Previously I prewashed but now just soak a bit and rinse in hot water. This seems to take care of chemicals my daughter and myself are quite sensitive to when sewing pieces together. Actually batiks go through so much in the dying process that they cannot be hurt by prewash or rinse and almost always release some color. Granted some of the newer batiks are looser weave than the first ones in my stash. They do NOT seem to tighten in a wash or rinse.