Soda washing is a technique where wick material is treated with sodium carbonate (also known as washing soda and soda ash) to improve its capacity and release characteristics. It is most often used with cotton twine but may apply to other materials as well. It does not seem to improve (or hurt) the performance of t-shirt yarn. Handling! Washing soda is a very alkaline powder. Dishwashing gloves or rubber gloves should be used when handling it, and care should be taken not to inhale any powder. if you have difficulty finding it, it is easy to make (as discussed below).