an object of cross - linked polymer , such as insulation for a cable conductor or a cable sheath , is formed by using a starting material containing a peroxide and a polymer material with the ability to undergo a cross - linking reaction in the presence of peroxide . the reaction is carried out during at least a part of the process in the presence of a substance which has the ability to undergo a color change during the cross - linking reaction of the polymer material so that a color change of the substance is obtained . there may also be added a dye which does not have the ability to undergo a color change during the cross - linking of the polymer material . the substance may be an aromatic compound containing at least one halogen atom and / or at least one heterocyclic ring in which ring there is at least one nitrogen atom to which no hydrogen atom is linked .