The need to move more and more towards natural and ecological textiles has in recent years brought hemp fabric, an ancient fabric with a low environmental impact but excellent breathability and high quality, back into the limelight. However, given a halt in hemp production that lasted for several decades, which also caused us to lose a lot of acquired knowledge on sowing, harvesting and processing, hemp sowing today is still very low compared to its potential. This situation is mainly related to the lack of development of specific machinery for harvesting and processing and, especially in Europe, its production still requires a high number of manual steps.
Hemp fabric is made from the long strands of fibre that make up the stem of the Cannabis Sativa plant. After harvesting, these fibres are separated from the bark through a process called 'retting' and are then spun together to produce a continuous thread that can be woven into garments. This is a very long and complex process that has been perfected over thousands of years to achieve the best results and create ever finer fabrics. Let us take a detailed look at the hemp processing steps that take us from the field to the finished fabric.
Sowing of hemp
Hemp needs a temperate climate to grow and its life cycle is very fast. In Europe, it is generally sown between March and April and harvested between the end of July and August, as it takes about 120 days from sowing to harvest (for more information on the botany of the plant, read here).
When its use is for textiles, it is sown very densely, also known as 'lime' sowing. This is a plant that is 'clean' of pesticides, fertilisers and herbicides, which does not impoverish the soil, but rather prepares it for subsequent crops. This aspect allows hemp to be sown on the same land for several years, thus not following the principle of rotation, which is common for other textile fibres.
Harvesting hemp for textiles
Harvesting is very delicate, as it is necessary to collect the hemp after flowering but before the seeds settle, as the seed formation roughens the fibres, which then makes it much more difficult to process the hemp fibre. Harvesting is done manually by cutting the plant at the root or mechanically with a combine harvester. Once harvested, the hemp is left to dry in the field, awaiting an initial drying process from which the leaves and inflorescences are lost [1].
For the part of the hemp that can be weaved, the leg of the plant is used (and therefore not the flowers, seeds and roots). Harvesting yields two types of stem length, namely [1,1]:
- long fibre that generally varies from 1.50 m to 3.00 m, is cut into pieces of approximately 70 cm and used for weaving;
- short fibre, i.e. made from the ends, is used instead for paper production and other applications.
Stem maceration
Once dried in the sun and divided by size, the stems are macerated. Maceration can be done using the following main techniques:
- Artificial technique: using chemicals (such as caustic soda) whose main purpose is to shorten the soaking time and thus speed up the breakdown of pectin. However, the chemicals can also attack the fibres and compromise the final quality of the fabric.
Dew technique: this is definitely the oldest technique, as the crop is laid on the ground for a period of 3 to 6 weeks on average, taking care to turn the plants so as to achieve uniform maceration. Through this exposure to weathering as the pectin, which binds the outer fibres to the inner fibres begins to break down and then the outer fibres separate into individual fibres. The stems can be either lying on the ground (in this case we speak of dew retting) or standing (so-called stand retting) [2] or
Soaking technique: the stems are soaked in water. Once upon a time, they were mainly left to macerate in streams or ponds, specially created to collect rainwater. In recent decades, the most efficient maceration is linked to the use of hot water tanks to which bacterial and/or chemical agents are added, which considerably speed up the maceration process [3]. In fact, with the water-soaking technique, the waiting time is halved.
Stretching and softening of stems
Once the stem and fibre have completed the maceration cycle, i.e. have started to separate, the stems are scutching, which allows the stem to be further broken up and thus the textile fibre to be separated from the stem. This operation can be carried out either with a manual, and obviously more laborious machine or, with an industrial debarker. The waste obtained from this process, i.e. the hemp, can also be used for further applications in the construction or paper industry. However, the yarn obtained from this initial processing is still very strong to be combed and spun, so the fibres are softened [4] by passing them through special rollers that further soften the yarn.
Combing the fibres
The long fibre is cut into homogeneous lengths, usually 70 cm, and fed into automatic machines, so-called 'combers', which form a continuous ribbon of flakes. In industrial production, the different slivers are woven into a single sliver by means of the coupling procedure, which also allows the thickness of the draft to be defined [5].
Hemp spinning
The next step is spinning and its procedure varies depending on the desired thickness, i.e:
- wet spinning, for thinner yarns, which is then followed by further stretching and passing the sliver in hot water,
dry spinning, i.e. without the use of water, for somewhat coarser yarns [6].
The mechanical weaving of hemp
Like all types of weaving thread, hemp thread can also be used mechanically. However, despite mechanisation, the mechanical spinning process is quite lengthy. For a fine yarn, in fact, the fibres have to go through several steps of doubling and stretching. Next comes 'drawing', the process of passing the fibres through a series of rollers to straighten the yarn. After several rounds of doubling and stretching, the fibres are attached to the roving frame for spinning and wet-drying.
It is customary to boil the fibre in caustic soda and bleach it with hydrogen peroxide to create finer quality yarns (a process known as degumming and bleaching). The final stage is winding, basically wrapping. This is the process of transferring the lengths of yarn properly wound or coiled from the spinning reels to a suitable spool. The natural characteristics of hemp fibres can be modified for finer yarns by facilitating efficient fluid diffusion. This is why the degumming and bleaching process is particularly critical.
Shuttle weaving
A shuttle is a tool designed to neatly and compactly store a carrier carrying the weft thread during weaving with a loom. Shuttles are thrown or passed back and forth between the warp threads to weave the weft.
The simplest shuttles, known as 'stick shuttles', are made from a flat, narrow piece of wood with notches at the ends to hold the weft thread. Shuttle hemp fabrics are made using a loom on which the warp threads are mounted, through which the weft thread passes. The operation seems complex, but the weft thread comes and goes through the warp threads passing over or under them.
We can recognise shuttle weaving by the series of curves (called selvedges) with which the weft exits and re-enters the warp. Shuttle fabric consists - in a nutshell - of two or more thread systems crossed perpendicularly: the warp threads (arranged vertically) and the weft threads (arranged horizontally).
Jersey weaving
When we speak of jersey weaving, we mean an elastic fabric suitable for creating close-fitting and comfortable garments. Jersey is made on circular knitting machines with a single row of needles, i.e. they only work the vertical needles of the cylinder. Jersey is mainly used for the production of clothing.
In this case, each stitch phases a bit of knitting. To fully understand its structure, we can think of Jersey as a sort of spiral pattern; the resulting fabric is compact, smooth on the outside which differs from the reverse which appears more 'rough'.
To sew jersey, it is necessary to use a special needle whose blunt, rounded point, similar to a knitting needle, does not damage the stitch structure of the fabric. To prevent seams from tearing when the fabric is stretched, a stitch with a larger thread width, such as a zigzag or triple stitch, is chosen.
The name of this incredible technique is thought to derive from Jersey, the largest and best known of the Channel Islands, an important exporter of textiles since the Middle Ages.
Insights and references:
https://allfiberarts.com/2021/weaving-with-hemp.htm
https://patents.google.com/patent/CN1130694A/en
https://recreator.org/blogs/hemp-101/hemp-101-a-traditional-method-of-hemp-textile-production
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Translated with Linguana