A Yarn of Chemistry: All Ewe Knit to Know-Part 2

In part 1, I dealt with the chemistry of animal fibers: wool and silk. But animals are not the only source of fibers that we use for making yarns. Another source is from plants; these are termed "cellulosic fibers," as they are all primarily comprised of cellulose. I plan on covering cotton, linen, and hemp in this section. (The photo on the left is me in a yarn shop in Heidelberg, Germany.)

To begin, let's talk about the structure of cellulose. It is a polysaccharide. This means that it is a polymer (made of many units) and the repeating units are "saccharides" also called sugars, or carbohydrates. In cellulose there is only one sugar that is repeated to make it a polysaccharide. That sugar is glucose. Now this might surprise some people because we use glucose all the time: it is in starches, and refined table sugar, and the bowl of candy on my desk, all of which we eat and digest. But we cannot digest cellulose. And yet the part of the potato we can digest is made of the same stuff that the part of the potato that  we peel off because we can't digest. This has to do with how the glucose units are strung together. 

The picture on the right shows two molecules of glucose put together: the bond between glucose molecules is called the glycosidic bond and is highlighted in red. On the top is an "alpha" bond-this is how glucose that we can digest is arranged, so like starch. On the bottom is a "beta" bond-this is how cellulose is put together. Most animals do not possess an enzyme that allows them to break beta bonds and therefore they cannot digest cellulose. By having beta bonds in place of alpha bonds, the chain of glucose molecules becomes more linear and more rigid. This is why cellulose is used in plant cell walls: the rigidity gives the plant strength. 

Probably the most important cellulosic fiber is cotton. This particular fiber, unknown in Europe until the Middle Ages, is most associated historically with the growth of slavery and the industrial revolution. The fruit of the cotton plant produces bolls, in which the seeds are wrapped up in a mass of cotton fibers. The cotton plant requires long, hot summers, well drained soil, moisture, and no frost: this is why Canada has never been known for its quality cotton. There are many countries around the world that manufacture cotton, but the conditions that it is grown in can impact the properties of the fiber. The cellulose chains in cotton long and linear, due to the beta bond, discussed vide supra; this allows many chains to pack closely together and interact with each other through the formation of hydrogen bonds, making it highly crystalline.These cellulose fibrils are then arranged in essentially three layers that are spiraled together, resulting in the high strength that cotton is known for. This is important to know when working with cotton yarn because they are less stretchy than wool, and will show any mistakes or irregularities in your work. This can be quite frustrating for new crocheters or knitters. The other thing I have found when working with cotton is that the individual plies of the cotton yarn don't stick together as well as they do in wools, making really easy to put your hook through the strand. There are different standards of cottons. The longer the length of the fibers, the softer and nicer (and consequently more expensive) the cotton is. Egyptian cotton fibers are between 25-65mm, this is what makes them so lovely. American cotton is between 10-25mm. Another interesting property is that the fibers are actually stronger when wet.

Cotton is easy to wash, breathable, absorbent, and less of an allergen than wool. It also dyes very well, meaning that it can be found in all sorts of great colours. One of the most common projects for cottons is dish clothes and towels, like the set I made on the left. Because you are going to be pretty hard on dish cloths, you don't want to use high quality cotton, go with shorter, rougher cotton. Now when making something for a baby, cotton is not a bad choice. I most recently used cotton in a baby blanket. Anything you are making for a baby, you want to make sure that it washes easily because it WILL get dirty and they aren't going to be gentle with it. Here is a great project for a nicer cotton. The longer, softer fibers make a nice blanket. 

The next type of cellulose fibers are called "bast fibers". These are ones that are derived from the stem of the plant. Unlike cotton, these fibers are part of the structural make up of the plant, and the job of holding it up requires a lot of reinforcements, meaning that these cellulose fibers are mixed with a bunch of other things like: pectins, gums, waxes, lignins, and hemicelluloses. 

Linen has to be one of the oldest, if not the oldest, cloth fibers. Seriously, hop in your time machine and head to Egypt in 8000 B.C. and you will find linens.  This prized cloth is made of fibers isolated from the flax plant. Flax fibers are found at the surface of the stem and run the whole length. Since the stem is about a metre in length, you can see how flax fibers are longer than cotton fibers, ranging from 6-65mm (average length is 20mm). Flax fibers are also stronger than cotton fibers; actually this is one of the strongest naturally occurring fibers. Like cotton, linen is light-weight and absorbs water readily. It is easily laundered and takes dye well. It is a good conductor of heat, which is why it is so nice to wear in hot climates. As a dense fiber, it drapes well, but it also wrinkles super easy. Look at it the wrong way and it will wrinkle. Its stiffness can make it a challenge to work with, especially if you are just learning, but the history makes it an interesting choice too. 

Hemp is another bast fiber that is used in textiles. But for those of you who would like to use hemp as a reason to legalise marijuana, I hate to inform you, but that is a different plant. While they are of the same genus, the hemp cultivar only contains a small amount of THC. The amount of cellulose in hemp fibers is lower than in cotton, and it tends to have lignin in it. This makes it rougher and stiffer than cotton. But being long, at a typical 15mm length, and strong, it lends itself well to the production of ropes. I have never seen a yarn made of hemp in the shops I frequent, but I am positive there are some out there. 

A relatively newer cellulosic yarn is that derived from bamboo. These fibers are quite long at 38-76mm. Bamboo is super absorbent. The fibers tend to be smooth and round, leading to the soft feel of the yarn, as well as its low irritability, making it ideal for projects for anyone with sensitive skin, like babies. It also has a lovely sheen to it.

Coming up in Part 3-synthetic yarns!

References:

Stoller, D. Stitch'N'Bitch: The Knitter's Handbook 2003, Workman Publishing Company, Inc. New York, NY.

Crowfoot, J. Ultimate Crochet Bible 2010, Sterling Publishing Co. New York, NY.

Pratt, C. W.; Cornely, K. Essential Biochemistry 2004, John Wiley & Sons, Inc. Hoboken, NJ.

Mather, R. R.; Wardman, R. H. Chemistry of Textile Fibers 2011 Royal Society of Chemistry.

Le Couteur, P.; Burreson, J. Napoleon's Buttons 2003 Penguin Group, New York, NY.