Like many other thermoplastics, acrylic is liable to notching. Structures must therefore be made without sharp internal corners. If a structure contains notches as well as other internal tensions, it will not take much for the acrylic to crack when being processed, transported or on first use. Internal tension is a common factor in extruded acrylic due to the way in which the raw material is fabricated, but tension can also arise through inexpert fire polishing, laser-cutting or use of the wrong tools.Internal tensions in acrylic can be removed through detensioning, which is a slow process of heating and cooling (annealing). Tensions can be seen in clear acrylic, using two pieces of Polaroid filter. Place the acrylic against a source of light, lay the filters one over another and observe the acrylic though them whilst turning the filters separately. The tensions will light up.