Here are increasingly close photos of an Argiope aurantia egg sac. This is the orbweaver known as the Black and Yellow Garden Spider. The egg sac is 18mm wide (3/4") and contains 400 to 1000 eggs (Levi 1968). The first close up spans 15mm of the surface of the egg sac, the second spans 10mm, and the third spans 3mm. The spider is quite a sculptor and abstract artist. Notice the threads of black silk. They are visible to the naked eye and appear to be on all Argiope aurantia egg sacs. I've seen them on the half dozen or so egg sacs that I've examined closely. I have not been able to determine what makes them black. It is not dirt or fungus, as they are found on new egg sacs. Brunetta & Craig 2012 reports that Nephila spiders create yellow silk with the addition of chemicals, so it's possible that Argiope aurantia is doing the same for black silk.