Over thousands of years, humans have refined and perfected the methods to cultivate, harvest and process hemp, to obtain fibre for making garments and much more. Today, we have countless types and blends of textiles available to us, to suit all requirements and applications. But how is hemp fibre obtained and processed?
A Summary of Fibre Hemp Cultivation
Fibre hemp is produced from low-cannabinoid varieties of Cannabis sativa that are planted close together to minimise branching and encourage tall, straight central stems. Unlike most psychoactive strains of cannabis, fibre varieties typically have hollow stalks, which contain far higher amounts of fibre (35% compared to 15%).
In the northern hemisphere, fibre hemp seed is planted when the ground temperature rises above 7.5°C, and is typically harvested in around August, once the pollen begins to be shed. Hemp can flourish in conditions considered less than optimum, and will usually produce more than competitor crops in such instances.
However, for optimum harvests, hemp should be cultivated in mild, humid conditions, in well-drained, non-acidic soil that is high in nitrogen. Soil must be moist but not excessively so, as overly-moist soil has been shown to produce weak fibres; cool summers are also said to assist the fibre in becoming fine and strong.
Harvesting & Separating the Hemp Fibres
If cultivating solely for fibre, males and females are both cut down as soon as the males begin to exude pollen. If cultivating for fibre and seed, the males are allowed to pollinate the females before being cut; the females are left to mature until the seeds are ripe, at which point the plants are cut and the fibre and seed separated.
Interestingly, traditional farmers of hemp in the UK held that male hemp plants produce fibre much finer and silkier than that produced by the females; this was borne out by a 1996 study conducted in Hungary, which concluded that male fibre was finer but female fibre slightly stronger.
Retting of Hemp Fibre
Once the plants have been cut, the stems are usually laid out along the ground for several weeks so that retting can occur. This is a process of decay whereby the pectin (the gel-like polysaccharide present in most plant cell walls) that binds the fibres together decomposes on exposure to light and air, and the long bast fibres are exposed. Bast fibres are those that occupy the phloem or inner bark of dicotyledonous plants such as hemp and flax.
Retting may also be done in water tanks, which speeds up the process, or in frost and snow, which is said to produce a whiter, finer fibre. Now, there are also chemical and enzymatic means with which to speed up the process of retting.
Decorticating Hemp Fibre
Decortication is the removal of the central woody core from the stem. This step can be performed immediately after retting, while the stems are still wet; in this case, the damp fibres are peeled off the core and then dried. Alternatively, the stems can be dried and then processed with specialised machinery, which breaks up the woody core and separates it from the fibres.
Modern decorticators often negate the need for long retting periods and separate decortication processes, instead combining the processes into one and producing ready-to-bale fibre within a few minutes of cutting the plant.
Treating the Hemp Fibres
Once the fibres have been separated, they are formed into bales and removed from the field to be processed into yarn. Often, the fibre is spun without further processing; however, some producers have developed chemical or mechanical processes that increase the softness or elasticity of the fibres.
For example, one involved process necessitates soaking the fibres in a near-boiling solution of soap and carbonate of soda, before being washed with water and soaked in dilute acetic acid. The fibres are washed in pure water once more, then dried and combed to produce an end result of exceptional softness and fineness.
Removing Lignin from Hemp Fibre
Lignin is a hard, woody biopolymer that makes up 8-10% of the dry weight of hemp fibre, and is responsible for the rough, scratchy feel of traditional hemp fibre. If the lignin is removed, the resulting fibre is much smoother and softer. The inability to remove lignin from hemp without reducing its strength led to other crops being favoured over it—yet another reason that its use began to decline so dramatically in the post-industrial period.
In the mid-1980s, researchers developed a new technique to remove the lignin through enzymatic & microbial means: the protein-digesting enzyme protease is first applied to the hemp fibre, which reduces the nitrogen in the stems; then, a species of fungus known as Bjerkandera is allowed to grow upon the fibres, where it consumes the lignin. The fibres produced with this technique were far more versatile, and hemp began to be used in garment-making once more.
Spinning Hemp Fibre into Yarn
Hemp yarn is spun similarly to other natural fibres; typically, the fibres are twisted together to form long, continuous threads, which are often sealed with wax or a similar agent to render the end result waterproof or more durable.
It is usually at this stage of the process that other fibres are added to the blend: rather than blended cloth being woven from threads made purely from one type of fibre, the thread itself is a blend of fibres that influence its final characteristics. However, this is not always the case: fustian, for example, traditionally referred to a textile made from a flax warp (lengthwise thread) interwoven with a cotton weft (transverse thread).
The Hand-Spinning Process
This process was traditionally performed by hand, with the help of nothing more than two simple tools, the drop spindle and the distaff. The drop spindle is a spike-shaped weight to which the raw fibre is attached, and the distaff is a wooden stick around which the lengths of raw fibre are looped.
The hand-spinner sets the spindle spinning, and slowly releases raw fibre from the distaff; the spinning motion and the pull of the weight as it slowly drops cause the fibres to be tightly wound into threads. Some hobbyists and specialist producers still spin by hand using these traditional tools.
When hand-spinning with hemp fibres, the best choice of spindle is a lightweight top whorl, a type that can spin very fast and produce a fine, smooth yarn. Most hemp yarn is dry-spun, but can be spun ‘wet’: the spinner merely moistens the fingers with water and strokes the yarn as it spins, smoothing down the flyaway fibres to produce the smoothest possible result.
Although hemp now has to compete with an array of alternative fibres, both natural and synthetic, improved processing techniques have uncovered ground-breaking new uses for hemp textiles; as well this, the need to find textile crops of low environmental impact is increasing rapidly. For these reasons, hemp is growing in significance once more after a long period of decline, although it may never regain its former status as the number one textile crop.