So far all the posts have assumed you have fine silk thread (like the 90 or 100wt) there is also much heavier silk threads out there. The heavier weight ones don't sink into the fabric like the 100wt does. Can you elaborate what weight ply your thread is? It should say on the spool somewhere. The weight and ply of the thread makes a difference for what application you want to use it for. The heavier weight ones are usually used for decorative stitching.
Also, don't be confused by 100% silk thread and cotton thread with a silk finish. Mettler makes one and the spool says "silk" on it but it isn't silk, it is 100% cotton with a shiny smooth finish on it.
Either way, by all means it is ok to sew with whatever you have, unless of course, it is a silk finish cotton and it may be brittle from being old. Do a snap test on it to make sure it is still strong enough to use, if you happen to have a silk finish cotton.
Last edited by feline fanatic; 03-07-2014 at 05:35 AM.