Ryan Cash
UAMCC Associate Member
How do you clean Limestone?
Can you use SH on Limestone?
Will Acids etch Limestone?
Limestone cleaning is something a lot of contractors tend to avoid. There are many stories out there of guys using the wrong chemicals or too high of pressure and damaging the limestone. Because of the nature of the stone, it’s typically used on more expensive or historic homes/buildings. This means that any damage caused is likely to be an expensive mistake.
First a little bit about limestone.
“Limestone is a sedimentary rock composed principally of calcium carbonate (calcite) or the double carbonate of calcium and magnesium (dolomite). It is commonly composed of tiny fossils, shell fragments and other fossilized debris. These fossils are frequently visible to the unaided eye on close examination of the stone surface, however this is not always the case. Some varieties of limestone have an extremely fine grain.” -USGSA
Before we get into the cleaning methods, let’s first look at the different finishes you’re likely to see on Limestone.
Polished – Glossy surface that brings out the stone’s full color and character. This is typically only used for interior surfaces.
Honed – Satin finish with little or no gloss. Generally preferred for floors, stair treads and other heavy-traffic applications
Sandblasted – Slightly abraded finish, most commonly used in exterior applications where extra traction is desired. This has a rough, unpolished look
Flamed (Thermal) – Natural-looking cleft, bringing out warmer tones such as corals and pinks (often with wide variation and unpredictable results). To achieve the style, an intense flame is held at the stone’s wet surface. While firing it, the surface becomes so hot it bursts and a layer flakes away to reveal a rough, even texture.
Bush Hammered – Heavily abraded surface, often used as an accent texture
Identifying the type of finish is pretty straight forward. However, there is a lot of mis-information and un-informed information about how to properly clean limestone.
For the purposes of this guide, we are not going to be talking about Polished or Honed limestone finishes. These finishes are typically found in the interior of the home and are best left up to stone cleaning professionals. However, for exterior limestone cleaning there are generally two types of stains that will need to be removed: Inorganic and Organic.
Inorganic staining occurs from natural dust, debris, and dirt found outdoors. These types of stains can typically be removed with a pressure washer however extreme caution need to be taken. Limestone is a softer, more porous stone that can easily be etched or damaged by high PSI. So when using a pressure washer, it’s recommended to use a 45-degree fan tip running around 800 to 1000 PSI (1200PSI being the absolute highest). However, any damage caused to the limestone can only be remedied by sanding and polishing the finish back to it’s original look. This will often require the entire surface to be re-sanded to maintain uniformity. To aid in cleaning, neutral pH soaps and detergents can be used to aid in the removal of inorganics.
The Indiana Limestone Institute states:
“ILI recommends that architects and their clients decide in advance how clean the building must be, keeping in mind that the dirt is not harmful, and that its complete eradication almost always removes remaining original surface and increases surface area. If 80% to 90% clean is the target, the relatively benign high-pressure…will perform satisfactorily in nearly every case. Note, however, that even high-pressure water can cause damage. Pressure, nozzle size and working distance are all controlled by the operator and can be abused.”
Organic Staining occurs from the build up algae, bacteria, and molds. It is much more difficult to clean. Typically the organic growth sinks deep into the pores of the limestone and can’t be removed by water pressure alone (at least not without damaging the stone).
Typically, stone manufacturers recommend AGAINST the use of Bleach (SH) on limestone because being so porous, limestone absorbs the SH and it eventually crystallizes within the stone. This can cause future discoloration and/or complete degradation of the stone. SH will often lead to efflorescence in the stone that will leave an un-desirable finished product. While older manufacturer guidelines allow for the use of light SH in cleaning, most updated guidance recommends against it.
Another option are products that use Peroxide to clean. These are much more expensive and, like bleach, won’t get deep into the pores of the stone. While safer to use on Limestone than SH, Peroxide won’t always be a sure-fire way to get the job clean.
Following SH and peroxide, the next thought is then to use some form of acid (hydrochloric acid, hydrofluoric acid, oxalic acid, etc.). In most circumstances, the use of acid is generally frowned upon because it can lead to etching, or damage to the stone. This is especially true for more polished finishes.
There are some modern acidic limestone cleaners (OneRestore and NMD 80) that have been formulated and blended to successfully incorporate strong acids coupled with surfactants to clean without adverse effects on limestone. These products typically do not cause discoloration or etch on limestone, however, it’s always important to perform test spots in inconspicuous areas.
Because of the high acidity and inherent danger of using acidic cleaners, it’s usually safer for a contractor to instead use a Biological detergent. An example of a biological detergent would be Simple Green. Most of these products from big box stores aren’t formulated specifically for stone cleaning so they aren’t extremely effective.
However, there are a few manufacturers that make stone specific biological detergents that have been shown to be effective in removing biological staining in Limestone
Cathedral stone products and Bonstone (D2 Biological Cleaner), are brands that that come recommended by the Indiana Limestone Institute. One of the greater benefits of these types of products are that that contain no/low VOC’s, making the cleaning process much less intrusive. The downside of these types of cleaners is that they will require much more elbow grease. Application, thorough brushing, high pressure rinsing (1000PSI), and repeating is pretty standard.
Other products like Prosoco 942 Limestone and Marble Cleaner have also been found to be an effective cleaner for limestone
Historical Buildings
Historical and Government buildings present their own set of problems. For these jobs, cleaning the stone comes secondary to preventing damage.
When cleaning historical buildings:
"AVOID OVERCLEANING. AIM FOR ACHIEVING 85% CLEAN. MOST DAMAGE OCCURS WHEN ATTEMPTING TO CLEAN THE LAST 15%.
TEST-CLEAN A SMALL AREA TO DETERMINE EFFECTIVENESS OF CLEANING METHODS, MATERIALS, EQUIPMENT AND WORKING PRESSURES SELECTED BEFORE PROCEEDING WITH CLEANING OPERATIONS ON LARGER AREAS. ADJUST METHODS, MATERIALS, EQUIPMENT, PRESSURES, ETC. AS NECESSARY. DO NOT PROCEED UNTIL AN ACCEPTABLE CLEANING OPERATION HAS BEEN APPROVED AND FULLY DOCUMENTED." - USGSA
Work with the local municipalities and engineers to determine the safe level of clean that is acceptable.
Manufacturer Guidelines
Can you use SH on Limestone?
Will Acids etch Limestone?
Limestone cleaning is something a lot of contractors tend to avoid. There are many stories out there of guys using the wrong chemicals or too high of pressure and damaging the limestone. Because of the nature of the stone, it’s typically used on more expensive or historic homes/buildings. This means that any damage caused is likely to be an expensive mistake.
First a little bit about limestone.
“Limestone is a sedimentary rock composed principally of calcium carbonate (calcite) or the double carbonate of calcium and magnesium (dolomite). It is commonly composed of tiny fossils, shell fragments and other fossilized debris. These fossils are frequently visible to the unaided eye on close examination of the stone surface, however this is not always the case. Some varieties of limestone have an extremely fine grain.” -USGSA
Before we get into the cleaning methods, let’s first look at the different finishes you’re likely to see on Limestone.
Polished – Glossy surface that brings out the stone’s full color and character. This is typically only used for interior surfaces.
Honed – Satin finish with little or no gloss. Generally preferred for floors, stair treads and other heavy-traffic applications
Sandblasted – Slightly abraded finish, most commonly used in exterior applications where extra traction is desired. This has a rough, unpolished look
Flamed (Thermal) – Natural-looking cleft, bringing out warmer tones such as corals and pinks (often with wide variation and unpredictable results). To achieve the style, an intense flame is held at the stone’s wet surface. While firing it, the surface becomes so hot it bursts and a layer flakes away to reveal a rough, even texture.
Bush Hammered – Heavily abraded surface, often used as an accent texture
Identifying the type of finish is pretty straight forward. However, there is a lot of mis-information and un-informed information about how to properly clean limestone.
For the purposes of this guide, we are not going to be talking about Polished or Honed limestone finishes. These finishes are typically found in the interior of the home and are best left up to stone cleaning professionals. However, for exterior limestone cleaning there are generally two types of stains that will need to be removed: Inorganic and Organic.
Inorganic staining occurs from natural dust, debris, and dirt found outdoors. These types of stains can typically be removed with a pressure washer however extreme caution need to be taken. Limestone is a softer, more porous stone that can easily be etched or damaged by high PSI. So when using a pressure washer, it’s recommended to use a 45-degree fan tip running around 800 to 1000 PSI (1200PSI being the absolute highest). However, any damage caused to the limestone can only be remedied by sanding and polishing the finish back to it’s original look. This will often require the entire surface to be re-sanded to maintain uniformity. To aid in cleaning, neutral pH soaps and detergents can be used to aid in the removal of inorganics.
The Indiana Limestone Institute states:
“ILI recommends that architects and their clients decide in advance how clean the building must be, keeping in mind that the dirt is not harmful, and that its complete eradication almost always removes remaining original surface and increases surface area. If 80% to 90% clean is the target, the relatively benign high-pressure…will perform satisfactorily in nearly every case. Note, however, that even high-pressure water can cause damage. Pressure, nozzle size and working distance are all controlled by the operator and can be abused.”
Organic Staining occurs from the build up algae, bacteria, and molds. It is much more difficult to clean. Typically the organic growth sinks deep into the pores of the limestone and can’t be removed by water pressure alone (at least not without damaging the stone).
Typically, stone manufacturers recommend AGAINST the use of Bleach (SH) on limestone because being so porous, limestone absorbs the SH and it eventually crystallizes within the stone. This can cause future discoloration and/or complete degradation of the stone. SH will often lead to efflorescence in the stone that will leave an un-desirable finished product. While older manufacturer guidelines allow for the use of light SH in cleaning, most updated guidance recommends against it.
Another option are products that use Peroxide to clean. These are much more expensive and, like bleach, won’t get deep into the pores of the stone. While safer to use on Limestone than SH, Peroxide won’t always be a sure-fire way to get the job clean.
Following SH and peroxide, the next thought is then to use some form of acid (hydrochloric acid, hydrofluoric acid, oxalic acid, etc.). In most circumstances, the use of acid is generally frowned upon because it can lead to etching, or damage to the stone. This is especially true for more polished finishes.
There are some modern acidic limestone cleaners (OneRestore and NMD 80) that have been formulated and blended to successfully incorporate strong acids coupled with surfactants to clean without adverse effects on limestone. These products typically do not cause discoloration or etch on limestone, however, it’s always important to perform test spots in inconspicuous areas.
Because of the high acidity and inherent danger of using acidic cleaners, it’s usually safer for a contractor to instead use a Biological detergent. An example of a biological detergent would be Simple Green. Most of these products from big box stores aren’t formulated specifically for stone cleaning so they aren’t extremely effective.
However, there are a few manufacturers that make stone specific biological detergents that have been shown to be effective in removing biological staining in Limestone
Cathedral stone products and Bonstone (D2 Biological Cleaner), are brands that that come recommended by the Indiana Limestone Institute. One of the greater benefits of these types of products are that that contain no/low VOC’s, making the cleaning process much less intrusive. The downside of these types of cleaners is that they will require much more elbow grease. Application, thorough brushing, high pressure rinsing (1000PSI), and repeating is pretty standard.
Other products like Prosoco 942 Limestone and Marble Cleaner have also been found to be an effective cleaner for limestone
Historical Buildings
Historical and Government buildings present their own set of problems. For these jobs, cleaning the stone comes secondary to preventing damage.
When cleaning historical buildings:
"AVOID OVERCLEANING. AIM FOR ACHIEVING 85% CLEAN. MOST DAMAGE OCCURS WHEN ATTEMPTING TO CLEAN THE LAST 15%.
TEST-CLEAN A SMALL AREA TO DETERMINE EFFECTIVENESS OF CLEANING METHODS, MATERIALS, EQUIPMENT AND WORKING PRESSURES SELECTED BEFORE PROCEEDING WITH CLEANING OPERATIONS ON LARGER AREAS. ADJUST METHODS, MATERIALS, EQUIPMENT, PRESSURES, ETC. AS NECESSARY. DO NOT PROCEED UNTIL AN ACCEPTABLE CLEANING OPERATION HAS BEEN APPROVED AND FULLY DOCUMENTED." - USGSA
Work with the local municipalities and engineers to determine the safe level of clean that is acceptable.
Manufacturer Guidelines
