Over time, brick and stone buildings weather and age, and often new owners decide to clean them up to make them appear newer. I happen to prefer the rich, soft colors and patina of naturally aged stone. By removing all signs of aging, they also end up not only removing much of it’s character, but also speed up further deterioration through harsh chemical cleaning or sandblasting.
On the other hand, there are cases when cleaning masonry is warranted. If you have good reason to suspect that the masonry is being damaged by pollutants, dirt paint or stains on it’s surface, cleaning should be done in order to preserve the stone and mortar surface.
For example, dirt or soot encrustations on stone may absorb moisture and make it’s surface more prone to decay from acid rain and freeze/thaw cycles. The ideal scenario for cleaning is to leave the masonry’s natural aged character intact while removing as much harmful staining, encrustation, soot and grime as possible. Several methods exist to achieve this.
Water cleaning techniques aim to soften the dirt and then remove it from the surface by rinsing. It is the safest, simplest and least expensive way to clean masonry, yet is also the least commonly used. The 4 techniques available are spraying, handscrubbing, steaming and pressure washing. Water cleaning is the best way to clean limestone and marble. Lime-based brickwork on which dirt has encrusted to the surface by gypsum is also easily cleaned by water techniques, since gypsum is water soluble.
Cleaning masonry with chemicals is at it’s best when removing old paint. It must be done properly, though, or it will cause efflorescence, staining pitting, and other damage. All due safety precautions must be observed when using the chemicals, as they can be very dangerous to the operator and passersby.
Graffiti and spray paint can be cleaned with varnish, acetone and commercial paint strippers. Oils and grease are removable using chlorinated solvents, ammonium hydroxide and petroleum solvents. For rust, oxalic acid, citric acid or sodium citrate are employed.
Commercial herbicides and household bleach work well on organic agents such as moss, lichen and ivy, while mold and algae respond better to chlorinated lime, soap, leaching powders and peroxides. Tars, asphalt and bitumen can be cleaned from masonry with solvents such as kerosene, tolulene, and asphalt cleaners for automobiles.
Mechanical abrasion techniques include sandblasting and power sanding. These should never be used on old or historic buildings, as they are destructive and will damage the appearance, materials and structural integrity of the building. Among other harmful effects of sandblasting, it can destroy mortar joint pointing, leading to moisture penetration, roughen and pit masonry surfaces, leaving masonry surfaces prone to accumulation of water and soot, remove the hard-fire exterior surface of brick, exposing the softer interior to quick deterioration, and damage nearby surfaces due to the inaccurate nature of the application method (overspray).
One exception is the use of microabrasive cleaning. This is a technique that was developed by museums for sculpture cleaning. It uses very fine powder abrasives, for example silica flour, aluminum oxide or crushed dolomite driven at pressures of 30-50 p.s.i. through a small pen-like applicator. It is not practical for use on a large-scale masonry restoration, but can be useful for decorative details.
Once sandblasting or other abrasive cleaning has been performed on limestone, sandstone or marble, there is little that can be done to mitigate the damage; they will decay at a rapid rate. Sandblasted brickwork can be repaired to a certain extent.
Skillful repointing with a softer mortar can help dry it out, and application of a breathable surface treatment or paint may also be able to slow down the deterioration. Replacement of particularly damaged individual bricks can also be undertaken, but matching brick size and appearance on some older buildings can be challenging.
Photo by Beige Alert, Creative Commons Attribution License