The difference between flat silk and twisted silk.
Here I use Japanese Embroidery Flat Silk as an example:
The silk comes on 60 meters tubes of flat silk. One strand of silk is made up of 12 suga (filaments). For optimum shine the silk is used flat, strands can be split for finer threads or combined if a thicker thread is required. The thread can also be twisted.
The above photo shows a tube of flat silk, a variety of threads, twisted and flat and (laying across the silk) a strand of DMC floss.
The first three threads from the left are all flat silk – half a strand, 1 strand and 2 strands. The next five threads are a regular twist – 1->2, 1->1, 2->1, 3->1 and 4->1 (the first number is the number of strands used to make a single twisted thread, e.g 4->1 is four strands twisted together to make one twisted thread). The final two threads are a 3->1 soft twist and (on the far right) a 1->1 S-twist.
The stitched samples above show the relative thickness of a different number of strands of flat silk and the coverage when used to stitch a foundation.
From left to right: 0.5 strands, 1 strand, 1.5 strands, 2 strands and 2.5 strands.
This sample shows the relative thickness of the twisted threads and the coverage when used to stitch a foundation.
From left to right: 1->1 twist, 2->1 twist, 3->1 twist, 4->1 twist and 3->1 soft twist.
Note that the 3->1 soft twist looks as full as the 4->1 regular twist and gives fuller coverage even though it uses fewer strands of silk. The tighter the twist the finer the finished thread.
The Japanese Embroidery silk thread is not the normal silk thread that I will use for cross stitch. This is only to show the difference between Stranded silk and Twisted silk.
The silk threads that normally used for cross stitch is Twisted Silk.
I'd love to know too, where to get reasonable priced good silk. I have a piece I would love to do in silk instead of cotton since it is very fine (1 over 1 on 32 ct fabric) but I've not used pure silk before and am not sure what a good brand is