Wall paintings: recovering lost colours is a matter of chemistry

The use of white, yellow and red lead-based pigments in wall paintings can be seen in various parts of the world and in different epochs. While their use was appreciated for their covering qualities, a tendency to darken often emerges that over time compromises the aesthetics and legibility of many wall paintings. Cimabue's Crucifixion in the Upper Basilica of Saint Francis in Assisi is the best known example. But cases are not lacking in Ticino. Patrizia Moretti is a researcher at the Institute for Materials and Construction at SUPSI-Department of Environment Construction and Design, where she is conducting a research project that aims to develop a method for the reconversion and stabilisation of altered lead-based pigments and thus recover the original colours of the works.

La Crocifissione di Cimabue nella Basilica superiore di San Francesco ad Assisi.

A method from which to start again

Towards the end of the 1970s, Prof. Mauro Matteini, a chemist at the Opificio delle Pietre Dure in Florence, developed a reconversion method based on the use of chemical reagents (hydrogen peroxide and diluted acetic acid) to recover the original colour of lead white (lead carbonate) altered to brown lead dioxide. However, Patrizia Moretti explains its limitations: "This method has been successfully applied on several wall paintings, demonstrating immediate recovery of the original appearance. However, in some cases, especially for paintings located in damp environments, the reconversion was only temporary and the browning recurred over time. This gave rise to the idea of trying to make it more stable. In this sense, our project aims to complement the treatment devised by Matteini, who is also part of our research group".

Stabilising the effectiveness of the treatment

Lead pigments were often used to impart colour to the flesh tones of characters' faces, but nowadays in many cases the darkening hinders their reading and visibility. This is what Patrizia Moretti - and her colleagues in the conservation and restoration department at the Materials and Construction Institute - noticed in the wall paintings of the Sacro Monte in Varallo, Piedmont. Here, during the 1990s, attempts were made to reconvert the altered lead-based pigments, which, although effective in the immediate term, did not 'hold' over time. "Our project aims to stabilise the reconversion treatment by applying phosphorus-containing compounds capable of forming lead phosphate - a compound that is also white in colour - but less soluble than lead carbonate and thus less prone to oxidation causing browning. Preliminary tests have shown that this is a promising method. We now need to conduct a more systematic study in the laboratory to implement the treatment and to confirm the efficacy of the process, so as to develop the most appropriate procedure for use in situ on wall paintings with lead-based pigments that have darkened".

Causes difficult to identify

Moisture seems to be one of the main suspects, but it is difficult to determine the exact causes behind the colour alterations. "Looking at the different case studies we can see that one of the main factors is dampness, but of course there are other causes, for example the alkalinity of plasters. We often speak of a 'concomitance of factors' and other research groups internationally are also trying to understand what these are. In this sense, our project also involves the Department of Environment Construction and Design's Institute of Microbiology, with which we want to assess whether the presence of specific microorganisms may also play a role in the darkening process of lead pigments".

Patrizia Moretti

On-site and laboratory experiments

The idea for this research project took its first steps at the Varallo site-school: "There, in 2017, we tested our stabilisation method for the first time, which is still effective today, the recovered white colour is stable; while other areas treated with the reconversion method alone have darkened again. This result prompted me to write a research project to participate in the Swiss National Science Foundation's Ambition competition, which I was able to acquire. The project started in 2023, in September, and at the moment we have completed preliminary studies on five case studies of altered mural paintings found in Ticino".
In parallel, the project team coordinated by Patrizia Moretti is working on the creation of painting models consisting of lead dioxide layers, on which various conversion and stabilisation methods will be tested. These tests will be useful to understand the most effective application methods and to optimise the method before applying it on site. The project has a duration of four years. "The tests performed on the pictorial models will then be subjected to accelerated ageing processes in the laboratory in order to assess their stability over time and to be able to select the most effective ones to be applied in situ during the second year. The other two years will be devoted to monitoring the applied treatments. The final aim of the project is to develop a treatment that can guarantee the stability of the reconversion over time and a monitoring methodology that can also be used in the following years".

The project led by Patrizia is called Darkened lead-based wall paintings: innovative treatments to stabilise the recovered original colours. It was the first at the Department for Environment, Construction and Design to be awarded the Swiss National Science Foundation's Ambition Grant, aimed at young researchers wishing to conduct an independent project at a Swiss higher education institution.

The project is being carried out at the Department for Environment, Construction and Design by a research team consisting of conservation scientists and conservators with expertise in wall paintings from the Institute of Materials and Construction (conservation and restoration sector) and biologists from the Institute of Microbiology, with the support and collaboration of the Office for Cultural Heritage of the Canton of Ticino. Involved as external collaborators are Prof. Mauro Matteini and other expert chemists studying pigment alteration processes from CNR-SCITEC (Perugia) and the University of Antwerp.