Carracci’s frescoes under the microscope

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Now, that the exhibition of the frescoes of the Herrera Chapel by Annibale Carracci has just finished, we take the opportunity to share some of the experiences and information, the result of observing samples of these paintings under the microscope.

Usually, the magnification of the combinations of pigment particles in works of art provide images of evident chromatic plasticity. And Carracci’s frescoes are no exception. Quite the opposite. The distribution of the mixtures that he and his assistants used to achieve each shade give rise to microscopic compositions worthy of admiration.

The superimposition of the different layers or strata that a painter generates during the process of creating a work is very well documented in the cross-sections of painting samples, that is to say, a tiny fragment of the materials, cut in such a way as if it were a cake, and seen through the magnifying lenses of a microscope. Although stratigraphies are a recurring tool in scientific analysis, which allow us to identify the materials used, layer by layer, particle by particle, they also, as we say, generate images that, when we discover them for the first time, we feel very lucky to have the work we do, for the opportunity of a “second” aesthetic enjoyment, this unpublished one, of the exhibited works. If you want, just “pour le plaisir des yeux”, we invite you to see this compilation sequence of the cross-sections of one of the scenes of this exhibition: Apostles around the empty sepulchre of the Virgin.

Stratigraphic analysis of paint samples from the work, Apostles around the empty sepulchre of the Virgin

The original colours

However, microscopy images are not silent. They tell us about the geographical and chronological origin of the work. About the modus operandi of a certain group of painters or the particularities of the different painting techniques. In this case, for example, because they are paintings carried out between the 1604 and 1605, we identify the pigment lead tin yellow. This is a pigment linked to the glass industry used in Europe during a very specific period and one which would be forgotten, around 1750, when it was replaced by other yellow ones like the one from Naples. We also identify, widely used in this set, green earth, a pigment with the minerals celadonite and/or glauconite, of which the best-known historical site is probably Monte Baldo, near Verona, which explains why it is very recurrent, the green earth, in Italian painting. The fact that the binding agent in these paintings is calcium carbonate – as it corresponds to the fresco technique – explains the colour intensity of the smalt blue pigment that we see in the stratigraphic images. When this pigment is mixed with other binders such as oil, it soon degrades and loses its colour.

Detachment and transfer of the paintings. A levelling mortar is required

But these stratigraphic images tell us, especially, of the remarkable change in the material composition experienced, after the transfer to a new support, these frescoes detached from the walls in poor condition of the church of San Giacomo degli Spagnoli in Piazza Navona, Rome. In the catalogue of the exhibition, and also in this blog, the restorers Paz Marquès and Mireia Mestre explain how the Italian painter/restorer Pellegrino Succi intervened between 1833 and 1835 in the frescoes painted by Annibale Carracci and Francesco Albani.

Detachment by stacco causes the paint layer to separate from the wall, accompanied by a significant – but very variable – thickness of the original lime and sand mortar, which must be levelled, before continuing with the entire transfer process. To achieve this, the gaps were filled with a thin layer of materials mixed with lime, as we see in the different cross-sections in the two videos that we present to you. On the back of the layers, of a more or less equal thickness, to give more consistency to the detached fragments, layers of an adhesive or transfer mortar were added, as seen in the following video. We did not know, until now, if the lineage of estrattisti Succi already used for this intervention calcium caseinate adhesives, similar to those used by the restorers of Bergamo, successors of Secco Suardo, when in the years 1919-1923 they intervened the Romanesque paintings of the churches of the Pyrenees. The analyses have served to resolve this issue.

Stratigraphic analysis of a painting sample from the work Everlasting Father

Chemical composition of the transfer mortar

We have already explained in the blog of the museum, that, to transfer paintings during the 1919-1923 campaign, a mixture of lime and cheese was used, components that interact and form calcium caseinate, and strange as it may seem, it has the ability to stick. The lime that is in excess reacts with the carbon dioxide in the atmosphere and ends up being transformed into calcium carbonate, that is to say, it undergoes a carbonation similar to that of fresco painting. The cheese, on the other hand, apart from incorporating the casein protein, also leaves nodules of phosphates distributed throughout the layer of the mortar.

In the mortar used by Succi in the detached paintings from the Herrera Chapel, the majority compound is also calcium carbonate, although accompanied by plaster, as we have just seen in the video. It has been very interesting to identify the crystalline forms in which this calcium carbonate occurs. The carbonation of lime is a process that takes place in stages – something we want to highlight because it has not traditionally been taken into account – during which different phases of calcium carbonate are formed: first amorphous calcium carbonate, then vaterite, aragonite and finally calcite. If there is casein in the environment during carbonation, then the reaction becomes much slower and the initial phases of the process are stabilised, together with the calcite. So, if we tell you that in Succi’s mortar we have identified vaterite and amorphous calcium carbonate, you will agree to conclude that this is information points to a more than probable presence of casein in the mortar. And indeed, by liquid chromatography analysis we have confirmed the presence of two proteins, both casein and collagen. We associate the second protein with the use of animal glue and the first with the addition of milk to the mixture of lime and plaster to make the mortar. In this case, why are we talking about milk and not cheese? Well, because we didn’t find the phosphate nodules that we would find if it were cheese.

Image of the current exhibition at the Museu Nacional d’Art de Catalunya

Image of the current exhibition Annibale Carracci. The frescoes of the Herrera Chapel at the Museu Nacional d’Art de Catalunya

The transfer of mural painting in context

We would like to point out that the ingredients of the mortar used by Succi match those of the recipe that the Italian restorer Stefano Barezzi (1789-1859) used around 1820 to glue removed fragments of mural painting on wood. Barezzi used precisely a mixture of lime, chalk, milk and animal glue. The temporal proximity to Succi’s intervention suggests that he could have used an adhesive formulated quite similarly. Later, the restorer Secco-Suardo himself, in his manual from 1866, proposed a similar recipe with the ingredients lime, plaster, milk and glue, but he recommended it for oil painting transfers. The adhesive in question is obtained by mixing 100 g of casein, 170 g of gypsum, 180 g of lime and 25 g of animal glue. On the other hand, the recipe that Secco-Suardo advises for wall painting transfers, and that was adapted by the Italian restorers in the 1919-1923 campaign, contains 100 g of casein, 300 g of lime, 25 g of strong glue and milk added, with no plaster!

We can conclude that milk and, therefore, casein was already present in the transfer mortars used by Succi when he removed the frescoes from the Herrera Chapel, but that the recipe deviates from the one used in 1919-1923 for the Romanesque painting.

Knowing the details of the chemical composition of the new layers added to prevent the disappearance and give new life to the paintings, now allows us to design the optimal conditions to ensure their conservation. The secrecy that surrounded these interventions  has led to the fact that there is not a lot of contemporary documentary information and that, therefore, the answer lies directly in the materials themselves, which must be made to speak through powerful microscopic analysis techniques.

Related links

The mural painting transfer kitchen

The fresco paintings by Annibale Carracci and his assistants, conserved in the Museu Nacional: removal, transferral and shipping from Rome to Barcelona

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Restauració i Conservació Preventiva

Núria Oriols
Restauració i Conservació Preventiva

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