[vorpallemur]

The book says not to use silk thread. That seems odd to me; lots of wool is sewn with silk. Plus silk is available in skeins of pre-waxed thread, which is very handy for hand sewing. If you cut them to a sensible length, they don't tangle, and you don't need a scissors when you're threading the needle.

What am I missing?

[paulhenry]

I don't think you are missing anything, Tailors often use silk thread, it's really very strong, so not for basting. It's not unususal to see silk thread on very high quality clothing, if it is fine silk, I don't see the problem.

[Dixiecat]

There are two reasons I wouldn't use silk thread for making a kilt.

1) The inherent strength of silk thread. Thread that is too strong can actually cut fabric, fiber by fiber. Silk is a long staple thread, wool is a short staple fabric and 'soft' in comparison to silk.

2) The inherent fragility of silk. While silk can last for many hundred years, it does so with great care. Not so with wool. The wool in a kilt will last a lot longer than the silk thread used to sew it.