Thanks for the reply Fraew. I use Gutermann thread that I buy at JoAnns. The spool doesn't say what weight it is so I did a search just now and all of the Gutermann silk thread is saying 30W. I think the reason I'm wondering about it being durable is that in old crazy quilts the silk fabrics seem to deteriorate faster than the cottons. However, that may be because of the chemicals used in those fabrics rather than the silk itself. If cotton is truly significantly more durable then I think I would rather use cotton on this labor of love quilt. But, I DO like the results of the silk thread! Thanks for your help.