Pérez Monserrat, E. And Baltuille Martín, J.M.
Area de Rocas y Minerales Industriales. Instituto Geológico y Minero
de España (IGME). Madrid. jm.baltuille@igme.es
The big Romanic and Gothic cathedrals when they were built had an organised
structure of workshops for their conservation and correct maintenance. Though
the cathedrals have never stopped being built, the age of the monumental cathedrals
of stone came to an end and began the era of restoring them.
The letter of
Restauro (1972) and that of Venice (1964) established criteria of intervention
in the face of an alarming situation in which many buildings and monuments were
found owing to the successive interventions they suffered during years without
there being any type of control, norm or system. These criteria were summarised
in the permanence of the historical-artistic message that the author wanted
to transmit, in attending to the causes of deterioration, so as to eliminate
or minimise them as much as possible and towards the appropriate direct intervention
on the stone. When we refer to the reform or restoration of cathedrals we do
it in generic terms that covers churches, hermitages, cathedrals, convents and
palaces for archbishops. The restorations do not affect only the facades and
claddings, but also the railings, kerbs, staircases, benches..... "and
stone is always the protagonist" (see Fig.1)
In recent years, the topic of restoration and the problem associated with its
execution has been on the minds of everyone, thus forming multi-disciplinary
teams with increasingly more knowledge and perfected techniques. The intervention
in monuments is not something exclusive to our time, but it is restoration as
science", which in spite of being in apogee, is under observation, and
we do not know how some of the interventions realised on stone are going to
evolve over time.
FACTORS OF ALTERATION OF THE ROCKS
A) EXTERNAL CAUSES
From the moment in which we extract the rocks in a quarry and the separation
from the whole rock in which it was formed, we are altering it. The rocks are
formed in conditions of pressure, temperature and humidity very different to
those reining on the surface and when entering in contact with the atmosphere,
tend to create an equilibrium with it, experimenting a series of reactions that
leads to structural changes and of its composition. Moreover, when the rocks
reach the exterior surface, they are exposed to the action of atmospheric agents,
suffering many physical and chemical changes coming under the term meteorisation.
Water
Water is the main agent of alteration, reacting with the stone substrate, dissolving
its components, and acting as a vehicle of transport. The freezing of ice or
condensation of vapour increases the volume of the rock, the tensions provoked
by the ice on the internal walls and capillaries of the rock can break it, and
the cycles of icing and melting lead to exfoliation. The effect provoked by
the ice depends on its volume and the place of formation.
Atmospheric agents
The atmospheric agents act as catalysts of the reactions intensifying the chemical
action of water. Since the nineteenth century, the development of industrial
activities and the increase in population has produced massive emission of particles
to the atmosphere. The main atmospheric contaminants are: nitrogen oxides, carbon
and sulphur proceeding from the combustion of hydrocarbons, the methane gas
emitted by the fertilisers and the burning of forests, and the combustion gases
liberated in the incineration of solid residues (Anguita, 1993).
The Stone sickness
If there is a widely used term and which creates great confusion, it is the
denominated "sickness of stone". To speak of only one sickness of
stone is not quite adequate, and in numerous occasions confusing terms are used
referring to an aspect of the rock: thus one hears of the sanding of granite,
the cancer of the stone or of marble, which is nothing but the loss of material
and the origin of which can be in different processes. The term of the "sickness
of stone" as such refers to a determined pathology occasioned by a concrete
process, which is one of degradation suffered by the limestone rocks owing to
the presence of atmospheric particles, forming a crust of calcine and sulphine
that is very damaging. Carbonell de Massy (1993) explains the process of formation
of these crusts.
.Crusts of calcine
The capacity of dissolving the rain water increases with CO2, it makes the water
more acid and produces a rapid dissolving of the limestone rocks. The carbonic
acid formed from water with the carbon dioxide, reacts with calcium carbonate
forming calcium bicarbonate which, being soluble, can be washed. But this reaction
is irreversible; the surface water evaporates and precipitates calcium carbonate,
forming a crust of calcine.
CO3H2 + Ca(CO3H2) <=> Ca(CO3H2)2
The formation of this crust is beneficial in the short run, since it strengthens
the surface of the rock and is impermeable. But with time it reaches a greater
degree of compactness than the substrate itself and retains the humidity in
the interior.
.Crusts of sulphine
Its process of formation is quite similar, the sulphurous acid formed from the
reaction of water with sulphur dioxide oxidises in the presence of atmospheric
oxygen forming sulphuric acid, which specially attacks the limestone rocks.
When the sulphuric acid reacts with calcium carbonate under water pressure,
calcium sulphuric hydrate is formed, and on the water evaporating a crust of
sulphine is formed, more damaging than that of calcine because the sulphuric
is more aggressive than the carbonic and because it retains more humidity, being
of greater volume. In the areas close to the sea the calcium sulphate formed
reacts with sodium chloride of sea water and forms sodium sulphate, which is
very corrosive.
Bio-deteroration
The bio-deterioration is the physical and chemical degradation of the rock
provoked by living organisms. The fungus and moss retain humidity, favour the
colonisation and produce acids that modify the colour of the rock. The most
damaging bacteria are: the autotrophs because they develop with light, the sulpha-bacteria
because they transform the silicon compounds in acids and oxidises the sulphur
to sulphate, and the nitrifies that produce nitrates that react with calcium
carbonate of the rocks can form calcium nitrate.
Before cleaning, these substances have to be eliminated with some solution that
opens the pores of the rock for taking them out later once they are dried or
dead. Insects and rodents mobilise material and the acids present in the excrement
of the birds such as nitrate and phosphoric, and these favour the chemical degradation
of the stone.
Others
Here we include the wind, which on its own has little damaging effects but
with the presence of salts and humidity aggravate the process of crystallisation
and alveole; and the temperature changes, which regulate the humidity of the
porous system, dissolution of gases and dissolved salts and the speed of the
chemical reactions.
B) INTERNAL CAUSES
Though its effects are not comparable to those produced by external causes,
we should take into account these types of causes. Esbert et al. (1997), in
his Manual of diagnosis and treatment of stone materials and ceramics, considers
the following causes:
Anthropogenics: the blows suffered by the rocks, the weight of the construction
that supports it, its superficial treatment, the work on it, etc.
Petrographics: the bigger the size of the grain, more separated is the
rock and vice versa.
Tectonic: During the raising of the rocks to the surface or the loss-
of the upper side material, these lose resistance to deformation and openings
appear, which are ways of entrance for the erosive agents.
FORMS OF DETERIORATION
They include all the modifications of colour, texture, aspect and mineral composition
caused by alteration factors. The majority of the pathologies are produced on
the surface and contribute to the penetration of corrosive agents.
PATINA
These are thin
surface films formed due to various reasons (Fig.2)
Ageing: due to the passage of time and the exposure to harsh weather.
De-colouring: Varies the natural tonality of the stone, it is called "noble
patina".
Chromatic: they are polychrome, very much in fashion in the Middle Ages.
Biogenic: the surface of the rock is covered by organisms.
Dirt: Blackening caused by contamination in the atmosphere.
Historical: in ancient times the traditional films were used, composed
of milk and blood, so as to make the colour of the stone uniform and to cement
the rock surface (Gárate, 1994).
FISSURES
All the construction materials present fractures of variable dimension. The
origin of the formation may be due to mechanical forces, the corrosion of mechanical
elements or that were present in the initial rock.
FLORESCENCE
They are white spots produced by the precipitation of dissolved salts when
the water on the surface of the porous rocks migrates and evaporates. The origin
of the salts is highly diverse; the source can be in the floor, in underground
water, excrement of the birds, older treatments, in used mortar or proceeding
from the original rock. The most common salts in the rocks are the sulphates,
chlorites, carbonates and nitrates. If the formation of these salts in the rocks
of the monuments take place below the surface of the stone they receive the
name of sub-florescence, and if they form in the interior- crypto-florescence.
The destroying effect is a function of:
-Type of salt formed and the location of its crystallisation.
-Environment conditions, the humidity and temperature control the process of
evaporation, dissolving and precipitation.
CRUST
They are laminates of strengthened material as a result of the superficial
transformation of stone substrate. These crusts are developed in layers, presenting
a determined morphology, strength and colour and its physical-chemical nature
has nothing to do with that of the substrate.
PLATES
Due to variation
in humidity and temperature or due to the mechanical action of ice and salts,
phenomenon of exfoliation parallel to the stone surface are produced, originating
plating when the thickness is in millimetre, and separation when it is in centimetres
(Fig.3)
OTHER
In this are included excrement of animals such as worms or pigeons, materials
of diverse nature not very cohesive and of easy cleaning, corrugates, pricking,
cleaning areas, etc.
In the next issue of LITOS we will publish the second part of this article,
which deals with cleaning, protection and conservation of stone and restoration.
Fig. 1.- Loss of whitewashing with appearance of wall of calcium masonry.
Church -Palace Goyeneche, Nuevo Baztán, Madrid.(Photo R. Fort ).
Fig. 2.- Types of patina over calcium based rocks: biological (black) and historical
(orange).
Church -Palace Goyeneche, Nuevo Baztán, Madrid (Photo R. Fort).
Fig. 3.- Separation in the surface of a sanded granite. Royal Palace, Madrid
(Photo E. Pérez Monserrat )
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Last Updated: July 2008
