Isabel and Janet, you've asked a good question. The only buttons I've made were for my niece, who was going to use them on a sweater made with non-machine washable yarn. I would try sewing some of your buttons onto some fabric and washing them several times. If you wanted to get really fancy, you could sew some on that you wash in hot water and dry on high, some that you wash inn cold water and dry on low, and some that you wash on delicate and hang to dry. Try washing them several times, each time you do a load of wash, and see how they hold up.
I agree with Alia that any finish you apply would be ruined in the washer. I expose my clay beads to water as part of the finishing process - i.e. I wet sand them - and there is no effect from the water itself. I've left finished beads soaking for days and weighed them on my postage scale before and after, and they do not soak up any water.
By the way, I consider www.glassattic.com the ultimate source for truth about working with polymer clay. You can see what Diane has to say about making and washing PC beads here:
http://www.dianeb.info/polymer/buttons.htmGood luck!
