ISO 2143 specifies a method of estimating the loss of absorptive power of anodic oxidation coatings that have undergone a sealing treatment, by dye absorption after acid pretreatment.The method is suitable for use as a production control method and can be applicable to anodic oxidation coatings which may be subjected to weathering or aggressive environments, or where resistance to staining is important.The method is not applicable to those coatings thata) are formed on alloys containing more than 2 % copper or 4 % silicon,b) are sealed by the dichromate process,c) have been given supplementary processing, e.g. oiling, waxing or lacquering,d) are coloured in deep shades, ande) are less than 3 μm thickness.The method is less appropriate where nickel or cobalt salts, or organic additives, have been added to baths used for hydrothermal sealing.