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Polymer Degradation and Stability 95 (2010) 2481e2485

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Polymer Degradation and Stability

journal homepage: www.elsevier .com/locate /polydegstab

Stability of chromogenic colour prints in polluted indoor environments

Ann Fenech a, Matija Strli�c a,*, Ilaria Degano b, May Cassar a

aUniversity College London, Centre for Sustainable Heritage, The Bartlett School of Graduate Studies, Gower Street (Torrington Place site), London WCIE 6BT, UKbDipartimento di Chimica e Chimica Industriale, Università di Pisa, Via Risorgimento, 35 e 56126 Pisa, Italy

a r t i c l e i n f o

Article history:Received 29 May 2010Received in revised form1 August 2010Accepted 6 August 2010Available online 17 August 2010

Keywords:Colour photographsDye stabilityColourimetryChromatographyIndoor environmentPollutionAccelerated degradation

* Corresponding author. Tel.: þ44 2076795994.E-mail address: m.strlic@ucl.ac.uk (M. Strli�c).

0141-3910/$ e see front matter � 2010 Elsevier Ltd.doi:10.1016/j.polymdegradstab.2010.08.009

a b s t r a c t

Chromogenic colour prints are known to be sensitive to storage environments. However, limited researchis available on the effect of atmospheric pollutants on these materials, especially pollutants generatedindoors. The stability of photographic dyes is of particular interest and the rate of their change can bebest described using the standard RGB colour model. Therefore, the colourimetric method was comparedto dye extraction and liquid chromatographic analysis to justify its use as a rapid, non-destructivemethod for quantitative assessment of the rate of change in dye content of colour photographs duringdegradation. The effects of typical indoor (acetic acid, formaldehyde) and outdoor (nitrogen dioxide)generated pollutants on chromogenic colour prints were then investigated at 80 �C, 60% RH. It wasidentified that acetic acid leads to the most pronounced changes in photographic dye concentrations,which is significant considering that acetic acid is often the most prominent pollutant in archivalenvironments. On the other hand, formaldehyde exhibited a slight protective effect in comparison to theblank experiment.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Very few colour photographs can be described as permanentand photographic dyes are known to fade appreciably in the dark[1]. This is particularly true for chromogenic prints, which accountfor more than 99% of all colour prints. Image stability, however, isnot only influenced by the dye structure, as dyes are involved ininteractions with the physical, chemical and biological environ-ment. Photographs have been found to be more sensitive to theirenvironment than most other indoor heritage materials [1e3].Indoor air quality, defined by temperature, relative humidity andpollutant content, is the principal external factor contributing todye degradation in darkness [4,5]. In particular, little is knownabout the effect of atmospheric pollutants on the stability andpermanence of photographic materials.

Very limited research has been carried out to determine atmo-spheric pollutant levels that are safe for storage of photographicmaterials; so far, most guidance notes request the “best controltechnology” for pollutant control inside archives [6]. However,recent work on pollutants present in archives has shown that whilethese strategies are aimed predominantly at reducing outdoor-generated pollutants, high concentrations of indoor-generatedpollutants may still build up. Particularly in microenvironments

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(archival boxes), acetic acid was shown to be by far the mostconcentrated pollutant, its concentration being ca. ten times higherthan that of the most concentrated outdoor-generated pollutant,NOx [7]. For this reason, it is essential to investigate the effect ofindoor-generated pollutants on the stability of chromogenic printimages.

Techniques commonly used for investigation of dyes in photo-graphic materials are gas or liquid chromatography coupled withspectroscopic techniques [8], as these methods offer quantitativeinformation about individual dyes. However, chromatographicmethods are destructive, requiring invasive sampling followed bysample destruction, which is rarely permitted by curators of heritagecollections. Ideally, analytical methods need to be non-destructive ornon-invasive. Such methods also allow for monitoring of changes inthe same measurement spot over time, which is valuable indegradation studies.

Dyes used in colour photographic paper absorb light of distinctwavelength intervals1 and are known to be limited in number(generally a maximum of two types of dye for cyan, magenta andyellow [9]). Thus, it should be possible to monitor changes in theirconcentration using colourimetry, at least in a comparativemanner.Previous studies [10,11] introduced the use of colourimetry andreflectance spectrophotometry (densitometry), however,

1 Magenta dyes have an absorption maximum at 540e560 nm, yellow dyes:430e460 nm and cyan dyes in the 630e700 nm interval [23].

Table 1LC gradient elution timetable.

Time (min) % 0.1 M Ammonium acetate % Methanol

0 26 7410 5 9518 5 9519 26 7422 26 74

Fig. 1. Scheme of the experimental set-up for pollution degradation experiments.

A. Fenech et al. / Polymer Degradation and Stability 95 (2010) 2481e24852482

colourimetric measurements aremore comparable to image qualityas observed by the human eye, and may thus be more suitable forthe evaluation of change in heritage conservation terms.

In this paper we justify the methodology of dye analysis inchromogenic photographic prints using colourimetry. Based onthis, the colourimetric method is used to investigate the effect ofselected pollutants (acetic acid, formaldehyde and nitrogendioxide) on the stability of dyes during accelerated ageing.

2. Methodology

2.1. Samples

A reference collection of historic photographs has been assem-bled by acquisition and donation at UCL Centre for SustainableHeritage. For the HPLC and colourimetric measurements, samplesof 3 mm diameter were cut out from several prints to checkwhether reflectance spectra correspond to the sum of spectra ofindividual dyes, as determined using HPLC-DAD. The photographsanalysed were Kodak, 1973; Konica, 2006; Kodak, 1993; 2 unas-signed, late 1980s; 2 Kodak, 2005; unassigned, 1998; Kodak, 1994;unassigned, 1997; Fujifilm, 1998; Agfa, 1987; unassigned, early1980s; Konica; date unknown; and 1 unassigned photo of unknowndate.

For the pollution degradation study, samples of 6 mm diameterwere used. All subsamples were selected from evenly colouredareas. The four typical samples discussed in this paper are: Kodak,2005; Fujifilm, 2000; 2 unassigned, 1990s.

2.2. Colourimetry

Colourimetric reflectance spectra were measured using an X-Rite 530 SpectroDensitometer (D50/2� observation conditions,white background). The reflectance minima between 630 and700 nm for cyan (C) dyes, between 540 and 560 nm for magenta(M), and between 430 and 460 nm for yellow (Y) dyes weredetermined.

2.3. Extraction procedure

Trifluoroacetic acid (TFA, for UVeVis spectroscopy, �99.0%) waspurchased from Fluka (Milan, Italy). Ethanol was from Carlo Erba(Milan, Italy, pesticide analysis grade).

150 mL of the 1:2 TFA:ethanol mixture was found to be the bestextraction solvent for dyes from photographic prints. After extrac-tion, the solution was filtered and then dried under a stream ofnitrogen. The extract was then re-dissolved in ethanol.

2.4. Chromatographic analysis

Ammonium acetate (HPLC grade) was purchased from Fluka(Milan, Italy). Methanol was from Carlo Erba (Milan, Italy, pesticideanalysis grade). Doubly-distilled water was used throughout. Allreagents and chemicals were used without any further purification.

An HPLC consisting of a PU-2089 Quaternary Gradient Pumpwith degasser (Jasco International, Japan), equipped with a 2-mLRheodyne 7125 injection valve and coupled to a DAD MD 2010detector (Jasco International) was used. The wavelength range ofthe detectorwas 200e600 nm,with 0.8 s intervals for sampling and4 nm resolution. The signal was processed by ChromNav software(Jasco International). The chromatographic separations were per-formed using a C-18 column (Phenomenex Synergi 4u Fusion RP 80A, 100 � 2 mm, Phenomenex), connected to a C-18 pre-column(1 mm Opti-Guard C18, Optimize Technologies, Oregon).

The chromatographic gradient elution method was adaptedfrom Bristow and Bumfrey [12] for the analysis of dyes fromundeveloped photographic paper (see Table 1). A flow rate of0.4 mL/min was used. Using this elution programme, all peaks ofinterest were suitably separated.

2.5. Pollution degradation experiments

The 6-mm samples were attached onto a stainless steel wireusing a cotton thread and placed in 100 mL closed glass vials. Thevials were placed in a ventilated laboratory oven (Carbolite, ModelPF200) at 80 �C. The vials were flushed with a stream of airhumidified at 60% RH (Instruquest, V-Gen� Model 1), containing250 ppb of a pollutant (VICI Metronics, Dynacalibrator Model 150),every second day for two weeks. Three pollutants were investi-gated: acetic acid (permeation device: VICI Metronics, p/n 120-005-2851-F56-U60), nitrogen dioxide (permeation device: VICIMetronics, p/n 140-693-0081-U30) and formaldehyde (permeationdevice: VICI Metronics, p/n 100-005-2300-T53-U70). A control wasalso set up where no pollutant was added. Colourimetricmeasurements of the samples were taken at regular intervals.

A schematic representation of the set-up is presented in Fig. 1.This set-up was devised to allow for different combinations ofenvironmental conditions, namely temperature, RH and pollutantconcentration to be controlled, and their effect investigatedsystematically.

2.5.1. Analysis of datasRGB (standard RGB colour model) values were calculated from

the reflectance spectra [13]. The sRGB colour space was found to bemost suitable to monitor changes in dye concentrations due to thecomplementarity of red, green and blue to the cyan, magenta andyellow dyes, respectively. The measured values were normalisedagainst the initial measurement and rates of degradationwere thendetermined for each colour coordinate by calculating the slopes ofR/R0, G/G0 or B/B0 vs. time.

3. Results and discussion

3.1. Comparison of chromatographic and colourimetric data

The reflection spectra were first compared with extracted dyespectra obtained using HPLC-DAD, using the same individualsample. As reference dyes are no longer available in chemicallypure forms, the chromatographic method could only be used for

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quantitative analysis after extraction, separation and character-isation of individual dyes from a large number of identical photo-graphs. However, this is not necessary if changes in colour areevaluated colourimetrically, and normalised to the initial dye (cyan,magenta, yellow) intensity. To use this approach, however, wemustbe certain that spectra of extracted dyes correspond with thereflectance spectra as measured on photographs.

In each analysed sample, the dyes were extracted and separatedusing the described chromatographic method, and the absorptionspectra of the coloured compounds of four samples are shown inFig. 2. The spectra of individual dyes obtained chromatographicallywere summed up to give the ‘Sum spectrum’ of the three (in somecases four) dyes in solution. This was done as it was assumed thatthe sum of the individual dyes corresponds to the overall visualappearance of the photograph. This spectrum can be comparedwith the reflectance spectrum, converted into an absorbancespectrum using the KubelkaeMunk model [14].

The sum spectrum of dyes in solution is evidently trimodal, notdissimilar to the reflectance spectrum obtained colourimetrically,despite the considerable, but consistent shift in peak maxima,which could be due to solvation-related shifts [15], as the twospectra, i.e. the sum spectrum and the reflectance spectrum wereobtained in solution and in the dry state, respectively. Havingidentified that photographic dyes absorb in specific intervals withminimal overlap and no secondary peaks, the use of changes inreflectance spectra for quantitative evaluation of the rate of changeof individual dye concentration is justified.

Fig. 2. Comparison of the reflectance spectra of four sample photographs, and of the indiviAfter extraction, chromatography was performed with diode-array detection enabling the inD: unknown, 1980s.

The above identified cyan, magenta and yellow (CMY) dyes arecomplementary to the red, green and blue of the sRGB colourmodel, respectively [16]. The sRGB model numerically representsan absolute colour space in terms of three coordinates: red, blueand green [17]. Thus, the sRGB colour coordinates can be used toexpress changes in dye concentration if expressed relatively to thesRGB values at t ¼ 0.

3.2. The effect of pollutants on colour photographs

The presence of pollutants, in particular indoor-generatedpollutants, and their effect on archival material is of concern toheritage institutions. Despite the existence of standards for long-term storage of photographic materials, colour photographic printsare often stored in archival folders and boxes containing mixed, butmostly paper-based, archival objects, for practical and curatorialreasons. Limited work has been carried out on the effect of indoor-generated pollutants on the stability of photographic materials.Recently, typical pollutants in archival microenvironments havebeen determined, showing that acetic acid is by far the mostprevalent pollutant [7].

The colourimetric non-destructive method described aboveallows for measuring data at regular intervals during a degradationexperiment, and thus to monitor relative changes in dye concentra-tion over time. The plot of relative sRGB values over time shows thatthe degradation reactions are zero-order, as the nature of the plottedgraphs is linear (Fig. 3). This could bedue to initial dye concentrations

dual dyes (and their sum) as extracted from the same sample area of 3 mm diameter.dividual dye spectra to be obtained. A: Konica, 2006; B: Fujifilm, 1998; C: Konica, 2006;

Fig. 3. Relative changes in sRGB values against time of degradation for a photographdeveloped in 2005 (Kodak) and degraded in air with 250 ppb acetic acid, at 80 �C, 60%RH.

A. Fenech et al. / Polymer Degradation and Stability 95 (2010) 2481e24852484

being high enough so that minor changes did not affect the overallconcentration significantly. This experiment confirmed that dyespresent in colour photographs do not degrade at the same rate. InFig. 3 it is evident that the yellow dye (represented by the B coordi-nate) was the least stable. This is important, as human perception isparticularly affected by non-equivalent fading [18,19].

Fig. 4. Rates of dye change (expressed as slopes of D(Rt/R0) vs. time, analogously for G and Bair (control) or 250 ppb acetic acid, formaldehyde or nitrogen dioxide, as indicated. The er

In the experiments, it has become evident that both positive andnegative changes are possible. A positive change in an RGB valuewould indicate a decrease in dye concentration (dye concentrationof the CMY dyes are complementary, and thus inversely propor-tional to RGB values), as typified by degradation of the dye mole-cule. However, negative changes in RGB, thus indicating an increasein colour intensity, are also possible. This is often the result of eitherthe production of coloured degradation products or alternatively ofthe residual couplers in the photographic paper reacting during thedegradation experiment, thus increasing the respective dyeconcentration [9,20,21].

Having calculated the rate of change of RGB for the samplesdegraded in the presence of different pollutants, a comparison canbe made of the effect of different pollutants of interest, on the rateof dye degradation over time. However, as evident from Fig. 4, notall photographs are affected in the same way. This can beexplained by the fact that different dyes have been used bydifferent manufacturers at different periods of time. Also, thequality and quantity of the gelatine layer, in which the dyes areembedded, may affect the susceptibility of dyes to degradation ina polluted environment [22].

It is clear that acetic acid causes the most pronounced degree ofchange in comparison with other pollutants of interest. Nitrogendioxide is another pollutant that may significantly affect thestability of colour photographs, although from Fig. 4 it appears thatits effect is less pronounced or similar to that of acetic acid.However, in light of the fact that acetic acid (emitted by archivalmaterials themselves) was measured in archival boxes at

), for photographic print samples degraded at 80 �C, 60% RH, in different environments:ror bars represent uncertainties in the slopes of individual curves, as plotted in Fig. 3.

A. Fenech et al. / Polymer Degradation and Stability 95 (2010) 2481e2485 2485

concentrations of ca. ten times higher than those of nitrogen oxides(the major outdoor-generated pollutant present in the archivalatmosphere) [7], the acid can be identified as the pollutant ofgreatest concern in archival conditions. However, in indoor envi-ronments where the concentration of outdoor-generated pollut-ants is more significant (such as in non-filtered environments withhigh air exchange rates) and with no significant source of indoor-generated pollutants, the effect of nitrogen oxides may beprevalent.

Formaldehyde was another pollutant of high interest. While it isnot present in high concentrations in archival and library envi-ronments, it is intensively emitted by paper during degradation[23]. Interestingly, formaldehyde can have a protective effect, i.e.the observed rate of change in the presence of this pollutant waslower than in the control experiment in all four case studies inFig. 4, which is of high interest. Most dyes, particularly yellow andmagenta dyes, are namely known to be susceptible to oxidation [5],and formaldehyde could contribute in the process as an autoxida-tion initiator. Its apparent protective effect is therefore significant,especially since formic acid, if formed from formaldehyde, couldhave a similar effect on the degradation of dyes as acetic acid.However, this appears not to be the case in our experiments, andwould deserve to be studied inmore detail. The regular exchange ofthe atmosphere in the vial seems to limit the damage, potentiallycaused by secondary pollutants resulting from formaldehydeoxidation. However, it is also worth noting that formaldehyde istraditionally used to cure, or harden, gelatine [24]. Thus, formal-dehyde could react with the photographic gelatine, forminga protective layer around the dyes, thus preventing their oxidation.

4. Conclusions

A quantitative methodology was developed to investigate thestability of dyes in colour photographs, based on colourimetry andjustified using chromatographic methods. In this contribution, wehave shown that:

� The sRGB coordinates in the reflection spectra of photographscan be used to evaluate the rates of change of dye concentra-tions during degradation.

� Themethodology can be used to investigate the effect of typicalatmospheric pollutants found in heritage institutions (aceticacid, NO2 and formaldehyde) on the degradation of colourphotographic prints.

� Under conditions of accelerated degradation, acetic acid hasa stronger negative effect than NO2 or formaldehyde. The lattermay even exhibit a slightly protective effect in comparisonwiththe blank experiment.

� Since in archival microenvironments, acetic acid can be up toten times more concentrated than any other (indoor- oroutdoor-generated) pollutant, it could turn out to be the mostimportant parameter controlling the lifetime of colour prints inheritage collections at room conditions.

Further research is needed to extrapolate the behaviour of colourphotographic prints to room conditions, using the describedmethodology and to investigate the effect of multiple degradationfactors (T, RH, acetic acid). This will allow us to build a comprehen-sive model of possible synergistic effects, allowing those entrusted

with collections of cultural heritage objects to develop suitablepreservation strategies.

Acknowledgements

The authors gratefully acknowledge the financial support of theUK AHRC/EPSRC Science and Heritage Programme (project CDA 08/412, additionally supported by The National Archives, UK). Financialsupport by COST Action D42 e Chemical Interactions betweenCultural Artefacts and Indoor Environment is also gratefullyacknowledged.

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