Fig 4 - uploaded by Lynn Chua
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Close-up images of the haze on Portrait of Mrs Tan Beng Wan. (a) 5-White haze at top left corner in background. (b) 6-White haze at left of chair. Upper row shows camera images. Bottom row shows digital microscopic images at 500x magnification.
Source publication
Insoluble and stable crusts, haze, and efflorescence that have developed on the surface of paintings can be visually
disturbing and are generally difficult to remove during conservation. Highlighted here are a pair of oil-based portrait paintings of the Peranakans Mr and Mrs Tan Beng Wan, dating to the late 19th to early 20th century, that presents...
Context in source publication
Context 1
... without risking damage of the underlying paint would be inevitable. The fact that both portraits were created using very thin paint, and with a uniform and flat painting technique, makes cleaning even more challenging, since any attempt to alter the surface would be easily noticeable. The haze on Portrait of Mrs Tan Beng Wan was less apparent (Fig. 4), but when viewed along the plane of the painting, the whitish haze could be seen tracing the painting's craquelure on the paint layer. ...
Citations
... The paint fragment consisted of different degraded compounds from zinc white oil paint. Previously, the surface haze developed on different parts of the painting had been analysed by transmission-FTIR microspectroscopy and were identified as zinc-type degradation products consisting of zinc soaps, oxalates, hydroxychlorides, sulphates, and carbonates, postulated to have formed at the paint's surface from the natural ageing of oil binder and interaction with atmospheric pollution [8]. Figure 1b shows the cross-section with a whitish haze over an underlying blue paint containing Prussian blue. ...
... The high water-solubility of zinc lactate implies that this painting is water-sensitive. Experimentally, this painting has exhibited paint loss when water swabs were applied in some areas and not others; however, the exact reasons for its water sensitivity are unclear [8]. Combining the results from ATR-FTIR and O-PTIR, it becomes clear that in some areas, zinc lactate is localised (Figure 2d) below the gordaite layer (Figure 3c). ...
ATR–FTIR (attenuated total reflection–Fourier-transform infrared) microscopy with imaging is widely used in the heritage field to characterise complex compositions of paint cross-sections. However, some limitations include the need for ATR crystal contact with the sample and the inability to resolve particle size below the IR diffraction limit. Recently, a novel O-PTIR (optical-photothermal infrared) spectroscopy technique claimed to open a new avenue for non-invasive, efficient, and reliable analysis at sub-micron resolution. O-PTIR produces transmission-like FTIR spectra for interpretation, without the need to touch the sample, which are highly favourable attributes for analysing heritage samples. This paper reports the comparison of O-PTIR and ATR–FTIR techniques applied to a cross-section embedding a thin paint fragment that delaminated from a late 19th to early 20th-century oil portrait. The hazy paint fragment consisted of zinc soaps (both crystalline and amorphous), gordaite (NaZn4Cl(OH)6SO4·6H2O), and zinc lactate, that could not all be well-resolved with ATR–FTIR imaging. With O-PTIR analysis, the degradation compounds could be resolved at sub-micron resolution with an equivalent or better signal-to-noise ratio. This case study shows how the two techniques can be used to obtain comprehensive information at a broad level with ATR–FTIR and a detailed level with O-PTIR
