Across the globe, historical structures have been subject to natural degradation from the build-up of biological growth on surfaces of stone and other masonry. This growth can eventually settle on the mechanical structure and significantly diminish the aesthetics and usability of historical buildings over time. Thus, initiatives are taken to conserve historical sites threatened by contamination from biological growths such as biofilms of algae as well as carbon deposits due to air pollution. However such projects are expensive, in the UK, £19 m was spent in the restoration of Piece Hall in Halifax, Yorkshire, an English heritage site1 which is now used as a commercial district.

Masonry cleaning and restoration

In masonry cleaning, a substantial portion of these costs go into the recovery of environmentally hazardous substances used, like biocides, toxic liquid solutions sprayed onto stone surfaces. Due to their toxic properties, excess liquid must be recovered from the environment to mitigate far reaching ecological damage. However pay backs to society come in the form of encouraging education about the historical significance of these sites as well as through tourism promotion. This results in job creation and are thus generally viewed to off-set such costs, however, demand for lower ecologically risky solutions are coming to the forefront.

As restorative work is increasingly sought out in locations across the world and governments introduce environmentally conscious policies, conventional masonry conservation methods are coming under scrutiny.

These methods are proving inadequate where buildings have a variety of surface contamination since biocides are typically organism specific. This subsequently leads to multiple biocides being used in order to treat for all offensive organisms, however, as ecologically friendly government policies discourage use of biocides, these conventional approaches are becoming increasingly unattractive.

Figure 1 "Sheffield gargoyles" covered in green algal growth2 by Dun.can is licensed under CC BY 2.0

The limits of transitional stone cleaning methods

In addition to biocides, traditional abrasive conservation methods used to remove various degrees of biological growth and carbon deposits have had challenges of their own. Abrasive methods like bristle brushes and pressurised water can attack both carbon and biological staining but have been known to damage the surface of stone. This reveals larger surface areas making the stone prone to accelerated build-up of biological growth after cleaning since spores have more places to settle on, which can undermine the effectiveness of the cleaning process and the economic argument for performing the cleaning to begin with. Abrasive methods are by necessity time intensive, since they are performed relatively slowly in hope of making the cleaning process gentler and limit damage to the parent stone. However this results in long project durations, increasing required management time and associated running costs of the cleaning project.

Figure 2 (a) Stone with a biofilm coating before abrasive cleaning with bristle brushes or pressurised water (b) Stone profile becomes rough because the abrasive method removes material unevenly, increasing the surface area and hence sites for biological spores to deposit on the wall, accelerating their regrowth.

The alternative to abrasive methods as previously mentioned are biocides, chemicals applied to stone which kill and can inhibit bio-films. Where inhibitors can be active on the stone surface from a range of months to some years. Unfortunately, since organism susceptibility can vary, copious volumes of biocide can be required to target just a small fraction of biological species that are distributed over a wide area. Therefore excessive use of biocides to treat a relatively small population of bio-film makes risks of leeching products of harmful toxicity levels into surrounding water systems unjustifiable. Alternatives to solution based biocides, are natural biocides such a copper. Copper metal is physically installed in strips into the mortar and oxidises over time to produce a natural biocide which despite being prone to leech from buildings is more environmentally benign. However, incorporation of foreign structures into the mortar can compromise the integrity of the masonry, moreover, the characteristic bluish-green of copper (III) oxide, can cause staining which in some cases is not aesthetically pleasing.

NOVEL LASER SOLUTIONS

In response to challenges faced by conservationists, high average power lasers have offered a new kind of capability. Masonry laser cleaning makes it possible to simultaneously clean carbon deposits and biofilms with a single laser system, unlike traditional solutions which may require multiple techniques. Moreover, the volume of residue produced during laser cleaning, the majority of which are airborne ablation by-products, are considerably less; lending themselves to capture and containment by deploying standard air extraction systems. Contrasting with the cumbersome collection of waste sand from sand blasting, or the enigmatic recovery of liquid biocides laser cleaning solution enables users to conveniently control and quantify air-borne products limiting the environmental impact.

The versatility of laser cleaning solutions offered by Powerlase Vulcan systems allows laser parameters to be adjusted on the conservation site to optimise it for different masonry surfaces such as marble, sandstone, granite etc. without damaging the stone surfaces. Laser cleaning acts locally at the surface of the stone, hence avoiding destructive probing seen in abrasive methods or insertion of foreign objects which can compromise their mechanical integrity. Figure 3 shows a diagram of the surface finish for a laser cleaned surface, it is significantly smoother than the stone surface after abrasive cleaning, and there is a lower surface area for accumulation of biological deposits. 

Figure 3 (a) Stone covered in biofilm (b) Removed using laser is a gentle consistent removal rate that creates a uniform profile which has greater resistant to biological deposition to the surface of the stone after cleaning.

The ability of lasers to remove both carbon deposits and biological growth has significant implications for cleaning speeds and ease of processing. A single solution is required to remove both types of contamination and time is not spent recovering waste products from the environment after cleaning. Hazards involved such as laser light (visual) and ablation products (airborne) are either transient or conveniently controlled using standard air extraction, meaning the risks associated with this technique can be reliably controlled and quantified. This makes laser cleaning solutions simple to regulate and means users can confidently operate according to government policies.

The image below shows the results of cleaning a concentrated carbon deposit from marble stone. The laser beam is rastered from side to side to create an effective ‘line of laser’ which is evident from the interface between the soiled and clean region in Figure 4. In Figure 4, the spacing between the lines of laser has been adjusted to be wide enough to reveal the trailing edges of carbon deposit at the soiled interface for illustrative purposes. Therefore the cleaned region in between the trailing edges provides the width cleaned by a single line of laser, the ‘effective laser width’. The spacing between lines of the laser can be brought close together to remove all residue and reveal a sharp interface as in Figure 5.

Figure 4 Concentrated carbon deposits on marble stone cleaned with a Vulcan 1600 infrared laser. The raster lines left by the laser reveal the effective width of the laser.

Biofilms as shown in Figure 5, are also effectively removed using Powerlase Vulcan systems. The reason why the carbon deposits and biofilms can be removed indiscriminately from stone is because of the large difference in the ablation threshold and thermal capacity of stone in comparison to surface contaminants. This means as the laser interacts with the surface of the stone, the contaminants strongly absorb the laser light heating up faster than the stone and become so hot they degrade from solids into gas rapidly through a process called sublimation. A close up of the biofilm interface and cleaned stone reveals a clear marble surface labelled ‘cleaned region’ good precision and control achieved on the clean surface.

Figure 5 Biofilm coated marble stone with one side cleaned with a Vulcan 1600 infrared laser. One region of the biofilm and cleaned region interface is focused in on to show the difference in surface contamination in the two regions.

Laser cleaning solutions are becoming increasingly sought by conservationists as their rapid cleaning capabilities offer a convenient and controllable method to preserve valuable structures. Governments are also moving toward reducing ecological damage and are introducing policies that are encouraging a shift towards cleaning solutions with lower environmental impact. Laser cleaning solutions are naturally rising to be a preferred alternative and are setting the standard for masonry cleaning solutions of the future.

References

1. Glen, J. Heritage Statement 2017 Department for Digital, Culture Media & Sport. (2017).
2. Robert Gordon University of Aberdeen. The Scott Sutherland School of Architecture & Built Environment:Biodam. Available at: http://www4.rgu.ac.uk/sss/research/page.cfm?pge=33373. (Accessed: 1st May 2019)

For more information about our Laser Cleaning technology and using them in the Conservation and Restoration of Architectural Structures, please contact us via the button.

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