Around the world, natural degradation has impacted historical structures due to the build-up of biological growth on stone surfaces and other masonry. Over time, this growth can settle on mechanical structures and significantly impact the aesthetics and usability of historical buildings.
Therefore, plans are made to maintain historical sites that are jeopardized by contamination from biological growths for example biofilms of algae, along with carbon deposits because of air pollution.
Nonetheless, these types of projects are costly. In the UK, £19 million was spent towards the restoration of the Piece Hall in Halifax, Yorkshire, an English heritage site¹ which is now utilized as a commercial district.
Masonry Cleaning and Restoration
A large proportion of the cost of masonry cleaning goes towards recovering the environmentally hazardous substances employed, such as biocides which are toxic liquid solutions sprayed onto stone surfaces.
Due to their toxic characteristics, excess liquid must be recovered from the environment to reduce far reaching ecological damage.
Nevertheless, such work can be beneficial to society by educating the public about the historical significance of these sites, along with boosting the tourism industry. This creates more employment opportunities, which is mostly viewed as counteracting the cost of the work. However, there is an increasing demand for solutions that pose less of an ecological risk.
As restoration work is more and more sought out in locations across the globe and governments put environmentally conscious policies into place, traditional masonry conservation practices are coming under scrutiny.
These practices are proving to be insufficient where buildings have a variety of surface contamination, as biocides are commonly organism specific.
This then results in several biocides being utilized to treat all harmful organisms. However, as ecologically friendly government policies hinder the use of biocides, these traditional approaches are becoming increasingly unappealing.
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
Along with biocides, conventional abrasive conservation methods employed to remove different degrees of carbon deposits and biological growth have had setbacks of their own. Abrasive methods such as pressurized water and bristle brushes can address the issues of carbon and biological staining but have been known to harm the surface of stone.
This uncovers greater surface areas making the stone vulnerable to accelerated build-up of biological growth after cleaning, as spores have more places to settle on, which can limit the efficacy of the cleaning process and the economic argument for investing in cleaning.
As they are performed slowly to ensure the cleaning process is gentle and reduces damage to the parent stone, abrasive methods are, by necessity, time intensive. However, this means the project duration can be lengthy which increases the required management time and the associated costs of running the cleaning project.
Figure 2: (a) Stone with a biofilm coating prior to abrasive cleaning with bristle brushes or pressurized water (b) Stone profile becomes rough due to the abrasive method removing material unevenly, increasing the surface area and hence sites for biological spores to deposit on the wall, boosting their regrowth.
Biocides, which are chemicals applied to stone which can kill and prevent against biofilms, are an alternative to abrasive methods. However, inhibitors can be active on the stone surface from a month to several years.
Unfortunately, as organism susceptibility can vary, an abundance of biocide can be needed to attack only a small fraction of biological species that are spread out over a wide area.
Consequently, the excessive use of biocides to tend to a relatively small population of biofilm makes the risk of leeching harmful and toxic products into local water systems unjustifiable.
Natural biocides such as copper are an alternative to solution based biocides. This is where copper metal is physically installed in strips into the mortar and oxidizes in time to create a natural biocide which, despite being prone to leech from buildings, is less harmful to the environment.
Nonetheless, the introduction of foreign structures into the mortar can jeopardize the integrity of the masonry. Additionally, the characteristic bluish-green of copper (III) oxide can cause staining which is not aesthetically pleasing in some cases.
Novel Laser Solutions
High average power lasers have offered a new type of capability in response to the challenges faced by conservationists. Masonry laser cleaning makes it possible to clean carbon deposits and biofilms with a single laser system all at once, dissimilar to conventional methods which may require several techniques.
Furthermore, the amount of residue created during laser cleaning, most of which are airborne ablation by-products, are significantly reduced; lending themselves to capture and contain excess residue by utilizing standard air extraction systems.
In contrast with the complicated collection of waste sand from sand blasting, the enigmatic recovery of liquid biocides in the laser cleaning solution allows users to simply control and quantify air-borne products, thus reducing the impact on the environment.
The ability to customize laser cleaning solutions provided by Powerlase Vulcan systems means that laser parameters can be adjusted on the conservation site to optimize it for various masonry surfaces like marble, sandstone, granite and more, without harming the stone surfaces.
Laser cleaning performs locally at the stone surface, thereby avoiding destructive probing seen in abrasive methods or the introduction of foreign objects which can compromise the mechanical structure of the building.
Figure 3 shows a diagram of the surface finish for a laser cleaned surface. It is extensively smoother than the stone surface following abrasive cleaning, and there is a lower surface area to reduce the risk of biological deposits accumulating.
Figure 3: (a) Stone covered in biofilm (b) Laser removal is a moderate and effective removal technique that creates a uniform profile which has greater resistance to biological deposition to the stone surface following cleaning.
As lasers can remove biological growth and carbon deposits, this has a significant effect on cleaning speeds and the ease of processing. A single solution is needed to remove both types of contamination and time is saved as waste products from the environment do not need to be recovered following cleaning.
Standard air extraction means that any hazards involved, for example laser light (visual) and ablation products (airborne), are either temporary or easily controlled. Therefore, the risks involved with this method can be reliably managed and quantified.
Laser cleaning solutions are therefore easy to regulate and allows users to confidently operate in line with government policies.
The image below displays the results of cleaning a concentrated carbon deposit from marble stone. The laser beam is rastered from side to side to make an effective ‘line of laser’ which can be seen from the interface between the soiled and clean region in Figure 4.
For illustrative purposes, the space between the lines of laser has been adjusted to be wide enough to show the trailing edges of carbon deposit at the soiled interface in Figure 4.
As such, the cleaned area 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 get rid of all residue and display a sharp interface as shown 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 shows the effective width of the laser.
As displayed in Figure 5, biofilms are also effectively removed utilizing Powerlase Vulcan systems. The large difference in the ablation threshold and thermal capacity of stone compared to surface contaminants is the reason why biofilms and carbon deposits can both be removed at once.
Therefore, as the laser interacts with the stone surface, laser light is strongly absorbed by the contaminants, leading them to heat up faster than the stone and become so hot that they degrade from solids into gas quickly through a process known as sublimation.
A close up of the biofilm interface and cleaned stone demonstrates a clear marble surface labeled ‘cleaned region’ showing the excellent 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 the cleaned region interface is focused in on to demonstrate the difference in surface contamination in both regions.
Conservationists are increasingly looking for laser cleaning solutions as their efficient cleaning capabilities provide a simple and manageable way of preserving valuable structures.
Governments are also shifting towards the reduction of ecological damage and are implementing policies to encourage the use of cleaning solutions with a reduced environmental impact. Laser cleaning solutions are naturally becoming the preferred alternative, and are setting the standard for masonry cleaning solutions of the future.
References and Further Reading
- 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)
This information has been sourced, reviewed and adapted from materials provided by Andritz Powerlase Limited.
For more information on this source, please visit Andritz Powerlase Limited.