A Fun Glow-In-The Dark Effect Thread! SULKY GLOWY is a special effect polyester thread size 40 and is suitable for all embroidery designs and decorative stitching. When light, sunlight or UV light hits SULKY GLOWY, the thread stores the light and releases it in the dark - the thread glows. The duration of exposure to light determines the intensity of the luminosity. Tests have shown NO loss of luminosity after 50 washings.