Information from the War Memorials Trust

Bronze will often be combined with stone, such as a plinth for statuary or bronze plaques on a stone memorial. Over time the bronze will turn green, but also this colour (copper sulphate) will be transferred onto surrounding masonry by rainwater. Although some metallic salts will be deposited onto the stonework it does not cause significant harm and is essentially an aesthetic issue which can be unsightly and distracting. The options available in these cases are limited. It is unlikely to be possible to completely remove this staining, although it can be reduced. The older the staining is the more difficult it is to remove. Simple cleaning will not remove the staining as it penetrates deep into the pore structure of the stone so the most effective method of removal is by ammonium chloride or EDTA-based poultice, the appropriate poultice will be dependent on the type of stone. Poultices draw out staining by capillary action, which is required for this type of staining. Porous stones such as limestone or sandstone are far more susceptible to copper salt staining than a more compact stone such as granite.
After an exhaustive series of cleaning applications there are still patches of copper sulphate visible on the Portland stonework of the Cenotaph. This as as much as can be done at present without causing lasting damage. It is hoped that the stains will naturally fade as the stone weathers over time. Now fully cleaned the rest of the stonework has an amazing glow to it and the bronze is to the original finish last seen at the unveiling in 1922. The cleaning work has been undertaken by a contractor on behalf of and funded by Leeds City Council with contributions from local associations.