Although some quilters never pre-wash their fabrics, I nearly always do, because I don't want any surprises when I wash a finished quilt for the first time.
Some Fabrics BleedSome cotton fabrics bleed, particularly reds, purples and other vivid colors. Bleeding dyes can stain sections of other fabrics during prewashing, but they're an even bigger problem when they transfer dyes onto patches in a finished quilt.
Make sure your fabrics are stable by performing a bleed test before you use them in a quilt or wash them with other fabrics.
Keep a laundry additive called Synthropol on hand. Just a capful added to each load of fabric keeps loose dyes from depositing on other fabrics during the wash. Synthropol won't prevent fabrics that touch each other--like patches in a quilt--from absorbing dyes that bleed and transfer from an adjacent patch.
Cotton Fabrics ShrinkWhen cotton 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. Part of what we see as shrinkage is actually the relaxation of the cotton fibers as they try to return to their natural state.
When fabrics are washed, the agitation of the washing machine and the wicking action that draws moisture into the fibers allow the coated threads to relax and return to a position more like that in which they grew. Putting the fabrics in the dryer gives fibers another opportunity to relax. How much? It depends on the fabric.
Cottons Shrink DifferentlyWhat if your quilt block contains several different unwashed fabrics--and some of them shrink more than others? Uneven shrinkage could cause puckers and distortions the first time the quilt is washed.
Quilters sometimes intentionally use unwashed fabrics to assemble a quilt because the puckers left over after the first wash give the quilt a vintage appearance. If vintage isn't the look you're going for it's best to pre-wash your fabrics.
Prewashing Removes ChemicalsQuilting fabrics arrive from the manufacturer coated with sizing, protectants and other chemicals that give them a crisp feel and make them easy to rotary cut. If you are sensitive to chemicals, handling coated fabrics or breathing small particles that might flake off of them could be a health risk.
Prewashing removes most of the loose chemicals from fabric. If you prefer to work with stiffer fabric, use spray starch or sizing to reintroduce body. Yes, that adds a chemical, but it's one you have control of. Read the ingredients list on the can to find out exactly what's in it.
How to Prewash Your Quilting FabricsWash your quilting cottons in cool water with a mild detergent or Orvus soap, which is available online and from most quilt shops. Keep wrinkles to a minimum by drying fabrics with low heat and removing them from the dryer as soon as they are dry.
Some quilters like to press fabrics right away. I prefer to press them when I'm ready to use them. After removing fabrics from the dryer I use clothespins to suspend segments from hangers until I'm sure they are completely dry, then I fold the fabrics and stash them away until I'm ready to make a quilt.
You'll develop your own preferences for prewashing as you work with fabrics and construct your quilts. Ask other quilters for their opinions on fabric care, listen to what they say, then experiment to determine which techniques work best for you.