by Frederick M. Hueston

Before any stone building is cleaned, several questions need to be answered before a successful cleaning operation can be started. 

Why does the building need to be cleaned?  There are many reasons a building may need cleaning. Primarily, it is for the appearance – dirty buildings are ugly. However, there are other practical reasons a building may need to be cleaned. If the stone has many cracks, decay, or is severely eroded, cleaning may be necessary to make proper repairs.

First Step: Identify the stone type. Before any cleaning is begun, identifying the stone type is very important. Is it marble, granite, limestone, sandstone, etc.? Identification is essential since cleaning chemicals effect each stone differently. For example, marble is very reactive to acid cleaners, whereas granite can be very acid resistant. If the building contains more than one stone type, they need to be identified and perhaps several cleaning methods will need to be used.

Next, identify the soiling.  Many substances can stain a stone building. Often, staining is not discovered until after the building is cleaned. This can result in a building that looks worse cleaned than it did dirty! Some materials that can cause staining are: metallic stains from copper roofs and down spouts; rust from iron, grease, and oils from industrial sources; roofing tar; lichens, algae, moss, and fungal growth; paint and wood stains; graffiti; caulking from windows; and the list goes on. Examine the building carefully and try to pinpoint where staining may be a problem.

These are just a few of the questions that must be answered before any cleansing is performed.  If the building is of historical significance, local historical societies, the National Historic Trust, and other agencies may have something to say about the method of cleaning. In some states, counties, and cities, the methods for cleaning a building are strictly regulated.

Cleaning Tests

Once it has been determined that the building will be cleaned, a series of cleaning tests should be done.  Several methods are usually tried to decide what method will produce the desired results without damage to the stone.

The cleaning tests should help determine the following: does the cleaning produce the desired results? Does it look better cleaned or does it look worse? Which method may be damaging? This method should be eliminated right away.

The cleaning test area should be observed over several weeks, since problems such as efflorescence and staining can take several weeks to develop.  

All cleaning tests should be done with the same equipment and procedures that will be used on the large-scale project.

Once a successful cleaning test is performed, that cleaning test should be used to design the specifications for the entire building.

Cleaning Types

Several methods for building cleaning are available. The following is a brief description of methods I have found effective

Water Misting

Many stone types can be cleaned with just misting the surface with water. Misting heads are set up on scaffolding and water is gently misted on the surface of the building. These misting heads are usually set on a timer so that they go on and off intermittently. For example, they mist for 30 minutes and are off for 30 minutes. This intermittent cycle allows for the building to dry and prevents oversaturation of the stone. This method is the safest, gentlest method for cleaning a building and is widely used for historical buildings. 

Pressure Washing

Pressure washing uses high-pressure water jets of up to 2,500 PSI or more. Pressure washing works by blasting the dirt off the surface of the stone. Pressure washing also can cause irreversible damage to a stone surface and should be used only by highly trained craftspersons. Pressure should never exceed 1,000 PSI and you should only use a fan tip spray nozzle.

Steam Cleaning

Hot water is generally believed to be more effective at removing dirt than cold water. Tests have proven that this is not true when removing atmospheric dirt. However, steam cleaning may be very effective in removing tar, caulking, and chewing gum.

Sandblasting

Sandblasting uses sand, grit, and many other abrasives to remove the soiling mechanically.  Sandblasting is very dangerous to the stone and should be avoided. However, several alternative blasting methods are available that use very gentle abrasives that cause no damage to the stone surface. 

These methods look very promising for cleaning stone, and include media such as baking soda, walnut shells and proprietary soft abrasives. Many of these abrasives are delivered with a special gun that creates a vortex or swirling on the surface of the stone.

Chemical Cleaning

Chemical cleaning is used in combination with one or several water washing methods. Often, a chemical agent will be needed to detach soiled particles. Chemicals can be very dangerous to the stone, the operator, and the surrounding landscape. 

It is extremely important that all chemicals used be tested and monitored. The following are some cleaning chemicals commonly used:

  • Acids – chemicals with a Ph less than 7. Use carefully on granite and sandstone, but acids should not be used on marble and limestone.
  • Alkali – chemicals with a Ph greater than 7. Safe for use on most stone types, but can cause spalling if not rinsed thoroughly.
  • Neutral cleaners –  chemicals containing surfactants with a Ph equal to 7. Safe for most stone surfaces.
  • Solvents – waterless chemicals. Mineral spirits, acetones, etc., are solvent cleaners. They are rarely used for building cleaning due to their high flammability, but are useful for spot stain removal.

Scheduling Cleaning

Scheduling the cleaning project is as important as choosing the cleaning method. Never schedule cleaning when there is a chance of freezing weather or frost. If water penetrates the stone and freezes, the stone will flake and/or spall. The best time to clean is late spring, early summer, and early fall. Hot weather can also cause cleaning problems. If a building needs to be cleaned in hot conditions, try cleaning early in the morning and late in the afternoon or shield the building from the sun.

The Big Picture

As you can see, many factors need to be considered before a stone building is cleaned. It is best to consult with a Conservator, building cleaning contractor, or specialist before doing any cleaning. You will be glad you did.

Fred Hueston has written over 33 books on stone and tile installations, fabrication and restoration and also serves as an expert for many legal cases across the world. 

If you wish to learn more about historic building cleaning there is an upcoming seminar on August 12-13 in Melbourne, Florida. 

For more information contact Dr. Fred at 321-514-6845 or visit www.stoneforensics.com.