that makes them very useful cleaning techniques for delicate or intricate
sculpture and stonework.
Bioremediation for buildings. The term bioremediation covers a range of
processes that utilise microorganisms to return contaminated environments to
their original condition. As discussed above, deterioration of building stone
begins from the moment it is quarried as a result of natural weathering
processes. Other factors, sometimes acting synergistically, including crystalli-
sation of soluble salts, pollution and biological colonisation can accelerate
natural deterioration. Internal pressures created by crystallisation, hydration
and thermal expansion of salts are a significant cause of damage to stone.
Accumulation of sulfates, derived from the oxidation of sulfur dioxide, and
nitrates, from oxides of nitrogen present in the atmosphere (N 2 O, NO, N 2 O 3 ,
NO 2 , N 2 O 5 ), is of particular concern. Whatever the cause of stone deteriora-
tion, buildings require remedial measures to stabilise the surface layer and
prevent further loss.
Conventional techniques for the cleaning and conservation of stone in
buildings have a number of disadvantages: they can cause colour changes in
the stone; excessively remove the original material; or adversely affect the
movement of salts within the structure of the stone. While microorganisms
have usually been associated with detrimental effects on stone, affecting min-
eral integrity or exacerbating powerful physical processes of deterioration,
there had been growing evidence that some types can be used to reverse the
deterioration processes on historic buildings and objects of art. Bacteria, such
as Pseudomonas and Desulfovibrio, have shown potential to remove harmful
salts such as nitrate and sulfate by denitrification and sulfate reduction and to
mineralise organic residues or pollutants like carbohydrates, waxes or hydro-
carbons which commonly occur in crusts on stonework. Research suggests
that bioremediation, may offer a safe, viable alternative to conservators and
restorers working to reverse or prevent further damage to stone buildings.
On the surface of the stone, particulate matter from the atmosphere can
combine with gypsum to leave unsightly black crusts containing a complex
mixture of aliphatic, aliphatic and aromatic carboxylic acids and polycyclic
aromatic hydrocarbons. The removal of black crusts is problematic for con-
servators since conventional cleaning techniques may remove a portion of the
underlying stone. Recently, microorganisms have been used to remove sul-
fate from black gypsum crusts. Sulfate-reducing bacteria (SRB) are able to
dissociate gypsum into Ca2+and SO 4 2+ions, and, in the absence of oxygen,
the SO 4 2+ions are then reduced by the bacteria to a gas, H 2 S. In some cases,
the Ca2+ions have been shown to react with carbon dioxide to form new cal-
cite. Desulfovibrio desulfuricans andD. vulgarisare two types of bacteria
that have been applied to gypsum crusts and used to successfully remove
black crusts from marble.
Stone 233