Investigación sobre morteros y sus tratamientos para su restauración en el Retablo del Altar Mayor de la Basílica del Pilar

Los morteros de reparación se han diseñado con una composición similar a la de los antiguos, con una mezcla de yeso y cal, y se estudió su compatibilidad con el alabastro, optimizándose igualmente la granulometría de los áridos a utilizar The repair mortars were designed with a composition similar to the originals, with a mixture of gypsum and lime, and their compatibility with the alabaster was studied, while also optimizing the granulometry of the aggregates used.

The present paper covers the guidelines for the design of repair mortars used to restore the altarpiece.They are based on compatibility criteria with the existing materials and treatments in the altarpiece, according to research carried out during the recent works of cleaning and restoration.
During the previous research, the composition of the original mortars was tested, the aggregate granulometry, and the composition of the organic additives used.The patina of the altarpiece was also analyzed and identified.
The original mortars were made up of a mixture of gypsum and lime with the addition of oil or fats.In many cases various types of fibre were detected in the mortar, used to improve its mechanical characteristics.
Los morteros de reparación se han diseñado con una composición similar a la de los antiguos, con una mezcla de yeso y cal, y se estudió su compatibilidad con el alabastro, optimizándose igualmente la granulometría de los áridos a utilizar The repair mortars were designed with a composition similar to the originals, with a mixture of gypsum and lime, and their compatibility with the alabaster was studied, while also optimizing the granulometry of the aggregates used.
En marzo de 1993 se inició el último proyecto general de intervención que culminó en julio de 1994.En él se abordaba una limpieza y restauración general con el fin de devolver a la obra todo su esplendor, y fue llevado a cabo por el Equipo de Restauración coordinado por Mercedes Nuñez y bajo la dirección técnica de Olga Cantos (ICRBC).

1.HISTORIC AL DATA
The present Basilica of El Pilar is built on the site where, according to tradition, Saint James the Apostle built a modest chapel following the miraculous appearance of the Virgin, and which received the name of Santa Maria La Mayor.Following that time, the building suffered numerous transformations and during the wars against the Sarracins was practically destroyed, as mentioned in a document of 1118 (1).Following the Reconquest it was rebuilt and the construction endured until the end of the XVII century, including the Altarpiece among its works of art.In 1681 the first stone was laid of the great building that today forms the Basilica, with the Altarpiece remaining in its original position until 1717, when it was moved to its present location (2).
The Altarpiece of the Assumption of Our Lady is located on the High Altar of the Basilica of El Pilar in Zaragoza, work of the master Damián Forment and finished in 1518, which makes it ten years prior to a similar work by him found in the cathedral of Huesca.The style is Flemish Gothic except for the socle which is in the Renaissance style.Sculptured completely in alabaster, according to recent studies it measures 16.75 x 9.70 m (2).

In march 1993 the last general works program was begun which was finished in July 1994. During this, cleaning and general restoration was undertaken to restore the work to its original splendour, carried out by the Restoration Team co-ordinated by Mercedes Nuñez and under the technical direction of Olga Cantos (ICRBC).
Since its removal to the Basilica, the Altarpiece has suffered various actions, and old metal attachments of lead can be seen; in 1928 the pillars were reinforced; during the sixties repairs were carried out with cement mortar; and in the seventies surface treatment and protection was applied to the whole work (2).
The restoration of the El Pilar Altarpiece has been carried out in collaboration with the E. Torroja Scientific Institute for the investigation of materials, mortars and treatments, taking as a starting point those existing in the monument itself By characterizing these, during the course of the work the guidelines were established for the design of the mortars to be applied in the restoration.This work covers the general aspects of the study carried out during the restoration.
Prior to this research, and with a notable relationship to the same, was that carried out on the alabaster façade of the Church of Santa Engracia, also in Zaragoza (3).
El traslado del Retablo en 1717 a su actual emplazamiento, realizado en un breve plazo de tiempo y la necesidad de adaptación al nuevo espacio físico han dado lugar a la mayoría de los defectos de montaje y daños que presenta en la actualidad.Los bloques de alabastro no se encajaron exactamente, esta deficitaria adaptación ha dado lugar a una gran diversidad en la anchura de las juntas y a desplazamientos en los planos vertical y horizontal, se produjeron, al mismo tiempo, pérdidas de material, fisuras y grietas y falta de unión de las piezas con el mortero.El mortero se aplicó en ocasiones en las juntas, sobrepasando su anchura y ocultando parte de la talla.
Con el primer examen sobre el estado de conservación del Retablo y de los morteros existentes, se establecieron los criterios para realizar una toma de muestras que permitiera la caracterización diferenciada de los morteros.El muestreo se apoyó en la amplia documentación gráfica de una publicación de The removal of the Altarpiece in 1717 to its present location, carried out in a short space of time, and the need to adapt it to the new physical space, gave rise to the majority of the defects in mounting and the damage seen today.The alabaster blocks do not fit together exactly, and this incorrect adaptation has given rise to great diversity in the width of the joins and the displacements in the vertical and horizontal planes, and at the same time there has been loss of material, fissures and cracks and lack of joining of the parts with the mortar.The mortar was occasionally applied at the joins overlapping their width and hiding part of the sculpture.

External factors such as humidity, temperature changes and pollution, have also caused deterioration to the alabaster and to the joint mortars of the A harpiece. Th e specific characteristics of the stone and the mortars, both composed principally of gypsum, means that between them water (humidity) is the main degradation factor
in the Altarpiece, since other contaminants, such as SO2, do not affect mainly its integrity in this case.

RESEARCH INTO THE OLD MATERIALS OF THE ALTARPIECE
The mortars that may be considered as original to the Altarpiece, at least in its present location, have three principal functions: . Union of each of the pieces of alabaster that form the sculptured groups. .Holding and laying of the whole Altarpiece for its placement and location inside the building, forming part of the bonding mortar.
. Reintegration of some alabaster elements.

Over time, various repairs have been carried out with the application of new mortars. Prior to the present restoration, repair mortars have been detected in the joints to repointing the loose pieces in the most external parts of the jointing mortars, in the re-carving of some individual pieces, and in the fill of fissures and cracks appearing in various zones of the overall sculpture.
During a first examination of the state of preservation of the Altarpiece and the existing mortars, the criteria were established for canying out a sampling that would allow a differentiation between the characteristics of the mortars.The sampling was supported by the extensive graphic principios del siglo XX (4), seleccionando con fiabilidad morteros de reparación en grietas y en desprendimientos antiguamente existentes y actualmente renovados, así como también juntas conservadas aún intactas y documentadas fotográficamente en dicha fecha.
documentation in a publication dating from the beginning of the XX century (4), reliably selecting the repair mortars in the cracks and in the existing old separations now renewed, and also the joints that were still intact and photographically documented at that time.
During the work of rehabilitation one of the plates of the predella was removed to give access to the bonding mortar used to attach the Altarpiece.Its characterization was used as a point of comparison with other mortars extracted, to establish analogies not for total identification, since the holding mortar had to have specific characteristics, particularly in the content and distribution of the sizes of aggregates.Furthermore, to select old mortars we searched in other zones inside the joins.Also, the base mortar offered a relationship between the origin and composition of the mortars and their chronology since it can be considered that they were applied during the work of setting the Altarpiece into its new location in 1717.
The studied mortars came from both the main body and the basement, chosen at different heights and depths.
The major component of the ancient mortars is gypsum, and even the aggregates are formed mainly by alabaster.This composition gives the original mortars a whitish tone.In all cases the binder used was a mixture of gypsum and lime.This practice of incorporating lime with the gypsum is habitual in the preparation of traditional gypsum mortars since the lime improves the properties of the gypsum, particularly its workability.The mortars of a mixture of gypsum and lime receive the name of trabadillos (5).

This identification of the mineralogical composition of the mortars was obtained by the application of various instrumental techniques, obtaining concordant results (X-ray diffraction, infrared absorption spectroscopy and scanning electron microscopy connected to microanalysis by X-ray dispersion)
. The laser granulometry applied to the study of the agglomerates in the mortars, aside from the granulometric analysis, also allowed us to detect the presence of more than one component and it was thus found that the fineness of the gypsum used as agglomerate was low.
As an example, the IR spectrum for one of the mortars studied is given below, in which the presence of gypsum and calcium carbonate can be seen (Fig. 1).

La proporción de finos en los morteros antiguos de junta y de unión es mayor con relación a los gruesos (áridos), en general del orden de 2:1 ; aunque dada la variación del tamaño de las juntas se encuentran valores
One of the specific characteristics of the old mortars present in the Altarpiece is the presence of fibres.These fibres prevent the retraction and cracking of the mortars and give it greater mechanical resistance.Their size and proportion depend on the function of the mortar, for example the base mortar has small fragments of wood that reach a size of almost 1 cm (Fig. 2).In the joint mortars the size of the fibres is in relation to the thickness of the joint, although fibres of a diameter between 0.16 and 0.5 mm are most frequently found (Fig. 3).Next to these fibres of vegetable origin others of a dark colour can be seen under an optical microscope, which have the appearance of hairs and with a very much smaller diameter, around 0.08 mm.hasta de 4:1 o bien en otros casos del orden de 1:1, es decir, con una proporción de finos inferior.En el mortero de fondo el porcentaje de áridos es mayor que el de finos y la proporción aglomerante/áridos es del orden 1:2, inversa a la de los morteros de junta.

The porosity of the original mortars varies depending on the aggregate size, the water content used in the mixing mortar and the binder/aggregate ratio in the mortar. Lower porosity is observed in the joint mortars than in the bonding mortar holding the Altarpiece; where the amount of accessible porosity reaches 24% due to the greater proportion of coarse aggregates. The proportion of fines in the old joint mortars is greater in relation to the large particles (aggregates), generally in the order of 2:1; although in view of the variation in size of the joins amounts
Respecto a los áridos, su granulometría está directamente relacionada con la fiínción del mortero.Los morteros de junta más fi"ecuentes poseen áridos de tamaños de 0,08 mm, en general los más abundantes, y de 0,16 mm.El mortero de fondo tiene áridos con una distribución completa desde tamaños superiores a 3,36 mm, hasta 7 y 8 mm, a tamaños de 0,08 mm, siendo el tamaño de áridos más abundante el que corresponde a 0,16 mm.Los tamaños de las fibras se ajustan a la granulometria correspondiente de los áridos del mortero.
Las adiciones orgánicas en morteros y pátinas se identificaron mediante espectroscopia de absorción infrarroja, tras extracción con mezclas de disolventes (6).Los morteros anfiguos presentan una adición de aceite, muy posiblemente secante, reaccionado en su práctica totalidad.El tipo de adición y el estado de reacción es idéntico al detectado en otros monumentos de España y de Europa.Antes de la restauración, los morteros existentes se encontraban aparentemente en buen estado de conservación, salvo en lo que a la suciedad superficial se refiere, a la falta de adherencia al alabastro en algunos lugares del conjunto escultórico y a la pérdida de material y fisuraciones ya referidas (Fig. 4) of up to 4:1 are found while in other cases it is 1:1, i.e. with a lower proportion of fines.In the bonding mortar the percentage of aggregates is greater than fine material and the binder/aggregate ratio is about 1:2, the inverse of the joint mortars.
Regarding the aggregates, their granulometry is directly related to the function of the mortar.Joint mortars most frequently have aggregates with sizes of 0.08 mm, generally the most abundant, and of 0.16 mm.The bonding mortar has aggregates with a complete distribution range from sizes of more than 3.36 mm up to 7 and 8 mm, to sizes of 0.08 mm, with the aggregate size of 0.16 mm being the most abundant.The fibre size is in accordance with the granulometry corresponding to the aggregates in the mortar.

The organic additives to mortars and patinas were identified by infrared absorption spectroscopy, after extraction with mixtures of solvents (6). The old mortars show the addition of oil, most probably as a dryer, almost all reacted. The type of addition and state of reaction is identical to that detected in other monuments in Spain and Europe.
Prior to the restoration, the existing mortars were apparently in a good state of preservation, except for the surface dirt, the lack of adherence to the alabaster in some parts of the overall sculpture and the loss of material and cracking already mentioned (Fig. 4).
Los morteros utilizados en el Retablo antes de la presente intervención y que pertenecían a reparaciones anteriores^ presentan mayor dispersión de unos a otros en sus características.Sin embargo hay semejanza entre diversos morteros que poseen una misma función de parcheados y remate de juntas, localizados a diferentes alturas en el cuerpo principal e incluso en la predela.

APLICACIÓN DE RESULTADOS A LA RESTAURA CIÓN: LOS MORTEROS DE REPARACIÓN
Los morteros para la obra de restauración del Retablo se diseñaron en base a las características de los morteros originales, teniendo en cuenta la compatibilidad con los materiales existentes y también las características del alabastro.
Para la ejecución de ensayos de compatibilidad con la piedra en los morteros de reparación, se tallaron probetas de alabastro de 40x40x160 mm, con el The mortars used in the Altarpiece before the present work, and which belong to later repairs, show greater dispersion of characteristics between each other.However, there are similarities between various mortars that have the same function of patching and completing joints, located at different heights of the main body and even in the predella.
The composition of these mortars is lime, now in the form of carbonates, and gypsum, but differing from the mortars considered as original in that the proportion of lime in this case is greater, and a larger content of silicates is detected in the mortar as a whole.In many cases the colour is greyish, particularly on the surface, with the appearance of Portland cement but has not been the object of this study and corresponds to work carried out in the sixties (2).Finally, in some replacements mortars have been found with a gypsum base as almost the sole component.
In the external treatments of the mortars studied, the existence of fats reacted with lime have been found, in greater or lesser proportion, with in general an abundant presence of fatty acids.All the patinas are quite possibly old (6), with the sole exception of the appearance of a fairly recent soap, and also the presence of resin is detected (rosin or rosin pitch).In the organic based mastics used to cover cracks, oils and resins are also detected.
Finally a study was made of the residue obtained from the cleaning of the alabaster of the Altarpiece as required by the restoration.It was found that acrylics or similar were not detected.The presence of fats, resins and, possibly, waxes was found, reacted with lime to a large degree.The tests were carried out by TLC (thin-layer chromatography) (8) and the presence of soaps and résinâtes was confirmed.

APPLICATION OF THE RESULTS OF THE RESTORATION: THE REPAIR MORTARS
The mortars for the restoration work on the Altarpiece were designed based on the characteristics of the original mortars, taking into account their compatibility with existing materials and also the characteristics of the alabaster.
To carry out tests for compatibility with the stone in the repair mortars, alabaster test specimens were taken 40x40x160mm, of material from the quarry material procedente de cantera seleccionado para las reparaciones, para efectuar en ellas la evaluación de las propiedades del material pétreo.Se obtuvieron datos sobre el comportamiento mecánico-resistente de la piedra de alabastro a utilizar en la restauración, previamente al diseño de los morteros de reparación, alcanzando los siguientes valores: 10,0 MPa para la resistencia a flexotracción y 24,5 MPa para compresión.
Para constatar las propiedades del mortero de reparación base, compuesto de yeso, cal y árido fino, y con objeto de seleccionar las proporciones adecuadas se realizaron también ensayos de adherencia entre los morteros y la piedra alabastro, comparando los resultados de la resistencia a la tracción para diferentes tipos de morteros.El equipo empleado fue Instron Universal Testing Instrument, floor model (TT) 1114 selected fi)r the repairs, to evaluate on them of the properties of the stone material.Data were obtained on mechanical performance of the alabaster stone to be used in the restoration, prior to designing the repair mortars, and reaching the following amounts: 10.0 MPa for flexural and 24.5 MPa for compressive strength.
The selected composition for the restoration mortars, based on mortars already found in the Altarpiece, was gypsum with a low proportion addition of lime to obtain, among other advantages, greater plasticity which was particularly important when filling the fine joints, improved workability and also adherence behaviour which was of great interest in view of the low values obtain in this test with mortars of gypsum only.
Regarding aggregates, granulometry was held at mixtures of sizes between 0.08 mm and 0.16 mm, according to the thickness of the joints or items to be repaired.In the case of small fissures and repairs the larger sizes were discarded.The dosage of the mixture to be used varied according to the application (Figs. 4 and 5).
In order to define the properties of the base repair mortar, composed of gypsum, lime and fine ^gg^^gcite, and to select the appropriate proportions, adherence tests were also carried out between the mortars and the alabaster stone, comparing the tensile strength for different types of mortars.The equipment used was an Instron Universal Testing Instrument, fioor model (TT) 1114 (1175) dotado de un sistema electrónico de pesada, provisto de células de carga y de un sistema de detección para la determinación de la resistencia la tracción y a la compresión de los materiales.Para ensayar los distintos morteros se prepararon cubos de alabastro de 40 mm de lado, uniéndolos de dos en dos mediante una capa de cada uno de los morteros a ensayar con un espesor medio de 65 mm.Los valores obtenidos con mezclas de yeso y áridos proporcionaron valores muy bajos.Se constató cómo el alabastro empleado como material soporte ofrece, en todos los casos, resultados bajos de adherencia.Si la proporción de árido en los morteros es menor la adherencia del mortero a la piedra alabastro se incrementa.La zona de rotura en el sistema alabastro + mortero + alabastro se produce en la interfase entre el mortero y la piedra, no provocando, en ningún caso, la rotura de la piedra.Por ello, los morteros, aunque débiles en su adherencia, resguardarán sin daño la piedra labrada de alabastro del Retablo.Eventualmente se podrian utilizar aditivos apropiados para mejorar la adherencia de los morteros.

AGRADECIMIENTO
Los autores agradecen a Olga Cantos y a Ana Laborde, del Instituto de Conservación y Restauración de Bienes Culturales (ICRBC) (Ministerio de Cultura) la participación en el Proyecto, a Mercedes Nuñez su colaboración en el estudio y a la empresa P3, S.A. y Dirección General de Patrimonio del Gobierno de Aragón la financiación de la investigación, así como a la Fundación Nueva Empresa y al Cabildo Metropolitano de Zaragoza.
(1175) with an electronic weighing system, with loading cells and a detection system to determine tensile and compressive strength in the materials.In order to test the various mortars, alabaster cubes 40 mm long were prepared, joining them two by two using a layer of each one of the mortars to be tested, with an average thickness of 65 mm.The results obtained with mixtures of gypsum and aggregates gave very low amounts.It was found that the alabaster used as support material offered low adherence results in all cases.If the proportion of aggregate in the mortars is lower, the adherence between the mortar and the alabaster stone increases.The fracture zone in the alabaster + mortar + alabaster system occurs in the interface between the mortar and the stone, and in no case led to the failure of the stone.Therefore although their adherence is weak the mortars will hold the carving alabaster stone of the Altarpiece without damaging it.Appropriate additives could be used eventually to improve the adherence of the mortars.
To evaluate the characteristics of the mortars according to the type of aggregates, mixtures were made with different dosages of aggregates which in turn different in composition.On the one hand aggregates were used from crushed alabaster and the results were compared with siliceous sand, with the result that mixtures of gypsum (E-35) and siliceous aggregate (standard sand according to , prepared in a ratio of 1:1 gave low mechanical strength (3.4 MPa for flexural and 9.8 MPa for compressive strength), although 4 times greater than those in which the aggregate selected came from alabaster stone.In any event the mechanical strength of the prepared mortars was lower than those obtained with alabaster stone, which guaranteed their compatibility.Therefore the choice of mortars was made from those with lower compressive strength than that of alabaster stone, to ensure the restoration did not damage the sculptured work

Figure 1 .
Figure 1.-Composition of a joint mortar from alabaster Altarpiece.Identification of gypsum and calcium carbonate by IR spectroscopy.

Figure 2 -
Figure 2-Woody fibers in the bottom mortar of the Altarpiece (optical microscopy, x 10.7).

Figure 4 .
Figure 4.-Central zone of the Altarpiece in 1922.Detail of the old cracks Sculpture of san Marcos in a pillar (3).

Figura 5 .
Figura 5.-Detalle del cuerpo central del Retablo en 1993 durante la restauración.Imagen de San Marcos en una de las pilastras.Figura 5.-Detail of the central parí of the Altarpiece in 1993 during the restoration.Sculpture of San Marcos in a pillar.