I have a pair of silk pajama pants that have come apart along one seam on the leg. There’s no damage to the cloth and it’s spreading very slowly, so there was just some flaw and that bit of seam was not fastened properly.
To fix it, what kind of thread and needle should be used? I’m leery of using the generic thread from a wardrobe repair kit (a couple dozen miniature spools in different colors) without at least checking first.
For a 4 inch repair I would use the yarn and needle in the repair kit, and do it now before it spreads further. Use the most matching colour and the finest thread if you have several options, and as thin/fine a needle as you can handle for the repair.
When sewing a whole project, it is good to get good quality thread and needles, but for a small repair you use what is at hand.
While silk thread is available, it is expensive and many people will still use polyester or cotton for the hidden seams and only buy silk if there is top stitching where it shows. When you do buy it for the top stitching you might well use it for the rest of the sewing as well.
Not all thread is equal. Very cheap thread will be fuzzy, hard to thread through a needle eye, knot easily as you sew and break easily due to short fiber length and cheap manufacturing. Cotton thread can also rot over time. Even polyester thread will degrade over time.
Test the thread before you spend a lot of time sewing with it. Pull a bit off of the spool, hold an end in each hand, yank it and if the thread breaks very easily, toss it out.
To make your hand sewing experience a little more hassle free, thread your needle with thread that is about 36 " long. Knot one end. Run the thread through a chunk of beeswax. Iron the thread to soak the wax in. This will discourage the thread from tangling. Don't use wax if you are sewing white fine fabric that will show wax marks.
My favourite hand sewing thread is Rice's silamide finishing thread but it is not available everywhere: https://www.youtube.com/watch?v=mkZ33TC5k_Y
In general, I sew with good quality polyester thread, e.g., Gutermann, it doesn't create a lot of "dust" from fiber breaking off the thread, and the colors are relatively "duller" (more like natural fibers) than cheaper polyester thread that can be shiney.
If I were a quilter, I'd use cotton thread because of the similar hand (feel, touch) and visual aesthetics of the cotton fabric and cotton thread.
A caution with cotton thread especially, is it can become brittle and break easily with age.
So if the thread in your wardrobe repair kit is cotton, do the strength test suggested by the previous answerer.
For special fabrics, like silk (which can also become brittle and degrade easily with age), I recommend using thread that is most like the fiber content of the fabric you want to repair, in this case, silk thread for sewing on silk fabric.
For delicate fabrics like silk, a strong cotton or polyester thread will be stronger than the silk fabric and can actually pull and tear the fabric (instead of the seam splitting), when the fabric is stretched or twisted in wear.
Since you are repairing silk pajamas that you clearly love and want to preserve, only you can decide if you want to spend the money on silk thread, or a good quality polyester thread.
When you do a lot of sewing, the price of good thread can be a consideration, but if you plan to buy only one small spool of thread, it will a very small extra expense, and you can take the item you want to repair with you when you shop for thread and get the color that goes the best.
