>silk conservation ?
>The fact is that some silks get broked after some years while other qualities
>remain fine with no problems.

 The way the silk was processed and colored is critical.  Contact a museum with
a flag collection for case studies.  I have examined American Civil War flags
where some colors of silk looked like they were burned while other colors still
looked fresh. A fairly large survey on flags that centered on conservation was
done a few years ago.