It is a wool that has been treated and processed so that it can be washed in the washing machine.
There are 2 ways to do the superwash treatment
- Using an acid bath that removes the "flakes" from the fiber.
- Coating the fiber with a polymer that prevents the flakes from sticking together and causing shrinkage.
It is an excellent option when choosing yarns since on the one hand you have a natural fiber and on the other hand the convenience of washing in the washing machine.
When you buy a superwash wool, you never know which of the 2 treatments has been used, but there are weavers who claim that they can be identified when weaving, since the treatment with a polymer makes it a more polished and shiny wool.
Any downsides to superwash wool?
One potential problem is that the project can stretch a lot when you wash it. This is because the scales help give structure and allow the fiber to adhere to itself.
When there are no scales, the fiber loses structure. This is highly variable between the different fibers, so it is always recommended to make a blocked sample of your yarn for each project.