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In the textile arts, plying is a process used to create a strong, balanced yarn. It is done by taking two or more strands of yarn that each have a twist to them and putting them together. The strands are twisted together, in the direction opposite that in which they were spun. When just the right amount of twist is added, this creates a balanced yarn, which is a yarn with no tendency to twist upon itself. Almost all store bought yarns are balanced, plied yarns.
A two-ply is thus a yarn plied from two strands, a six-ply is one from six strands, and so on. Most commercial yarns are more than a two ply. Embroidery floss is generally a six ply yarn, for example.
The creation of two-ply yarn requires two separate spools of singles and either a lazy kate or something to hold the spools in place. On a wheel, two-ply is created by taking two spools of singles, placing them on a lazy kate, tying the ends together onto the spool attached to the wheel, and spinning the wheel in the opposite direction that the singles were spun in while also feeding it onto the spool on the wheel. On a drop spindle, two-ply is created by placing the spools on a lazy kate, tying the ends together onto the drop spindle, holding equal lengths of singles together and dropping the spindle. The weight of the drop spindle combined with the twist in the singles, causes the drop spindle to turn in the opposite direction that the singles were twisted in until the two singles are plied together.
Plying handspun yarns
When hand-spinning, there are two common ways to ply a balanced yarn: regular and Navajo.
When spinning fleece into yarn, you must first scour the fleece, remove the vegetable matter, card or comb it, and then spin it into singles. These singles are then used to create the finished yarn in a process known as plying. The purpose of plying singles is to strengthen them so that they do not break while knitting or crocheting them.
Most spinners (who use spinning wheels) ply from bobbins. This is easier than plying from balls because there is less chance for the yarn to become tangled and knotted if it is simply unwound from the bobbins. So that the bobbins can unwind freely, they are put in a device called a Lazy Kate, or sometimes simply kate. The simplest lazy kate consists of wooden bars with a metal rod running between them. Most hold between three and four bobbins. The bobbin sits on the metal rod. Other lazy kates are built with devices that create an adjustable amount of tension, so that if the yarn is jerked, a whole bunch of yarn is not wound off, then wound up again in the opposite direction. Some spinning wheels come with a built in lazy kate.
Regular plying consists of taking 2 or more singles and twisting them together, the opposite way. This can be done on either a spinning wheel or a spindle. The most important thing to remember though is that the twist must go the opposite direction. If in spinning the single the wheel was spinning clockwise (which is called a "Z" twist, as on any given side the fibres appear to cross diagonally in the same direction as the diagonal of a "Z"), in order to ply it the wheel must spin counter-clockwise (an "S" twist). This is because otherwise you are not balancing the twist, just twisting it more. The concept is similar to when a heavily twisted piece of yarn is folded, and it twists up on itself. It is most common for singles to be spun with a "Z" twist, and then plied with an "S" twist.
FORMULA FOR S / Z TWIST :
CONSIDERING PRIMARY TWIST IS S AND SECONDARY TWIST IS Z, The relationship between S and Z twist is as follows : S:Z = 1:0.8 in regular plying, this formula will make the final thread balanced.
When plying, the singles are kept separate, either with the fingers or with a tool. This tool can be anything from the top of a salt dispenser, and the singles threaded through the holes, or a specially carved piece of wood with holes in it. The singles are kept separate to ensure that they do not get tangled and so the tension can be controlled.
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Navajo plying (also known as chain plying) consists of making large loops, similar to crocheting. Only one single is necessary, and if the single is already dyed this technique allows it to be plied without ruining the colour scheme. First make a loop about 8 inches long through the loop on the end on the leader. (A leader is the string left on the bobbin, which the new yarn is spun from.) Start spinning all three strands together in the opposite direction than that they were spun in. When only 2 to 3 inches (76 mm) remain of the loop, pull a new loop of yarn through the loop, and continue spinning. The new loop should be around 7 inches (180 mm) long. Repeat this process until the yarn is all plied. This technique allows the spinner to try to match up thick and thin spots in the yarn, thus making for a smoother end product.
Machines that ply yarn use the 'regular' method mentioned above. The main difference is that instead of humans, gears control the intake making sure that the strands all have the same tension and the same length. Other than that, the process for plying is exactly the same.
Many novelty yarns make use of special plying techniques to gain their special effects. By varying the tension in the strands, or the relative sizes of the strands, or many other factors different effects can be achieved. For example, when a soft, thick strand is plied against a tightly twisted thin strand, the resulting yarn spirals. Another example is bouclé, which is a yarn where one strand is held loosely and allowed to make loops on the other yarn while plying.