Patina

Over the last years of my work as a conservator and microbiologist
in the field of biodeterioration, I have encountered the term
"patina" many times, however, sometimes in different contexts with
different applications. Whereas I, as a conservator, was under the
impression that "patina" is a surface alteration, usually with a
colour shift, that is stable and therefore not considered a hazard
to the artefact. In some cases, it might even have a protective
nature.

However, I have heard the term used, in more microbiologically
oriented presentations on biodeterioration of cultural heritage, in
a context that included all surface changes of cultural heritage
artefacts and in several cases even surface deposits that included
microorganisms that were potentially detrimental to the artefact.

The only published essays on the topic of "patina" from a
microbiologist was by W.E. Krumbein and his research group (e.g.
"Patina and cultural heritage--a geomicrobiologist's perspective"
and " Life on stone--an endless story?"). I am citing here some of
his explanations and understandings of the term patina and I would
be interested to hear if the conservation community feels that this
reflects their understanding of the term. I would be very interested
to know if there is a recent definition of the term "patina" from a
conservators perspective.

I hope that this posting will not be misunderstood. I am not trying
to criticise my colleague publicly--it is a person who contributed
tremendously to the field of biodeterioration. However, I would like
to clarify for myself if this is yet another case where different
professions have a their own jargon, which may lead to significant
misunderstandings.

Patina after Krumbein "Life on stone" and "Patina and cultural
heritage":

Krumbein defined Patina in its current application as "the sum of
material and textural changes that occur in the surface zone of all
materials, especially in objects of physical cultural heritage.
These changes are caused by aging, material decay and environmental
impact, including the biological environment". Involved are a
surface layer of a few micrometers to several centimetres, where
material and energy exchange between two open heterogeneous systems
occurs. After the formation of a patina an intermediate
stabilisation is reached. "If the patina formation leads to
considerable mass increase of the substrate (deposits, subaerial
biofilms, microbial mats, microstromatolite, sinter, silica skins,
crusts black crusts, internal consolidation, cementation etc.) the
mechanical and chemical influence of the mass increases may produce
fissures and cracking, exfoliation, desquamation and other
alteration processes".

Further, he introduced the term bio-patina for a biologically
induced patina, formed by microorganisms. This term includes two
forms of aesthetic changes of rock surfaces: (1) "brightly pigmented
crusts covering rock surfaces" (2) "a pigmentation that is directly
incorporated into the fabric of the uppermost layer of rock
surfaces". He continues that "chemical alterations such as the
dissolution of the rock material itself, are not necessarily
connected with these phenomena". Later he claims that "since patina
is obviously the oldest term for surface changes of any material
that is exposed to the environment (atmosphere), all other terms
related to surface changes may be subordinated to this general term.
Such terms are oxalate films, lacquer, crust, deposit, karst,
rock-varnish, micro-stromatolite, efflorescence, carbonate, gypsum,
iron, manganese, oxalate, silica skins, as well as other
descriptions of environmentally induced surface changes."

As I mentioned before I am interested to hear the currently
applicable definition for the term "patina" in cultural heritage
conservation or information of where I can find such a definition.

Stefanie Scheerer
Departamento de Microbiologia Ambiental y
Biotecnologia
Programa de Corrosion del Golfo de Mexico
Universidad Autonoma de Campeche
Av. Agustin Melgar s/n. Col. Lindavista
C.P. 24030, Campeche, Campeche
Mexico.


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                  Conservation DistList Instance 20:16
               Distributed: Wednesday, September 27, 2006
                       Message Id: cdl-20-16-013
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