Place pack in a chilling unit or freezer. Packs remain pliable and soft even below freezing. Pack maintains the therapeutic level for up to thirty minutes. Chilled pack should be applied to treatment area using either a light towel or a Relief Pak(R) cover. ColPaC(R) re-usable packs are available with either a vinyl or urethane coating. Use the blue vinyl pack or the new black urethane pack. The black pack is more supple. It maintains its flexibility at a lower temperature than the blue vinyl pack.