Crazing is the phenomenon that produces a network of fine cracks on the surface of a material, for example in a glaze layer. Crazing frequently precedes fracture in some glassy thermoplastic polymers. As it only takes place under tensile stress, the plane of the crazing corresponds to the stress direction. The effect is visibly distinguishable from other types of fine cracking because the crazing region has different refractive indices from surrounding material. Crazing occurs in regions of high hydrostatic tension, or in regions of very localized yielding, which leads to the formation of interpenetrating microvoids and small fibrils. If an applied tensile load is sufficient, these bridges elongate and break, causing the microvoids to grow and coalesce; as microvoids coalesce, cracks begin to form.