to cut two dresses from the same pattern--one using a straight-grain or cross-grain cut, the other a bias cut--what differences would be observable if they were worn by two models standing next to each other?

If my use of any terminology is mistaken or imprecise, please correct me!

posted by Gordion Knott to Clothing, Beauty, & Fashion (8 answers total)

If you take a piece of fabric and pull it straight along the warp or weft - that is, you're pulling straight on the threads themselves - the fabric can't stretch (unless the threads themselves are elastic). But if you pull the fabric diagonally, it will stretch some - more if it's woven loosely, less if it's a tight, dense fabric. That's not because the threads themselves are stretching, but because they have some freedom to move along the diagonal (aka the bias). Normally you can imagine the warp and weft threads as intersecting at right angles, forming tiny squares or rectangles in the space between them. Pull on the diagonal, and those angles change and the space between the threads becomes more diamond-shaped. Also, especially when the edges of the fabric are bound so the threads are slightly constrained in how they can move, the fabric will often go 3D a bit to accommodate the pulling. Note that the fabric doesn't actually expand - it just changes shape.

If you have a piece of loosely-woven fabric at hand, try pulling on it in the direction of one of the threads to see for yourself why it can't stretch, and then pull at an angle to see how the threads shift a bit to make stretch possible. The looser the fabric, the more easily you'll see the effect.

If you don't have any, try this: take two sheets of paper and cut and weave them into a paper mat like this - only don't adhere the strips to each other. Pull straight down along one of the "threads" and see what happens; and then pull from the corners and see how the mat goes 3D to accommodate stretch. You can also cut off the frame to see what happens when the "threads" aren't constrained by it. (If you're short on time, you can see the same principle by just taking a sheet of paper and cutting out the middle so you're left with only a rectangular frame. Pull the sides of the frame themselves - nothing happens. Pull from the corners, and the frame warps and stretches into a diamond shape.)

With clothing, you have to think about all the ways the fabric is going to be pulled on when it's worn, and how you want it to respond to that pulling. If you want it to maintain its shape, you'll try to make your seams not on the bias. (For woven fabric, anyway.) If you want it to be able to stretch some, then your seams will often be on the bias. Also, sewing on the bias lets you incorporate stretch and 3D into the garment to begin with - useful, for one example when you want to connect a slightly longer edge with a slightly shorter one and have the ends line up (which is one way to create a 3D wavy, flowing effect); for sewing curves together; for things like binding necklines and hems, where making the binding a bit shorter than the edges it needs to cover helps keep things a bit taut and neat and, again, accommodates the curves; and so on.
posted by trig at 2:56 AM on December 2, 2021 [31 favorites]

Pull the sides of the frame themselves

Just to be really clear - this means pulling on both ends of a single side, not pulling the sides of the frame apart.
posted by trig at 3:07 AM on December 2, 2021 [1 favorite]

In terms of actual physical forces—if you are pulling with the grain on a woven fabric, you have your hands on the same threads on either end going pretty nearly in a straight line, so you can’t have much more stretch than is present in the threads themselves. The result is not a lot of elasticity in that direction, assuming traditional fibers and threads. Pulling on the bias does not have that constraint; you’re no longer pulling on two ends of the same thread. Instead, the constraint is in how closely spaced the threads are and how much friction they have in all those little spots where warp and weft rub against each other.

Knitted fabric, made of one thread tortuously zigzagging through a fabric, doesn’t have this warp/weft constraint, either, which is why it can be stretchy even if the thread used isn’t. So an imperfect model for your last question, but one that may be more intuitive, is to imagine a typical t-shirt made of knitted fabric vs. a woven button down shirt. The woven shirt has so little give that it needs buttons to fit, even with the same measurements. But the knitted fabric, which gently warps and bends around the wearer, has more folds in the fabric, and ends up looking a little sloppy as a result. Something sewn on the bias is not quite as stretchy as knitted fabric because it does still have warp and weft in it. I imagine that a non-expert wearing a piece of clothing with biased and non-biased fabrics in the wrong places would feel that wrongness in tightness/bunching in the wrong places, less fluidity in others, but have a hard time articulating why.
posted by tchemgrrl at 3:44 AM on December 2, 2021 [4 favorites]

I found this tutorial on sewing a round ball on grain vs. on bias illuminating. The author uses a measuring-tape-print fabric for both examples, so you can easily see how much longer (√2 ×) the constituent threads are in each bias-cut panel. In the straight-cut examples, the lack of length/elasticity in the narrow ends of each lemon-shaped panel results in a sphere with flattened "poles."

There are YouTube examples of the same garment cut straight and on bias, but in many instances I suspect the difference in your hypothetical two-model example wouldn't be so much how the garments looked, but whether the model in the straight-cut version would be able to move.
posted by wreckingball at 5:31 AM on December 2, 2021 [8 favorites]

The ability of woven fabric to stretch on its biases is one of the simplest examples of fabric as a metamaterial or one with emergent properties, usually described in knits, as previously.

Other famous examples: cloth cut on the bias to make tight fitted hose (imitated with difficulty in argyle knitted socks); circle and half-circle skirts putting the bias over the hips where the highest curvature usually is. (Exercise for the reader: for what proportions of a circle is this possible?)
posted by clew at 10:21 AM on December 2, 2021 [2 favorites]

A scrap of window screening is my favorite demonstration of bias stretch. Metal screening if you can get it. Threads not stretchy, not grippy of each other, and everything is big enough to see the deformation.

Another example, vanishing into a "technology parenthesis ": worn-in non-elastic jeans use the bias to go around hip and butt curves, with seaming to limit the deformation. Washing tends to reset the fabric, but eventually it stays deformed. If you trace a pair to make new jeans they won’t be quite right because you’re tracing the stressed shape- which won’t lie flat to be traced anyway.

Most things pressed over a ham (instead of a sleeve roll) are also using bias stretch.
posted by clew at 10:38 AM on December 2, 2021

You can also demonstrate on any non stretch clothing you own. But a square bandana is a good one. If you tried wrapping or tying it around you folded flat in half it has no to very little give. But if you fold it to a triangle it has enough give that it can be worn as a hair wrap or around a neck comfortably. That’s also why you can wrap a lot in it and tie the corners together. All the force in a bindle pulls on the bias.

This quilting tutorial explains grain bias well as a baseline. And here’s a pattern drafting analysis of that exact dress. (I didn’t watch all of it but I presume it answers some of your questions.) And here’s a video on the history of that designer and how revolutionary garments cut on the bias are.
posted by Crystalinne at 12:00 PM on December 2, 2021

Oh and another thing - a finger trap is an example of bias. It only works because it’s woven at a bias grain. If the strips were horizontal and lengthwise it would just be a tube with no stretch.
posted by Crystalinne at 12:05 PM on December 2, 2021 [4 favorites]