Brick and stonework cleaning is of great importance and requires the diverse skills, knowledge and craft experience of times past. Brick and stonework can be irreversibly damaged by inappropriate repairs, replacement, repointing and cleaning.
The cleaning of brick and masonry walls can enhance the appearance of a building a great deal. However, great care must be exercised to ensure the process does not damage the face of the brick and stone, or alter the appearance and character of the building.
Freshly cleaned brick and stonework should be treated with a clear masonry water repellent sealer that will allow the fabric to breath to its full natural potential. This will also reduce the affect of further pollution hazards.
Repairs and repointing should be carried out using materials that are compatible and compliment the original fabric of the building.
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CLEANING OF BRICKWORK
The cleaning of brickwork is a much debated issue in conservation circles. While the appearance of a building can be enhanced by cleaning, a great deal of damage to the brickwork can be caused by inappropriate cleaning methods. Very careful consideration must be given to the need for cleaning and specialist advice should always be sought.
Before embarking on a cleaning programme it is important to consider the type of brick, the condition of the brick and the nature of the dirt deposits. Georgian brick tends to be softer and more vulnerable than industrial Victorian brick. Ill-considered cleaning will often destroy the subtle patina of Georgian brick, and may also destroy original pointing finishes and old colour washes.
Great care must also be taken when cleaning Victorian brickwork, especially polychrome work, terracotta and elaborate moulded details. These materials may differ in texture, hardness and colour, and each material may react differently to cleaning.
TYPES OF SOILING
Vegetation such as moss and algae grow on the surface of damp brick. Organic growths can be removed by applying a biocide to kill the plant, so it can be brushed off after drying out although they will usually die off naturally if the moisture source is eliminated.
Metal stains from corroding iron. These can be particularly difficult to remove successfully.
Graffiti. Removal will normally require the use of solvent paint removers, possibly combines with the use of abrasive cleaning.
Chemical deposits. The build up of chemical deposits such as calcium sulphate in the form of black gypsum crusts occurs through atmospheric pollution. It is visually unattractive and in extreme cases can lead to deterioration of the brick itself.
Brick and stonework can be irreversibly damaged by inappropriate repairs, replacement, repointing and cleaning.
Any decision to clean stonework should not be taken lightly as it can alter the appearance of stonework dramatically and can seriously damage stone if inappropriate techniques are used. Specialist advice should always be sought, particularly as the cleaning techniques are constantly changing as are opinions of these techniques.
Before embarking on a cleaning program it is important to consider the condition of the stone, the type of stone and the nature of the dirt deposits. This is necessary to ensure that the most appropriate cleaning technique is used and the damage is minimized. Only experienced operatives should be employed, and samples of all cleaning methods should be carried out prior to widespread cleaning to ensure that the cleaning method does not cause damage.
Sufficient time should be given to allow for appraisal of the test results, and to look for side-effects from cleaning such as staining and salt efforescence. In some cases several months may be needed for this and it is therefore important to plan such work ahead. It is also important to realize that more than one cleaning technique may be necessary to deal with differing stone types and dirt deposits. Care should also be taken to ensure that any scaffolding used is painted, as rusting scaffolding can cause staining of stone.
It should be noted that the use of abrasive grit or sandblasting in particular can cause a great deal of damage to stone, and careful consideration needs to be given to its use as inappropriate specification can leave stone with a pitted finish and can increase the rate of soiling in the future. The type and size of aggregate, and the pressure, can be varied and should be specified after an evaluation of the stone type, condition and nature of deposits by a specialist. Abrasive cleaning should never be used on carved stonework such as doorcases or capitals.
A wide variety of other stone cleaning systems are available including the JOS system, laser cleaning, ultrasonic cleaning and micro-blasting, but these tend to be very expensive and their use may not be economical in many situations.
Weather-struck and Ribbon or Strap pointing – where the edge of the mortar is proud of the surface of the brick. These are generally inappropriate as they are visually unattractive and because they are usually executed using hard cement mortars which can accelerate brick decay.
Repointing should only be carried out when the existing pointing has failed and where water penetration is taking place. Where repointing is necessary, it is vital that the correct style of pointing is used as well as the appropriate materials. The use of inappropriate joint finishes and cement mortars can seriously alter the appearance of a historic building, will detract from its character, and can result in accelerated decay of the stone over time.
When repointing, decayed mortar should be raked out using appropriate hand tools. The use of mechanical equipment such as angle grinders should be avoided at all times as they are difficult to control and can damage the stone arises and widen joints. Mortar that proves resistant to hand tools should be regarded as sound and should be left in-situ.
Joints should be raked out to the full width of the joint and should be square in profile. The depth will depend on the friability of the pointing but should be at least twice the width of the joint.
Careful consideration needs to be given to the style of joint finish. Raised or weatherstruck finishes should never be used as they spoil the appearance of stonework. Instead joints should be either flush with the stonework or be slightly recessed where the arises are weathered back or rounded.
When repointing rubble stone walls, small stones known as pinnings should be inserted into any wide joints to reduce the amount of mortar needed and the apparent size of the joint, and the mortar should be contoured to follow the contours of the stone.
Particular care needs to be taken when repointing ashlar work where the joints are very fine. Loose mortar should be raked out using a hacksaw blade. New mortar should never be coated over the surrounding stonework, and very fine jointing tools are required to maintain a precise finish. It is recommended that masking –tape is placed over the surrounding stonework to protect it from mortar staining. In some cases it may be necessary to insert the new mortar with the aid of a syringe or mastic gun.
Flush – mortar is pointed flush with the surface of the facing brick but avoids covering the edges or arises of the brick.
Recessed – mortar is set back slightly from the face of the brick. Generally used on late Victorian and Edwardian buildings where the brick is machine made and has well defined arises.
Double struck – mortar is given a double trowel indent where it meets the brick on both sides.
Penny or Trowel struck – mortar is laid flush with brick and is then incised to a depth of about 1-2 mm. with a trowel edge in the centre of the joint.
Tuck pointing was a technique commonly used during the 18th and 19th centuries to give the impression of a neat and precise mortar joint. Mortar joints were often relatively wide owing to the unevenness of hand-made brick. Tuck pointing disguised the width of these joints with the use of brickcoloured mortar.
During the later part of the 19th century, the use of more uniform machine-made bricks meant that neat joints were easily formed and a flush or slightly recessed joint was most commonly used. Nowadays, tuck pointing is often used when a building is being repointed as a way of disguising repairs and to give a neat finish to weathered brickwork. These are two types of tuck pointing.
Irish tuck pointing is achieved by applying brick-coloured mortar over the brick and the joints while leaving the ‘tuck’, a thin strip of white lime mortar, exposed in the centre of the joint. The application of brick-coloured mortar in this way is known as wigging and it is this that gives the appearance of a neat joint.
English tuck pointing was achieved by pointing the joint flush with the brick face using a brick-coloured mortar. A groove was then cut along the centre of the joint and a thin strip of white lime mortar was tucked into the centre groove.