Crepeline is a super, super fine silk that is used by conservators to protect and stabilize worn textiles. I buy both the natural and the brown. You can see that, while they change the color of my hand a bit, they are still incredibly sheer. The words "gossamer" and "fairy wings" come to mind.
True conservators, which I am not, or people who are good at fabric dying, which I am not, can dye their crepeline to match the fabric being covered. I experiment with the dark or light, and choose the one that alters the look of the original fabric the least.
I just bought a new yard of brown from Talas, a conservation supply store. It is far from inexpensive. I think it is totally worth the cost, because it is virtually invisible when applied. This is the second yard of brown I have bought in a couple of decades of repairing quilts, and I'm on my second yard of the natural. A little goes a long way, in other words. Thankfully. Most people send quilts to me for restoration, i.e. patching, and not stabilizing and conservation.
Here's my little packet of silk soaking in the sink.
And now, on to the process of stitching it onto a quilt. This quilt is a marvelous collection of early-mid and mid 19th century fabrics. It's owned by a collector who understands the historical value of these fabrics and chose to cover them with crepeline rather than patching over them.
Here's one of the blocks that needed help.
Here's how it looks with the crepeline pinned over the worn triangles. I cut the crepeline very large, and cut to size as I stitch. Silk in general, and especially a gossamer silk like this one, is notorious for scuttling away from the scissor blades, almost impossible to cut straight. This is even more of an issue in applying to patchwork, where there are pretty much always going to be some bias edges. So I give myself plenty of lee-way fabric.
I gently cut the silk back to a generous 1/4" as a go along each side. That gets turned under and stitched with a running stitch, no less than 1/4" long under the old weak fabric so the stitches don't catch just a few threads and break them. It's very fussy work, especially on those bias edges. Be careful and gentle with the fabrics.
I use extra fine 100% cotton machine embroidery thread, a small quilting needle (which is easier than usual to thread, what with the finer embroidery thread!), and "insect pins", the slender pins museums use to mount insects in display boxes.
Here's the finished look.
As you can see, the end result doesn't look very different then the original look. The dark crepeline has the nice side effect of dulling out the white of any batting that's showing through.
I pinned the patch back down while the quilt lay flat to avoid introducing any puckers....
....and then I stitched with a ladder stitch, one half of the stitch into the background and one half running through the fold of the appliqué. These stitches also are sewn larger than what quilters are used to doing, to avoid breaking the fabrics.
Photos of this entire quilt and its wonderful collection of fabrics can be found here.