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GENERAL TECHNIQUES FOR THE CHEMICAL CLEANING OF MASONRY
The following is intended as a very basic guide to chemical cleaning techniques, and chemical applications to various substrates.
Before you undertake chemical cleaning it is first necessary to identify the building materials present, i.e. sandstones (red, buff, cream), brick, terracotta, granites, calcited stones ( bath,portland , slate, limestone & marble ), metals ( copper, aluminium , brass, iron etc), and paint protected surfaces.
We must consider the type of dirt and degree of soiling, products available, safety and environmental issues, costs , time and efficiency.
Perhaps most important of all are the safety aspects ; not only for the operative, but also for members of the public . Unless proper care is taken, damage to the person, or to the masonry on which you are working can be irreparable. If in any doubt, then it is advisable to leave the job alone.
TOPIC HEADINGS
- TYPES OF SOILING - Where the various types of soiling matter evolve, how they adhere , and the problems they cause.
- A REVIEW OF CLEANING METHODS AVAILABLE - The various advantages and disadvantages of each method.
- A GENERAL CLASSIFICATION OF TYPES OF MASONRY - The characteristics of various substrates, the chemicals normally required and those which to avoid.
- THE SAFETY ASPECTS OF CHEMICAL CLEANING - A look at damage which can be caused to ourselves or buildings/masonry. Unless we use the correct materials and handle them correctly serious consequences can arise.
5. THE ENVIRONMENT - Environmental considerations. With sections on. hydrofluoric acid and methylene chloride.
- OTHER TYPES OF MASONRY, ARTEFACTS AND SPECIAL TREATMENTS. A look at treatment and protection of special structures , including paint removal, graffiti removal, anti-graffiti coatings, efflorescence, insoluble white deposits, algae and mosses, marble, copper and copper stains, iron staining, linseed oil, and oil stains.
OIL & GREASE - Oil drips from vehicles and small particles of tar from road surfaces floaton rainfall water , then get splashed into a fine spray which carries on to buildings where it is absorbed and dries. Part burnt petrol and diesel emerges from engines , as well as chimney smoke. Dirt bonds very strongly in this traffic film.
FUMES - From engines, houses, factories and industrial buildings – we get carbon dioxide and sulphur dioxide gases which dissolve in rain to form acids. The acids can attack calcited sandstones and limestones to cause serious erosion, but, more importantly, the chemicals formed from these dissolving parts, if they dry on the masonry surface set like plaster, and in doing so bind in grease, oil, soot and dirt. In some cases deposits forming under sills etc. can be several centimetres thick.
DIRT - Arrives in many forms; road dust, cement dust, sand, coaldust, brakedust, tyre and factory dust all carried on the wind from possibly many miles away and consequently stoked up by passing vehicles and local breezes can adhere to masonry surfaces.
SOOT - Smoke from chimneys, boilerhouses, and vehicle exhausts all contain black soot particles. Soot, ( carbon ) deposits on masonry to leave black carbonaceous deposits. It is often greasy too, and can layer up to deposits of 2 – 4 mm thick. These can be particularly difficult to remove and will only respond to prolonged treatments with caustic based degreasants.
PAINTS - These are applied as surface treatments to masonry, and the degree of difficulty in removal depends on the binding agents holding the pigments and the absorbency of the substrate. Most modern paints are oil or water based, but in past years people used to mix their own coatings….. linseed oil with soot for a black waterproof coating. Melted wax , fat, shellac etc with various dyes or lead distemper to try and colour and waterproof buildings.
LEAVE IT ALONE - If in doubt as to what you are doing , and if your actions may lead to damage, then it’s best to leave it alone. Some buildings however may need a clean-up for maintenance purposes, i.e. dirt and contamination may be causing corrosion, spalling and fractures. Situations can therefore arise where fine-line decisions must be made as to whether it is best to clean, or leave alone.
WASH WELL WITH CLEAN WATER ONLY – Can sometimes work, but if the surface is greasy, then we are unlikely to obtain a good result without using soap/detergents, ( and even this is chemical cleaning). Many salts present in stone , such as calcium sulphate are not very readily soluble in water, and even if greasy will take a lot of water to shift.
In freezing weather if water gets inside stone, bursting can be expected.
Using hot water can help to cut through some oil/grease resistance.
Steam cleaning can lead to patchy results.
ABRASIVE BLAST CLEANING - Involves projecting various media with compressed air, sometimes with water added to soften the impact . Can work well in the correct hands when cleaning large flat areas of hard stone. However many buildings and monuments have been devastated by unconscientious , unskilled labour not knowing when to stop. Most damage is caused to soft sandstone and ornate fine detail which is simply eroded away for ever.
PAINT OR RENDER THE SURFACE - If a surface is painted , the pores in the stone are often closed. Dampness trapped in the stone cannot dry by natural ‘ breathing. ’ Damp patches may occur on the interior or further away from the damp area. If it freezes, surface damage by bursting can be expected. The treatment is likely to need expensive maintenance and re-doing every few years , and to remove old coatings can also be expensive.
CHEMICAL CLEANING METHODS - When dirt first goes onto the surface of a building it washes into the pores and sticks as a film. If certain chemicals could loosen this dirt and grease, even if necessary by dissolving the outer layer, then washing, using high pressure cold or perhaps hot water, would probably remove that dirt. We must be sure that the chemicals do not destroy the strength or colour of the stone. We must leave no residues in the stone which will cause future damage. For this reason it is normal to pre-wet masonry with water prior to applying chemicals to greatly reduce the absorption into the stone.
Structure – Very abundant sedimentary rock. Formed from the mineral calcite (calcium carbonate) and sediment.
METHODS -
1) If the surface is not too greasy wash with prolonged water sprays.
Or 2) Pre-wet the surface, brush or spray on a caustic based degreasant ( Centurywise CW5 gel or CW5a liquor). Allow contact time of normally 1 - 3 hours depending on degree of soiling. Pressure wash off thoroughly with copious amounts of water. Brush or spray on a weak acid neutraliser ( Centurywise CW17 ). Pressure wash thoroughly.
WARNINGS -
1) Strong acids must not come into contact with limestones or calcited stone or they can be ruined. ( fizzing can be observed).
* To test a substrate where it is unclear if it is a form of limestone, mix a little washing up liquid with a 10% hydrochloric acid solution, and spot it onto the stone. Any calcium carbonate present will form gas bubbles . Non-calcited stone such as sandstone or granite will not release bubbles.
2) Caustic based degreasants will attack skin, aluminium, brass, tin, painted surfaces and polished wood.
3) Incomplete washing off of the cleaning chemicals may lead to a breakdown in the stone strength.
4) Even if limestone is cleaned only by water washing, brown staining can sometimes occur due to resins present in the stone. With time they fade.
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SANDSTONES - Cream, brown, grey & MILLSTONE GRITS
Structure – Formed by sands of various colours depositing and compressing.Sandstone is a sedimentary rock composed mainly of sand- sized grains cemented together by clay, silica, carbonate, or iron oxide. Sandstone often contains other mineral grains such as feldspar or mica. When cemented by silica , sandstone has great strength, making it suitable for structural use. Millstone grit looks like sandstone, but much stronger and can be difficult to clean.
METHODS -
1) Pre-wet with water to reduce absorption.
2) Degrease with a caustic based detergent such as CW5 gel or CW8 (liquid), and pressure wash off debris. THIS STAGE CAN BE OMITTED IF SURFACE IS NOT TOO GREASY.
3) Brush or spray on CW10 ( hydrofluoric/phosphoric acid blend). Allow approx. 10 minutes contact time and pressure wash off.
WARNINGS -
If the hydrofluoric acid blend used is too strong, or is left on for too long, it could cause streaking/ bleaching effects.
If hydrofluoric acid is used too strong without added phosphoric acid, patchy iron stains may appear on the surface.
Caustic based degreasants will attack skin, aluminium, brass, tin, painted surfaces , and polished wood.
Hydrofluoric acid mixtures will attack skin, glass, polished granite, and metals.
Some millstone grits when cleaned still have a slightly dirty looking surface thet will not clean with chemicals.
Some sandstones are silver or grey when newly quarried and weather to form a golden surface around 6mm deep on the face. After chemical cleaning the colour may revert to the silver grey state.
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RED SANDSTONES
Structure – Formed by red sands depositing and compressing .Red sandstone is a sedimentary rock composed mainly of sand- sized grains cemented together by clay, silica, carbonate, or iron oxide.
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METHODS -
1) Pre-wet with water to reduce absorption.
2) Degrease with a caustic based detergent such as CW5 gel or CW8 (liquid), and pressure wash off debris. THIS STAGE CAN BE OMITTED IF SURFACE IS NOT TOO GREASY.
3) Brush on CW12 red sandstone cleaner and pressure wash off after approx. 15 minutes.
WARNINGS -
Do not use a phosphated hydrofluoric blend ( CW9 or CW10 ) , as this often leads to the formation of heavy white efflorescences.
Caustic based degreasants will attack skin, aluminium, brass, tin, painted surfaces , and polished wood.
Hydrofluoric acid mixtures will attack skin, glass, polished granite, and metals.
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METHODS -
1) Pre-wet with water to reduce absorption.
2) Degrease with a caustic based detergent such as CW5 gel or CW8 (liquid), and pressure wash off debris. THIS STAGE CAN BE OMITTED IF SURFACE IS NOT TOO GREASY.
3) Brush or spray on * CW10 ( hydrofluoric/phosphoric acid blend). Allow approx. 10 mins. contact time and pressure wash off.
WARNINGS -
*If flint or granite has a glass like or polished finish, hydrofluoric acid mixtures must not be used as it will attack and etch the surface. The surface is so hard that it often does’t absorb dirt, so degreasing and power washing is often adequate for an acceptable finish.
Caustic based degreasants will attack skin, aluminium, brass, tin, painted surfaces , and polished wood.
Hydrofluoric acid mixtures will attack skin, glass, polished granite, and metals.
METHODS -
1) Pre-wet with water to reduce absorption.
2) Degrease with a caustic based detergent such as CW5 gel or CW8 (liquid), and pressure wash off debris. THIS STAGE CAN BE OMITTED IF SURFACE IS NOT TOO GREASY.
3) Brush or spray on CW9 or CW10 ( hydrofluoric/phosphoric acid blend). Allow approx. 10 minutes contact time and pressure wash off.
WARNINGS -
If the hydrofluoric acid blend used is too strong, or is left on for too long, it will cause streaking/ bleaching effects.
Caustic based degreasants will attack skin, aluminium, brass, tin, painted surfaces , and polished wood.
Hydrofluoric acid mixtures will attack skin, glass, polished granite, and metals.
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The dangers may arise from several sources. From an unskilled person with lack of training, awareness, or understanding to over-familiarity which can lead to experienced operatives taking shortcuts, calculated risks, or ignoring correct procedures.
The main points of consideration on site are –
- Store the chemicals in a safe place.
- Do not handle or use a chemical until you have read and understood the product information, & health and safety data sheets .
- Provide site agent with relevant data sheets and risk assessment.
- Have supply of clean water readily to hand, and know nearest accessible water supply point for emergencies.
- If using hydrofluoric acid, have burns gel readily available.
- Make sure you have all the correct equipment, and that machinery is properly seviced.
- Properly mask off items which require protection.
- Make certain that other site personnel , or the public are not in any danger.
When the cleaning has been completed there are still safety duties to perform - the work area should be thoroughly washed down , including plants, scaffold boards & tubes, brushes, buckets, wetsuits and equipment.
Everything then needs to be put away in safe places.
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Modern chemical cleaning formulations are biodegradable , so damage to plant and fish life is minimal , especially after the cleaning agents have been mostly used up when cleaning the masonry, and the rest massively diluted during washdown.
Consider the situation where a square metre of masonry has been treated with a chemical.
The small amount of material present is typically washed down with over 100 litres of water, ensuring a high degree of dilution, and so damage to the drains or the environment is negligible.
Most of the solutions have been in use for over 40 years.
HYDROFLUORIC ACID
1.2 What happens to Fluorine, Hydrogen Fluoride, and Fluorides When They Enter the Environment?
Fluorides occur naturally in the earth’s crust where they are found in rocks, coal, clay, and soil. They are released into the air in wind-blown soil. Hydrogen fluoride is released to the air from fluoride-containing substances, including coal, minerals, and clays, when they are heated to high temperatures. This may occur in coal-fired power plants; aluminum smelters; phosphate fertilizer plants; glass, brick, and tile works; and plastics factories. These facilities may also release fluorides attached to particles. The biggest natural source of hydrogen fluoride and other fluorides released to the air is volcanic eruptions.
Fluorine cannot be destroyed in the environment; it can only change its form. Fluorides released into the atmosphere from volcanoes, power plants, and other high temperature processes are usually hydrogen fluoride gas or attached to very small particles. Fluorides contained in wind-blown soil are generally found in larger particles. These particles settle to the ground or are washed out of the air by rain. Fluorides that are attached to very small particles may stay in the air for many days.
Hydrogen fluoride gas will be absorbed by rain and into clouds and fog to form aqueous hydrofluoric acid, which will fall to the ground mainly in precipitation. The fluorides released into air will eventually fall on land or water.
In water, fluorides associate with various elements present in the water, mainly with aluminum in freshwater and calcium and magnesium in seawater, and settle into the sediment where they are strongly attached to sediment particles. When deposited on land, fluorides are strongly retained by soil, forming strong associations with soil components. Leaching removes only a small amount of fluorides from soils. Fluorides may be taken up from soil and accumulate in plants, or they may be deposited on the upper parts of the plants in dust. The amount of fluoride taken up by plants depends on the type of plant, the nature of the soil, and the amount and form of fluoride in the soil. Tea plants are known to accumulate fluoride in their leaves.
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PAINT REMOVER – METHYLENE CHLORIDE
1.1 What is methylene chloride?
Methylene chloride, also known as dichloromethane, is a colorless liquid that has a mild sweet odor, evaporates easily, and does not easily burn. It is widely used as an industrial solvent and as a paint stripper. It can be found in certain aerosol and pesticide products and is used in the manufacture of photographic film. The chemical may be found in some spray paints, automotive cleaners and other household products. Methylene chloride does not appear to occur naturally in the environment. It is made from methane gas or wood alcohol. Most of the methylene chloride released to the environment results from its use as an end product by various industries and the use of aerosol products and paint removers in the home.
1.2 What happens to methylene chloride when it enters the environment?
Methylene chloride is mainly released to the environment in air and to a lesser extent in water and soil, due to industrial and consumer uses.
Many chemical waste sites, including NPL sites, contain methylene chloride and these might act as additional sources of environmental contamination through spills, leaks, or evaporation.
Because methylene chloride evaporates readily, most of it is released into the air. In air, it is broken down by sunlight and by reaction with other chemicals present in the air. About half of the methylene chloride disappears from air in 53–127 days.
Although methylene chloride does not dissolve easily in water, small amounts may be found in some drinking water. Methylene chloride that is present in water is broken down slowly by reactions with other chemicals or by bacteria. More than 90% of the methylene chloride in the environment changes to carbon dioxide (CO2), which is already present in air. It takes about 1 to 6 days for half the methylene chloride to break down in water. When released into a river , methylene chloride levels were non-detectible 3 – 15 miles from the source. It is reported to be completely biodegradable under aerobic conditions with sewage weed or activated sludge between 6 hours and 7 days.
When methylene chloride is spilled on land, it attaches loosely to nearby surface soil particles. It evaporates rapidly from the soil into the air. Some may also move into groundwater. We do not know how long it remains in soil, very little research has been done . We do not expect methylene chloride to build up in plants or animals.
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6. OTHER TYPES OF MASONRY, ARTEFACTS & SPECIAL TREATMENTS
1) Paint Removal
2) Graffiti Removal & Anti-graffiti coatings
3) Efflorescence
4) Insoluble White Deposits
5) Algae and Mosses
6) Marble
7) Copper , Bronze & Brass
8) Iron Staining
9) Linseed oil
10) Oil Stains from stone
11) Disclaimer
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Paint strippers contain organic solvents which soften the binding agents and resins which hold the film together in solid form. These solvents evaporate away easily leaving a soft debris which can be scrapped and pressure washed off.
CW3 Paint remover is best applied to a dry surface by brush as a generous thick film. After 5 to 10 minutes more stripper should be worked in to further loosen up the already softened paint and to contact the remaining hard paint below. After a further 10 minutes pressure wash off……… hot water washers greatly aid the process.
At this stage, if there is any paint residues or oils left, it is often beneficial to apply CW5 gel which besides being a limestone cleaner is also an excellent caustic stripper . Leave CW5 on for 30 minutes and jet off.
If circumstances allow, CW5 can be used as a long contact time stripper – brush on and leave for approx. 90 minutes , work in more, and leave for another 90 minutes. The debris can now be pressure washed off – again preferably hot. The surface now needs to be neutralized with CW17 acetic neutralizer and jetted off .
This whole long contact method may need to be repeated several times for stubborn coatings.
The effective removal of graffiti requires the use of specific chemical strippers designed to dissolve unwanted markings without negatively impacting the masonry surface. There are many variables ; - type and condition of stone, type and thickness of paint, type of oils, resins and pigments, length of time since application.
Before starting, check that any nearby surfaces are protected from damage by your localised treatments. Work to distinct boundaries as it is highly likely that the area you have just cleaned will look much cleaner than adjacent areas.
Apply thick layer of CW3 Paint Remover and leave on for 5 –10 minutes. Work in more stripper , leave for a further 5 minutes and clean off debris with pressure washer ( hot water is preferable). The main layers of paint should now have been removed, and in most cases you will be left with a “ghost” outline of the graffiti wording.
The “ghosting” is a mix of residual oil and pigments. This can be readily removed by applying CW5 Limestone Cleaner and H.D. degreaser. Leave on for 5 – 10 minutes and pressure wash off.
Neutralize the area with an appropriate acid for the type of masonry. If stubborn pigments are still on the surface, then it is likely that this final treatment will dissolve and remove them.
POULTICING
In some special cases , such as felt tip on limestone or marble, where the normal method of graffiti removal would probably push the offending marks deeper into the substrate, it may be necessary if circumstances allow to apply a poultice to try to draw out the markings.
There are a number of complex kaolin based products available, but for the purpose of general graffiti removal try either CW5 or CW21 Eco-stripper . Apply generously, cover in cling film and tape edges, leave for 12 – 24 hours , and pressure wash off. CW5 treated area will need to be neutralized.
And finally… On occasions none of the above may work very well. Specialist graffiti removal firms usually carry on board a small bottle of just about every chemical that their firm stocks. There are so many different types and combinations of paint that sometimes the most unlikely chemical works. i.e. CW1 mortar stain remover on thin limewash. Bleach or ammonia based floor cleaners may be required to break through wax coating. Meths or cellulose thinners can sometimes work wonders.
ANTI- GRAFFITI COATINGS
Over the past few years a variety of protective treatments have been developed to help reduce the penetration of graffiti into the pore structure of the masonry. These fall into three general categories : high density polymeric films (i.e. polyurethanes ), sacrificial films ( i.e. acrylics) , and penetrating paint repellants ( i.e. modified siloxanes ).
High density polymeric films are clear or pigmented coatings with extremely high chemical resistance and low porosity to resist graffiti penetration and stripping solutions. The disadvantages of this type of product is the low permeability ( which leads to moisture entrapment behind the film) , and the fact that they radically alter the appearance of the masonry ( gloss or matt sheen).
Sacrificial coatings are as the name implies, consumed in the protection process. The disadvantages are that they require retreatment after stripping, they are fim formers which are not very vapour permeable, and usually slightly alter the appearance of the masonry.
Penetrating water repellants this is a fairly new technology based development, which works on a similiar principle to water repellants. It alters the surface energy properties of the masonry so that graffiti has the tendency to agglomerate, rather than sink into the surface. The graffiti then cures without having penetrated the pores, and can be removed relatively easily with the appropriate stripper.
The chemical structure of this protectant makes it inert to most chemical stripping solutions.
It does not form a continous film, and can also be water based making it enviro-friendly.
Although some surface darkening can occur during application as it penetrates the masonry, it is not as pronounced as with high density or sacrificial coatings.
Efflorescence is water soluble salts carried from within the substrate to the surface by water.
This evaporates and leaves the powdery salts on the surface. It is usually white, but efflorescence may be green, yellow, or brown.
Causes
Salts enter the wall from various sources. New bricks seldom contain soluble salts but mortar and concrete have relatively high soluble salt contents. Ground waters that are naturally salt-bearing, can be drawn into base brickwork. A faulty damp proof course or a damp course bridged by mortar will allow the salts to migrate up the wall. Render which has been applied over a damp proof course can also allow salt to migrate up the face of the brickwork.
The amount of efflorescence that occurs is directly related to the amount of water in the bricks, and their drying time. The more water in the bricks, and the longer it is there, the more chance salts will have to dissolve in it and be brought to the surface as the bricks dry out. Water allowed to enter uncovered cavity walls during construction is likely to cause efflorescence so brickwork must be protected from water entry during construction.
The denser a material, whether brick, stone or concrete, the more difficult is is for water to transport the salts to the surface. Conversely, the lense dense the material, the greater the likelihood of efflorescence.
Salts enter the wall from various sources. New bricks seldom contain soluble salts but mortar and concrete have relatively high soluble salt contents. Ground waters that are naturally salt-bearing, can be drawn into base brickwork. A faulty damp proof course or a damp course bridged by mortar will allow the salts to migrate up the wall. Render which has been applied over a damp proof course can also allow salt to migrate up the face of the brickwork.
The amount of efflorescence that occurs is directly related to the amount of water in the bricks, and their drying time. The more water in the bricks, and the longer it is there, the more chance salts will have to dissolve in it and be brought to the surface as the bricks dry out. Water allowed to enter uncovered cavity walls during construction is likely to cause efflorescence so brickwork must be protected from water entry during construction.
The denser a material, whether brick, stone or concrete, the more difficult is is for water to transport the salts to the surface. Conversely, the lense dense the material, the greater the likelihood of efflorescence.
Persistent efflorescence should be taken as a warning that water is entering the wall through faulty copings, flashings, or pipe leakage. If allowed to continue unchecked the salts carried to the face of the wall may eventually attack and cause deterioration of some bricks.
Remedies
Efflorescence can be minimised by laying dry bricks and by speeding up the drying process after the bricks have been laid by providing good ventilation. The salts that cause efflorescence are soluble in water. Hosing with water will cause the salts to dissolve and be re-absorbed into the brickwork, and then reappear when the brick wall dries out again.
Acid or alkaline treatments are not recommended as they increase the salt content of the wall.
The best method is simply brush off the deposit with a stiff dry bristle brush after the wall has dried out. Then sponge the surface with a damp synthetic chamois or high suction sponge. Use very little water and rinse sponge frequently in fresh water.
Remedies
Efflorescence can be minimised by laying dry bricks and by speeding up the drying process after the bricks have been laid by providing good ventilation. The salts that cause efflorescence are soluble in water. Hosing with water will cause the salts to dissolve and be re-absorbed into the brickwork, and then reappear when the brick wall dries out again.
Acid or alkaline treatments are not recommended as they increase the salt content of the wall.
The best method is simply brush off the deposit with a stiff dry bristle brush after the wall has dried out. Then sponge the surface with a damp synthetic chamois or high suction sponge. Use very little water and rinse sponge frequently in fresh water.
Hard white deposits that are insoluble in water sometimes appear in brickwork. Don’t confuse these deposits with efflorescence which is water soluble.
Causes
Most commonly this staining arises from products of the setting reactions of portland cement. These are leached out of concrete elements such as sills, lintels, copings, cement render or insufficiently-dense mortar. They combine with carbon dioxide from the atmosphere to form insoluble white deposits. A second form of insoluble white scum can occur after acid cleaning of smooth-faced bricks, especially the reds and darker colours.
However the main cause of these insoluble white deposits is bad cleaning practice, allowing mortar made from sand containing too much clay to remain too long on the surface of the bricks and then removing it with too much acid and too little water. When not enough water is used before and after the acid wash, the products of the reaction between the acid and the mortar are absorbed into the faces of the bricks instead of being washed off the wall.
Kaolin, a clay mineral present in most bricklaying sands, can also form a hard deposit. It is insoluble in most acids except hydrofluoric acid.
The combination of clay from the mortar with calcium and silica residues from the cement, form calcium silicate which could also produce the insoluble white scum. Calcium silicate is highly insoluble in most acids and is white in colour. When wet these calcium deposits are invisible.
Most commonly this staining arises from products of the setting reactions of portland cement. These are leached out of concrete elements such as sills, lintels, copings, cement render or insufficiently-dense mortar. They combine with carbon dioxide from the atmosphere to form insoluble white deposits. A second form of insoluble white scum can occur after acid cleaning of smooth-faced bricks, especially the reds and darker colours.
However the main cause of these insoluble white deposits is bad cleaning practice, allowing mortar made from sand containing too much clay to remain too long on the surface of the bricks and then removing it with too much acid and too little water. When not enough water is used before and after the acid wash, the products of the reaction between the acid and the mortar are absorbed into the faces of the bricks instead of being washed off the wall.
Kaolin, a clay mineral present in most bricklaying sands, can also form a hard deposit. It is insoluble in most acids except hydrofluoric acid.
The combination of clay from the mortar with calcium and silica residues from the cement, form calcium silicate which could also produce the insoluble white scum. Calcium silicate is highly insoluble in most acids and is white in colour. When wet these calcium deposits are invisible.
Any building , whether it’s been blast cleaned, chemically cleaned , or left untreated can grow algae and mosses and turn green.
In the past, factories, industrial buildings , and even domestic properties used to belch out smoke and fumes which subsequent rain would turn to acid , making it very difficult for these organisms to survive. Today’s cleaner air is perversely providing a perfect enviroment for algae and mosses.
Most available control agents are water soluble, and therefore depleted by weathering.
Quaternary ammonium compounds ( Centurywise CW4 ) , phenols, tin complexes , chlorinated blends, and even alkali and strong acid cleaners destroy the living organisms and spores and leave the surface sterile. But wind-borne spores can quickly re-infect a surface, and the growths re-appear. On a newly cleaned building the colour contrast is usually more objectionable than on a dirty building.
By treating a cleaned surface ( after it has dried out ) with a silicone water repellant like Centurywise CW2, the moisture level in the masonry surface is kept to a very low level, where organisms find it difficult to survive. All organisms need water to survive.
The most frequent requests for cleaning marble come from churches , stately homes & office blocks , and involve cleaning ornate internal structures , staircases and gravestones.
Problems arise because marble is often finished by a polished coating of beeswax , or other coating which is constantly replenished as part of a cleaning and maintenance plan. Over a period of time, this becomes dirty and discoloured. It responds to caustic based degreasers, but care must be taken to ensure that it is thoroughly washed down. In most internal situations pressure washing is simply not possible. Washing down with water and a sponge requires frequent water changes and lots of mopping up , but the results can be very acceptable. Unless a final wash is carried out using a weak acid , the surface of the cleaned marble tends to dry with a white powdery finish, as if coated with talc. Washing down with plenty of clean water in the final stages is necessary to prevent this.
bronze is copper alloyed with tin
brass is copper alloyed with zinc
Advice is often requested for cleaning wall plaques, statues, rainwater goods, vessels etc.
The most important point to remember is to ensure that it is clearly understood that some chemical cleaning methods will remove the patina.
The patina is the oxidation which forms on the surface as a brown or green deposit. It is sometimes considered to be aesthetically important, and it’s removal could be thought of as desecration.
If acids are used to clean soiled copper , bronze or brass, they will attack the metal and so cause damage as they remove the patina. Abrasives will also remove the patina and can damage the underlying metal. Alkali cleaners will remove greasy dirt only, and will probably not clean sufficient of the remaining surface debris to provide an acceptable finish.
Ammonia based cleaners will remove the patina and leave bright metal , as log as the surface is free from protective varnishes and greases. On the downside though, ammonia is extremely unpleasant to use.
A common situation is where a statue covered in bird droppings needs to be cleaned, with the patina retained . ……… The thick deposits should first be removed with a scraper after perhaps being softened with a hot water pressure washer. Further jetting and scrubbing with a small stiff brush may be all that is required to produce acceptable results. If more dirt needs to be removed use Centurywise CW7 Traffic film remover with further scrubbing and jetting.
If further cleaning is necessary, then it may be that the statue has been coated in the past by varnish for protection. Apply CW3 Paint Remover , wash off , apply further CW7 , scrub and wash off.
If further cleaning is still necessary, then either acids or ammonia will be required and the patina will be removed.
If required the patina can be reformed by brushing over the statue with a solution of “ Liver of Sulphur “ ( ammonium sulphide ). This smells of bad eggs , but it immediately returns the surface to a brown/black finish.
Green copper stains in limestone , under a plaque or statue, can be very difficult to remove.
Acids cannot be used. If ammonia is used it will turn the surface deposits to a deep blue, and subsequent pressure washing leaves the stone looking clean. However , the staining has been caused by copper salts running down for many years and after a few hours the staining will re-appear and migrate to the surface again. Even after repeating process a dozen times , the stains always re-appear.
Poulticing with ammonia and kaolin is the recommended method. It is simply pasted on and left to draw out the copper deposits as it drys out. Brush off when completely dried out.
On sandstones and granite the appropriate acids are usually adequate to remove the stain when jetted off in the normal manner.
Iron, or rust stains , as they are more commonly called, are usually yellow to deep brown.They are more noticable on cream or light coloured bricks.
Causes
Insufficient pre-wetting of walls prior to applying too strong a mix of hydrochloric or especially hydrofluoric acid based cleaners, causing the acid to react with the iron oxides present.
Also caused by rusting iron or steel objects , such as, nails, bolts, clips, lintels and railings.
Remedies
Complete removal of stains can be difficult when present in porous stone.
On sandstone , acid ( with added rust inhibitor) clean usually removes sufficient fo an acceptable result.
On limestone a solution of sodium hydrosulphite can be applied on the pre-wetted stone and pressure washed off after 30 minutes.
People sometimes apply linseed oil to stone and brickwork to “waterproof”, and to add a “ wet-look” type sheen to the surface. However, in a short space of time they usually regret their actions because the linseed oil is a “magnet” for dirt , which binds strongly, making the property look unsightly.
Fortunately linseed oil usually responds very well to caustic based degreasants ( CW5, CW5a & CW8 ).
Remedy
Apply degreasant , leave on for 10 – 15 minutes, and pressure wash off. The surface now needs to be neutralized with the appropriate acid for the substrate.
If oil has been allowed to soak into stone, brick or flags for a long time, whilst it may be possible to remove the surface film stains, the underlying stains usually migrate to the surface again. Several repeat treatments should further reduce stains, but it’s almost impossible to completely eradicate them. Indeed if the stains are on concrete flags or brick paviors, then it is normally far more cost effective, as in the case below, to replace them with new ones.
Use either CW7 Traffic Film Remover, or a hydrocarbon solvent based detergent.
Apply, work in , and pressure wash off , preferably with a hot water washer.
NB. Do not use a hydocarbon solvent , or highly alkaline cleaner on pitch, tarmac, or composite floors, as they are likely to soften the binding agents.
THE ABOVE INFORMATION IS PROVIDED AS A BASIC GUIDE TO VARIOUS ASPECTS OF CLEANING ONLY, AND DOES NOT IMPLY ANY CHEMICAL SPECIFICATION. THE DATA HAS BEEN COMPILED FROM SOURCES CONSIDERED ACCURATE.
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Date of Preparation : April 2007
