Smocking stitches are done with normal embroidery thread. On silk apparel use silk thread and on cotton material use cotton thread. Spandex covered yarn is usually composed of a Main yarn made out of resources like polyester, nylon which is then covered or encapsulated by a thin layer of spandex. https://sethxcdeg.myparisblog.com/32212481/the-smart-trick-of-smoking-thread-that-no-one-is-discussing