Envejecimiento natural y artificial para evaluar los tratamientos para impermeabilización del mármol Natural and artificial ageing for evaluating waterproofing treatments for marbie

La exposición de muestras a envejecimiento artificial se lleva a cabo comúnmente para chequear la duración de las piedras naturales y artificiales y la eficacia de los productos usados para su cuidado. Las correlaciones entre el envejecimiento artificial y natural, sin embargo, no siempre se entienden bien principalmente porque el envejecimiento trata de simular, de una manera rápida pero sencilla, los efectos de las muchas causas que actúan en el ambiente natural.


RESUMEN
La exposición de muestras a envejecimiento artificial se lleva a cabo comúnmente para chequear la duración de las piedras naturales y artificiales y la eficacia de los productos usados para su cuidado.Las correlaciones entre el envejecimiento artificial y natural, sin embargo, no siempre se entienden bien principalmente porque el envejecimiento trata de simular, de una manera rápida pero sencilla, los efectos de las muchas causas que actúan en el ambiente natural.
En este estudio, muestras de mármol de las canteras de Carrara y Proconnesio, han sido tratadas con cuatro productos o mezclas de productos, para su impermeabilización y expuestos a envejecimiento artificial.Mientras, un juego análogo de piedras se exponía al exterior en el centro de la ciudad de Roma, por un período de dos años.

SUMMARY
Exposure of samples to artificial ageing is commonly carried out to test the durability of natural and artificial stones and the effectiveness of producís used for their conservaron.Correlations between artificial and natural ageing, however, are not always well understood mainly because the former can only try to simúlate, in a faster but simplified way, the effects of the many causes interacting ín natural environments.
In the present study, marbie samples from Carrara and Proconnesion quarries, were treated with four waterproofing products (or mixtures of products) and exposed to artificial ageing while an analogous set was exposed outdoor, in the town center of Rome for two years.
Several physical paramenters were measured before and añer the ageing and the experimental results enabled us to compare the effects produced by the different types of ageing and to estímate the correlations between them.
As for the durability of the tested waterproofing products, the two systems are in good agreement even if two years of natural ageing resulted to be slightly more severe than 33 days of exposure to saline fog, to UV radiation and to wetting-drying cycles. (

INTRODUCCIÓN
La eficacia de los productos usados para la conservación de piedras se evalúa comúnmente por medio de la medición de varios parámetros físicos de muestras tratadas y no tratadas, antes y depués de la alteración artificial.Esto permite acelerar el estudio de la durabilidad de los tratamientos y por eso reducir el tiempo de la experiencia.
Los resultados de la experiencia permiten una comparación de los efectos totales de los dos métodos de desgaste y un chequeo de la conducta de los tratamientos estudiados bajo las condiciones del campo.Todavía necesitamos un entendimiento más profundo de los mecanismos de alteración y de la posibilidad de reproducirlos en ensayos de corto plazo.

INTRODUCTION
The effectivenness of producís used for the conservation of stone is commonly evaluated through the measure of severa!physical parameters of treated and untreated samples, before and after artificial wathering.This enables US to accelerated the study of the durability of the treatments and thereby reduce experimental time.
Artificial weathering is carried out through cycles, such as freeze-thaw, sait crystallization, acid spraying, etc.These cycles are aimed at the simulation of the effects of outdoor exposure in a given situation.
Nevertheless, natural ageing is a very complex phenomenon in which it is difficult to individualize all the components and their relativo importance.So, artificial ageing can hardly simúlate a real condition.It only tries to reproduce in a simpler way the effects of the most important factors.
Several studies are being carried out in order to improve our knowledge in this field (Zender, Arnold, 1988) (Heiman, 1988) but the probiem is still far from a satisfactory solution.
In the present study, an attempt has been made to compare the effects of natural ageing with those of artificial ageing on samples of the same stones untreated or treated with the same waterproofing products, applied with the same methodology.
Experimental results allow a comparison of the total effects of the two weathering methods and a check on the behaviour of the studied treatments under field conditions.Yet we need a deeper comprehension of the mechanisms of alteration and of the possibility to reproduce them by shortterm testing.
The products used for the treatments are choosen among those commonly used in Italy, which were suggested for the protection of polychromed sculptures of the "Arconi di San Marco" in Venice (Phot. 1 and 1a) (Fassina et Al., 1987).

EXPERIMENTAL
Two different quarry marbles, Carrara and Proconnesion, are used for the study.Both are very frequently utilized for architecture and sculpture, not only in ancient monuments but also for present use in many Countries.
While the white Carrara can be considered a puré calcitic marbie, Preconnesion has a Calcite/Dolomite ratio of about 4-5 and a Sulphur content of around 0.1 % (Lazzarini, Mariottini, 1987).
Tile ir structure and the average grain size are also rather different, < 1 mnf for Carrara and > 1 mnf for Preconnesion (Lazzarini et Al., 1980) The following products have been applied to the marbie samples: -SCO.Acryl-silicone mixture: "Surface Clear Opaco" by Raccanello, Padova.
The resin SCO, AS and S have been applied by a coater which gives a film of 30 micron thickness.F doesn't form a film as it remains in the liquid State and was applied by brush.
Ten samples of each type of marbie have been treated with each of the tested products.The preparation of marbie samples and the application of products were carried out by V. Fassina and L Lazzarini at the Scientific Laboratory of the Soprintendenza ai Beni Artistici e Storici di Venezia (Fassina et AL, 1987).
Five samples have been submitted to artificial weathering and the other five to the natural weathering, for each type of marbie and for each treatment.
Artificial weathering has been carried out by submitting the samples to a solution spray containing NaCI (5 g/100 mi of deionized water) and H2SO4 at a pH of 5,0 ± 0,1 and, cyclically, to the exposure of a UV lamp at fixed Temperature and Relativo Humidity.
As a whole the weathering lasted 33 days; the experimental details have already been described (Fassina et AL, 1987).
All the water soluble salts produced during the weathering were washed out before the measurement of the physical parameters.
For natural ageing an analogous set of samples has been exposed for two years, from October 1984 to October 1986, in the small balcony of the en el pequeño balcón de "Casa Borgia" (foto 2), frente a Vía Cavour, una de las calles de más tráfico en el centro de Roma.Las muestras han estado expuestas a la lluvia, con una inclinación de aproximadamente 30 grados; el balcón mira hacia el norte.
"Case Borgia" (Phot.2), facing Via Cavour, one of the streets with the heaviest traffic congestión in the centre of Rome.The sampies have been kept exposed to rain, with an inclination of about 30 degrees; the balcony is facing north.
The monthly average minimum and máximum valúes of Temperature, and the monthly total amount of precipitation in Rome during the above period are given in Table II.
The physical parameters measured to evalúate the effect of the weathering are: -colour, -water absorption by capillarity, -water vapour permeability.
Foto 2.-El balcón de la "Casa Borgia", en el centro de Roma, en donde las muestras fueron expuestas por 2 añ Phot 2.-The balcony of the "Case Borgia", in the centre of Rome, where the samples were exposed for 2 years.En el caso de muestras desgastadas naturalmente se ha llevado a cabo una evaluación cuantitativa del polvo depositado en sus superficies.
A menos que se especifique claramente, los resultados se dan como valores medios de cinco muestras por cada tratamiento.

Color
El color se ha medido por comparación con la Munsell Colour Charts (ASTM, 1968); los resultados se dan en la Tabla NI, expresados como valores promedio aritméticos, aunque no siempre correspondan a los escalones de la Munsell Neutral Scale.
Como se puede ver en las fotos 3 y 4, las muestras expuestas en el exterior aparecen bastante sucias debido a los depósitos de polvo.Este hecho ha tenido una clara influencia en el color de la superficie.
La alteración artificial no cambia el color del mármol de Carrara no tratado o tratado con AS y S. En el caso del Fomblin se ha comprobado sólo un muy pequeño oscurecimiento, mientras que las muestras tratadas con SCO se han vuelto amarillas después de la primera etapa de alteración superficial; al término del envejecimiento el color amarillo desaparece, debido a la desaparición de la resina de la superficie.
Después del envejecimiento natural, el Carrara no tratado y las muestras tratadas con S se han oscurecido ligeramente: ambos valores de Neutro (N %) y Reflexión (R %) se han reducido.En comparación con los valores medidos antes del desgaste, la exposición en el exterior produce una mejora con el tratamiento F, bien respecto de los valores iniciales y bien de los valores posteriores al desgaste artificial (N y R % se han vuelto más parecidos a los valores de las muestras no tratadas).SCO y AS muestran el más fuerte oscurecimiento, debido a un considerable depósito de polvo, y se han vuelto un poco In the case of the samples naturally weathered a quantitative evaluation of the dust deposited on their surface has been carried out.
Except for colour, al I the measures have been carried out on washed and dried samples.

RESULTS AND DISCUSSION
In the following paragraphs the experimental results are given, making a comparison among the samples before the weathering and those which are artificially and naturally aged.The first two seried of results were already published in a previous paper (Fassina et AL, 1987).
Unless differently stated, the results are given as average valúes of five samples for each treatment.

Colour
The colour has been measured by comparison with the Munsell Colour Charts (ASTM, 1968); the results are given in Table III, expressed as arithmetic average valúes even if they don't always correspond to the steps of the Munsell Neutral Scale.
As shown in photographs 3 and 4, the samples exposed outdoor appear rather soiled due to the deposition of dust.This has had a clear influence on the surface colour.
Artificial weathering doesn't chango the colour of Carrara marbie untreated or treated with AS and S. In the case of Fomblin only a very slight darkening is measured, while the samples treated with SCO turned yellow after the first step of weathering.At the end of the experiment the yellowing disappears, due to the disappearance of the resin from the surface.
After natural ageing the untreated Carrara and the samples treated with S are slightly darker: both Neutral valué (N) and Reflectance % (R %) decrease.In comparison with the valúes measured before the weathering, the outdoor exposure produces an improvement on treatment F, both in respect to the initial valúes and to the valúes after artificial weathering (N and R % become more similar to the valúes of the untreated samples).
SCO and AS show the strongest darkening, corresponding to a heavy deposition of dust, and some yellowing.Therefore, the colour amarillas.Sin embargo, las mediciones de los colores han sido posibles sólo en comparación con la Tabla Munsell Y (Yellow).En lo que se refiere al Proconnesio, la evaluación de los cambios de colores es más difícil debido a la variedad de los tonos de gris del mármol.Es sin embargo posible comprobar que el envejecimiento artificial no da una variación importante para todos los tratamientos, salvo para el SCO, el cual muestra el mismo comportamiento antes detallado para el mármol de Carrara.
Las muestras de Proconnesio aparecen un poco más sucias que las muestras de Carrara.En particular, SCO y AS muestran el mismo oscurecimiento fuerte y color amarillo, ya detallados para el mármol de Carrara, mientras que otros tratamientos dan sólo pequeñas variaciones que son difíciles de evaluar, debido a la variabilidad de color de este mármol.
La conductividad específica de las soluciones se ha medido y los residuos insolubles se han recogido por medio de filtrado con filtros calibrados.Los resultados se recogen en la Tabla IV.
La cantidad total de las partículas insolubles recogidas de las muestras de Carrara es levemente más alta de la cantidad del Proconnesio, cosa que está de acuerdo con la aparencia más sucia de las primeras.Además, los valores de la conductividad específica de los productos estraidos del agua del mármol de Carrara son un poco más altos que los correspondientes valores del de Proconnesio.
El peso del "polvo" y la conductividad específica deí extracto de agua ofrecen sólo una primera indicación del grado de deterioro de los dos mármoles después del desgaste natural.Los cambios de color son, por el contrario, de la máxima importancia en la evaluación de los productos utilizados para la protección de las obras de arte.
measurements have been possible only by the comparison with the Y (yellow) Table .As far as the Proconnesion is concerned, the evaluation oí colour changos is much more difficult due to the variability of the gray tone of the marble.It is, however, possible to ascertain that the artificial ageing doesn't give any significant variation for all the treatments, except for SCO which shows the same behaviour as that previously described for Carrara marble.
The Proconnesion samples appear lightly less soiled than the Carrara samples.In detall, SCO and AS show the same strong darkening and yellowing already described for Carrara marble, while the other treatments give only slight variations which are difficult to evalúate because of the variability of colour in this marble.This is due to the presence of grey veins in the marble.
After the colour measurements all samples have been immersed in a fixed amount of deionized water for 96 hours on a shaking table in order to wash them and to extract the water soluble salts possibly produced by pollution.
The specific conductivity of the solutions has been measured and the insoluble residuo has been collected by filtration on calibrated filters.The results are given in Table IV.
No correlation has been found between the colour variations and the weights of the insoluble residuo probably because the black colour is mainly due to carbonaceous products with low bulk density, scarcely influencing the weight of the insoluble residue, and, moreover, because the latter can be formed not only by the atmospheric dust but also by powder coming from the sample and from the applied product (phot.5).The above hypothesis, however, should be confirmed by the chemical analysis of the insoluble residue.
The total amount of insoluble matter collected from the Carrara samples is slightly higher than the amount from Proconnesion, which is in accordance with the dirtier appearance of the first ones.Moreover, the valúes of the specific conductivity of the Carrara water extracts are a little bit higher than the corresponding valúes for Proconnesion.
The weight of the "dust" and the specific conductivity of the water extract give only a first, rough indication of the degree of deterioration of the two marbles after natural weathering.The colour changos are, on the contrary, of the utmost importance in the evaluation of products to be used for the protection of works of art.
Para el mármol de Carrara el ángulo de contacto de los tratamientos S y F muestra siempre una rápida disminución, mientras que para el SCO la disminución no es tan marcada.Muy extrañamente, el tratamiento AS mejora después del desgaste artificial y queda prácticamente invariable después de la exposición en el exterior.En el caso de las muestras sin tratar, debido a una cierta heterogeneidad de las superficies naturales, los valores del ángulo de contacto, antes y después de los dos tipos de ensayos, se pueden considerar prácticamente constantes.
La tendencia de la absorción por capiiaridad en los dos tipos de mármol, tanto después de la alteración natural como de la artificial, es bastante parecida.El aumento de los valores de M es mayor para las dos piedras después del

Contact angle
A Lorentz & Wettre Instrument (Sweden) was used which allows a direct reading of the height and diameter of the drops, without taking photos.The readings were always carried out 15 seconds afther the deposition of the drop on the sample surface.Thirty measures were taken for each set of samples; the average valúes and their standard deviation are given in Table V.
From a general point of view, the effects produced by the two weathering conditions can be considered rather similar, however, the results of this test are difficult to interprétate, probably because of their high dispersión.
For Carrara marbie the contact angle of treatments S and F shows always a sharp decrease, while for SCO the decrease is not so accentuated.Strangely enough, the treatment AS improves afther the artificial weathering and remains practically unchanged afther outdoor exposure.In the case of the untreated samples, due to a certain heterogenity of natural surfaces, the valúes of contact angle, before and afther the two types of weathering, can be considered practically constant.
In the case of Proconnesion, the effects of natural weathering on contact angle are in good agreement with those obtained with the artificial ene, except for treatment SCO which remains practically unchanged in the first case, whereas it is strongly affected by the laboratory weathering.

Water absorption by capillarity
The measures have been carried out following the Normal methodology (NORMAL 11/85) and the results are given in Table VI, where the Capillarity Coefficient C.A. (g/crrf • s^^^) and the asymptotic valúes of water absorption per unit surface M (g/crrf) are given, before and after weathering.
These valúes have been obtained from experimental graphs of the average valúes of water absorbed per unit surface versus the square root of time.As a consequence, error is not indicated.
The trend of capillarity absorption of the two types of marbie, after both natural and artificial weathering is rather similar.The increase of the valúes of M is higher for the two stones after natural ageing.The increase of the C.A. valúes is envejecimiento natural.El aumento de los valores de C.A. es mayor sólo en el caso de las muestras de Proconnesio expuestas en el exterior.
El aumento más marcado de C.A. y M se ha registrado con el tratamiento F aplicado a las muestras de Carrara.
Los dos tipos de envejecimiento producen un aumento en los valores de las muestras tratadas, mientras que aquéllas no tratadas quedan constantes o hasta disminuyen un poco, como en el caso del mármol de Carrara.La influencia de la exposición natural produce una variación más fuerte en comparación con la exposición artificial sobre las muestras de Proconnesio, en tanto que el efecto sobre el mármol de Carrara no es tan unívoco.Para los tratamientos AS y F el desgaste natural no ha sido tan agresivo como el desgaste artificial, pero la situación es al revés en el caso del tratamiento AS.
Cuando se comparan los resultados de cada uno de los parámetros medidos, después de los dos tipos de alteración (natural y artificial), el orden de calidad de los cuatro tratamientos queda el mismo para el mejor y para el peor, mientras que alguna variación se ha observado para los otros, como se muestra en la Tabla VIII.Una excepción se observa con los resultados de la permeabilidad de las muestras de Carrara tratadas con AS, y por aquéllos del ángulo de contacto para las muestras de Proconnesio tratadas con SCO y S.
higher oníy in the case oí Proconnesion samples exposed outdoors.
The sharpest increase oí C.A. and M has been registered with the treatment F applied on Carrara samples.

Water vapour permeability
The Normal methodology has been followed (NORMAL 21/85), using a "wet cup" exposed to the dry atmosphere on 1 cm thick samples wlth the treated surface facing upwards.The results given in Both the two types of ageing produce an increase in the valúes of the treated samples, while the untreated ones remain constant or even slightly decrease, as in the case of Carrara marble.The influence of the natural exposure produces a stronger variation compared to the artificial exposure on Proconnesion samples, while the effect on Carrara marble is not so univocal.For the treatments AS and F natural weathering has been not so aggressive as the artificial weathering, but the situation is reversed in the case of the AS treatment.

CONCLUSIONS
The above results allow us to consider some of the behaviour of the studied treatments when exposed to real conditions.The results also offer US a comparison of the effects produced by artificial and natural weathering.
After two years in outdoor exposure and after artificial ageing the protective action of all the tested treatments decreases and in some cases even disappears.
When comparing the results of each of the measured parameters, after the two weatherings, the ranking order of the four treatments remains the same for the best and the worst one, while some variation is observed for the other ones, as shown in Table VIH.An exception is given by the results for water vapour permeability for the Carrara samples treated with AS and by those of contact angle for the Proconnesian samples treated with SCO and S.     Cuando se evalúan globalmente los resultados de todos los parámetros medidos después de las dos exposiciones, la conducta más satisfactoria es siempre la mostrada por los tratamientos S y AS, mientras que F y SCO muestran una baja duración.Sin embargo las muestras tratadas con AS (y también con SCO), cuando se exponen en el exterior, se vuelven más polvorientas que las otras.La remoción del depósito es difícil y, en todo caso, el color resultante ha cambiado mucho respecto del inicial.
Es sin embargo de una cierta dificultad reproducir las condiciones que llevan a la adhesión de las partículas suspendidas a la superficie de la piedra hasta la formación de la costra negra, que es tan común encontrar en los monumentos en nuestra área (Foto 4).Una posibilidad puede ser dada por la cámara climática planeada para conseguir la entrada de cantidades controladas de partículas y su captura por la superficie de las muestras de ensayo.
When comprehensively evaluating the results of all the measured parameters añer the two exposures, the most satisfactory behaviour is always given by the S and the AS treatments, whiie F and SCO show a lower durability.However, the samples treated with AS (and SCO as well) when exposed outdoors become much dustier than the other ones.The removal of deposit is difficult and, in any case, the resuiting colour has changed much more in respect to the initial one.
In both the AS and SCO treatments an acrylic copolymer is present and it seems reasonable to ascribe to that resin the property of attracting and retaining dust.
Especially in the case of monuments and sculptures exposed outdoors the property of attracting and retaining dust represents an important drawbac¡< which results in a negativo evaluation of the treatment even if other properties, such as those related to water, are good.
Taking into account the above consideration, the S treatment can be judged the most effective among those tested.
Corning to the comparison between the two ageing systems, it is possible to say that they produced rather similar effects, even if natural weathering was slightly more severo.
The only serious, basic difference is due to the presence of dust in the natural environment, chemically aggressive and rich in carbonaceous matter in urban polluted áreas.
On the other hand, artificial weathering produced in 33 days results similar to those given by two years of natural exposure.
This experimental result confirms the usefulness of the laboratory simulations but stresses the necessity to improve them by adding the dust, particularly the suspended particulated matter, to the factors of artificial óyeles.
It is certainly difficult to reproduce the conditions which bring about the adhesión of s.p.m. on the stone surface up to the formation of the black crust which is so frequently found on monuments in our climate (Phot. 4).One possibility may be given by a climatic chamber planned in order to have the inlet of controlled amounts of particulate matter and its capture by the surface of the test samples.
Such a type of chamber is beeing planned by the Central Instituto for Restoration in Rome (Laurenzi Tabasso, Marabelli, 1988).

TABLE V
a -2 arctang (2h/d), where h is the height of the droplet and d its diameter.