In warm temperatures the scales on wool expand. If the wool is also tumbled and manipulated when it has been heated with warm water, the scales get caught on each other. This is what causes wool to felt. In other words, it felts together and becomes thick and hard. When wool felts together it shrinks by between 25–30%. Many of us have experienced this if we’ve thrown our favourite woollen sweater in the washing machine or tumble dryer. Once wool has felted, nothing can be done to “unfelt” it again.

Long service life and possibility of machine washing

So why can certain woollen clothes be machine washed while others need to be hand washed? It is because the wool has been treated with a superwash treatment which makes the wool extra durable and resistant. People started doing this in the 1950s when washing machines grew in popularity and the demand for materials that could be machine washed increased. It was actually the introduction of washing machines that made synthetic fibres such as polyester popular. To be able to stand up to these competing synthetic clothes, a technique was developed that prevented wool from felting in the washing machine.

There are different ways to treat wool so that it can be machine washed. The most common method is called Hercosett which was developed in the mid-1960s by an Australian wool company. This method entails the wool being treated in two stages. First the wool is dipped in large basins that contain a chlorine solution of 3% chlorine. The chlorine bath causes the surface of the wool fibre, the epidermis scales, to even out somewhat and become a bit softer. The chlorine is washed off the wool after the chlorine bath. Thereafter Hercosett 125 is added to flatten out the fibres even more. Hercosett 125 is a water-resistant nylon-based polymer that covers the epidermis scales like a thin membrane, which means they cannot expand thus stopping the wool from felting. After that the wool is dried and wrapped together into something called bumps* of about 10kg.

*picture how candy-floss looks when it is spun on a stick. That’s what a wool bump looks like, more or less.

Some people think that Hercosett 125 makes the wool lose its unique moisture-absorbing and warming properties, but the treatment creates such a thin layer over the wool fibre that the fibre’s unique properties still work.

Pros and cons of superwash treatment

Treating wool with a superwash treatment has both advantages and disadvantages. The advantages of superwash are that the wool becomes more durable and resistant and that it can be machine-washed in high temperatures. This is most important in clothes that are used in various professions where there is a risk of noxious bacteria that can make humans ill getting stuck in the clothes, for example in the Armed Forces.

On the downside, this treatment is not the best from an environmental perspective. This is mainly due to the much-debated first part of the treatment, i.e. the chlorine bath and primarily the release of adsorbable organic halides (AOX). The greatest risk of AOX emissions comes from the paper and pulp industry where Hercosett is used to strengthen things like nappies and tea-bags so that these don’t turn into a sticky mass when wet. There is also a risk of emissions from waste management, wastewater treatment and chlorination of drinking water and swimming pools. Like most chemicals, AOX have a large environmental impact if they get into nature. Which is why chlorine needs to be handled safely and within a closed system. We maintain a continuous dialogue and make regular visits to our producers to ensure that the wool we use comes from sustainable processing where no dangerous waste is released into nature.

We have an ongoing investigation into alternatives to the superwash treatment. The challenge for us is to identify a method that allows us to retain the same quality and washing instructions for our products as we have today.