AIC's 52nd Annual Meeting

The revenues from the salt trade were of great significance to the Bavarian electors until the late 18th century. The 'white gold' was transported by waterway from Hallein and Berchtesgaden to Stadtamhof near Regensburg, with so-called 'salt ship trains' being used on the Danube. These were convoys of ships specially designed for transporting salt, which were pulled upstream with the help of horses, a process known as 'towing'. The only known sculptural representation of a Palatinate salt ship convoy is housed in the Bavarian National Museum (BNM) in Munich. The group of objects has so far been dated to the second half of the 18th century. As an object, it is culturally a piece of regional historical significance.

This representation of a salt ship convoy is made of glazed ceramics, leather, metal, and wood. Initial investigations revealed a notable pattern of damage on all figures and ships: sporadically growing "needles" from the surface. Initially, it seemed likely that these could be salt efflorescences, but a closer examination held a surprise. The white material always had its origin in a metallic core. Knowledge of the exhibition conditions during recent years led to the assumption that the white material is indeed lead corrosion, which was confirmed by SEM-EDX and X-ray diffraction. The lead had reacted with formic and acetic acid emissions from display cases to form lead hydroxycarbonate (lead white) and lead formate. The question of the origin of the lead accumulations was investigated through CT scans. It became evident that the lead did not originate from impurities in the body but was formed from the lead glaze during firing.

The restoration of the salt ship convoy involved dealing with various options for treating active lead corrosion and discussing their applicability to an object made of mixed materials. The decision was made to mechanically remove the lead corrosion products and apply a protective coating to the lead accumulations. Aspects of preventive conservation were also considered because the new display case is made again of wooden materials that can lead to an accumulation of acetic acid. Therefore, options for retrofitting an existing display case to reduce emissions are also being discussed.