How you ply your singles can dramatically change the nature of your yarn, but how many plies is best? It depends.
I decided to conduct an experiment with my Nebula hand spun yarn, which is a blend of Merino, silk, and Stellina. I was whipping up a sample skein (50 grams, 2-ply) in a heavy lace/thin fingering weight for an auction donation, so while I was at it, I spun 50 grams of singles the same weight as my 2-ply.
The results are pictured below, with the singles on top and the 2-ply on the bottom. Both of these are fresh off the bobbin, and neither has had the twist set yet.
When I plied the bottom yarn, the singles had been resting for a while. Adding the ply twist caused them to be energized again, which resulted in a smooth twist that ran down the length of the hank. If I had plied energized singles, it would have looked balanced. Never fear, though! It just needed a good soak for everything to even out.
The singles on top were freshly spun. You can see how they curled and twisted as soon as the hank was no longer under tension. I prefer yarns with a higher twist, so it was challenging to only use enough to get the fibers to stay together.
You can really see the difference between the two hanks below.
Then it was time for a soak! I filled the kitchen sink with hot tap water and submerged my yarns. I like to be a little rough with them so that they full nicely without felting together. I read a suggestion to shock singles with a dip in cold water following the hot to encourage some felting, but I didn't try that here. I wonder if that would enable me to have a stronger yarn with less twist.
After about 20 minutes, I squeezed out as much water as possible and hung the hanks up to dry. You can see in the picture that the middle hank, which is my two ply, has a quarter twist in the direction of my plying, and the right hank, which is a single, has a quarter twist in the other direction. My blue yarn, on the other hand, was perfectly balanced!
You can weight your yarn to make it hang straighter, but you run the risk of stretching it, and the twist can become active again once you block your final project, resulting in an unwelcome surprise!
If yarn is unbalanced, anything made from it can skew to one side. While this can be a cool design feature if you plan for it, it will ruin anything that is meant to lay flat. No amount of blocking will make it right. I put anything with more than a half twist when dry in the "for felting" pile. Anything with a half twist I keep for my own projects, just in case. If it has a quarter twist or less, it's good to go!
While I was pretty happy with my wet singles (look at that quarter twist!), as it dried, I ended up with a half twist. So I decided to practice another skill, chain plying! This time, perfection.
Finishing your singles instead of plying them together can preserve distinct color changes. I often find pops of color that are more pronounced in singles become more subtle or muted when plied. Singles also tend to stay where you put them when blocking, and because they will flatten, your project looks softer. Having singles as your finished product means that you only spin that length of yarn once, so spinning a hank is much faster.
On the other hand, because more surface area is exposed, singles are less durable and pill more than plied yarn. You also need to make sure you use just enough twist to hold the fibers together. Too much twist can cause your project to twist, too.
Two Are Better Than One
A 2-ply yarn handles wear and tear better than a single, and plying yarn can enhance color placement. Fractal spinning, anyone? It is also much easier to balance your yarn, as plying introduces twist in opposite directions. 2-ply yarn is also lighter than a single of the same size, since more air is trapped between the strands. I love how lace opens up with 2-ply, and stockinette stitch takes on an interesting, organic texture. In my spinning experiment, the two plies also helped tame the Stellina, which stuck out with wild abandon in my single.
Even though 2-ply is a hands-down winner in my book, like anything else it does have its downside. Plying takes time that you could be using to spin more singles. While it showcases texture better than singles, the stitches aren't as crisp as they could be, and cables fall a little flat. It's also tougher to make colors line up in gradient or self-striping yarns, and some people don't care for the marled, barber pole effect.
Good Things Come In Three
The only time I spin 3-ply yarn is when I use chain plying. It makes it easier to preserve color changes in gradient or self-striping yarns, but you can also play with color if you ply 3 different singles. In terms of shape, 3-ply yarn is rounder, which makes for a smoother yarn. Because it is so crisp, color changes have the cleanest edges, and it creates the best stitch definition and most dimensional cables.
Of course, the more plies you have to create for the same length of yarn, the longer the process takes. If you want a thicker yarn, you can just spin thicker singles. And the same structure that makes stitches distinct also causes the holes in lace to close.
So What's Best?
Only you can answer that question. Different projects call for yarns with different natures, so try the dreaded swatch and choose what plays best!