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The Superwash Process

The superwash process turns wool that needs to be carefully hand washed into something that can be tossed into the washing machine. In theory it seems fantastic, but what exactly happens to make wool superwash?

The Makeup of Wool

To understand how the superwash process works, you need to understand the structure of wool. This can vary a little depending on different breeds of sheep, however the general structure is the same.

Each wool fibre is made up of the cortex and the cuticle. The cortex is the core of the wool, which makes up 90% of the fibre. Different cells are contained within the cortex, and these determine various characteristics of the wool such as the crimp, moisture absorption levels and how tough it is.

The cuticle is the outside part of the wool fibre. Cuticle cells overlap similarly to the scales on a fish, although exactly how they do so is dependent on the type of wool. You can clearly see the cuticle in the image below.

A microscope image of a Bluefaced Leicester Fibre

These scales are what makes wool felt when agitated. Wool naturally has a waxy coating called lanolin that makes it water repellent and stops the wool from felting on the sheep. The scales of the cuticle are all laid in the same direction, allowing dirt to wash off, and again, stopping the wool from felting on the sheep. During sheep shearing, the wool gets gathered together and the scales of each fibre end up pointing in different directions. When we wash wool, we remove the lanolin, exposing the cuticle cells. Agitation of the wool causes the edges of the cuticle cells to rub together, get stuck on each other and cling together. The more it is agitated, the more they cling, so the more the wool felts.

The Superwash Process

Most superwash wool is treated using the Chlorine-Hercosett process. There are various other treatments available but none produce as good a product as the Chlorine-Hercosett process.

During superwash treatment, wool goes through the following:

  1. Chlorine treatment (using chlorine gas or sodium hypochlorite) – This removes the surface of the scales
  2. Reduction of chlorine using sulphite in the same bath – This reduces the chlorine levels
  3. Rinsing
  4. Neutralisation with sodium carbonate – This lowers the acidity levels back to a normal PH
  5. Rinsing
  6. Resin application – A polyamide-epichlorohydrin polymer, called Hercosett 125 is applied. This coats the wool fibre with resin, smoothing it further
  7. Softener application
  8. Drying and polymerisation – This stabilises the Hercosett 125 so it is safe for use

Treated wool should be almost impossible to felt, meaning it can be washed in a washing machine rather than by hand. Some superwash wool can even be dried in a tumble drier. The wool will also take up dye easily and quickly, and pilling of the finished fabric will be significantly reduced.

Comparison of Superwash & Non-Superwash Fibre

The picture below shows the differences between the untreated and treated Bluefaced Leicester fibres fairly clearly. On the left is the untreated fibre. You can clearly see the scales of the cuticle, and if you look closely to the right of the fibre you can see how they make the edge uneven. On the right is the superwash treated fibre. Unfortunately I don’t know which method was used to treat it, but it is likely to be the Chlorine-Hercosett process. You can see that the scales look smoothed out and are shinier, and the fibre looks like it’s been encased in plastic. This is likely the resin coating described in step 6 of the superwash process above.

The Problems With Superwash

While there are obviously numerous positive aspects to superwash wool, there are various known problems with it:

  • The treated wool loses some of its natural characteristics;
  • Most superwash wool has the same smooth, almost synthetic feel. This is because the fibres have been coated in a thin layer of synthetic resin;
  • Due to the resin coating the wool loses some of its ability to breathe and absorb moisture;
  • The natural colour is often yellowed;
  • Ecological aspects: Waste water from the process contains high levels of adsorbable organohalogens (AOX) toxins, which are created when chlorine reacts with carbon-based compounds. A group of AOX toxins, dioxins, are highly dangerous. More information on dioxins can be found on the World Health Organisation website.

Personal Preference

For me, the cons of superwash wool outweigh the pros, but that’s definitely not the case for everybody.

For me:

  • Hand washing wool is a slight inconvenience, but because it doesn’t need washing as often as most other fibres I am happy to do so.
  • Untreated wool retains its desirable characteristics.
  • I am trying to make as minimal an impact on the environment as possible, in both my personal life and business, so it seems sensible for me to stick to untreated wool.


The original patent filed for Hercosett

A blog post by Sue Blacker of Blacker Yarns on the subject of superwash wool

A paper comparing the usual Chlorine-Hercosett method of superwash processing and a different, much lesser used method

Some figures on the Chlorine-Hercosett process and a brief discussion of an alternate process. A project by the EU to reduce environmental impact

A review of the sustainable methods in imparting shrink resistance to wool fabrics