Blanching of paint and varnish layers in easel paintings: contribution to the understanding of the alteration

Abstract

The blanching of easel paintings can affect the varnish layer and also the paint layer with a blurring effect. The understanding of the physicochemical and optical phenomena involved in the whitening process remains an important challenge for the painting conservation. A set of ca. 50 microsamples from French, Flemish, and Italian blanched oil paintings, from sixteenth to nineteenth century, have been collected for in deep investigations. In parallel, the reproduction of the alteration was achieved by preparing some paint layers according to historical treatises and altering them in a climatic chamber in a humid environment or directly by immersing in ultrapure water. The observation of raw samples with a field-emission gun scanning electron microscope revealed for the first time that the altered layers have an unexpected highly porous structure with a pore size ranging from ca. 40 nm to 2 lm. The formation mechanism of these pores should mostly be physical as the supplementary analyses (Fourier transform infrared spectroscopy, gas chromatography-mass spectrometry) do not reveal any noticeable molecular modification. Considering the tiny size of the pores, the alteration can be explained by the Rayleigh or Mie light scattering.

On the stability of mediaeval inorganic pigments: a literature review of the effect of climate, material selection, biological activity, analysis and conservation treatments

“…They were as well used as lake substrates [74]. Green earth oil layers exposed to humidity and heat showed discolouration, pinpointing differences based on the binder composition, and ageing conditions [75].…”

Partially perfluorinated derivatives as powerful tool for the restoration of blanching easel paintings

“…A series of paint mock-ups were made according to the recipe reported in [5], which successfully reproduces the blanching after artificial aging. Binder was prepared from walnut oil , water, and PbO in the mass proportions 10:10:1.…”

“…Blanching as an alteration in easel painting, is induced by excessive moisture or aqueous conservation treatments, affecting natural varnish layer and oil paint layer [2][3] [4]. Recently, Genty-Vincent et al revealed by field emission gun scanning electron microscopy (FEG-SEM) the highly porous structure in the altered paint and varnish layers, with pores ranging in size from few nanometers to few micrometers [5] (Fig. 2).…”

Warm white LED lighting in museums: Ageing effects on terpenoid resins assessed through Raman spectroscopy and chemometrics

“…Natural resins and varnishes used in paintings as adhesive and protective films consist of terpenoid polymers, which undergo natural deterioration over time. The ageing process can affect the integrity of the artefact due to the formation of cracks and detachments in the pictorial layers or cause changes and dulling of the colours, which can definitely alter the aesthetic perception of the work of art . Among the most common deterioration mechanisms (i.e., thermal, hygrothermal, chemical, and mechanical), photochemical ageing is the most widely studied and recognised .…”