Salt efflorescence on leather objects from the Vasa ship

Paper in proceeding, 2023

The Swedish warship Vasa sank in Stockholm harbour in 1628. She was raised in 1961, excavated and conserved, and is displayed in the Vasa Museum in Stockholm. A large collection of leather objects was found on board, mostly personal belongings such as shoes and gloves, but also book covers, pump packings and others (Hocker 2018, 76). The objects excavated from the ship were conserved in a facility at Beckholmen, in Stockholm, continuously from 1961 to 1996.

During an inventory of the leather collection in 2017, salt outbreaks were noticed on some of the objects. A 2018 documentation of these salt outbreaks showed that, out of roughly 600 leather objects, over 100 were visibly affected. Those objects are partially or completely brittle, hard, fragile and often have a disturbing odour. The salt outbreaks are seen as coloured deposits, commonly red, yellow, white or grey, and often glitter. Salt outbreaks on wooden objects from the Vasa and on the ship itself have been well documented and researched, and were found to consist of various compounds of iron and sulfur (Hocker 2018, 105). While the salt outbreaks on the leather objects are similar in appearance and odour, their chemical composition was unknown.

A research project to identify the composition of the salt outbreaks on the leather was initiated in 2019. Samples of salts and leather were analysed by X-ray fluorescence (XRF), scanning electron microscopy with energy- dispersive X-ray spectroscopy (SEM–EDX), X-ray diffraction (XRD), and Fourier transform infrared (FTIR) and Raman spectroscopy. Samples from over 20 leather objects have been analysed, as well as samples from textile and wooden objects for reference and comparison. The focus of this article is the results of the analysis of four leather objects, as between them they show the full breadth of the characteristics of salt efflorescence.