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.
Climate change: UK commits to cut greenhouse gas emissions to almost zero by 2050 https://t.co/WFn7T78NTF— BBC Politics (@BBCPolitics) 12 June 2019
Moreover, 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.
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 pressurized 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 pressurized water (b) Stone profile becomes rough because the abrasive method remove 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 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 depositson 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, since only one solution is required to remove both staining types. In addition to this, the 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 images below show the results from cleaning a concentrated carbon deposit from marble stone. The laser is rastered from side to side which is evident from the interface between the soiled and clean region in Figure 4, the trailing edges of carbon deposit at the interface have been left for illustrative purposes. The width of material one line of rastered laser beam interact with is approximately 2 mm and these can be overlapped to remove the entire 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.
For biofilm the residue is also effectively removed using the Vulcan 1600 infrared laser. The reason why the carbon deposits and biofilm can be removed indiscriminately from stone at
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.
- Glen, J. Heritage Statement 2017 Department for Digital, Culture Media & Sport. (2017).
- 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 ANDRITZ Powerlase directly (Email: email@example.com or call +44 (0)7943 VULCAN / +447943 885 226 and speak to us more infroamtion) visit powerlase-photonics.com.