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ATR-FTIR imaging of albumen photographic prints
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Journal of Cultural Heritage 8 (2007) 387e395 http://france.elsevier.com/direct/CULHER/
Original article
ATR-FTIR imaging of albumen photographic prints Camilla Ricci a, Simon Bloxham b,1, Sergei G. Kazarian a,* b
a Department of Chemical Engineering, Imperial College London, London SW7 2AZ, UK Department of Conservation, Royal College of Art, Kensington Gore, London SW7 2EU, UK
Received 31 October 2006; accepted 17 July 2007
Abstract The preservation of early 20th century, late 19th century albumen prints is of great concern to collection managers and conservators of photographic materials. Attenuated Total Reflection Fourier Transform Infrared (ATR-FTIR) spectroscopic imaging is presented for the first time as analytical methodology for the study of albumen photographs. This paper shows the feasibility of obtaining FTIR images of samples from albumen photographs with a high spatial resolution using a Ge ATR objective coupled with an infrared microscope. The improved spatial resolution compared to FTIR images obtained by the reflection method is due to the high refractive index of the ATR crystal, which gives a high numerical aperture and hence, a higher spatial resolution. The technique reveals detailed information on the organic functional group distribution in the individual layers of embedded cross sections and is used complementary to visual microscopy and scanning electron microscopy/energy dispersed X-ray spectroscopy. The main results of the study are discussed with regard to their historical and artistic significance, and they are compared with data from historical and conservation literature. Ó 2007 Elsevier Masson SAS. All rights reserved. Keywords: Chemical imaging; Albumen photographs; Conservation study; Infrared spectroscopy; SEM-EDX; Optical microscopy; FT-IR
1. Introduction Early 20th century, late 19th century albumen photographs are multi-layer systems made up of complex mixtures of inorganic and organic materials- their only definite structural feature is that egg-white was used in their production [1]. From an historical point of view, the first mention of albumen, or strictly speaking albumenized [2], paper was on 11 May 1839 [3]. This predates, by 11 years, Louis BlanquartEvard’s presentation of albumen paper printing to the French Academy of Sciences on 27 May 1850 and the first accepted English description by Robert Hunt, in 1853, by 14 years [4]. Albumen paper became the most widely used photographic * Corresponding author. Department of Chemical Engineering and Chemical Technology, Imperial College London, South Kensington Campus, London SW7 2AZ, UK. Tel.: þ44 207 594 5574; fax: þ44 207 594 5604. E-mail addresses: [email protected] (C. Ricci), s.k.bloxham@qmul. ac.uk (S. Bloxham), [email protected] (S.G. Kazarian). 1 Present address: Interdisciplinary Research Centre in Biomedical Materials, Queen Mary, University of London, Mile End Road, London E1 4NS, UK. 1296-2074/$ - see front matter Ó 2007 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.culher.2007.07.002
printing support (from 1855 until around 1890) and it can be thought of as the first ‘‘ready-made’’, shop-bought photographic material [5,6]. Their tonal range and lack of graininess made them exceptionally popular: especially when they were used in conjunction with fine-grained high contrast wet colloidon negatives [7]. Albumen paper was made by coating a single sheet of paper with egg white mixed with ammonium or silver chloride. Photographers would do this themselves until the late 1850s, as by then the process had popularised and commercial production of albumen paper had begun. A number of modern recipes for albumen paper production are around and these do not appear, in principal, to be too different from the original 19th century processes [3,8]. The only difference appears to be the scale of production: it was estimated (in 1866) that six million egg whites were used in England annually for its production. In summary, the production has, essentially, nine steps [4]: 1. Separation of the egg whites. 2. Beating egg whites to a froth in the presence of chloride (ammonium and/or sodium- NH4Cl, NaCl).
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3. 4. 5. 6. 7. 8. 9.
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Allow the froth to settle. Strain the resulting suspension. Age the suspension. Float the paper support on the suspension. Hang the coated paper up to dry. Allow the coated paper to cure. Sensitise the moistened paper by floating it, coated side down, in a solution of silver nitrate (AgNO3).
Steps six and seven were frequently repeated, with inversion during the drying stage, in order to produce a more even albumen coating that gave a much glossier final result [3,9]. One of the most important steps in the process was the ageing or curing (step eight), this was when the soluble egg white ‘‘hardened’’ to insoluble albumen [5,8]. Most collections of nineteenth-century photographs in museums, anthropological archives, etc., are predominately composed of albumen photographs. This is partly due to superb resolution of detail, long tonal range, and high contrast of the albumen print which is crucial for achieving optimum results with colloidon wet-plate negatives. Prior to albumen prints, printing paper was limited to low contrast salted paper or early forms of albumen printing paper which had a lower sensitivity. Because of their predominance, the preservation of albumen prints is a matter of great concern to collection managers and to conservators of photographic materials [10]. In particular, a previous study showed that aqueous treatments increase the cracking of albumen layer [10]. In fact, the moisture can hydrolyze the peptide linkages, and it improves the growth of fungi and bacteria which also break the peptide bonds. Light also provokes the breakdown of the peptide bond and the modification of structure of the protein [11].
The nature of the photographic prints, i.e. their heterogeneity, the minimal albumen layer thickness and the inorganic particle size, requires any instrumental technique used to study them to have a high spatial resolution. The imaging techniques used in this study include visible microscopy, ATR-FTIR imaging and scanning electron microscopy with energy dispersive X-ray analysis (SEM-EDX); one advantage of this approach is the same cross-section can be used for multiple investigations with all these imaging techniques. The techniques are complementary in the way that each technique reveals different additional information on the print composition. The simplest imaging technique is optical microscopy. The spatial resolution of optical microscopy in the visible range is ca. 1 mm. It helps to reveal the layer build-up and morphology and particle characteristics such as shape, colour, and size. The spatial resolution of the electron microscope is better than 1 mm. It allows structural features, of the order w0.5 mm, to be identified (and good quality images to be obtained) in addition to obtaining qualitative and quantitative information concerning the elemental composition (particularly inorganic elements). A backscatter image (BSE) of the cross section shows the compositional contrast between and within the layers according to atomic weight. A heavy element like silver is a greater scatter (looks lighter grey) than a lighter element like for example carbon (looks darker grey). Apart from the highly informative backscatter images, the EDX elemental maps show the distribution of the individual elements. Combining microscopy with molecular recognition represents a further step in the study of heterogeneous materials. This combination has been further enhanced into a chemical imaging method using FTIR spectroscopy with an array rather than a single element detector [12,13]. IR array
Fig. 1. The albumen photographs ‘‘Old’’ (left) and ‘‘New’’ (right) (both from around 1895e1905) showing the approximate locations of the sampling sites (doubleedged rectangles) used to generate the black/white (light/dark) samples.
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Fig. 2. Micrographs (50) of the ‘‘Old’’ (left) and ‘‘New’’ (right) albumen photograph samples showing: (A) the card support, (B) white crystalline layer, (C) paper, (D) albumen layer and (E) epoxy matrix.
Fig. 3. BSE images of the ‘‘Old’’ and ‘‘New’’ albumen photographic samples: ‘‘Old’’ white region (top left, 1380), ‘‘Old’’ black region (bottom left, 1380), ‘‘New’’ white region (top right, 2890) and ‘‘New’’ black region (bottom right, 2840). Indicated on the images: photographic paper (A), albumen layer (B) and epoxy support (C). The square areas in the centre of the ‘‘Old’’ samples are sites where EDX analyses were performed and the white crystalline solids were identified as kaolin.
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detectors (also known as IR focal-plane arrays) have, only recently, begun to be adopted outside the remote sensing and astronomical communities because of the greater availability and reduced costs of these devices [14]. Using focal plane array (FPA) infrared detectors, the spatial and spectral information is arranged in a three dimensional data cube, usually configured as spatial information in the x and y direction and spectral information in the z direction [15]. This high-information technique has been implemented for analyzing a wide range of systems, including pharmaceutical applications [16,17], polymer systems under high pressure [18], polymer dissolution [19e22], forensic materials [23e28], works of cultural significance [29,30], biological systems [31,32] and high throughput analysis [33,34]. Although infrared imaging is becoming more commonplace and publications demonstrating its utility are on the rise, the applicability of this method is often restricted by the diffraction limit of light in air and by the necessity to microtome samples for transmission measurements. Also, reflectance measurements can present large spectral distortions in both the band shape and absorption frequency, which may depend on the band strength, on the concentration of the sample, or on the optical layout of the measuring system [35]. Therefore, it is difficult to compare reflectance spectra with those collected in the transmission mode and, consequently, with the available databases. In order to overcome these obstacles and fully realize the potential of FTIR imaging, our group has applied of ATR crystals coupled with a focal-plane array (FPA) detector [36]. According to the Rayleigh criterion, the high refractive index of the crystal increases the numerical aperture of the objective and therefore increases the spatial resolution. It has been demonstrated that the achievable spatial resolution in microATR with a 64 64 array detector and a germanium crystal
Fig. 4. Micro ATR image of the PMMA film on a silicon wafer generated by integration of the absorption band between 1753 and 1672 cm1.
Fig. 5. Absorption profile of the n(C]O) band of PMMA across the line A in Fig. 4.
is on the order of 3e4 mm without the use of expensive synchrotron source [36]. 2. Experimental 2.1. Sample preparation A thin poly(methyl methacrylate) (PMMA) film approximately 0.5 mm thick on a silicon wafer and patterned using electron beam lithography was used to measure the achievable spatial resolution. Small sections of the film were removed to expose part of the silicon to form circular patterns of polymer 4 mm in diameter with a separation of 2 mm between each circle. The pattern dimension was measured by viewing the sample under a visible microscope with a 50 objective. The photographic samples (labelled ‘‘Old’’ and ‘‘New’’ Fig. 1) were studied in cross section. The ‘‘Old’’ has been accurately dated to 1895e1905, from the subject’s dress and discernable details of the photographer’s studio on the image and ‘‘New’’ is thought to be a contemporary of ‘‘Old’’ from
Fig. 6. Infrared spectrum of the PMMA on the silicon wafer.
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Fig. 7. (a) Micro-ATR-FTIR images of the ‘‘old’’ cross section obtained integrating the spectral range 1680e1610 cm1 (Amide II). The size of the image is approximately 63 63 mm2. (b) The absorbance profile of Amide II band across the line perpendicular to the interface protein-resin of (a).
the subject’s clothing. The samples were removed from regions of high contrast and thus a single sample has a dark (black) and a light (white) section. They were placed in steel clips, had their positions recorded and mounted in w20 ml of epoxy resin. After setting for 24 h, the epoxy block was then ground, using increasingly finer abrasives to expose ‘‘smooth’’ cross sections (between changes of paper the samples were washed with 20 ml of isopropyl alcohol)- this was monitored by assessing the surface to see if the samples were visible through the epoxy using an optical microscope at 10 magnification. Once a ‘‘smooth’’ surface was obtained,
with the sample cross sections visible through the resin, the epoxy sample was stored, for 14 days, in a desiccator (containing silica gel). 2.2. Optical microscopy Microscopic studies on the albumen photograph cross sections were performed using a Meiji Techno Instrument microscope (Meiji Techno UK Ltd, Axbridge, Somerset) with long-working distance objectives and polariser-analyser/ filter with camera attachments.
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2.3. Micro-ATR-FTIR imaging A FPA detector (Santa Barbara, USA) comprising 16,384 small pixels arranged in a 128 128 grid format was used to measure FTIR spectra with a spectrometer operating in continuous scan mode. Spectra were collected with 8 cm1 spectral resolution in the range 4000e900 cm1 using 225 scans. The chemical images were obtained by attributing a colour to each pixel according to the absorbance of a spectral band characteristic of a given compound. In the micro ATR configuration, the spectrometer and the FPA detector were coupled with an infrared microscope with a 20 cassegrainian objective and a Ge ATR crystal. The imaging ATR spectrometer is patented by Varian [37]. 2.4. SEM-EDX Scanning electron microscopy studies in combination with energy dispersive X-ray analysis (SEM-EDX) were performed
on a Leo 1525 FEGSEM with an Oxford Instruments INCA energy dispersive X-ray analytical system, fitted with a FEG using 10 kV (or 15 kV) accelerating voltage. The samples were coated with a thin layer of carbon to improve surface conduction. 3. Results and discussion Looking carefully at the ‘‘Old’’ and ‘‘New’’ samples cross sections in the microscopic images of Fig. 2, both of the photographic samples share identical structural features. Both of the photographic prints are mounted upon card and both of the samples’ card is coated with a white, powdery substance (around 10 mm thick, identified by EDX as BaSO4), although the ‘‘New’’ sample’s layer appears to be thicker. Old’s’’ paper is thinner (around 60 mm) whilst ‘‘New’s’’ is about 80 mm. Finally, just discernable towards the top of the micrographs are the albumen layers (although ‘‘New’s’’ is more prominent than
Fig. 8. Micro-ATR-FTIR images of the dark region of the ‘‘old’’ albumen image cross section obtained integrating the spectral range 1680e1610 cm1 (amide II). The size of each image is approximately 63 63 mm2. The representative spectra at the location indicated by the letters are shown below. b. Micro-ATR-FTIR image of the light region of the ‘‘old’’ albumen image cross section obtained integrating the spectral range 1680e1610 cm1 (amide II). The size of each image is approximately 63 63 mm2. The representative spectra at the location indicated by the letters are shown below.
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‘‘Old’s’’) it should be apparent that both of the layers have widely varying thicknesses. To further investigate the albumen layers, they were examined by SEM-EDX. Slight differences between the ‘‘Old’’ and ‘‘New’s’’ paper structure is readily apparent- evidenced by the wider spacing between the paper fibres of ‘‘New’’ than in ‘‘Old’’ (Fig. 3). The change in thickness of the albumen layers is not thought to be related to whether the sample is a ‘‘black’’ nor ‘‘white’’ end as the optical images indicate that the thickness of the albumen is highly variable in each of the samples. The width of the discrete albumen appears to vary in thickness 8e12 mm for both samples and the inhomogeneous nature of the coating is particularly apparent for the ‘‘New’’ sample where there are a number of ‘‘breaks’’ visible in the coatingthis is not an unsurprising feature as extensive cracking of the proteinaceous layer is one of the key means of identifying albumen prints [9]. EDX analysis appeared to demonstrate that there were some slight differences between the inorganic chemical content of ‘‘black’’ and ‘‘white’’ ends of the albumen cross sections and that the paper in the both of the samples contained fine particles of kaolin (Al2Si2O5(OH)4) which
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was a commonly used additive in the making of glossy paper [9,38]. EDX line scans across the black and white ends of the albumen samples appear to show differences between the Au and Ag contents, in that the Ag in the white ends appears to be localised within the centre of the albumen layer, whilst in the black samples it appears throughout the layer and at a higher concentration (this appears to be ‘‘mirrored’’ by the concentration profile for S and Cl) a similar scenario is repeated for Au but is at a much higher concentration in both samples. The precious metal concentration difference is not untowardly surprising as it is Ag that gives albumen photographs their ‘‘colour’’ and Au salts were commonly introduced into the photographic fixation formulae to increase tonal depth [39]. The organic components of the cross sections were investigated by ATR-FTIR spectroscopic imaging. This study used a 128 128 array detector and it was needed to assess the spatial resolution achievable with this particular detector using our system (this assessment was previously performed with a 64 64 detector). Therefore, the achievable spatial resolution in micro-ATR with a 128 128 array detector and
Fig. 9. Micro-ATR-FTIR images of the dark region of the ‘‘new’’ albumen image cross section obtained integrating the spectral range 1680e1610 cm1 (amide II). The size of each image is approximately 63 63 mm2. The representative spectra at the location indicated by the letters are shown below. b. Micro-ATR-FTIR image of the light region of the ‘‘new’’ albumen image cross section obtained integrating the spectral range 1680e1610 cm1 (amide II). The size of each image is approximately 63 63 mm2. The representative spectra at the location indicated by the letters are shown below.
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a germanium ATR crystal was tested by imaging the previously used polymer film on the silicon wafer [36]. Fig. 4 shows that circles 4 mm in diameter with 2 mm separation have been spatially resolved by this FTIR imaging method. An integrated carbonyl absorbance profile across the axis A in Fig. 4 has been plotted and is shown in Fig. 5. It should be noted that the integrated absorbance does not reach its maximum and minimum values expected for the circles and the gaps, respectively. In fact, the absorbance at 1723 cm1 (the infrared spectrum of PMMA is shown in Fig. 6) does not decrease to zeroeven at the centre of the gap; this is apparently due to the fact that the spatial resolution of this system is w4 mm. This test also allowed estimation of the size of the imaged area: which is ca. 63 63 mm2. Next, we examined the achievable spatial resolution with an albumen photographic sample. A measurement of the spectra near this interface generates a step in the absorption profile and the width of that step determines the achievable spatial resolution. The Amide II band in the range 1680e1610 cm1 was used to generate the image which is shown in Fig. 7. The absorption profile shown in Fig. 7b was obtained by plotting the absorbance of the Amide II band along the line perpendicular between the protein and the epoxy (along axis A, Fig. 7a). Using the same method as was used by Chan and Kazarian [36], ascertaining the distance between points corresponding to 95% and 5% of the maximum absorbance, the spatial resolution using the Amide II band for this sample was found to be ca. 5 mm. This achievable spatial resolution appears worse than the value measured by Chan and Kazarian [36], apparently because the albumen print cross section may present some interdiffusion between the epoxy resin and the albumen layer. After having evaluated the capability of the experimental set up, the analytical procedure has been applied to the study of both dark and light regions of the ‘‘Old’’ and ‘‘New’’ albumen images. No differences have been detected between the ‘‘Old’’ and ‘‘New’’ images. But, it should be noted that a constant absorbance scale has been utilised throughout the study. Comparison of the ‘‘Old’’ and ‘‘New’’ light and dark regions reveals that the absorbance of the Amide II vibration appears to be greater in the ‘‘black’’ regions compared to the ‘‘white’’. In Fig. 8 the results obtained on the on the ‘‘Old’’ cross section are shown. The spectra extracted form the supporting paper show a very strong absorption of cellulose at 3340, 1160, 1110, 1060 and 1030 cm1 [40]. The most notable features in the spectra of Fig. 8, from the albumen layer, are the bands at 1640 and 1530 cm1 corresponding to the C]O stretching and NeH in plane bending of Amide II of protein. Similar results have been obtained for the ‘‘New’’ image cross section, as shown in Fig. 9. 4. Conclusions Firstly, this is believed to be the first study to examine by spectroscopy the organic chemical makeup of the entire thickness of the albumen photographic layer e not just a surface region [41,42]. The utility of ATR-FTIR spectroscopic imaging for the elucidation of structural and compositional details of works of cultural significance, particularly works of art, is
amply demonstrated in the present study of the 19th century albumen photograph samples. The ability of ATR-FTIR spectroscopic imaging to combine non-destructive quantitative chemical analysis and visualisation of the spatial distribution of each component in the sample is unmatched by other analytical techniques. Furthermore, the results of ATR-FTIR spectroscopic imaging approach were combined with the data obtained by SEM which provided a powerful approach to characterise the molecular structure of albumen prints from both the organic and inorganic point of view. These techniques were used to investigate the cross sections of dark and light regions of nineteenth century albumen photographs. SEM-EDX analysis showed slight differences between the inorganic chemical content of ‘‘black’’ and ‘‘white’’ regions of the albumen cross sections, particularly in the levels of silver and gold. With regards to ATR-FTIR imaging results, the different absorbance of Amide II band for the light and dark regions of the same sample appear to indicate different protein contents, possibly because of the increased amount of metallic silver present in the dark region that resulted in a reduced overall amount of protein in that region- although the increased absorbance of the Amide II band in the black regions of the photographic sample may result from the increased Ag concentration necessary to give the black and white image its necessary ‘‘blackness’’. Further work should be undertaken to investigate fully any differences in chalcogen/Ag bonding between black and white regions. Acknowledgment CR and SGK thank EPSRC (Grant EP/C532678/1) for support. SB’s work was carried out as part of his studies in the Royal College of Art/Victoria & Albert Museum Conservation Department, in collaboration with The National Archives (TNA), and supported by the Ronald E. Compton Scholarship. SB acknowledges the supervision of Nancy Bell, Head of Research Strategy, TNA, and Dr David McPhail, Dept. of Materials, Imperial College London and William Lindsay, Head of RCA/V&A Conservation. SB also thanks Dr Mahmmod Ardakani, Dr Sarah Fearn and Nick Royall at the Dept. Of materials at Imperial College London, and Dr Alex Ball at the Natural History Museum for their guidance and assistance in making the SEM substrates and obtaining SEM images. Thanks are due to Stephen Harwood (TNA) for the donation of the ‘‘Old’’ sample image. References [1] http://albumen.stanford.edu/. [2] Anonymous, The Athenaeum 11 May 1839. [3] J.M. Reilly, The Albumen and Salted Paper Book: The History of Practise of Photographic Printing 1840e1895, Light Impressions Corporation, Rochester NY, 1980. [4] W. Crawford, The Keepers of Light: A History and Working Guide to Early Photographic Processes, Morgan & Morgan, New York, 1979. [5] J.M. Reilly, The manufacture of use of albumen paper, The Journal of Photographic Science 26 (1978) 156e161. [6] S. Rempel, The Care of Photographs, Nick Lyons Books, New York, 1987.
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Journal of Cultural Heritage 8 (2007) 387e395 http://france.elsevier.com/direct/CULHER/
Original article
ATR-FTIR imaging of albumen photographic prints Camilla Ricci a, Simon Bloxham b,1, Sergei G. Kazarian a,* b
a Department of Chemical Engineering, Imperial College London, London SW7 2AZ, UK Department of Conservation, Royal College of Art, Kensington Gore, London SW7 2EU, UK
Received 31 October 2006; accepted 17 July 2007
Abstract The preservation of early 20th century, late 19th century albumen prints is of great concern to collection managers and conservators of photographic materials. Attenuated Total Reflection Fourier Transform Infrared (ATR-FTIR) spectroscopic imaging is presented for the first time as analytical methodology for the study of albumen photographs. This paper shows the feasibility of obtaining FTIR images of samples from albumen photographs with a high spatial resolution using a Ge ATR objective coupled with an infrared microscope. The improved spatial resolution compared to FTIR images obtained by the reflection method is due to the high refractive index of the ATR crystal, which gives a high numerical aperture and hence, a higher spatial resolution. The technique reveals detailed information on the organic functional group distribution in the individual layers of embedded cross sections and is used complementary to visual microscopy and scanning electron microscopy/energy dispersed X-ray spectroscopy. The main results of the study are discussed with regard to their historical and artistic significance, and they are compared with data from historical and conservation literature. Ó 2007 Elsevier Masson SAS. All rights reserved. Keywords: Chemical imaging; Albumen photographs; Conservation study; Infrared spectroscopy; SEM-EDX; Optical microscopy; FT-IR
1. Introduction Early 20th century, late 19th century albumen photographs are multi-layer systems made up of complex mixtures of inorganic and organic materials- their only definite structural feature is that egg-white was used in their production [1]. From an historical point of view, the first mention of albumen, or strictly speaking albumenized [2], paper was on 11 May 1839 [3]. This predates, by 11 years, Louis BlanquartEvard’s presentation of albumen paper printing to the French Academy of Sciences on 27 May 1850 and the first accepted English description by Robert Hunt, in 1853, by 14 years [4]. Albumen paper became the most widely used photographic * Corresponding author. Department of Chemical Engineering and Chemical Technology, Imperial College London, South Kensington Campus, London SW7 2AZ, UK. Tel.: þ44 207 594 5574; fax: þ44 207 594 5604. E-mail addresses: [email protected] (C. Ricci), s.k.bloxham@qmul. ac.uk (S. Bloxham), [email protected] (S.G. Kazarian). 1 Present address: Interdisciplinary Research Centre in Biomedical Materials, Queen Mary, University of London, Mile End Road, London E1 4NS, UK. 1296-2074/$ - see front matter Ó 2007 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.culher.2007.07.002
printing support (from 1855 until around 1890) and it can be thought of as the first ‘‘ready-made’’, shop-bought photographic material [5,6]. Their tonal range and lack of graininess made them exceptionally popular: especially when they were used in conjunction with fine-grained high contrast wet colloidon negatives [7]. Albumen paper was made by coating a single sheet of paper with egg white mixed with ammonium or silver chloride. Photographers would do this themselves until the late 1850s, as by then the process had popularised and commercial production of albumen paper had begun. A number of modern recipes for albumen paper production are around and these do not appear, in principal, to be too different from the original 19th century processes [3,8]. The only difference appears to be the scale of production: it was estimated (in 1866) that six million egg whites were used in England annually for its production. In summary, the production has, essentially, nine steps [4]: 1. Separation of the egg whites. 2. Beating egg whites to a froth in the presence of chloride (ammonium and/or sodium- NH4Cl, NaCl).
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3. 4. 5. 6. 7. 8. 9.
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Allow the froth to settle. Strain the resulting suspension. Age the suspension. Float the paper support on the suspension. Hang the coated paper up to dry. Allow the coated paper to cure. Sensitise the moistened paper by floating it, coated side down, in a solution of silver nitrate (AgNO3).
Steps six and seven were frequently repeated, with inversion during the drying stage, in order to produce a more even albumen coating that gave a much glossier final result [3,9]. One of the most important steps in the process was the ageing or curing (step eight), this was when the soluble egg white ‘‘hardened’’ to insoluble albumen [5,8]. Most collections of nineteenth-century photographs in museums, anthropological archives, etc., are predominately composed of albumen photographs. This is partly due to superb resolution of detail, long tonal range, and high contrast of the albumen print which is crucial for achieving optimum results with colloidon wet-plate negatives. Prior to albumen prints, printing paper was limited to low contrast salted paper or early forms of albumen printing paper which had a lower sensitivity. Because of their predominance, the preservation of albumen prints is a matter of great concern to collection managers and to conservators of photographic materials [10]. In particular, a previous study showed that aqueous treatments increase the cracking of albumen layer [10]. In fact, the moisture can hydrolyze the peptide linkages, and it improves the growth of fungi and bacteria which also break the peptide bonds. Light also provokes the breakdown of the peptide bond and the modification of structure of the protein [11].
The nature of the photographic prints, i.e. their heterogeneity, the minimal albumen layer thickness and the inorganic particle size, requires any instrumental technique used to study them to have a high spatial resolution. The imaging techniques used in this study include visible microscopy, ATR-FTIR imaging and scanning electron microscopy with energy dispersive X-ray analysis (SEM-EDX); one advantage of this approach is the same cross-section can be used for multiple investigations with all these imaging techniques. The techniques are complementary in the way that each technique reveals different additional information on the print composition. The simplest imaging technique is optical microscopy. The spatial resolution of optical microscopy in the visible range is ca. 1 mm. It helps to reveal the layer build-up and morphology and particle characteristics such as shape, colour, and size. The spatial resolution of the electron microscope is better than 1 mm. It allows structural features, of the order w0.5 mm, to be identified (and good quality images to be obtained) in addition to obtaining qualitative and quantitative information concerning the elemental composition (particularly inorganic elements). A backscatter image (BSE) of the cross section shows the compositional contrast between and within the layers according to atomic weight. A heavy element like silver is a greater scatter (looks lighter grey) than a lighter element like for example carbon (looks darker grey). Apart from the highly informative backscatter images, the EDX elemental maps show the distribution of the individual elements. Combining microscopy with molecular recognition represents a further step in the study of heterogeneous materials. This combination has been further enhanced into a chemical imaging method using FTIR spectroscopy with an array rather than a single element detector [12,13]. IR array
Fig. 1. The albumen photographs ‘‘Old’’ (left) and ‘‘New’’ (right) (both from around 1895e1905) showing the approximate locations of the sampling sites (doubleedged rectangles) used to generate the black/white (light/dark) samples.
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Fig. 2. Micrographs (50) of the ‘‘Old’’ (left) and ‘‘New’’ (right) albumen photograph samples showing: (A) the card support, (B) white crystalline layer, (C) paper, (D) albumen layer and (E) epoxy matrix.
Fig. 3. BSE images of the ‘‘Old’’ and ‘‘New’’ albumen photographic samples: ‘‘Old’’ white region (top left, 1380), ‘‘Old’’ black region (bottom left, 1380), ‘‘New’’ white region (top right, 2890) and ‘‘New’’ black region (bottom right, 2840). Indicated on the images: photographic paper (A), albumen layer (B) and epoxy support (C). The square areas in the centre of the ‘‘Old’’ samples are sites where EDX analyses were performed and the white crystalline solids were identified as kaolin.
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detectors (also known as IR focal-plane arrays) have, only recently, begun to be adopted outside the remote sensing and astronomical communities because of the greater availability and reduced costs of these devices [14]. Using focal plane array (FPA) infrared detectors, the spatial and spectral information is arranged in a three dimensional data cube, usually configured as spatial information in the x and y direction and spectral information in the z direction [15]. This high-information technique has been implemented for analyzing a wide range of systems, including pharmaceutical applications [16,17], polymer systems under high pressure [18], polymer dissolution [19e22], forensic materials [23e28], works of cultural significance [29,30], biological systems [31,32] and high throughput analysis [33,34]. Although infrared imaging is becoming more commonplace and publications demonstrating its utility are on the rise, the applicability of this method is often restricted by the diffraction limit of light in air and by the necessity to microtome samples for transmission measurements. Also, reflectance measurements can present large spectral distortions in both the band shape and absorption frequency, which may depend on the band strength, on the concentration of the sample, or on the optical layout of the measuring system [35]. Therefore, it is difficult to compare reflectance spectra with those collected in the transmission mode and, consequently, with the available databases. In order to overcome these obstacles and fully realize the potential of FTIR imaging, our group has applied of ATR crystals coupled with a focal-plane array (FPA) detector [36]. According to the Rayleigh criterion, the high refractive index of the crystal increases the numerical aperture of the objective and therefore increases the spatial resolution. It has been demonstrated that the achievable spatial resolution in microATR with a 64 64 array detector and a germanium crystal
Fig. 4. Micro ATR image of the PMMA film on a silicon wafer generated by integration of the absorption band between 1753 and 1672 cm1.
Fig. 5. Absorption profile of the n(C]O) band of PMMA across the line A in Fig. 4.
is on the order of 3e4 mm without the use of expensive synchrotron source [36]. 2. Experimental 2.1. Sample preparation A thin poly(methyl methacrylate) (PMMA) film approximately 0.5 mm thick on a silicon wafer and patterned using electron beam lithography was used to measure the achievable spatial resolution. Small sections of the film were removed to expose part of the silicon to form circular patterns of polymer 4 mm in diameter with a separation of 2 mm between each circle. The pattern dimension was measured by viewing the sample under a visible microscope with a 50 objective. The photographic samples (labelled ‘‘Old’’ and ‘‘New’’ Fig. 1) were studied in cross section. The ‘‘Old’’ has been accurately dated to 1895e1905, from the subject’s dress and discernable details of the photographer’s studio on the image and ‘‘New’’ is thought to be a contemporary of ‘‘Old’’ from
Fig. 6. Infrared spectrum of the PMMA on the silicon wafer.
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Fig. 7. (a) Micro-ATR-FTIR images of the ‘‘old’’ cross section obtained integrating the spectral range 1680e1610 cm1 (Amide II). The size of the image is approximately 63 63 mm2. (b) The absorbance profile of Amide II band across the line perpendicular to the interface protein-resin of (a).
the subject’s clothing. The samples were removed from regions of high contrast and thus a single sample has a dark (black) and a light (white) section. They were placed in steel clips, had their positions recorded and mounted in w20 ml of epoxy resin. After setting for 24 h, the epoxy block was then ground, using increasingly finer abrasives to expose ‘‘smooth’’ cross sections (between changes of paper the samples were washed with 20 ml of isopropyl alcohol)- this was monitored by assessing the surface to see if the samples were visible through the epoxy using an optical microscope at 10 magnification. Once a ‘‘smooth’’ surface was obtained,
with the sample cross sections visible through the resin, the epoxy sample was stored, for 14 days, in a desiccator (containing silica gel). 2.2. Optical microscopy Microscopic studies on the albumen photograph cross sections were performed using a Meiji Techno Instrument microscope (Meiji Techno UK Ltd, Axbridge, Somerset) with long-working distance objectives and polariser-analyser/ filter with camera attachments.
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2.3. Micro-ATR-FTIR imaging A FPA detector (Santa Barbara, USA) comprising 16,384 small pixels arranged in a 128 128 grid format was used to measure FTIR spectra with a spectrometer operating in continuous scan mode. Spectra were collected with 8 cm1 spectral resolution in the range 4000e900 cm1 using 225 scans. The chemical images were obtained by attributing a colour to each pixel according to the absorbance of a spectral band characteristic of a given compound. In the micro ATR configuration, the spectrometer and the FPA detector were coupled with an infrared microscope with a 20 cassegrainian objective and a Ge ATR crystal. The imaging ATR spectrometer is patented by Varian [37]. 2.4. SEM-EDX Scanning electron microscopy studies in combination with energy dispersive X-ray analysis (SEM-EDX) were performed
on a Leo 1525 FEGSEM with an Oxford Instruments INCA energy dispersive X-ray analytical system, fitted with a FEG using 10 kV (or 15 kV) accelerating voltage. The samples were coated with a thin layer of carbon to improve surface conduction. 3. Results and discussion Looking carefully at the ‘‘Old’’ and ‘‘New’’ samples cross sections in the microscopic images of Fig. 2, both of the photographic samples share identical structural features. Both of the photographic prints are mounted upon card and both of the samples’ card is coated with a white, powdery substance (around 10 mm thick, identified by EDX as BaSO4), although the ‘‘New’’ sample’s layer appears to be thicker. Old’s’’ paper is thinner (around 60 mm) whilst ‘‘New’s’’ is about 80 mm. Finally, just discernable towards the top of the micrographs are the albumen layers (although ‘‘New’s’’ is more prominent than
Fig. 8. Micro-ATR-FTIR images of the dark region of the ‘‘old’’ albumen image cross section obtained integrating the spectral range 1680e1610 cm1 (amide II). The size of each image is approximately 63 63 mm2. The representative spectra at the location indicated by the letters are shown below. b. Micro-ATR-FTIR image of the light region of the ‘‘old’’ albumen image cross section obtained integrating the spectral range 1680e1610 cm1 (amide II). The size of each image is approximately 63 63 mm2. The representative spectra at the location indicated by the letters are shown below.
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‘‘Old’s’’) it should be apparent that both of the layers have widely varying thicknesses. To further investigate the albumen layers, they were examined by SEM-EDX. Slight differences between the ‘‘Old’’ and ‘‘New’s’’ paper structure is readily apparent- evidenced by the wider spacing between the paper fibres of ‘‘New’’ than in ‘‘Old’’ (Fig. 3). The change in thickness of the albumen layers is not thought to be related to whether the sample is a ‘‘black’’ nor ‘‘white’’ end as the optical images indicate that the thickness of the albumen is highly variable in each of the samples. The width of the discrete albumen appears to vary in thickness 8e12 mm for both samples and the inhomogeneous nature of the coating is particularly apparent for the ‘‘New’’ sample where there are a number of ‘‘breaks’’ visible in the coatingthis is not an unsurprising feature as extensive cracking of the proteinaceous layer is one of the key means of identifying albumen prints [9]. EDX analysis appeared to demonstrate that there were some slight differences between the inorganic chemical content of ‘‘black’’ and ‘‘white’’ ends of the albumen cross sections and that the paper in the both of the samples contained fine particles of kaolin (Al2Si2O5(OH)4) which
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was a commonly used additive in the making of glossy paper [9,38]. EDX line scans across the black and white ends of the albumen samples appear to show differences between the Au and Ag contents, in that the Ag in the white ends appears to be localised within the centre of the albumen layer, whilst in the black samples it appears throughout the layer and at a higher concentration (this appears to be ‘‘mirrored’’ by the concentration profile for S and Cl) a similar scenario is repeated for Au but is at a much higher concentration in both samples. The precious metal concentration difference is not untowardly surprising as it is Ag that gives albumen photographs their ‘‘colour’’ and Au salts were commonly introduced into the photographic fixation formulae to increase tonal depth [39]. The organic components of the cross sections were investigated by ATR-FTIR spectroscopic imaging. This study used a 128 128 array detector and it was needed to assess the spatial resolution achievable with this particular detector using our system (this assessment was previously performed with a 64 64 detector). Therefore, the achievable spatial resolution in micro-ATR with a 128 128 array detector and
Fig. 9. Micro-ATR-FTIR images of the dark region of the ‘‘new’’ albumen image cross section obtained integrating the spectral range 1680e1610 cm1 (amide II). The size of each image is approximately 63 63 mm2. The representative spectra at the location indicated by the letters are shown below. b. Micro-ATR-FTIR image of the light region of the ‘‘new’’ albumen image cross section obtained integrating the spectral range 1680e1610 cm1 (amide II). The size of each image is approximately 63 63 mm2. The representative spectra at the location indicated by the letters are shown below.
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a germanium ATR crystal was tested by imaging the previously used polymer film on the silicon wafer [36]. Fig. 4 shows that circles 4 mm in diameter with 2 mm separation have been spatially resolved by this FTIR imaging method. An integrated carbonyl absorbance profile across the axis A in Fig. 4 has been plotted and is shown in Fig. 5. It should be noted that the integrated absorbance does not reach its maximum and minimum values expected for the circles and the gaps, respectively. In fact, the absorbance at 1723 cm1 (the infrared spectrum of PMMA is shown in Fig. 6) does not decrease to zeroeven at the centre of the gap; this is apparently due to the fact that the spatial resolution of this system is w4 mm. This test also allowed estimation of the size of the imaged area: which is ca. 63 63 mm2. Next, we examined the achievable spatial resolution with an albumen photographic sample. A measurement of the spectra near this interface generates a step in the absorption profile and the width of that step determines the achievable spatial resolution. The Amide II band in the range 1680e1610 cm1 was used to generate the image which is shown in Fig. 7. The absorption profile shown in Fig. 7b was obtained by plotting the absorbance of the Amide II band along the line perpendicular between the protein and the epoxy (along axis A, Fig. 7a). Using the same method as was used by Chan and Kazarian [36], ascertaining the distance between points corresponding to 95% and 5% of the maximum absorbance, the spatial resolution using the Amide II band for this sample was found to be ca. 5 mm. This achievable spatial resolution appears worse than the value measured by Chan and Kazarian [36], apparently because the albumen print cross section may present some interdiffusion between the epoxy resin and the albumen layer. After having evaluated the capability of the experimental set up, the analytical procedure has been applied to the study of both dark and light regions of the ‘‘Old’’ and ‘‘New’’ albumen images. No differences have been detected between the ‘‘Old’’ and ‘‘New’’ images. But, it should be noted that a constant absorbance scale has been utilised throughout the study. Comparison of the ‘‘Old’’ and ‘‘New’’ light and dark regions reveals that the absorbance of the Amide II vibration appears to be greater in the ‘‘black’’ regions compared to the ‘‘white’’. In Fig. 8 the results obtained on the on the ‘‘Old’’ cross section are shown. The spectra extracted form the supporting paper show a very strong absorption of cellulose at 3340, 1160, 1110, 1060 and 1030 cm1 [40]. The most notable features in the spectra of Fig. 8, from the albumen layer, are the bands at 1640 and 1530 cm1 corresponding to the C]O stretching and NeH in plane bending of Amide II of protein. Similar results have been obtained for the ‘‘New’’ image cross section, as shown in Fig. 9. 4. Conclusions Firstly, this is believed to be the first study to examine by spectroscopy the organic chemical makeup of the entire thickness of the albumen photographic layer e not just a surface region [41,42]. The utility of ATR-FTIR spectroscopic imaging for the elucidation of structural and compositional details of works of cultural significance, particularly works of art, is
amply demonstrated in the present study of the 19th century albumen photograph samples. The ability of ATR-FTIR spectroscopic imaging to combine non-destructive quantitative chemical analysis and visualisation of the spatial distribution of each component in the sample is unmatched by other analytical techniques. Furthermore, the results of ATR-FTIR spectroscopic imaging approach were combined with the data obtained by SEM which provided a powerful approach to characterise the molecular structure of albumen prints from both the organic and inorganic point of view. These techniques were used to investigate the cross sections of dark and light regions of nineteenth century albumen photographs. SEM-EDX analysis showed slight differences between the inorganic chemical content of ‘‘black’’ and ‘‘white’’ regions of the albumen cross sections, particularly in the levels of silver and gold. With regards to ATR-FTIR imaging results, the different absorbance of Amide II band for the light and dark regions of the same sample appear to indicate different protein contents, possibly because of the increased amount of metallic silver present in the dark region that resulted in a reduced overall amount of protein in that region- although the increased absorbance of the Amide II band in the black regions of the photographic sample may result from the increased Ag concentration necessary to give the black and white image its necessary ‘‘blackness’’. Further work should be undertaken to investigate fully any differences in chalcogen/Ag bonding between black and white regions. Acknowledgment CR and SGK thank EPSRC (Grant EP/C532678/1) for support. SB’s work was carried out as part of his studies in the Royal College of Art/Victoria & Albert Museum Conservation Department, in collaboration with The National Archives (TNA), and supported by the Ronald E. Compton Scholarship. SB acknowledges the supervision of Nancy Bell, Head of Research Strategy, TNA, and Dr David McPhail, Dept. of Materials, Imperial College London and William Lindsay, Head of RCA/V&A Conservation. SB also thanks Dr Mahmmod Ardakani, Dr Sarah Fearn and Nick Royall at the Dept. Of materials at Imperial College London, and Dr Alex Ball at the Natural History Museum for their guidance and assistance in making the SEM substrates and obtaining SEM images. Thanks are due to Stephen Harwood (TNA) for the donation of the ‘‘Old’’ sample image. References [1] http://albumen.stanford.edu/. [2] Anonymous, The Athenaeum 11 May 1839. [3] J.M. Reilly, The Albumen and Salted Paper Book: The History of Practise of Photographic Printing 1840e1895, Light Impressions Corporation, Rochester NY, 1980. [4] W. Crawford, The Keepers of Light: A History and Working Guide to Early Photographic Processes, Morgan & Morgan, New York, 1979. [5] J.M. Reilly, The manufacture of use of albumen paper, The Journal of Photographic Science 26 (1978) 156e161. [6] S. Rempel, The Care of Photographs, Nick Lyons Books, New York, 1987.
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