The base material consists of thin, flexible polystyrene plastic. Prior to heating, the plastic sheets can be coloured with permanent markers or colour pencils and cut into shapes. Oily or waxy substances are not suitable because they melt or burn in high heat. When heated with a conventional oven (160C for 3 minutes or until flat), or a heat gun (on low heat), design shrinks and becomes thicker and more rigid, while retaining the coloured design. Use a Silicone Mat underneath to protect working surface. As a general guide, work out what your desired finished size is, then multiple the length and width by 2. 25 to establish artwork size. Note that shrinkage is approximate and design will not always remain proportionate please allow for some distortion. Pigment will intensify with shrinking.