The reddish-brown, brown or yellowish stains of circular or irregular shape known as foxing spots have been fully described in conservation literature but still, this phenomenon does not find any scientific agreement since many hypotheses have been raised concerning their origin. In this work a contribution to foxing definition not only focussed on its appearance but also reported on its chemical information. For this purpose foxing stains present in drawings from two Portuguese artists dated from the eighteenth to nineteenth centuries were observed under ultra-violet light and optical microscope and analysed by three non-invasive spectroscopy techniques. The observations carried out on the stains provided information on their surface morphology. The use of energy-dispersive X-ray fluorescence revealed a variation on the elemental content between foxing and paper region. Although the results from X-ray diffraction analysis showed no signs of cellulose degradation in foxing stains, Fourier-transformed infrared analysis revealed the presence of oxide groups. Both the information on the chemical nature and surface morphology of the stains achieved in this study will contribute to increase foxing formation information and develop future protocols for conservation purposes.
This work comprises the use of a multi-analytical approach combined with microbiological studies to characterize six paper samples, containing foxing stains, from the 20th century, regarding their cellulose matrix, fillers, and sizing materials, and to evaluate possible paper degradation that might have occurred during the foxing stains. Photography under different illuminations and optical microscopy were used for morphological characterization of the paper samples and foxing stains. Scanning electron microscopy coupled energy dispersive spectroscopy (SEM-EDS) was of particular importance for defining the presence of fiber disorder and disruption on the surface of some of the stains, and localized accumulations of mineral-like particles on the surface of others. SEM-EDS, attenuated total reflection Fourier transform infrared spectroscopy (ATR-FT-IR), and energy dispersive X-ray fluorescence (EDXRF) were used for the identification of mineral fillers, whereas sizing agents were analyzed using ATR-FT-IR. EDXRF results showed that no differences, within the standard deviation, were found in iron and copper contents between the foxed and unfoxed areas. Fungi belonging to the genus Penicillium spp. were found in all the paper samples. Unfoxed areas presented lower contamination than the foxed areas.
Three surveys on the occurrence of foxing stains were carried out on papers dating from 1560 to 1975 in three Portuguese collections. Foxed papers were found to be more intensely and intrusively stained in certain time periods of each collection. Based on historical data and on the professional paper conservation experience, the authors linked the increased occurrence of foxing stains in certain time periods to the new papermaking processes and materials, which began to be introduced in the late 18th century, and in particular to a synergistic effect between three factors: the use of deficient gelatine sizing which began to present a poorer quality and homogeneity, the presence of iron-containing impurities throughout the paper leaf and a distinct sorbency of moisture. Observing batches of morphologically similar papers by using photographic imaging with different types of light sources and incidences, the authors verified that similar papers present similar foxing stains.
Deterioration of graphical documents can occur naturally as a result of aging, but it can be accelerated by poor manufacturing constituents or conditions. One indicator of deterioration can be discoloration or staining of the surface [1][2][3]. Among these paper discoloration processes is foxing, which has been actively researched since the 1930s. Our attention herein is focused on trying to elucidate its causes and to establish protocols for detection, prevention and treatment. This foxing phenomenon occurs in the form of small isolated patches of discoloration that are typically rust, brown or yellowish toned and limited in size, with sharp or jagged edges; it is often punctiform and sometimes circular [4]. A long discussion about the origin of foxing has been undertaken, being the major causes attributed to fungal activity or to chemical origin. In fact, sometimes biological attack is the only factor observed, while in others metal-induced degradation is present and interaction between both factors can also be found [5]. During the last few decades, different analytical techniques have been used for the study of the foxing processes; nevertheless there is still much to be done [4,5]. Four different foxed papers from the XX century, labelled P1, P2, P3 and P7, were studied. Characterization of paper samples and morphological aspects of foxing was undertaken by analytical techniques and imaging: photography and macro-photography under standard light, UV/Vis fluorescence light and raking light; light microscopy; variable pressure scanning electron microscopy with energy dispersive X-ray spectrometry (VP-SEM/EDS) and attenuated total reflection Fourier transform infrared spectroscopy (ATR-FT-IR). Biological colonization was also studied in order to evaluate the influence of microorganisms in foxing. Photographic imaging allowed to evaluate the natural colours of paper samples and their state of degradation, namely, the presence of dirt, foxing stains, wear, and others (Table 1). The foxing stains of P3 sample were the only ones to exhibit fluorescence under UV light. Morphological aspects of the foxing stains were observed by SEM/EDS operating in variable pressure conditions (pressure of 20 Pa in the chamber). Two different typologies were devised. In samples P1, P2 and P3, fibre disruption was observed in the foxed areas (Figure 1), when compared with the non-affected areas of those samples. As to sample P7, a localized accumulation of particles was observed in the foxed areas, which were confirmed to be a calcium salt due to Ca detection by EDS analysis. Different fillers used in paper production were also estimated through EDS. ATR-FT-IR complemented the study and was particularly adequate to evaluate the existence of biotic attack in the foxed areas of all the papers. ATR-FT-IR was also able to supply information about the fillers and the FT-IR bands related to cellulose suggesting alterations of the cellulose structure in the foxed areas (Figure 2). Fungi belonging to the main genera Penicillium sp. were iso...
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