Intricacies of Air Abrasive Cleaning
The decision to preform cleaning on a historic building is not one that should be made lightly. Cleaning can have significant physical and aesthetic results both positively and negatively. The first step must be to undertake a detailed investigation to determine whether cleaning should be preformed and, if so, how this should be done. The characteristics and condition of the masonry must be understood, including the stone, brick and mortar, as well as the make up of the soiling. The latter can include atmospheric deposition, paint, limewash and metal oxide staining. Each requires a different approach, there is no single system or product that will safely and effectively remove everything.
Once a fundamental understanding of the a cleaning system and technique must be chosen. Every system and product no matter how gentle can be used right and wrong. Usually poor cleaning is due to the incorrect process being selected for cleaning. Regardless what the literature from suppliers may say, restorative cleaning is deceptively complex requiring experience and expertise.
The goal of cleaning is to remove soiling while causing little to no disruption of the underlying masonry. This can be extremely difficult to achieve due to the fundamental relationship between masonry and its soling, as the soiling can be embedded deeply in the substrate.
Several published sources exist which outline the fundamental constituents of various historic masonry materials and the susceptibilities of these to particular cleaning procedures. Experience must come in to play when assessing surface conditions and characteristics particular to the cleaning job. Work must be undertaken by skilled masons specializing in working with and cleaning historic masonry.
Air abrasives cleaning systems, apart from having a bad wrap do have a place and are often considered when soiling is not water-soluble and when chemical processes are inappropriate or less preferable. Although wide range of abrasive blasting techniques are currently available. Machines that can safely be used for cleaning are few and far between and are all imported from European companies. These systems include a variety of machines, nozzles and abrasives for micro abrasive techniques. Some large scale equipment can also be used in a very versatile and sensitive manner. The fundamental action of an air abrasive system is the direction of abrasive particles onto the soiled masonry by a stream of compressed air. Cleaning is accomplished by the impingement of the particles which dislodge or pulverize the surface layer of the masonry. Most systems also involve the use of water, either additional to the air and abrasive stream or combined as a slurry with the abrasive. The main effect of the introduction of water is to reduce the dust generated, although the mist produced is still a health hazard.
Air abrasive cleaning techniques are most successful on surfaces of even profile and consistent surface texture and hardness. An air abrasive stream cannot by its self, regardless of abrasive used, differentiate between the removal of soiling and the removal of masonry. It also cannot distinguish portions of masonry which are closer or farther or areas which are harder or softer. When using air abrasive systems damage can only be avoided through the skill and ability of the operator to make the necessary adjustments in technique. Air abrasive cleaning is usually most successful on plain stone surfaces of even hardness. Careful use can can allow the technique to be employed on moulded or carved stone surfaces. However for brickwork, it is nearly impossible to successfully clean historic brickwork using abrasive blasting without any damage. Bricks are very intolerant to abrasive cleaning and it is not a good choice from the removal of paint.
When your professional is utilizing air abrasive cleaning, two factors are crucial; the velocity and the concentration of the particles which impact on the surfaces. The levels are controlled by the pressure and volume of air and the concentration of the abrasive feed into the line. Specifying pressure alone is not adequate to ensure safe cleaning. Commonly available abrasives for facade cleaning include, calcium carbonate, glass powder and crush dolomite (limestone) powder. The are countless variables to take into consideration when setting up for historic abrasive and specific advice cannot be given without properly assessing the building. However the following general principles can be applied.
1.) Smaller particles of the same abrasive type can be less damaging than larger ones, used int he same manner.
2.) Water increases the inertia of any individual pieces of abrasive making up the stream.
3.) Harder abrasives are more damaging than softer abrasives of the same size, used in the same manner.
4.) A higher concentration of abrasive particles can be more damaging than a lower concentration, all other factors being equal.
5.) High air pressure and volume can be more damaging than lower air pressure and volume, all other factors being equal.
6.) A closer working distance between the output of the nozzle and the masonry can be more damaging than a greater one, all other factors being equal.
7.) Depending on how they are used, some small scale abrasive systems can be more damaging than larges scale systems.
8.) Modification of technique is required for plain and carved surfaces, consolidated and deteriorated conditions.
General recommendations cannot be made in relation to air abrasive cleaning, any more than with any other cleaning approach. Site trials are always necessary for cleaning historic masonry. Trials and the work should be preformed and overseen by an experienced professional who can observe and assess the effects of each procedure and produce a detailed plan for the work.