My understanding from a sewing dealer was that if the thread is old - throw it out, and I doubt that you would want to use rayon thread in quilting. Good qualities thread is best to use for your new quilting projects. You definitely would not want to ruin something you spent lots of time on by having thread that may not hold up. I would err on caution. Have little kids? Let them sew with it.

The poly isn't something that you would use in you quilt unless you were using it for basteing where it would be removed.
As to the rayon--I use 40 wt. for machine embroidery on my quilts often but would ? if it were old. Jayne

Personally, I would toss it, but that's just because I would never buy thread from a garage sale - especially if it were dusty. I want to know the age of my threads.

But what brand is it? Many standard sewing threads are cotton-covered polyester, and they are perfectly fine for sewing and piecing (sorry cotton-only people, but I don't share the prejudice!).

I do the actual quilting with whatever thread meets my criteria for the project in looks and weight. I've used cotton, poly-cotton, rayon embroidery, poly embroidery in weights from 60 (very fine, almost invisible) to 30 (heavier, and usually used for dense embroideries).

Just curious why the age of the thread matters? As long as you test the strength of it, it shouldn't deteriorate too much should it since it has never been washed, exposed to sunlight, etc. And the dust is mostly superficial which doesn't show in sewing seams.

I recently read a post by Caryl Bryer Fallert where she answered some questions about what type of thread she uses - just found it interesting if anyone else would like to see it.
Karlene

Here is a link that might be useful: Caryl Bryer Fallert

I use old thread too, and as far as dust.......by the time you get half a bobbin's worth unspun, it's pristine underneath.

I guess it depends on the machine. My Viking is really picky about the thread I feed her. Old thread and Coats and Clark would split and break when I've used them in the past. The old thread I just use for hand basting, and the Coats and Clark (if it's not too old) I use in my old mechanical Pfaff--she'll eat anything!

I don't use exclusively 100% cotton thread. I would test this for strength and if it seems fine then I would use it and be pleased with the bargain!

Kate

I agree with Mary - I'd toss it ~ in fact I did get rid of most of my own old thread, and it wasn't dusty or dirty, and I knew where it had been. Just not won't using old thread, in my opinion.
Anita ~ my old Pfaff 1222E never, ever has a problem with any brand or size of thread. It has sewn with thread that I would never dream of sewing on my newer electronic Pfaff.