the tank this being particularly undesirable accordingly use in conventionally made of board 11 11 39 11 34 usually referred to as a 34 capping board 34 for supporting the electrodes 5 protecting the top surfaces of the partitions 3 3 39 3 34 preventing shocks from occurring when handling the electrodes 5 and avoiding damage caused by the heavy weight of the electrodes these boards 11 11 39 and 11 34 must be acid resistant and perfectly insulated to the high electric charges used for electrolytically ionizing the metals they also must not absorb moisture these boards 11 the width of which depends on the thickness of the partitions 3 further must resist the combined weight of the electrodes 5 which may amount to several tons in addition they must be resistant to the chemical compounds used as electrolyte such as sulfuric and hydrochloric acids finally they must resist peak temperatures up to 230 u00b0 c or sometimes up to 260 u00b0 c which can occur in case of a short circuit during the electrolytic operation as a result of many tests carried out with different materials and additives a capping board has been found which meets all the above mentioned requirements this capping board is made of a plastic resin selected from the group consisting of polytetrafluoroethylene and the acid resistant polyester polyvinylester epoxy and phenolic resins to which from 10 to 20 of fiberglass from 2 to 10 of silica from 2 to 10 of mica and from 2 to 10 of feldspar have been added as the plastic resin use can be made of polytetrafluoroethylene use can.