Fabric Specs - Part 2, Construction
In doing the homework I gave you last week, you might have discovered that while different fibers certainly feel different from one another, the same fibers also feel different from each other. How can this be? It has to do with the fabric construction, which we’ll cover this week, and the yarn, which will be next week’s subject.
There are a myriad of fabric constructions, but for now we’ll just cover the basics. First things first, a fabric is simply defined as a material constructed with fibers, and there are 3 main umbrella categories: wovens, knits, and nonwovens. Of these, we’ll start with the smallest category - nonwovens.
Nonwovens are pretty much anything that’s not knit or woven, like fusible interfacing or felt. In a knit or a woven fabric, fibers are spun into yarns and then constructed into a fabric, but not so with non-wovens. In this case, the fibers are held together in a random matrix to form a fabric. Some common ways this is done are by mixing the fibers with glue, fusing the fibers to a scrim, or needlepunching the fibers so they interlock together, like in the felted heart below.
This results in a useful material for a variety of industries (medical supplies, filters, etc.), but for the fashion industry, nonwovens are typically only used to provide internal structure to a garment. To get a drape-y, fabric-like nonwoven, the fibers would have to be loosely connected, which would mean they would also easily pull apart when wearing the garment. To prevent this, the fibers could be more densely intertwined or more glue could be used to hold them together, but this would result in a stiffer material. It would still have a bit of “give,” in the way that you could say a piece of paper has “give” – you can bend, twist, roll, or fold it, but it’s not going to drape. But because of these properties, nonwovens are perfect for providing a somewhat flexible structure to garments.
Now that we know what a nonwoven is, let’s talk about woven fabrics. Woven fabrics are made by yarns that perpendicularly cross over and under each other. The yarns that run up and down (vertically) are called the warp (or ends), and the yarns that run side to side (horizontally), are called the weft (or picks). To understand the construction of fabrics, you need to first have a rough idea of the machinery that makes them.
image via https://commons.wikimedia.org/wiki/File:Warp_and_weft_2.jpg
All woven fabrics start with the warp yarns. First, yarns are wrapped onto a giant beam, usually with the help of a creeling machine. This now becomes the warp beam, which is then attached to the weaving loom. Next, each warp yarn is threaded through a heddle, which is attached to a harness (the heddles keep the yarns organized, and the harness moves the yarns up and down). After this, the yarns pass through the reeds, which help keep the yarns organized. Once threaded, the warp yarns are tied off under tension on the cloth beam, which uptakes the resulting woven fabric (because warp yarns are under so much tension, they tend to be stronger than the weft yarns). This process is called warping a loom and it’s done by hand – it can take a full day or two to complete. Because of this, industrial weavers try to keep each warp on the loom for as long as possible to avoid the downtime (and production loss) of re-warping.
creeling packages of yarn onto a warp beam. credit: https://search.creativecommons.org/photos/3e0f9458-9dc5-46e6-bd2c-27a24442e557
two warp beams on a towel loom. credit: https://search.creativecommons.org/photos/69492da0-6ec0-40dd-bac7-c9e92fc5ae71
With the warp set up, the weft yarns are then attached to a shuttle (which is a tool that helps slide the weft yarns between the warp yarns). In the most basic weave – appropriately called the plain weave – a weft yarn will go under one warp yarn, then over the next, then under the third, over and over again. But unlike your youthful days of making potholders, the shuttle doesn’t have to pick its way under and over each warp yarn. This is where the loom harnesses come in – they can raise certain threads of the warp, so the shuttle can slide right through to the other side. For a quick half-second or less the shuttle then pauses, while the harnesses are lowered and then raised again with a different set of warp yarns. The shuttle again slides through the resulting shed (the space between the raised and lowered warp yarns), and so on until the fabric is complete.
And now we can get fancy with our woven fabrics. We can have the weft yarns go over three warp yarns, and then under one, and then over three again, and we get a twill weave (a diagonal pattern; this is the construction you see in denim). We could have the weft yarns go over fifteen warp yarns all at once, and then under just one warp yarn, to get a satin weave. We could use two yarns together as one in the weft and get a basket weave. We can even have two warp beams on a loom and get a double-woven fabric – literally, two pieces of fabric woven together and connected at once. You can make so many different fabrics by just weaving yarns over and over each other in different patterns – velveteen, corduroy, sateen, poplin, chambray, gabardine, gauze, lawn, and georgette are just some examples.
printed poplin fabric. credit: https://search.creativecommons.org/photos/389c411e-ac2e-4fbe-9760-c7ab8089f5a3
gauze fabric. credit: https://search.creativecommons.org/photos/a12f2863-ed81-477d-ae37-de99fc8fa8e5
We can get even fancier by just playing with the tension and size of the yarns we use. For example, poplins and gauzes are both made in the basic plain weave structure, but they are vastly different fabrics. By tightly weaving very fine weft yarns with slightly coarser warp yarns, we get the light and crisp fabric we know as poplin (often used in shirting). In contrast, gauzes are so loosely woven together that you can see the open squares formed by the yarns, like a grid (this is super nice for light and airy beach cover-ups). We can quantify this difference to some extent by counting the number of warp and weft yarns in a square inch of fabric. Assuming that all yarns are the same size in two fabrics up for comparison, the fabric with fewer warps and wefts per inch will be more loosely woven than a fabric with a greater number. Of course, this all changes with yarn size – a thicker yarn naturally takes up more space than a thinner yarn (we’ll get in to this more next week).
No matter the tension, yarn, or fiber used, though, woven fabrics always have one thing in common – two sets of yarns (a warp and a weft) that go over and under each other at right angles (perpendicularly). This is the most important thing to remember about woven fabrics, and it’s in stark contrast to knits.
Knits are made by yarns that loop together. Unlike wovens, a knit fabric can be made with just one yarn (think of hand knitting), but most industrial machines use several different spools of yarn. Interestingly, hand weaving and industrial weaving follow pretty similar mechanic, while hand and industrial knitting aren’t quite the same as each other. To me, it’s easier to visualize the process of knitting a fabric by imagining hand knitting – if we don’t have one, we’ve all at least seen a picture of a grandmother and her knitting needles!
To hand knit, you start by casting on the yarn (looping the yarn around the left hand needle). This is the first row (or course in knit-speak). The yarn “feed” and second needle are held in the right hand. To knit, the right needle is poked through the first loop on the left hand needle, and more yarn is wrapped around that right needle to form a new loop. The right needle then comes back out of the loop on the left hand needle, but in doing so, slides that loop off the left needle. Now the right needle has one loop on it (and the left needle has one less loop), but that one new loop is still connected to the old loop from the left hand needle – so vertically, you have two loops. This is your first column (which is called a wale), and also the start of your second row (course). As the right needle continues to make new loops and removes old loops from the left needle, the fabric grows.
In industrial knitting, hundreds of needles are used, and they look more like a cross between a sewing needle and a crochet hook than they do hand knitting needles. To start, each needle on an industrial machine holds a loop. A contraption called the carriage holds the yarn, and acts a bit like the right hand needle in hand knitting. The needles are made with a little hinge, so that as the carrier approaches the needle, the needle eye can be opened to accept the new yarn from the carrier while at the same time the old loop is pushed forward and out of the eye. As the carrier passes the needle, the hinge is closed, holding on to the new loop, while the old loop is pulled off the needle.
industrial circular knitting machines. credit: https://search.creativecommons.org/photos/fca39a7c-b9fd-4bd7-8b2c-efe523a41c68
upclose image of industrial knitting machine needles. credit: https://search.creativecommons.org/photos/1642141e-096a-45fc-b836-2f42053d848c
Different types of knits can be made in how loops are formed. There is a “ridge” (also called a bar) at the bottom of each loop where it joins the below loop (see above picture of basic knit structure). Depending on if the ridge is made on the front or the back of the fabric, you can get different knit structures and patterns - hand knitters will know this as “knit” (ridge on the back) and “purl” (ridge on the front) stitches. For example, a jersey fabric has all the ridges on the back, while a rib knit alternates the ridges from front to back to create the rib lines.
top diagram: forming a knit stitch, with ridges on the back side of the fabric. bottom diagram: forming a purl stitch, with ridges on the front side of the fabric. credit: https://search.creativecommons.org/photos/266cdb80-7d17-475c-9df3-ca45928f014b
To get more complicated knits, you can also choose to not knit certain needles on a given row (course). This is called a hold (or skip) stitch – the stitch is literally held until the next row is made, and it forms a small hole in the fabric. You can also do the opposite and knit a certain needle twice, forming a thicker loop (called a tuck stitch). Alternate between these two stitches, and you can get unique textures and patterns (see below picture). And just as wovens can have a double warp beam to make a doubleweave, you can also have a double needle bed knitting machine which creates doubleknits (for example, interlocks and pontes are varieties of doubleknits).
Lastly, if you wanted to have a knit-woven fabric mash-up, you can do that with a warp knit machine. Technically classified as a knit because the fabric is formed with loops, the warp knit machine acts more like a weaving loom. There is a warp beam, but instead of the yarns being threaded through the warp, they’re threaded through knitting needles. This is in contrast to the knitting described above (weft knitting), where the same yarn goes through many needles – in warp knitting, each needle gets its own yarn. Like in weaving, these warp yarns can be moved, but instead of up and down, the yarns are moved side to side to connect the loops together to form a knitted fabric.
raschel knit structure, a type of warp knit. credit: https://search.creativecommons.org/photos/9e1ab8d0-5e82-49dd-9152-824c76e97b3a
Similarly to wovens, some differences in knits can be quantified by the number of courses and wales per inch (rows and columns, or for wovens, warp and weft). Likewise, yarn size and type can make a vast difference in the resulting knit fabric. One important difference between knits and wovens, though, is in the naming and categorizing of knits – for example, while a loose plainweave can be a gauze and a tight plainweave can be a poplin, a loosely knit jersey is still just a jersey, as is a tightly knit jersey.
So, what does this all mean? Basically, the physical construction of a fabric impacts its texture, drape, and strength (in collaboration with the yarn). Going back to last week’s blog, cotton fibers tend to usually feel dry (grab some cosmetic cotton balls to see what I mean). But if you’re wondering how it is your cotton jeans have a dry hand but stiff drape and slightly ridged texture, while your cotton poplin shirt also has a dry hand yet a soft drape and smooth texture – it’s 50% because of the fabric construction. The other 50% has to do with the yarn, the subject of next week’s blog :)