I designed a bluestone coping on an exterior wall near a swimming pool. The masons got some mortar on the bluestone and cleaned it off with muriatic acid. Since it was adjacent to the pool, though, they didnÆt want to rinse off the acid, fearing it would get into the pool. Later, the places that were cleaned with acid turned orange. Why did they turn orange, and how can I get the orange out?
Bluestone is a type of sandstone that is bluish green in color. Bluestone is classified as a flagstone, which means that it will split along flat planes. These splits occur along shale-rich layers. Shale contains iron sulfite or pyrite. Pyrite reacts with acids, causing the stone to turn orange. The bluish-green color comes from clays within the sandstone. These clays may also contain iron compounds. The acid cleaner will also accelerate the corrosion of these compounds. Chemical cleaners should not be used on any stone products without first testing the cleaners on a piece of unused stone or in a concealed area. Although the iron in the stone will eventually oxidize as the stone weathers, the appearance may be very different from that of the acid-accelerated corrosion.
The level of success in cleaning the orange stains will depend on how well-distributed the iron compounds are in the stone. If the iron compound causing the orange color were primarily on the surface of the stone, phosphoric acid or oxalic acid may be successful in removing the stains. There are also several proprietary rust-stain removers that could work. Remember that using any chemicals on stone is always risky. Before beginning to clean the stains, you should perform a test in an inconspicuous area to evaluate the effectiveness of the approach.