The spatially resolved characterisation of Egyptian blue, Han blue and Han purple by photo-induced luminescence digital imaging
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- Verri, G. Anal Bioanal Chem (2009) 394: 1011. doi:10.1007/s00216-009-2693-0
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The photo-induced luminescence properties of Egyptian blue, Han blue and Han purple were investigated by means of near-infrared digital imaging. These pigments emit infrared radiation when excited in the visible range. The emission can be recorded by means of a modified commercial digital camera equipped with suitable glass filters. A variety of visible light sources were investigated to test their ability to excite luminescence in the pigments. Light-emitting diodes, which do not emit stray infrared radiation, proved an excellent source for the excitation of luminescence in all three compounds. In general, the use of visible radiation emitters with low emission in the infrared range allowed the presence of the pigments to be determined and their distribution to be spatially resolved. This qualitative imaging technique can be easily applied in situ for a rapid characterisation of materials. The results were compared to those for Egyptian green and for historical and modern blue pigments. Examples of the application of the technique on polychrome works of art are presented.
KeywordsArcheometry/fine artsFluorescence/luminescenceEgyptian blueHan blue/Han purpleImagingInfrared
Photo-induced luminescence imaging
The study of luminescence can provide important information used in a variety of fields, ranging from physics to medicine, from forensic applications to the conservation of cultural heritage . A particular application in the last of these fields is the focus of the present study. Photo-induced luminescence photography or imaging records the steady-state emission from a chemical compound continuously excited by electromagnetic radiation. The term ‘imaging’ is used hereafter to refer to both traditional photographic processes and digital imaging techniques. However, in this study, all the images were made using a digital camera. Generally, imaging techniques are crucial in the characterisation of original materials in works of art as well as of past conservation treatments since they can provide spatial information on the distribution of different materials.
Among photo-induced luminescence imaging techniques, ultraviolet (UV)-induced luminescence imaging in the visible (VIS) range is the most commonly exploited technique for the examination and characterisation of painting materials such as organic binders and varnishes .
Although much less common than UV-induced luminescence, VIS-induced luminescence imaging is a non-invasive investigative technique that can help in the characterisation of painted surfaces. In this case, the excitation wavelength(s) lies in the visible range and is governed by the types of radiation sources used. A few inorganic pigments, among which are Egyptian blue, Han blue, Han purple, cadmium red and cadmium yellow, show strong photo-induced luminescence properties, with emissions in the IR when excited in the visible range [3–8].
The present study is intended to provide a qualitative, non-invasive, inexpensive and portable tool for spatial localisation of Egyptian blue, Han blue and Han purple on works of art. Several light sources were tested with the aim of identifying an experimental setup, which could be used in a museum environment as well as in situ on historic or archaeological sites. In general, the nature of museum objects is such that sampling is always kept to a minimum and is often unacceptable. Similarly, large, immobile or difficult-to-access artworks such as wall paintings or statues may be challenging to analyse. A method such as this therefore has many advantages in this field.
The photo-luminescent properties of Egyptian blue, Han blue and Han purple pigments
Egyptian blue is one of the earliest known synthetic pigments. The pigment, a calcium copper tetrasilicate (CaCuSi4O10) corresponding to the rare natural mineral cuprorivaite, was extensively used from the fourth dynasty in Egypt (c. 2500 bc) up to the end of the Roman period in Europe and beyond (c. 800 ad) . Han blue and Han purple, both artificial pigments, the synthetic barium copper tetrasilicate (BaCuSi4O10) and barium copper silicate (BaCuSi2O6), respectively, were found on Chinese artefacts from the Han dynasty (208 bc–220 ad), hence their names [10, 11]. All three pigments show strong luminescence properties. When excited in the blue, green or red range of the electromagnetic spectrum, Egyptian blue shows an intense and broad emission (full width at half peak height of c. 120 nm) in the IR range, centred at about 950 nm. Han blue and Han purple emit photons in the IR range (c. 980 nm) when excited in the visible range. According to Pozza et al. , the absorption and emission spectra of synthetic cuprorivaite show three different transitions from the ground to the excited states (2B1g → 2B2g, 2Eg and 2A1g); the transitions are attributed to Cu2+ ions, which are the only photo-active units. The emission spectrum is related to the lower energy electronic transition (2B2g → 2B1g), which is symmetrically prohibited. The differences between the emission spectra of the pigments can be attributed to a ligand-field change . In contrast, Egyptian green (parawollastonite, a CaSiO3 polymorph with as much as 2% copper as an impurity), which is manufactured in a similar way to Egyptian blue and often occurs in the same contexts, does not seem to show luminescence properties. The colour of Egyptian green, which is often very similar to that of Egyptian blue, is due to a silica-rich copper glass .
Other commonly used historical blue pigments such as indigo and the naturally occurring mineral lazurite have been reported to show luminescence properties when exposed to visible light. The emission of indigo is reported to be at c. 750 nm while that of lazurite at c. 830 nm . However, as shown in the following chapters, their emission—at least when captured with the experimental setup presented in this paper—is negligible (not observable) when compared to that of Egyptian blue, Han blue and Han purple.
Excitation: Two identical radiation sources (fluorescent, LED, tungsten-halogen or Wood's light, as discussed above) symmetrically oriented at approximately 45° to the camera focal axis.
Emission: The luminescence emission from the pigments is captured in fully manual mode using a Canon 40D camera that had been modified by the removal of the inbuilt IR-blocking filter. Once the filter is removed, it is possible to select the spectral range under investigation by placing a filter or a set of filters in front of the lens. The camera is sensitive to approximately 1,000 nm, and the lower wavelength cut-off is selected by use of an appropriate filter—in this case, a Schott RG830 with a cut-on (50%) at 830 nm (see Fig. 2). This gives an IR sensitivity range of approximately 800–1,000 nm in the red channel, the information from which is extracted and represented as a grey scale image. The gamma response of the camera was not corrected for this study.
Reference grey scale: A set of Lambertian grey references is placed in the same plane as the object under investigation . These Spectralon® references, manufactured by Labsphere, have certified flat reflectance properties in the UV–VIS–IR spectral range under investigation and do not show luminescence properties—a crucial property for this study. The reference grey scale should always contain a 99% reflectance standard, as the evaluation of the presence of luminescence is carried out by comparison with such a reference.
List of the radiation sources tested
Dulux 36W 12-950
Philips TLD 58W/35
MASTER TLD 58W/840 Super80
Warm white de luxe
Blue (major peak at c. 421 nm)
Green (major peak at c. 528 nm)
Red (major peaks at c. 619 nm and 700 nm)
Yellow (major peak at c. 586 nm)
Blue (peak c. 470 nm)
Green (peak c. 525 nm)
Red (peak c. 629 nm)
Yellow (peak c. 594 nm)
Non-visible (peak c. 365 nm)
The radiation sources (fluorescent, LED, tungsten-halogen and UV) were tested on the pigment test-set keeping the distance of the lamps from the object and the capture parameters for the camera fixed. A visible (400–700 nm) image of the objects was also always taken in order to allow comparisons. For this study, all images were acquired as raw images and transformed into 4,256 × 2,848 pixel resolution images in 16-bit.tif (tagged image file) format with Canon dedicated software. The spatial resolution of the imaging system is dictated by the optics used and is, in this case, approximately 100 μm.
Results and discussion
Among the blue pigments analysed in this study, Egyptian blue, Han blue and Han purple were the only ones showing luminescence emission in the 800–1,000 nm range. Therefore, the following discussion will be related to those pigments only. The images relative to the other pigments will not be reported here.
For the setup described above, the excitation wavelength(s) (λexc) and stray IR radiation from the source reach the surface of the object under investigation. While part of the exciting radiation is absorbed by the particles of the luminescent pigments and re-emitted as IR radiation, part will be reflected, transmitted or absorbed according to the optical properties of the pigments. Similarly, the stray IR radiation will be reflected, transmitted or absorbed by the surface. During capture, the reflected exciting radiation will be blocked by the RG830 filter in front of the camera, while both luminescence and reflected stray IR radiation will be recorded by the digital sensor to generate the photo-induced luminescence image in the IR range. Assuming a uniform distribution of the incoming radiation, the reflective properties of the 99% reflectance standard allow an accurate estimation of the intensity of the stray IR radiation generated by the source or present as ambient radiation in the dark room where the measurement is undertaken; as the targets do not show luminescent properties in the range under investigation, the average grey level measured on the 99% reflectance standard represents the intensity of the stray IR radiation. Therefore, in the photo-induced luminescence images, the presence of ‘real’ luminescence is evaluated by comparing the sample under investigation and the 99% reflectance standard; as the reference standard is non-luminescent and reflects 99% of the incoming radiation, any grey values exceeding that measured on the 99% reflectance standard can be considered as real luminescence from the sample. In other words, luminescence will appear as ‘glowing white’ .
LED radiation sources
Figure 2 shows the uncalibrated and normalised emission spectra of the LED radiation sources used in this study. The LEDs show no emission in the IR range. Figure 1b shows the VIS-induced luminescence image of the samples of Egyptian blue, Han blue, Han purple and Egyptian green (λexc = red LED). Only the red-induced luminescence image is reported as the images obtained using the blue, green and yellow LED sources showed comparable results. In Fig. 1b, the 99% reflectance standard appears black (background grey level), indicating that there is no stray IR radiation in the region under investigation. Grey levels greater than zero are therefore to be considered luminescence generated within the samples by the excitation wavelength(s). All the LED radiation sources tested excited a strong luminescence in the Egyptian blue, Han blue and Han purple specimens, which appear ‘bright white’. By contrast, with the setup described in this paper, none of the other blue pigments showed any appreciable emission and appeared black (background grey level); for this reason, their images are not reported. For the three pigments under consideration, the relatively narrow blue, green, yellow and red bands generated by the LED sources proved that the luminescence can be efficiently excited anywhere in the visible range. Although intense and easily detectable, the emission generated by the sample of Han blue is less intense than that of Egyptian blue and Han purple. The behaviour of the Chinese blue pigment will be further investigated in the near future. In all cases, as the emission by the pigments is intense and the spatial resolution of the camera is high, even single particles of pigment can be observed. This aspect of the technique will be exploited in the investigation of the case studies presented below. In addition, Fig. 1b confirms that there is no IR emission by the LEDs and that the grey scale reference does not show any luminescent properties in the range under investigation. The use of visible LED radiation sources may prove useful to provide a preliminary, non -invasive indication of the spatial distribution of Egyptian blue, Han blue or Han purple and to distinguish their presence from that of all the other blue pigments investigated in this study.
Fluorescent radiation sources
Figure 3 shows the uncalibrated emission spectra of a selection of fluorescent light sources tested for this study. All fluorescent radiation sources tested showed some emission in the IR range. However, such emission was found to be small when compared to that in the visible range. Figure 1c shows the photo-induced luminescence image of the set of pigments obtained using a Philips TLD 58 W/35 fluorescent lamp. The Philips TLD 58 W/35 fluorescent lamp was also used in some of the case studies described below. Although there were different amounts of stray IR, all the fluorescent light sources tested in this study behaved similarly. Therefore, only one example is reported here. In Fig. 1c, the grey scale shows levels greater than zero. This confirms that fluorescent lamps emit stray IR radiation. However, the grey levels observed for the samples of Egyptian blue and Han purple are higher than that of the 99% reference standard. This behaviour implies that the emission of the pigments, when excited by fluorescent lamps, is stronger than the stray emission of the fluorescent lamps in the IR range. By contrast, the grey levels for the samples of Han blue and Egyptian green (and of all the other blue pigments) were found to be less than that for the 99% reference standard and cannot therefore be attributed to luminescence, but rather to IR-reflected radiation. In some cases, as described below, the presence of a small amount of stray IR radiation from the source may be considered beneficial, allowing the user to observe where the pigments are situated within the scheme of the work of art more easily.
Tungsten-halogen radiation sources
The uncalibrated emission spectrum of the tungsten-halogen radiation source used in this experiment can be seen in Fig. 2. This type of radiation source emits a considerable amount if IR radiation. Figure 1e shows the IR image obtained when the tungsten-halogen lamp is used. In this case, all pigments showed levels of grey considerably lower than those on the 99% reference standard. As in the case of Han blue excited by a fluorescent lamp, while the visible component of tungsten-halogen radiation sources excites luminescence in the samples of Egyptian blue, Han blue and Han purple, the strong IR emission from the radiation source completely masks the luminescence. Therefore, the images recorded with the tungsten-halogen radiation source mainly illustrate the reflective behaviour of the pigments in the IR range (IR-reflected), while the images recorded using LED or fluorescent sources define the luminescence properties of the pigment in the same range (VIS-induced luminescence). It should be noted that in order to characterise the reflective properties of luminescent pigments in the IR range precisely, a radiation source that emits outside their absorption curve would need to be used; in this case, a source that emits only in the 800–1,000 nm range would be required.
Ultraviolet radiation sources
The uncalibrated emission spectrum of the ultraviolet radiation source used in this experiment is reported in Fig. 2. This ultraviolet radiation source does not emit IR radiation. Figure 1e shows the UV-induced luminescence emission in the 800–1,000 nm range. As can be observed on the 99% reflectance standard, there is no stray IR emission from the excitation source. UV radiation (λexc = 365 nm) seems capable of exciting a strong luminescence emission in the Han purple sample, but not in the Han blue powder. A faint luminescence can be observed in the Egyptian blue specimen.
The use of LED radiation sources excites an easily detectable luminescence in Egyptian blue, Han blue and Han purple samples, while the absence of IR straylight allows an easy preliminary characterisation of this group of pigments and their distinction from a variety of historical and modern blue pigments.
The use of fluorescent light sources with a limited emission of IR radiation can be used to excite luminescence in Egyptian blue and Han purple. In this case, the small amount of stray IR radiation allows the user to locate the presence of these pigments easily within the scheme of a work of art.
When UV radiation sources are used, Han purple is excited. Egyptian blue shows a weak emission, while there is no detectable emission from Han blue.
When tungsten-halogen radiation sources are used, the luminescence is completely masked by a high IR background. Pure IR emitters should be used to capture ‘true’ IR-reflected images.
In conclusion, when a preliminary, non-invasive characterisation of Egyptian blue, Han blue and Han purple is sought or when single grains of the pigments are to be characterised, the use of LED radiation sources is recommended. When the spatial distribution of Egyptian blue and Han purple on large or difficult-to-access artefacts is of interest, the use of fluorescent radiation sources is probably most practical since, as discussed above, the presence of a small amount of stray IR radiation allows the user to locate the luminescent pigments within the scheme of the work of art easily (see examples below). Therefore, a possible sequence of investigations may start with LED sources and, once the presence of one of this group of pigments is ascertained, could move subsequently to fluorescent radiation sources.
It should be underlined that organic materials such as binders or varnishes are not expected to show luminescence properties comparable to those of Egyptian blue, Han blue or Han purple. Many complex biopolymers such as amino acids, oils and resins emit distinctive visible wavelengths when excited in the UV [4–6]. However, the luminescence properties in the IR range of painting materials other than the above-mentioned pigments are currently under investigation.
This section illustrates examples of the application of the methodologies described above to real case studies. The details of the experimental setup used in each case are reported in the figure captions. All pigment identifications were confirmed by Raman spectroscopy.
The photo-induced luminescence of Egyptian blue, Han blue and Han purple can be easily recorded by means of IR imaging. The equipment required consists of commercially available visible or UV LEDs or, in the case of Egyptian blue and Han purple, fluorescent light sources, a cut-on IR filter and a digital camera capable of recording IR radiation in the 800–1,000 nm range. The strong response to visible radiation of these pigments allows the characterisation of their spatial distribution and the detection of small quantities of these pigments within a painted scheme. This methodology may allow the distinction between Egyptian blue and Egyptian green, a pigment which can often show similar reflective properties in the visible range. The setup required for the measurement is relatively inexpensive and can be easily used in a museum environment as well as in situ. Large objects, which cannot be moved (e.g. wall paintings or statues), or rare works of art, which cannot be sampled, can be usefully investigated by this portable, inexpensive and non-invasive investigation technique, which, although not intended as a substitute for analytical investigations, offers a valuable tool in the understanding of ancient polychromy. In addition, imaging techniques such as that introduced here can inform sampling strategies.
Further avenues of investigation include the measurement of the quantum yield, lifetime and absorption spectra of Egyptian blue, Han blue and Han purple and investigations of the behaviour of Egyptian green. The luminescence properties of other pigments, binders and other painting materials in the IR range are also under investigation.
The pigments analysed were artificial ultramarine, atacamite, azurite, blue verditer, cerulean blue, chrysocolla, cobalt blue, dioptase, indigo, lazurite from lapis lazuli, manganese blue, Maya blue, Prussian blue, monastral blue, shellfish, smalt and vivianite.
This study was partially sponsored by the Andrew F. Mellon Foundation and was made possible thanks to the invaluable collaboration of the scientists, conservators and curators of the British Museum, the Courtauld Institute of Art and the National Gallery in London, particularly David Saunders, Janet Ambers, Catherine Higgitt, Tracey Sweek, Michelle Hercules, Thorsten Opper, Peter Higgs, Sharon Cather, Maram Na'as and Marika Spring. Dr Emily Murray kindly provided a sample of shellfish lake. Pro-Lite Technology LLP kindly provided samples of the Spectralon® references.