When fabric is manufactured, threads are stretched on a loom, pulling fibers into an unnaturally straight position. Coatings are added to help stabilize the threads and keep them taut. Fabrics arrive from the manufacturer coated with sizing and other chemicals that give them a crisp feel and make them easy to rotary cut. Pre-washing relaxes the fibers and removes most of the chemicals from fabric.
How to Pre-wash Your Fabrics: Wash your fabric in cool water with a mild non-alkaline soap (like Ivory Liquid or baby bhampoo). If you have hard water, you can add a spoonful of borax. While rinsing, add a few tablespoons of distiled white vinegar to dissolve soap residue.
For cotton keep wrinkles to a minimum by drying fabrics with low heat and removing them from the dryer as soon as they are dry. Either press fabrics right away or press them when you are ready to use them. After removing fabrics from the dryer, use clothespins to suspend segments from hangers until you are sure they are completely dry before they are folded and stashed away. If you prefer to work with stiffer fabric, use spray starch or sizing to reintroduce body.
For silk fabrics, do not dry in your dryer as it can cause damage; heat dulls the fabric and may shrink it, and friction from the dryer drum might cause yarn breaks or white streaks. Most wrinkles can be removed by hanging the fabric in the bathroom during a shower. The humidity will do the work for you. If necessary, pres on the back side while the fabric is damp with the iron on the low heat or silk setting. Do not wet in spots as that will leave rings.
You will be glad you did not skip this step!