This is the feature of yarn most likely to trip up knitters trying to substitute yarns for a pattern. While every pattern tells you the thickness of the yarn you should buy, none of them particularly address density. When handling individual balls of yarn, density isn’t obviously an issue– most yarns are sold in balls of 50 grams, and when they’re all wound up it’s not obvious to the eye or hand that Yarn A will give you 120 yards per ball while Yarn B will only give 82 yards. The label says so, of course, but the yarns don’t feel that different…
There’s good psychophysical reasons why we’re not equipped to evaluate differences in density when comparing small quantities, but that doesn’t mean they don’t matter. In these modern days, most projects ask you for N yards of x-weight yarn (they used to ask for N grams of x-weight yarn, which created different problems). So if you’re knitting a sweater that asks for 1,500 yards of sport-weight yarn and you use Yarn A, your final product will weigh 625 grams, or 1.4 lbs. If you use Yarn B, your sweater will weigh 915 grams, or 2 lbs.
Your gauge will probably be the same, and obviously .6 lbs isn’t going to take you down, or prematurely bow your back, or anything. But using a denser yarn, and therefore getting a denser final product that weighs more per square inch, can substantially affect the look and feel of your finished product.
Projects knit from dense yarns are more likely to sag. Cotton is your prototypical dense yarn. In fact, almost all plant fibers will be more dense than animal fibers, barring some pretty heavy work on the manufacturing end. And as you’ve probably already discovered for yourself, heavy cotton things sag. More weight per square inch means the pull of gravity is felt that much more, and unless you edit the pattern to prevent it, your cute, baggy, hip-skimming sweater will end the day as a tightly-fitted, knee-brushing sack.
Projects knit from dense yarns have more drape. Again, it’s the gravity thing– denser yarns are pulled downward, stretching vertically and contracting horizontally. The result is that they tend to lie against the vertical planes of whatever they’re draped over, be that a human body or a stair railing. Less dense yarns, on the other hand, will float out or hang straight down, leaving space between you and them.
Denser yarns have less air inside. When playing around with yarn, you’ve probably found that some yarns are “squooshy,” and some aren’t– when you squeeze the ball of yarn, some yarns give more than others. Dense yarns are not very squooshy, because less air was trapped among the fibers when the yarn was spun. This means there’s less space for the yarn to compress later on. Low-density yarns can generally be knitted in a wider variety of gauges without sacrificing comfort, while high-density yarns will only go so small before the fabric becomes stiff and rigid. Some patterns deliberately use a smaller needle size than typically called for by a yarn, to achieve a denser fabric (my Spiral Hat pattern, worked in 100% wool, is one of those). Depending on the density of your starting yarn, you may or may not want to follow that recommendation.
Denser yarns mean heavier products. I know I said earlier that this isn’t very important, but in certain situations, the weight of the completed knitted object does matter. For one, most knitted objects are already heavier than their machine-manufactured counterparts, because knitters typically work at much larger gauges than mass-produced products. Even if our yarns aren’t more dense, our fabrics are thicker. Density comes in when working on projects that create a very dense fabric anyway, such as heavily-cabled sweaters like Alice Starmore’s Na Craga (pictured above). Na Craga is knit using Aran-weight yarn, which typically takes a size 9 needle, but the pattern calls for a size 7 needle. Knitting Na Craga in a dense yarn would be piling density upon density, resulting in a stiff, heavy sweater that seems even stiffer and heavier when compared to the store-bought article.
Dense yarns experience more friction. Within fabric knitted from a dense yarn, the stitches pull more heavily on one another. Over time, this friction results in increased wear-and-tear.
Projects knit from dense yarns tend to be less warm, per inch, than projects knit in less dense yarns. This may seem counter-intuitive, since many knitting patterns use smaller needles to maximize fabric density for heavy-duty winter outerwear. However, fabric density and yarn density are different things. Dense fabric tends to be warmer than non-dense fabric because the fabric has fewer holes through which cold outer air can enter and warm inner air escape. Dense yarn, on the other hand, usually isn’t as warm, because it lacks the air pockets that are necessary to insulate a person within the knitted object. Not only are there no air pockets spun into the fibers of a really dense yarn, but the resultant fabric will cling closely to your skin, so there will be no air between you and the garment to act as additional insulation. Air is what typically holds your body heat, even as the garment itself gets chilled. A lack of insulation is a great quality in the summer; not so great in winter.
Aside #1: Fabric density and yarn density operate as opposites to one another. Ideally in the summer you’d use larger needles than called for to knit a loose fabric with a dense yarn, probably a plant fiber. The resulting fabric would lie close to the skin, letting your body heat escape and keeping you cooler. In the winter, you’d take a lighter, less-dense yarn (probably an animal fiber) and knit it with smaller needles than called for, resulting in a dense fabric that will skim over your body, leaving space between you and the garment for warm air to accumulate, while the dense fabric keeps the warm air from escaping.
Aside #2: I’ve just recently learned that handspun yarns tend to be denser than machine-spun yarns. As a spinner, I see why– it’s a lot safer to err on the side of adding too much twist (squishing out more air between fibers) than too little!
When I get to plying and texture (which may not be until Saturday), I’ll get into why it is that animal fibers are usually more airy and plant fibers more dense.