An old French medieval saying goes: “Des goûts et des couleurs, on ne discute pas”: one doesn’t argue about taste and colors. The saying is generally used within the context of differing opinions. Most of us would agree that everything is a matter of taste. And we also know that whenever opinions can collide, they will collide. The challenge is the outcome … any gain or only casualties? If it is because of such differences that conflicts arise, it is also due to such differences that progress is possible.
When one considers the wars that have plagued mankind over the centuries, uncompromising clashes of opinions have essentially been responsible. There exists another type of warfare, one that is carried out not on the battlefield, but in old churches between architects and art restorers. In this case, the casualties are not human lives, but include the frescoes of Michelangelo and the wall paintings of the Ming Dynasty. In a world that is becoming more technological and versatile by the day, many architects opt for the usage of modern materials such as cement and glass for their constructions. On the contrary, art restorers strive to secure the conservation of historic materials, such as lime-based mortars serving as surfaces for frescoes.
These two approaches collide when the task is to incorporate new materials into historic structures. The mechanical resistance of cement, for instance, shows an increase in both flexural and compressive strength, by far exceeding that of historic mortars based on limestone or pozzolanic components. Both cement and lime-based mortars can efficiently absorb the mechanical vibrations caused by traffic to a building that has been constructed solely with either the one or the other. The problems have arisen when both have been used contemporarily in a same building.
The church of Sant’Alessandro di Lasnigo, northeast of Como in northern Italy, suffers from mild, but regular traffic on the Via Provinciale connecting the historic towns of Lasnigo, Bellagio and Canzo. Due to fungal deterioration of the church’s interior wooden ceiling, the roof has progressively been losing proper support. Small cracks have long been forming in the southeastern wall. The damaged area was eventually reinforced with cement containing calcium chloride (CaCl2) – a salt often added by architects in order to increase the hardening rate of the mortar without affecting its positive properties such as strength and mechanical resistance. The restorers, however, did not approve of this choice.
The fungal deterioration of the wooden beams supporting the roof had also caused fissures in the upper extremity of the wall, creating an inlet for rain. One of the fissures exceeded 2 m in length, and stretched across a fresco wall-painting. The unknown artist had used azurite – a deep blue pigment comparable to the more modern ultramarine – to color the garment of his featured Madonna. Over time, rain water oozed through the entire wall, dissolving the calcium chloride contained in the more recently applied cement. When the soluble salt ions finally reached the fresco beneath, a chemical reaction ensued that caused the azurite to turn into a bright green compound known as malachite.
The chemical formulas of azurite and malachite are very similar. Both minerals consist of copper, carbonate and hydroxide units. Azurite is known to be highly sensitive to both rain and saltwater. Under such conditions, some of its carbonate units are replaced by hydroxide units, thereby altering the balance between the two and transforming the mineral into malachite. In the case of the fresco, the chloride ions had served as a catalyst in the chemical reaction. Similar mishaps have been observed for Michelangelo’s frescos in the Sistine Chapel, and the wall paintings of Central China dating back to the Ming und Sung dynasties.
An old French medieval saying goes: “Des goûts et des couleurs, on ne discute pas”. But Michelangelo and the Madonna of Sant’Alessandro draped in her – now – green garments would surely beg to differ.