Cleaning the stones before mortaring, or even as part of maintaining a structure is often necessary. The question becomes, what is the preferred way to clean the stones?
Washing
Washing the stones is always the safest bet where possible, but there are some factors to consider. First, if the stones were washed shortly before being set into a mortared wall, the mortar will need to be made dryer, and it would be preferable if the stones were allowed to at least dry enough to eliminate any standing puddles. Washing with high-pressure water is fine, within reason. Full out pressure washing the stone can be a risky proposition, especially for historic structures. The high-pressure water tends to scour the surface of the stone, and, if the stone is soft, will degrade it. For a historic structure, overzealous pressure washing can lead to all manner of damage, including a thorough scouring of mortar joints and possible damage to historic inscriptions. Unfortunately, low-pressure washing of stone may not be able to eliminate all the dirt buildup.
Scrubbing with a Wire Brush
Scrubbing the stones with a wire brush is generally a very bad idea. The wire brush scratches the surface of the stones, sometimes pretty heavily, which is an obvious no-no for historic structures, and even for new construction. Besides the cosmetic harm, this practice will cause the stones to be more vulnerable to the elements. This scrubbing with a wire brush is common when cleaning excess mortar off a new structure, but the builders need to be aware of the possible damage to the stone, and, if there is no other option, need to be careful.
There is one exception where a wire brush is a good choice. If you use it to clean the dirt off the surfaces of a stone to be mortared, the scratches on the stone will actually help the mortar bond and, since these surfaces are not exposed to the elements anyway, the brush won’t seriously damage the stones.
Light Scrubbing
Gentle cleaning of stones with a rag and water is an effective way to remove grime and algae, and is the safest method of cleaning existing masonry. The use of chemicals should be avoided altogether where possible, as anything that can dissolve the stone on any level will do serious damage long-term.
Conclusion
The key to safely cleaning a stone is to avoid damaging the outer surface of the stones that are exposed to weather. The reason is that the stone surfaces oxidize over time, forming a hard, thin barrier that protects the stone from erosion and being dissolved by acidic environments. Scrubbing this surface off means the stone will have to weather all over again in order to become impenetrable. This extra weathering, of course, amounts to deterioration. On new construction, heavy scraping scores the stone, opening up places for frost to act, and generally detracts from the appearance of the stone.
Stone Arch Bridges 