It is known that the physical properties and the conservation rate of a wooden artefact are strictly related to its surface treatment, especially in order to control mechanical and rheological properties and to increase the durability of the wood. The pictorial film of painting forms a structure that, adhering to the wood surface, plays both a protective and an aesthetic role. In order to restore artefacts, it is important to know the original function of the external coating. The aim of this paper is to define the nature and the function of a white coating covering all the surface of a bas relief made by Giacomo Bertesi, one of the most important artists and wood carvers in northern Italy, in the second half of the seventeenth century. Three microsamples were taken from the surface. Micro-Fourier transform infrared and scanning electron microscopy-energy dispersive X-ray spectroscopy analyses were performed to characterise the materials and the different layers.