The un-talked of enemy of those who work with silk. Well, any fabric really. But silk is particularly prone to it, given that it’s a filament, not a fiber. So what is it? Seam Slippage occurs when the seam is solid, but the threads/fibers to either side of the seam start to pull away, resulting in a gap in the fabric.
This typically occurs when not enough stitches per inch are used during crafting the seam, and are more likely to occur on seams that run parallel to the selvage, along the warp of the fabric.
So why is silk so prone to it? Fabrics like cotton, wool, and linen are fibrous, meaning they are actually shorter bits of themselves, spun in to a longer thread for weaving. Silk, however, is one long unbroken piece. This means there are no other pieces of it to grab onto itself. This filament nature contributes to it’s lustrous quality. But is also makes silk slick, and that slippery tendency includes having the filaments migrate away from the seam stitches, especially at stress points. But not all is lost. There are actually several steps you can take to avoid this catastrophe.
First, shorten your stitch length. The average stitch length for commercial sewing machines is 2.5 mm or 10-12 stitches per inch. Shorten that to 2 mm or 12-13 stitches per inch. May not seem like much, but it makes a big difference in seam strength for silks. Always make sure your seam allowance is at least 1/2 inch. This is so you can do the next step: flat fell your seams. Or use French seams. Really any double row of stitching is effective in combating seam slippage. Binding the edges is NOT effective against seam slippage due to the binding occurs on the outer edge to prevent fraying from the outside in, but does not really strengthen the seam itself. However, I have experienced some luck in lining flat lining a silk piece with silk organza. This seems to also strengthen the seam.
And preventing slippage is that easy. Seam allowance, stitch length, flat felling. And however much you may hate flat felling seams (I HATE flat felling seams...I prefer pinking shears and call it good. I am a lazy seamstress in that regard), you will hate more having poured your heart and soul in to making the perfect gown, only to have the characteristics of the fiber destroy your efforts from the inside out.