Nebulous Spray or intermittent mist spray
Low-pressure water washing is probably the least aggressive form of cleaning. Its application is particularly useful where water-soluble dirt is present or water-soluble chemical compounds bind the dirt. Thicker encrustations of soiling which tend to form in protected areas of a building not regularly washed by rain may be softened by the water and subsequently mechanically removed. However, it cannot be used to remove soiling or staining which is insoluble in water.
Nebulous spray, also known as intermittent mist spray, is a development of low-pressure water washing. The aim is to apply the minimum amount of water for the minimum duration to soften the dirt, thereby enabling its removal by scrubbing or other relatively gentle treatment. Ordinary low pressure water washing, by comparison, risks saturating the masonry, causing damage to the wall by mobilising salts and causing fixings to corrode for example, as well as damaging other features fixed to the wall such as internal plasterwork, timber or decorations. It can also lead to dry rot.
Only once all the investigations have been carried out, questions answered, options considered and the conclusion drawn that nebulous water spray cleaning fulfils all the criteria, should cleaning be commenced by those trained and skilled in the use of this cleaning method and following the guidelines established during trials.
The system of nebulous sprays is based on the principle of passing water through a very fine mesh or filter to create a mist that is then passed through fine nozzles. The mist spray system can be set up with nozzles at intervals along the building, concentrating on areas of greater need and reducing the level where less dirt is present. The level of water may be controlled electronically or by timers, allowing pulse or intermittent spraying, to avoid ever having water running down the face of the building. Before starting, the porosity of the stone can be assessed in order to balance the amount of water and duration required.
As the system produces such a fine mist it is important to place the nozzles close to the building’s surface in order to ensure the water is directed correctly. Depending on the location and exposure of the elevation it is frequently necessary to erect a screen to reduce the risk of wind disturbance.
Nebulous spray systems can be designed to be incredibly flexible, directing the spray only where needed. Straight or flexible hoses may be employed depending on the requirements of the surface being treated and the nozzles from the hose may be grouped or spaced according to the severity of the dirt or encrustation being treated. Flat surfaces often require less water than a carved heavily soiled detail, which may require a cluster of nozzles positioned on an articulated hose to the profile of the carving.
The most obvious advantages of cleaning with water are that water is cheap, readily available, safe and environmentally friendly. It is also particularly effective for cleaning limestone and marble.
The impact of the mist on the surface is negligible, reducing the risk of mechanical damage unless the surface is extremely friable. Consequently the risk of washing away weak pointing material or decaying stone is almost entirely eliminated.
Encrustations and dirt are softened progressively, reducing the risk of mechanical damage, and allowing greater control over removal and permitting more frequent monitoring of the surfaces. This ensures that the right levels of clean are achieved and reduces the risk of over cleaning. It also gives greater opportunity to re-evaluate the method or levels of cleaning than with many other cleaning methods.
Where the use of harsher methods of cleaning are unavoidable, prolonged use may be reduced by first cleaning with the nebulous spray system.
Removing softened material by brush between spraying cycles may accelerate the cleaning process and has the added advantage of enabling progress to be monitored.
A further advantage is the ability to control the quantity of water used. Excess run off, which this method avoids, is a particular problem with traditional water washing methods where weathered wash patterns formed by rainwater may channel the spray, avoiding adjacent areas of the masonry. As mist sprays use less water, a more even wash is achieved, avoiding the weathered wash channels and reducing the probability of saturation as the stone does not get so wet.
Disadvantages and Risks
- Although the nebulous spray system reduces the risk of saturation enormously, this problem may still arise as a result of a failure in the timer, switch or in judging the porosity of the stone which can mean damage to internal finishes, hidden timber and ferrous fixings.
- Water cleaning methods may exacerbate deterioration when used on badly deteriorated masonry. The risk of water penetration through defective joints or fractures is still present with the nebulous spray system, illustrating the importance of carrying out a thorough survey externally, and continuous monitoring of the interior as cleaning progresses.
- As with all water treatments, the work should not be carried out when there is potential for frost damage.
- The network of hoses and bars situated close to the face of the building can restrict access and make monitoring or brushing down awkward.
- Efflorescence on the surface is possible where water treatments are carried out. Generally it is possible to estimate the risk of this prior to commencement.
- Water cleaning is less effective on siliceous stones such as granite and sandstone where the soiling is tightly bound to the silicate surface in insoluble compounds. Dirt on limestone is generally bound to relatively soluble chemical compounds.
- A frequent problem with many limestones and some sandstones is brown or orange staining caused by naturally occurring free iron within the stone being mobilised and carried to the surface. Consideration must also be given to the possibility of previous treatments, which may have been carried out, such as the application of a solution of copperas (ferrous sulphate) to Portland limestone in the 19th century in order to emulate the more fashionable Bathstone. Earlier conservation or cleaning treatments may also have a detrimental effect on the success of water cleaning.
- Finally the set-up and cleaning time required for the nebulous spray is greater than many other cleaning methods, however, this must be weighed against the increased control and gentleness of this type of spray.