to about 50 secs per micron of thickness of metal layer so as to ensure that the metal is completely etched from the exposed areas of the web not protected by the pattern of etchant resistant material the rate of movement of the web 114 through the tank may vary widely but generally higher speed operations are preferred in the interests of economy generally up to about 350 meters min preferably about 150 to about 250 meters min although lower speeds down to 10 meters min may be used the web 114 is driven through the tank 134 by providing driven power to at least some of the lower and upper sets of rollers 124 126 as etching of the web 114 continues in the tank 118 etched aluminum becomes dissolved in the aqueous sodium hydroxide solution and tends to build up in the solution the bath may be intermittently or more preferably continuously rejuvenated by appropriate processing of the solution to remove the dissolved aluminum and regenerate the alkali as described in more detail below the process may be operated with a dissolved aluminum concentration in the bath 134 which may vary widely generally from about 5 to about 95 of aluminum saturation of the bath preferably towards the lower end.