Wool is a natural material. As such, it shares much in common with other natural textiles materials such as organic cotton and hemp and shares many of the same characteristics as these materials when it comes to sustainability. Yet unlike these materials, wool is derived not from plants, but, of course, from animals. This means that it comes with a whole range of different considerations with regards to sustainability and ethics that do not apply to these other raw materials for textiles.
In this article, we will look at the properties of wool fabric in more depth, looking at how it is used in weighted blankets; what its different physical characteristics are and what these mean for its general use and application. We will take a look at the history of the material, and examine the processes involved in each stage of the process – from rearing sheep right through to the creation of the finished fabric. In doing so, we will build up a picture of the sustainability credentials of this particular textile material.
How and Why Wool Fabric is Used in Weighted Blanket
Wool is extremely good at keeping people warm, whether worn as clothing or used in weighted blankets on a bed. Its thermal performance beats many other textiles, both natural and synthetic. Yet while it can keep us warm, wool is also a breathable material. It retains its insulative properties even when wet, and can absorb a large proportion of its own weight while remaining dry to the touch. What is more, wool does not support the growth of mold and has natural antibacterial and antimicrobial properties. It also performs well in case of fire.
As a natural fibre, wool is often also used as a sustainable option (though its sustainability credentials, as described in much more detail below, do vary considerably). It does have a range of positive characteristics with regards to the environment, it’s recyclable, and will biodegrade in soil or compost. However, though easily biodegradable, wool is also an extremely durable material and can create a range of long-lasting textiles.
Where Wool Comes From
Wool is the term usually applied to the textile fibre which is obtained from the fleeces of sheep. However, it is worth noting that the term can also be applied to fibres obtained from the coats of a range of other animals. For example, wool can also be used to refer to cashmere and mohair obtained from goats, qiviut from muskoxen, angora from rabbits, and other types of wool from camelids. In this article, however, we will be focussing on the wool that comes from sheep. This is the most common animal fibre used in the textile industry.
We all know that wool comes from sheep. Most of us are familiar with the appearance of these animals, and what their fleeces look like. Looking in a little more depth at how the woolly fleece is actually produced, however, can yield some fascinating insights that allow us to understand the material in a little more depth.
Wool is produced by small follicles (skin cells) located in the skin of the animals. Located in the epidermis, or upper layer of skin, theses push down into the second layer of skin, which is known as the dermis, as the wool fibres grow. Follicles are of two different types – primary and secondary. Primary follicles produce three different types of fibre – kemp, medullated fibres, and true wool fibres. Secondary follicles produce only true wool fibres. Hemp fibres are very coarse and shed out, and medullated fibres are long, like hair, but lack crimp and elasticity (1).
The proportions of these different types of fibre in a fleece depend on the variety or breed of sheep. They will determine how best a fleece can be used.
The Structure of Wool Fibre
Sheep’s wool has three distinct parts or layers. On the outside of the fibre is an outer layer comprised of scales, which absorbs water. Within is the main part, the cortex, which accounts for 90% of the fibre’s mass. At the centre is the medulla, which has a honeycomb-like structure with air pockets.
Unlike cotton and other plant-derived fibres, wool is made up largely of protein rather than cellulose. Approximately 33% of wool fibre is composed of keratin. Carbon, hydrogen, oxygen, nitrogen, and sulfur are combined in amino acids which then link together to form polypeptide chains. Another important component in wool is lanolin, an oil secreted from sebaceous glands. (It is this which accounts for the allergic reactions some people experience when exposed to the fibre.)
Like other hair fibres, it is largely composed of different amino acids along with basic monomeric units and therefore cannot crystallise well.
Wool fibre differs from hair or fur in that it is crimped and elastic (2). Different wool fibres can vary considerably depending on the type of sheep from which they are derived. The amount of crimp wool fibre is one of the characteristics that will determine its fineness. A fine sheep’s wool like Merino will have up to 100 crimps per inch, while coarser wools will have far fewer.
The crimp of wool fibre is the property that allows it to be bound into yarn. This is not possible with hair, which has no crimp. In addition to crimp, the quality of a wool fibre is determined by its fibre diameter, yield, colour and staple length. Fibre diameter is, of all these things, the most important factor in determining the quality and price of wool.
The finest and most valuable wool comes from Merino hoggets (sheep of this breed that are yearlings, or between weaning and first shear). Merino wool is typically very fine – between 12 and 24 microns) and has a length of 3-5 inches.
Generally a creamy white in colour, wool fibre can also come in other natural shades, such as black, brown, silver or random mixtures of hue.
The Physical Properties of Wool
In order to understand wool fibre, we can examine a number of its most important physical properties. These properties define and determine the range of uses and applications for the material within the textile industry.
The Fibre Density of Wool
The fibre density of a material has a direct correlation with the weight of a fabric. Wool has a fibre density when dry of 1.30 (g/cm3, Mg/m3), and a specific volume of 0.77 (cm3/g) The density of wool, like that of other materials, alters when it absorbs water.
The Thermal Properties of Wool
Wool has a number of beneficial thermal properties.
The specific heat of dry fibres can be informative about how a fibre will behave at given temperatures and in certain scenarios. Wool has a specific heat of 1.36 J/(g K) at room temperature. This is similar to that of silk, at 1.38 J(g K). It is also just a little higher than cotton, the upper end of the range for which is 1.35 J(g K), and at the lower end of the range for rayon, which is the same figure.
Wool has a thermal conductivity of 54 (mW/(m K)). This is comparable with the figure for silk -50 (mW/(m K)) – and compares favourably with the thermal conductivity of cotton – 71 (mW/(m K)). Synthetic fibres have much higher figures for thermal conductivity (3).
It is also worth noting that wool behaves well when introduced to higher temperatures. It will ignite at a higher temperature than cotton and some synthetic fibres and has a lower rate of flame spread, heat release, and combustion. Furthermore, it does not melt or drip and forms a char which is insulating and self-extinguishing. It contributes less to smoke and harmful gases in the case of fire.
Water Absorption and Retention in Wool
Wool fibre has a hydrophobic exterior and a water absorbing interior. Wool absorbs high levels of moisture, readily, and has high regain levels for the reabsorption of water from the air. The recommended allowance, also known as the commercial regain of wool (the figure which determines what allowance should be made for water when calculating weights of the fibre) is set at around 14-19%, which is higher than all other common natural or synthetic textile fibres (4). But though it has a high moisture absorbing capability, the exterior of the fabric can still feel dry to the touch.
Another interesting thing about wool is that its extremely high water absorption and moisture regain figures is that this has an implication with regard to heat. When a fibre absorbs water, heat is evolved. Wool has extremely high heats of wetting when compared to other fibres, and will evolve more heat than many other materials – for example when 1kg of wool goes from 40% to 70% relative humidity, it generates 159 kJ of heat (5).
The evolution of heat has a considerable effect on the rate of conditioning for textile fabrics. It is also an important feature when it comes to human comfort. To give an example, a woollen man’s suit, weighing 1.5kg, when going from an outdoors atmosphere of 18 degrees C, with a relative humidity of 45%, to an indoors environment of 5 degrees, 95% relative humidity, would give out 6,000 kJ owing to this change – as much as the human body’s metabolism produces in 12 hours (6). Of course, there are implications not only for wool clothing but also for weighted blankets containing this material. As temperatures and humidity change, wools ability to absorb water and release heat can be of great physiological advantage, allowing time for the body to adjust to external changes.
Mechanical Properties of Wool
Wool, like other hair fibres, is characterised by low strength but great extensibility. How wool behaves under stress, however, can vary considerably depending on the variety and quality of wool considered, as well as on other factors such as whether the wool is wet or dry when tested, relative humidity, and the temperature. Broadly speaking, however, the tenacity of wool fibres from a given lock of wool increase with fibre diameter, and there is a slight positive correlation between breaking extension and fibre diameter (7).
Another interesting thing to note is that the physical properties of wool change remarkably in alcohol and in acid conditions, and also when placed in a salt solution. The mutability of the mechanical structure of wool fibre has a range of implications with regards to its application and use.
One final thing to note with regard to the physical characteristics of wool is that it lends itself to the creation of both woven and non-woven fabrics, which means that it has a wide range of different uses and applications within the textile industry (as well as other industries).
Like other natural materials, wool has the environmental advantage that it is biodegradable in the natural environment. Due to its high nitrogen content, wool will biodegrade within one year, significantly faster than many other materials.
During the biodegradation process, fungi first destroy the ends of the wool fibre. Bacteria then digest the weakened fibre by secreting enzymes which have evolved to break down the natural ingredients of which the fibre is composed. When placed in soil/ compost in test conditions, woollen textiles have been shown to break down in around 6 months or so (8).
The History of Wool
Wild sheep originally had mostly hair rather than wool. Sheep were first domesticated around 9,000 to 11,000 years ago, although it is believed that selection for woolly sheep began around 6,000 BC. Archaeological evidence to support this theory has been found in Iran (9). However, while selection for woolly sheep may have begun around this time, the earliest woven garments date from 2-3 thousand years later.
Woolly sheep were first introduced to Europe from the Near East in the early 4th Millennium BC. The earliest European wool textile that we know of dates from 1500 BC, and was discovered preserved in a Danish bog. During the early stages of wool use, it would have been plucked out by hand or recovered with the aid of bronze combs. Shears are believed to have been invented sometime during the Iron Age.
By Roman times, wool, along with linen and leather, clothed the European population. Imported cotton and silk were luxury goods, and there was little use amongst the general populace. According to Pliny the Elder, writing in his ‘Natural History’, the finest wool came from Tarentum (modern-day Taranto, southern Italy), where selective breeding had produced sheep with superior fleeces – though these sheep required special care.
Through the Middle Ages, trade connections and commercial links increased, turning the wool industry into a serious business and important generator of commercial capital. Annual Champagne fairs in small centres such as Provins spread woolens further afield around Europe. In the 13th Century, wool drove the economies of the Low Countries and central Italy, and Italy dominated the trade during the following century. Both industries were based on the export of English raw wool.
The importance of wool to the English economy can be seen in the fact that since the 14th Century, the presiding officer of the House of Lords has sat on the ‘Woolsack’ – a chair stuffed with wool. By the 15th Century, the English textile trade had grown such that the export of wool was discouraged. Smuggling of wool out of the country, known as ‘owling’ was at one point punishable by the cutting off of a hand. Partly aided by the Navigation Acts, after the Restoration, English woolens began to compete with silks in the international marketplace. In 1699, American colonies were forbidden from trading wool with anyone other than Great Britain. Much of the value of wool textiles lay in the dyeing and finishing of the woven product.
Before the Renaissance, in Florence, Italy, the wool guild, Arte della Lana, were instrumental in guiding Florentine policies. In Spain, the sheepwalks of Castile shaped the development and fortunes of the Iberian Peninsula. In the 16th Century, Spain allowed export of Merino lambs only with royal permission. The German wool market – based on sheep of Spanish origin – only overtook British wool relatively late.
During the Industrial Revolution, mass production technology was introduced to the wool industry and wool textile manufacture. Australia’s burgeoning colonial economy was founded on sheep rearing, and by 1845, the Australian wool trade overtook that of the Germans. Australian wool was sent to Bradford, in Yorkshire, England, which became the wealthy heart of industrialised wool textile production.
After its boom time in the late 19th and early 20th Century, Bradford, along with the wool textiles industry in general, began to decline. Demand for wool significantly declined with the introduction and increased use of synthetic fibres. In 1966, the price of wool experienced a 40% drop, and with small fluctuations, the general trend in the price of wool has been following a downward trajectory ever since. This in spite of innovations such as washable wool in the 1970s. Production of wool has decreased considerably, and sheep growers have often moved away from wool production and into the meat industry.
In recent years, there has been some small resurgence of interest in natural fibres. Some small-scale producers have had commercial success in developing new and innovative or more sustainable woollen textiles. Some sheep farmers have also looked into applications for wool outside of the textiles industry, such as for wool home insulation, or other eco-friendly products.
Today, however, wool comprises just 3% of the global textile market. (Though its value is higher due to dying and other modification of the material) (10). Global wool production is around 2 million tonnes per year, around 60% of which goes into apparel. Australia is still the leading producer of wool – largely from Merino sheep, and accounts for 25% of the global wool clip, although China now produces a greater amount of wool in terms of total weight. The United States is responsible for 17% of the global wool clip. New Zealand is another of the largest wool producers in the world, and the largest producer of cross-bred wool, responsible for 11% of the global wool clip. Amongst the smaller wool producing countries, the UK is notable for having the largest and most diverse range of sheep breeds. Though accounting for only 2% of world production, the UK has over 60 different breeds of sheep (11).
Organic wool is becoming more and more popular, as people wake up to the harm to the environment and to human and animal health involved in non-organic textiles production. Most organic wool on the market comes from Australia and New Zealand (12).
How Sheep Are Reared For Wool
In order to understand whether wool can be viewed as a sustainable textile, it is very important to look not only at the intrinsic properties of the material but also at the entire life cycle of the product – from initial sheep rearing right through to the point of sale – and beyond.
First of all, let us consider sheep farms, and take a look at the processes involved, and the implications these have for the environment, and in terms of sustainability.
The first thing to note is that, in terms of the carbon cycle, choosing wool can have a net positive result. Wool is a short term store of natural, renewable carbon. Pure, organic carbon makes up 50% of the weight of wool, which is higher than cotton (40%) and wood-pulp derived cellulose fabrics like viscose (24%). This carbon comes from the plants grazing sheep eat. Plants convert carbon through photosynthesis, and sheep use this fodder to grow wool. This is a natural, renewable, ongoing process by which carbon is sequestered from the atmosphere. In 2014, for example, the global wool clip of 1.05 tons of clean wool sequestered 1.9 million tons of CO2. All of this carbon is removed from the atmosphere during the time that the wool remains in use, thereby helping to mitigate climate change (13).
However, while the formation of wool through sheep grazing is a renewable and natural process, in basic terms, the environmental and sustainability credentials of sheep farming will largely depend on the processes and practices employed on a particular farm.
Many sheep farms do not implement sustainable land management practices. This means that ecosystems can become degraded and diversity can drop as land is over-grazed. Sheep farming in certain parts of the world has also been implicated in deforestation, soil erosion, and a range of other environmental disasters. In Britain, for example, sheep have done more damage to the environment than all the building that has ever taken place (14).
It is important, therefore, to approach with extreme caution when considering wool as a sustainable and eco-friendly textile, as its credentials in these regards are directly linked to the practices employed in the farming – before the raw material is even collected.
Sustainable sheep farming must involve:
- Mixed livestock systems which encourage livestock diversity (incorporating cattle as well as sheep, for example). Since different livestock grazes differently, such systems can help promote landscape diversity and help to maintain soil and ecological systems.
- Rotation of livestock to different grazing areas, and diverse fodder for sheep and other livestock.
- The use of land not suitable for other (arable) forms of agriculture (or as part of systems to grow both livestock and arable crops).
- Low-impact farming techniques/ machinery and minimisation of the carbon cost/ fossil fuel emissions/ pollution.
- Acknowledgment of waste and systems to avoid waste/ effective waste management.
- Avoidance of the use of artificial growth hormones and other harmful environmental pollutants.
- Avoidance of antibiotic overuse.
Since much of the sheep farming in the wool industry does not adhere to these environmental credentials, it is difficult for end-consumers to find wool textiles that are truly sustainable at the point of farming.
The treatment of sheep on sheep farms is another key issue in sustainability and ethics. Wool can only say to be a truly ethical choice where the sheep that have produced it are treated well throughout the entire process. Most sheep farmers are truly dedicated to the welfare of their animals and do their best to look after their animals and keep them healthy. Transparency in the wool industry is key, as it can allow consumers (and producers further along the chain) to confirm that sheep have been treated well.
In 2016, over 500 companies working in wool, which represent more than a million people employed in the industry, pledged commitment to the highest standards of animal welfare by signing the Dumfries House Declaration. This declaration set forth the principles of best practice in the wool industry. Those signing the declaration committed to the IWTO Specifications, which are premised on the Five Freedoms of Animal Welfare as set forth by the World Organisation for Animal Health (OIE): freedom from hunger and thirst, freedom from discomfort, freedom from pain, injury or disease, the freedom to express normal behaviour, and freedom from fear and distress. Further to this, the IWTO specifies guidance for best practice in sheep for wool farming, pertaining to the environment, nutrition, health, behaviour and handling (15).
One concerning practice (found most commonly in Australian wool production) is mulesing. Mulesing involves the removal of skin around the backside of sheep to reduce the incidence of a condition known as flystrike. Flystrike is a problematic condition, in which flies lay eggs in the folds of skin which burrow into the flesh as they hatch, and is very painful for sheep. Mulesing is a cruel and painful practice however, and there are alternatives. Flystrike can be humanely controlled by regular shearing of the hindquarters and inspection, potentially alongside other measures involving diet and/or spray washing. The New Zealand wool industry banned mulesing in 2018, though, in spite of promises that the practice would be phased out by 2010, it is still practised in Australia.
Sheep shearing is the process by which the woollen fleece of a sheep is cut off. The shearing is usually done by hand, by skilled farmers or other workers. In some cases, shearing is necessary for the welfare of the sheep, whose long fleeces would otherwise be an impediment to them. Shearing does not kill sheep, nor does it usually cause them harm. However, the process can cause minor cuts and some say that it can be traumatic for the animals. Due to time and economic pressures, shearers are often paid by the sheep, not by the hour, which encourages fast work without regard for animal welfare. Vegans will generally choose to avoid wool, even where sustainably and ethically farmed, and one of the reasons for this is that it impinges on the animals, even where, otherwise, animal welfare is of the highest standards. Animal welfare groups often also cite shearing as an ethical issue when it comes to wool.
Once shearing has taken place, the wool is separated into four main categories – fleece, which makes up the vast bulk, broken, bellies and locks. Wool straight off a sheep is known as ‘greasy wool’ (16) or ‘wool in the grease’. It contains a high level of lanolin, as well as the sheep’s dead skin and sweat residue. It will generally also contain pesticides and vegetable matter from the environment in which the animal was reared.
What Happens to the Sheep After Wool is Sheared?
When their fleece production declines, most sheep are sent to be slaughtered for meat. Whether or not consumers are concerned by this in general terms, sadly, the conditions in which sheep are sent to slaughter often raise a whole host of further ethical concerns. Sheep often face long journeys in crowded transportation and are often slaughtered by having their throats cut while they are still conscious, often in countries with poor animal welfare legislation (17).
How Fleece Becomes Wool Fabric
In Australia, wool from shearing is measured for micron, yield, staple length and sometimes colour and comfort factor before being auctioned. Wherever it is produced, greasy wool must next undergo a process of cleaning, known as scouring.
Scouring can be as simple as a bath of warm water, or far more complex in large commercial operations. Sometimes, scouring can involve an industrial process which uses detergent and alkali in specialized equipment. In commercial wool, vegetable matter is often removed through chemical carbonization (18).
Next, the clean wool undergoes a process of carding. Depending on what the end product of the process is intended to be, this process can have a number of steps, which will organise, de-tangle and align wool fibres so that they can be made into a nice uniform strand or yarn.
One the wool fibres have undergone this organising and de-tangling process, they are spun and twisted to form a single yarn. Often, finer single yarns are combined with a second strand, and the two are twisted together.
The resultant yarns can then be used in a wide variety of different ways, to make a range of different woven and knitted textiles.
Wool fibre can also be felted – a process by which the fibres are locked together and form a tight and tangled mat of non-woven fabric. This works due to the natural surface scales on the outer layer of the fibres, which mechanically lock together. More recently, wool has also been utilised in a range of other non-woven textiles, with properties that cannot be achieved with woven or knitted textiles, such as improved stretch or windproofing (19).
Dyes and Treatment for Wool
Like cotton, wool is a natural material which will happily take natural dyes. This is another reason why it is often considered a more sustainable choice than certain synthetic fabrics.
However, just because wool is natural, and can be organic and sustainably produced, this does not mean that harmful chemicals or dyes were used during the fabric manufacture. Looking for certifications and standards that take into account the entire process involved in creating a finished product, like Oeko Tex standards, can give consumers confidence that no harmful substances were used.
That said since the processing of wool for fabric is generally less intensive and complex than that required for many other textiles, post-fabric creation processing can be far less environmentally damaging than processes involved with other textiles.
Standards & Certifications in the Wool Industry
In addition to looking at general textiles standards and certifications, those buying woollen textiles can also consider looking out for specific wool related certifications, such as the Responsible Wool Standard (RWS)
The RWS is a Textile Exchange certification which focusses on wool. This standard is designed to provide the industry with information on best practice for farmers and to allow consumers to recognise when wool comes from farms with a progressive approach to managing their land, and from sheep that have been treated well and responsibly. The RWS protects animal welfare, preserved land health and provides for traceability within wool supply chains (20)
Wool is natural, renewable and has a wide range of desirable characteristics that make it ideal for a range of textile applications, including weighted blankets. However, as you have read above, the origin of the wool and processes it has gone through must be clearly determined before wool can be considered a truly sustainable choice.
(1)Simmons, Paula (2009). Storey’s Guide to Raising Sheep. North Adams, MA: Storey Publishing. pp. 315–316
(2) D’Arcy, J. B., Sheep and Wool Technology, NSW University Press, Kensington, 1986
(3)’Physical Properties of Textile Fibres’, Textiles Institute, Woodhead Publishing.
(4)J. E. Ford, ‘Fibre Data Summaries’, Shirley Institute, Manchester, 1966.
(5)P. Alexander and R. F. Hudson. ‘Wool: Its Chemistry and Physics’, Chapman & Hall, London, 1963.
(6)’Physical Properties of Textile Fibres’, Textiles Institute, Woodhead Publishing.
(7)S. L. Anderson and D. R. Cox, J. Text. Inst., 1950, 41, T481
(8)Elisabeth van Delden, ‘IWTO Fact Sheet: Wool and Biodegradability’ International Wool Textile Organisation.
(9) Ensminger, M. E.; R. O. Parker (1986). Sheep and Goat Science, Fifth Edition. Danville, Illinois: The Interstate Printers and Publishers Inc.
(10)Braaten, Ann W. (2005). “Wool”. In Steele, Valerie. Encyclopedia of Clothing and Fashion. 3. Thomson Gale. pp. 441–443
(11)IWTO Specifications for Wool Sheep Welfare, IWTO, 2017
(12) Speer, Jordan K. . “Shearing the Edge of Innovation”. Apparel Magazine (2006-05-01)
(13)International Wool Textile Organisation (IWTO) ‘Sustainability’, https://www.iwto.org/sustainability
(14)George Monbiot, ‘Meet the greatest threat to our countryside: sheep’, The Spectator, 2013.
(15)IWTO Specifications for Wool Sheep Welfare, IWTO, 2017
(16) Preparation of Australian Wool Clips, Code of Practice 2010–2012, Australian Wool Exchange (AWEX), 2010
(17)Ashley Palmer, ‘What’s Wrong With Wool’, PETA, 2018
(18)Wu Zhao (1987) ‘A study of wool carbonizing‘ PhD Thesis. University of New South Wales. School of Fibre Science and Technology
(19)’Biodegradable and Sustainable Fibres’, Textles Institute.
(20)Textile Exchange, Responsible Wool Standard Implementation Manual 1.0, 2016