The length and quality of the fiber in a yarn determines its texture, luster, strength and hand. Knitting yarn made from long fibers will pill less, be smoother, stronger, more lustrous and more elastic. Yarns containing a mixture of fiber lengths are softer, fuzzier and less strong. Tightly twisted yarns display the texture of a pattern to its best advantage.
ANIMAL YARNS:
Wool yarn comes from a domesticated sheep. Wool accepts dye well, is flame-retardant by nature, remains warm even when wet, sheds water better than other yarns. Natural wool should be hand-washed. ‘Superwash’ wool has been treated to allow machine washing. Wool will usually resume its proper shape when washed correctly; if it is mistreated and washed in too-hot water, it will shrink or felt.
Mohair yarn comes from an Angora goat. Mohair is durable, sheds dirt, dyes well and does not felt easily. Despite its hardiness, it is usually spun into knitting yarn used for fluffy garments and scarves. This knitting yarn is abraded, roughing its fibers to create that ‘fuzzy’ look.
Angora yarn comes from rabbits. Fabric made from this yarn is inelastic (no stretch), very fluffy, soft and warm.
Silk is the yarn produced by silk moths. Silk knitting yarn is made from damaged silk cocoons and broken fibers. ‘Raw’ silk still has the original moth secretions in it. ‘Tussah,’ silk obtained from wild moths is brown. The food fed to domesticated moths determines their silk’s natural color; this can white, green or yellow. Silk retains heat, absorbs moisture, pills less than wool, is very strong and very stable when knit, neither shrinking or stretching.
Cashmere yarn comes from the undercoat of a Cashmere goat. It is so expensive because only a few ounces are obtained from each goat per year. It is such a delicate yarn, more fragile than wool and more susceptible to abrasion, that it is usually blended with wool to make it more durable.
Alpaca Like cashmere, angora, and mohair, alpaca is a luxury fiber that is available in very limited quantities. Actually, most alpaca fleece goes into the production of tops which are used in blends with other fibers. One hundred percent alpaca garments are much rarer. For these reasons, alpaca is expensive. Like other luxury fiber garments, alpaca knitwear should be dry-cleaned or hand washed in cold water and dried flat.
VEGETABLE YARNS:
Cotton yarn comes from the surrounding of the seeds in a cotton pod. Cotton is heavy, dense and inelastic; although it will regain its shape after washing, its ability to do so decreases over time. It is comfortable to wear in a cool climate but not a hot one (the opposite of wool) and is slow to dry once wetted. It makes a weaker knitting yarn than silk or linen but is stronger than wool.
Linen yarn comes from the flax plant. It is durable and stronger than any other yarn. It absorbs moisture better than cotton and dries more quickly, making it more comfortable to wear than cotton in hot temperatures. It is easier to wash than wool and does not stretch or shrink.
Ramie yarn is made from nettles. It is often used as a substitute for linen since it is less expensive but shares linen’s good qualities.
Rayon is a yarn produced from natural ingredients by artificial means.
Rayon yarn is a weak fiber but it is absorbent, dries quickly, and stretches (although it does recover some when dried in a automatic dryer.)
Great News!!! 
(affordable isn’t it?)
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