Pérez Monserrat, E. & Baltuille Martín, J.M.
Area de Rocas y Minerales Industriales. Instituto Geológico y Minero de España (lGME). Madrid.
jm.baltuille@igme.es
STAGES OF INTERVENTION
At the time of intervention in a building or historical monument, one has to consider the stages of cleaning, protection-conservation and restoration, the three not always being necessary. Each one of the phases is as important as continuous maintenance, preventive conservation, that minimises or avoids the factors of alteration, and the active conservation, in which the environmental parameters and atmospheric contaminants around the building are measured, so as to guarantee a positive evolution of the interventions and achieve the acceptable durability of the applied treatment.
The realisation of maps of litologies, pathologies or interventions over plans of the building or monument to be intervened, is advisable. In the map of litology, all the typology of construction materials which have been employed, are represented. The map of pathologies puts together the deterioration that is present in each one of the litologies, and for their realisation we will pay special attention to the corners, to the areas where the greatest humidity is concentrated and the salt elements: it is highly recommendable to use these two maps. And last, the map of interventions represents in a highly synthetic form the operations realised.
The degree of detail of each one of the maps depends on the knowledge of the specialist who does it; they should be accompanied, moreover, by a memory of the corresponding legend and normally several maps of each one of the types is made.
Previous studies need to be done on the stone support before deciding on one or the other method or a product. According to Parrot (1990), the rock should be characterised and the pH determined of the cementing agent, and tests should be done for determining the possible secondary effects of the product.
1. - CLEANING
Its object is to eliminate the surface dirt and the harmful products. This is a stage of vital importance since sometimes it is the only intervention that is done, and the main problem of cleaning is its irreversibility.
The cleaning conditions and must be compatible with the later stages of intervention. The cleaning should improve the aesthetic perception, but above all should avoid or stop the deterioration of the stone material.
Before proceeding with the cleaning, two operations should be realised to see if the state of the rock makes it advisable. One is the pre-consolidation of the rock if it is very decohesioned, but lightly so as not to consolidate also the dirt or pathology. The other is desalting, because not eliminating the salts completely would be in detriment to the behaviour of the stone in later treatments.
Types of dirt
The dirt on the stone material appears as a persistent layer and of more or less regular thickness where the products of distinct types are mixed without distinction. Carbonell de Massy distinguishes between the following products:
Smoke and dust: particles formed by ash, solid oils that are not burnt and proceeding from the erosion of solid materials.
Spots: florescence, areas of colour loss, spots due to dissolution of metallic structure elements, to sprays and paints, etc. Substances of biological origin: vegetation and micro fauna.
Crusts and remains of previous treatments.
Cleaning methods
Before the intervention, one has to consider the aspects that will determine the choice of method, and these are:
The historical-artistic value or interest of the monument and the state of conservation.
Factors relative to the rocks: its physical-chemical nature, texture, technical and petro-physical properties and environmental behaviour in the area where it is located.
Factors relative to the substance to be eliminated: nature, type, extension, and thickness.
Speed of cleaning action so that the worker can control its effects.
The method used must not generate products damaging to the stone and the worker, neither there should be surface modifications that facilitate its deterioration.
There is no universal cleaning product which exists, for each type of material specific products should be used, causing the minimum desegregation of petreo-material and dissolution of cementing agent (Parrot). We will now go to the main methods of cleaning, we will focus on those mechanisms that are the most well known and frequently used.
Mechanical
These techniques separate the dirt of the petreo-material to be cleaned employing the mechanical energy that is generated when projecting abrasives. It is very important that the separation takes place right on the interface dirt - surface of the stone.
Sand jet
This method has, over time, lost prestige because it was used before in an uncontrolled manner and the abrasive particles had angled edges.
The mechanical action of the method is a function of:
a. - The abrasive particle
type: pumice stone, aluminium oxide, glass and sand.
strength: minimum 5 in the Mohs scale.
form: spheres, hollow or solid
size: the thinner it is, better penetration and precision.
b. - The abrasive jet.
pressure and density
application time.
distance between mouth of the jet and the surface to be cleaned.
Micro-jet of sand
Method very similar to the previous one but the particles are of less strength and their size is less than 60 micras, they are normally of glass and aluminium oxides. They are very effective for removing thick and strong incrustations, thin crusts and black crusts which cover stones with polychromes. The main advantage is that the jet pressure and the quantity of abrasive projected can be regulated, thus the cleaning can be regulated and can be used on all types of rocks. An inconvenience is that it is slow, a lot of dust is taken out which has to be collected and the apparatus destined for this end is expensive, and the cost of sand is also high.
Others
Here other simple methods are included, which can be:
Manual: sandpaper, pumice stone, knife, bronze and phosphorous brushes, glass paper, etc.
Electric: these are small rotating machines equipped with different points and perfectly controlled.
Pneumatic tools.
The efficiency of the method depends on the skill of the worker, and being a slow method it is used for pieces of small dimensions.
Specials
These are techniques which are still being experimented and in spite of their effectiveness having been proved, they are not yet easily available. Among the most important ones are, the microwaves, the ultrasound, and the laser, the last one being a technique the use of which is on the rise.
Laser
Laser is characterised by its constant intensity, the radiation not being dispersed, the waves coinciding in phase and that it is a monochromatic radiation. Since in each pulse the energy that is liberated is very low and the duration of light rays is very short, the material does not become hot; moreover, the wavelength is such that it propagates on the stone without altering it.
It has resulted as being very effective for pulverising black crusts produced from environmental contamination, liberating them when mechanical micro-resonance is produced on the surface of the stone. In spite of being a method that does no damage to the stone and which can also be used over not consolidated supports, it needs to be used with great care and precaution.
Chemical
Tensioactive products, alkaline cleaners, acid cleaners and organic solutions.
Water based
Waterjet at low and high pressure, water in the form of vapour, cloudy water and water applied with poultice.
2.- PROTECTION - CONSERVATION: CONSOLIDATORS AND WATER REPELLENTS
After the cleaning, the action of the agents of alteration continues to modify the porous system of the rocks and its capacity of water absorption. The product employed must avoid the degrading action of water on the rock, creating an impermeable barrier to water but permeable to water vapour.
Different authors indicate that the treatments have to be applied when the rock is dry, they should penetrate right up to the rock that is sane, covering walls of pores and fissures, should be reversible and adhere to the substrate, they should not generate harmful sub-products such as salts, modify substantially the porous system of the original material and neither its permeability to water vapour, so as to permit the respiration of the rock. One should also take into account the chromatic incidence (they are usually transparent so as not to modify the colour and natural shine of the stone), the expiry date, toxicity, resistance to acids and alkaline and ultraviolet radiation, ease of handling or the economic cost (Carbonell de Massy, 1993; Esbert et al., 1997; Fort, 1996b).
Since the ideal product does not exist, a majority of the times it is necessary to adopt a compromise solution that does least damage to the rock. The behaviour and efficiency of treatment depends on the porosity of the rock, of the tests to be done and the product itself.
Consolidation
With this action the aim is to improve the mechanical resistance of the rock, increasing the cohesion of the grains of the surface area and avoiding its coming apart. The consolidator should be applied in layers, taking care not to form a much stronger and resistant layer than the petreo substrate and which comes apart.
To achieve a good penetration and adherence between the deteriorated part and the sane one, the consolidator should be liquid, not viscous, have low surface tension and solidify in the interior of the rock. Two groups of products exist.
Inorganic:
they resist the harsh weather better. The calcium and inert mineral loads have been employed so as to diminish the excess of porosity. Nowadays the ethil silicates are used, these form a silicon gel when reacting with the stone that is deposited on the walls of the macro-pores. Their effectiveness in sandstone has been proved, but they do not have good adherence on calcium-based materials.
Organic:
they present better mechanical characteristics and permeability. Synthetic waxes such as paraffin and resins have been used. The latest products to be used are thermoplastic polymers; they also have water-repelling properties.
Water repellents
These serve to increase the resistance to penetration of water and not in making it waterproof. The products called water repellents, in liquid state fill the pores and on drying form a repelling layer that insulates the surface of the rock. In the interphase between a solid and liquid an angle of contact is created; if the value of the angle is between 0-90º (see Fig.4ª), the liquid wets the solid, and between 90-180º
(seeFig.4b), the liquid behaves as if it does not wet. With the water repellents the aim is that the angle of contact between the drop of water and the surface be the greatest possible.
The water repellents most used are of silicon, which when evaporating the solutions become polymers and transform into polysiloxans. To evaluate the repellent effect one needs to know the value of angle of contact stone-water.
3.- RESTORATION: REINTEGRATION AND SUBSTITUTION
This intervention is made necessary when the degree of alteration of a rock is such that cleaning or consolidation does not improve its state. In this third stage the location of quarries plays an important role, so much that some are protected, but it is a difficult task that does not always ensure positive results. Here the same conflict appears since the new material has to have an aspect and characteristics similar to the original, but at the same time sufficiently distinct so that the restoration realised is recognisable.
Previously, the factors that determined the use of one or the other rock were the proximity and the ease with which the material could be worked upon; it would be that the quarry supplying the original material has exhausted or that we cannot identify it because it is covered, we then have to look for another quarry that can provide the material of the same geological formation with the help of aerial photos, geological maps, geo-techniques, and industrial rocks. Moreover, the location of the new material is essential for realising tests because in numerous occasions we cannot extract samples of the construction to be restored. These tests allow one to know if the new material accelerates the deterioration of the old one, select the stone variety most adequate for each situation and evaluate how the different treatments which will be applied on it will evolve with time.
We can know the degree of deterioration and the possible causes of alteration attending to the differences existing between the stones of the monument and that of the proceeding quarry (Fort, 1996 a).
The mortars used in this stage have to have an aspect, mechanical resistance and permeability similar to the stone. They have to be sufficiently plastic so as to be easy to work with and to avoid tensions that impede their adherence They cannot modify the rock, introduce harmful agents and their strength cannot be superior to the stone, and always they must allow its respiration. (Carbonell de Massy, 1993; Fort, 1996b).
Normally the mortars carry grains of the same stone as the building or pigments which imitate the colour of the rock; the pigments can be natural or artificial, the iron oxides are the best because they are highly resistant to ultraviolet radiation.
At times the flaw is in the structure, the framework of the structures can increase by 8 times in volume due to corrosion and/or oxidation; to avoid the oxidation, the space between steel sprout and the stone is filled with cast lead, or these are substituted by steel rods or fibreglass, highly resistant.
Reintegration and substitution
The first operation is a delicate intervention consisting in recovering volumes and architecture forms lost partially or totally. The second is when the original stone is changed for another of adequate aspect and behaviour, and the aim is to raise the durability of the conjunct conserving to the maximum extent possible the original material.
Both operations can be done with natural or artificial stone. In the substitution by natural rock it is observed that the stone tends to behave better in the place where it is formed, which means the substitutes of limestone from Colmenar by that of Novelda or Bateig in numerous constructions, or that of dolomite of Boñar by the limestone from Hontoria in Burgos have not given the expected result, and, equally, the sandstone of Villamayor out of Salamanca has deteriorated with greater ease. Normally, natural stone is used as a substitute of elements in balusters, cornices, etc. (Fig.5).
The substitution by natural stone can result economically very interesting if the material used has the adequate benefits. If inorganic materials are used, such as mortars and cements, and organic polymers, the results of which have not been to the level desired because with time they develop cracks and end up separating from the stone substrate. With artificial stone also moulds are made; normally in a mould the piece is of mortar, and the counter-mould of silicon so as to permit a slight movement and avoid tensions that break the piece, a shell of polystyrene armed with fibreglass covering the counter-mould.
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Last Updated: July 2008
