evaporation is a function of the energy level used in plating and the surface area of the parts being plated the evaporation of water in the process naturally is greater when the plating barrel is of the open top type revolving on an inclined axis than when it is a closed barrel revolving on a horizontal axes as shown in fig1 according to the present invention the liquids required to make up this loss in the plating effluent stored in tank 90 are preferably made up with liquid from the first rinse stored in tank 91 or with fresh water from an external water source the first rinse loss both from evaporation and from liquid used to replenish the plating effluent is made up from the second rinse or with fresh water when necessary etc in any event the make up liquid required for the last rinse is clear fresh water taken in from an external source through rotatable valve 29 according to the present invention plating of metal parts in consecutive cycles and recycling of process liquids from one cycle to a later one continues indefinitely as described until the plating effluent level reaches the following contaminant levels ______________________________________ total contaminant content in suspension metal and solution ppm______________________________________zinc or cadmium 130 000 180 000copper inconsequentialtin inconsequentialiron 12 000 18 000______________________________________ generally speaking the lower contaminant levels are preferred when a very bright plating is desired but a good quality plating can be obtained at or even above the upper limits stated if a high degree of brightness is not required copper and tin residues usually are no problem because the copper and tin salts are added to the process in relatively small amounts in the first place and are as a rule substantially completely consumed in each cycle thus.