What Is the Superwash Process, and What Does It Do to Wool?

Wool is a miracle fiber. It is insulating, breathable, and resilient. It is fire-, UV-, odor-, stain-, and wrinkle-resistant. It is renewable, sustainable, and biodegradable. For thousands of years, humans have been happily clothed in sheep’s wool.

But in the mid-twentieth century, we started asking more from our clothes. We demanded easy care, in the form of machine washability. Textile scientists went to work developing treatments that would allow wool to be machine washed without shrinking or felting. Today, much of the wool sold as yarn and as finished goods is marked “superwash.”

What Is Superwash?

Superwash was originally a product certification trademark owned by the Wool Bureau, but that trademark expired in 2006. Superwash is now a generic term applied to shrink-proofed wool that has been treated with the “chlorine-Hercosett” process.

The superwash treatment is the final step before wool is shipped off to the spinning mill, after it has been scoured, dried, carded, and combed into top. The wool is passed through a chlorine solution (similar to what you’d find in a swimming pool). The chlorine erodes the overlapping scales on the surface of the fiber. The wool then passes through a neutralizing solution to stop the action of the chlorine. Then the wool is coated with a layer of a polymer resin called Hercosett to smooth and seal the eroded surface. Finally, a softener is applied, the wool is dried, and it’s packed up and shipped to spinning mills.

This doesn’t sound very eco-friendly . . .

It’s not. The chlorine-Hercosett process uses a lot of water, and the wastewater requires careful treatment to prevent all those chemicals from polluting the environment. The Hercosett polymer is essentially a petroleum-based plastic, so it’s inherently unsustainable.

Wool treated with the chlorine-Hercossett process cannot be certified as organic under the Global Organic Textile Standard (GOTS), regardless of how the fiber was grown.

How Does the Superwash Process Impact Performance?

Any process that alters the natural surface of wool fibers will change its ability to manage moisture and provide insulation. If you’ve ever washed a superwash wool sweater, you’ve probably noticed that it absorbs a lot of water, becoming limp, heavy and stretched out. Blocking it back into shape can be a challenge.

The resin coating on superwash yarn makes it less effective at retaining warmth compared to untreated yarn. It can also impair the natural elasticity of the fiber, making the yarn less resilient.

You’ll find that most superwash yarns are composed of many plies spun with lots of twist. This yarn structure is intended to compensate for the reduction in elasticity caused by the superwash process.

Is Superwash Evil?

No! In fact, it’s been argued that superwash saved the United States wool-growing industry. Since 2011, Congress has mandated that all American military apparel must be made in the US, and the US military now purchases 20% of the American wool clip. The strict specifications and performance requirements for military apparel demand the use of superwash wool.

Before 2011, there were no commercial superwash facilities in the US. The sheep industry worked with the Chargeurs wool scouring and processing facility in South Carolina to install a chlorine-Hercosett superwash line. Because their wool can now be superwash-treated onshore, American ranchers can provide wool for military apparel. Chargeurs remains the only commercial superwash facility in the country, and the scouring mill achieved RWS certification in 2022.

Your requirements for hand-knitting are not the same as those for military apparel. You can choose to celebrate the environmental and performance qualities of untreated wool in every knitting project.

Are There Sustainable Alternatives?

Over the past decade, textile scientists have been working to develop more eco-friendly processes for shrink-proofing wool.

A Japanese company called Life Fiber has developed a polymerization process that eliminates the chlorine stage. They call this Life Fiber EFT (for “Eco Friendly Treatment”).

Südwoole Group from Germany has developed a process called Naturetexx Plasma that modifies the surface of the fiber by passing it through an electrical plasma field. This process uses only air and electricity (obtained from renewable sources). Wool shrink-proofed using the Naturetexx Plasma process can be certified organic by GOTS.

These processes are being used by some high-end outdoor apparel brands to make washable wool clothing, but neither has made significant inroads in the handknitting yarn market.

What’s a Knitter to Do?

When you can, choose natural wool. The inherent qualities of wool fiber mean it need not be washed often. A gentle handwashing once or twice a year is probably all your sweaters will need. (Some knitters say that even wool socks probably don’t need washing after every wearing.) The joy of knitting and wearing untreated wool more than compensates for that small amount of effort.

There may still be times you choose to knit with a machine-washable yarn. For a parent with a full-time job, handwashing baby blankets or garments might seem overwhelming. You may want to knit for people who you know can’t be trusted to handwash. Better to use superwash wool than to have your gift languish in a drawer or be ruined in the laundry.

What about all that superwash wool already in your stash? Use it or donate it, but don’t throw it away. Adding yarn to the landfill doesn’t help anybody. But for future purchases, enjoy the wonder that is natural wool.