Edinburgh Napier University

  Historic buildings and monuments are a precious finite asset and powerful reminders for future generations of the work and way of life of earlier cultures and civilisations.
The stone cleaning and restoration of historic buildings is a crucial element in keeping the good look, integrity and quality of the fine art, method of construction and architecture of previous civilisations. Stone cleaning is one of the most noticeable changes a building can be subjected to, which changes its appearance, persona and environmental context. In this study, a series of physical and chemical tests were conducted to further investigate, evaluate and improve the efficiency of building cleaning. Seven different abrasives were adopted for air abrasive cleaning, including copper slag (fine, medium and coarse), recycled glass (fine, medium and coarse) and hazelnut/almond shell (natural abrasive), on a total of eight masonry stones and clay bricks, including yellow sandstone, red sandstone, limestone, marble, granite, white clay brick, yellow clay brick and red clay brick.
Physical investigations included sieve tests and impact tests on the abrasives, greyscale image analysis, thickness reduction measurements, Vickers surface hardness tests, Charpy impact tests and water absorption tests. Chemical investigations included Scanning Electron Microscope (SEM) and Energy-Dispersive X-Ray Spectroscopy (EDX) analyses.
Sieve tests and impact tests confirmed that the abrasives utilised were fairly reliable, and the abrasives with high bulk densities were stronger and tougher than those with
low bulk density.
Greyscale digital image analysis indicated a lower greyscale value corresponded to a dirtier masonry surface. In general, the greyscale continuously increased with the
increasing cleaning time and tended to be stable when the surface became fully cleaned. The cleanness was also introduced for assessing the effectiveness of the
building cleaning. Similar trends could be observed. Both parameters proved to be significantly useful.
For most of the samples, monotonic increase trends were observed between the greyscale and thickness reduction. The image analysis on greyscale and the thickness
measurement were two useful methods for assessing the cleaning degree of a masonry stone or clay brick. Based on the analysis on all the testing data, it is possible to recommend a more suitable abrasive for each masonry stone or brick. For granite and red clay brick, medium glass produced the best performance, while for
limestone, marble and red sandstone, fine glass was promising. For yellow clay brick,
fine slag could be the best option, while for yellow sandstone the natural abrasive
was found to be the most suitable.
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The Vickers hardness test results indicated that a larger hardness corresponded to a
harder masonry surface. Also the surface hardness continuously increased with the
increasing cleaning time but at a decrease rate. Most of the increasing trends of the
surface hardness could be approximately expressed using parabolic relationships.
Granite was found to be the hardest, and followed by marble and limestone. However,
there were no big differences in the surface hardness between yellow clay brick,
yellow sandstone, red sandstone and white clay brick.
The impact resistances of seven masonry stones and bricks were obtained by
conducting the Charpy impact resistance tests. Granite showed the highest impact
resistance among all the stones and bricks and was followed by marble, limestone,
clay bricks and sandstones. The stones and bricks with higher impact resistances also
had higher hardness values but lower water absorptions.
The water absorbing capacity of the seven masonry stones and bricks was
quantitatively determined. Two types of clay bricks showed the highest water
absorptions, and the water absorptions for limestone, yellow sandstone and red
sandstone were also quite high. However, the water absorption of marble and granite
was found to be very low. Larger water absorption corresponded to a softer stone or
brick, while smaller water absorption corresponded to a harder stone or brick.
The chemical investigations by using the SEM and EDX techniques showed that the
chemical substances on the masonry surface varied largely for different types of
stones and bricks. This study showed the way to detect such soiling using chemical
analysis by monitor the changes in chemical elements and compounds during the
building cleaning.
Finally, comprehensive conclusions were presented, together with useful suggestions
for future work.