Pigment Grains Research Articles

Synthesis Temperature Effect on the Structural Features and Optical Absorption of Zn1−xCoxAl2O4 Oxides

Zinc/cobalt aluminates with spinel-type structure were prepared by a polymeric route, leading to a pure phase with controlled grain size. The prepared pigments were characterized by powder X-ray diffraction Rietveld analyses in order to determine structural features, scanning electron microscopy for morphological investigation, helium pycnometry and (27)Al MAS NMR in order to highlight the occurrence of defects inside the structure, and UV-visible-near-IR spectroscopy to identify electronic transitions responsible for the compounds' color. The green-blue coloration of these pigments is known to be dependent on the sample thermal history. Here, for the first time, the Zn(1-x)Co(x)Al(2)O(4) color is newly interpreted. The pigment is green once synthesized at low temperature (i.e., with diminution of the pigment grain size); this variation was attributed to the appearance of a new absorption band located at about 500 nm, linked to a complex network feature involving Co ions in octahedral sites as well as oxygen and cationic vacancies. Hence, this work shows the possibility of easily getting a nonstoichiometric network with an abnormal cationic distribution from "chimie douce" processes with moderate synthesis temperature, and so various colorations for the same composition.

Surface-Enhanced Raman Spectroscopy: A Direct Method to Identify Colorants in Various Artist Media

Surface-enhanced Raman spectroscopy (SERS) has been developed as a direct, extractionless, nonhydrolysis tool to detect lake pigments and colorants of various classes used in a variety of artist materials. Presented first is the SERS analysis of the natural colorant turmeric (Curcuma longa L.), main component curcumin, as present in dry lake pigment grains, dyed textile yarns, and reference paint layers containing the lake pigment bound in animal glue painted on glass. This experiment demonstrated that it is possible to detect the chromophore in various matrixes of increasing complexity, allowing its unambiguous identification in a wide range of artists' materials, even at very low concentration and in the presence of binders such as glue. In addition, removal of the colorant from the complex with the inorganic substrate or mordanted yarn was not necessary for identification. This proof-of-concept study was then extended to include analysis of several pastel sticks from a historical pastel box and two samples from a pastel artwork, both attributed to American painter Mary Cassatt (1844-1926). This study represents the first extractionless, nonhydrolysis direct SERS study of multiple artist materials, including identification of natural and synthetic colorants and organic pigments contained in historic artists' pastels spanning a broad range of chemical classes: polyphenols, rhodamines, azo pigments, and anthraquinones. Successful identification is demonstrated on samples as small as a single grain of pigment.

Histopathological study on retinal lesions and restoration induced by micropulse diode laser and argon green laser in rabbits

Objective To study the histopathological effect of retinal lesions and restoration induced by micropulse diode laser and argon laser in rabbits. Methods Under the parameter of diabetic macular edema micropulse diode laser and argon laser was used to produce retinal lesions in rabbit eyes. The histopathological alterations were observed. Results The subthreshold spots were undetectable and argon spots were detectable. The photoreceptor cells did not change but there was vascular engorgement in the choroid and it was disappear in 4 week after photocoagulation. The outer membranous discs were arranged disorder and the hydroncus of retinal pigment epithelial （RPE） and nodal cells was observed in argon spots by transmission electron microscope. The heterochromatin became muhy in outer nuclear layer. The hyperplasia of RPE and collagen were obviously in 4 week and the hyperplasia of pigment grains were seen in chorioid. But no changes were seen in micropulse spots. Conclusion The retinal lesions produced by micropulse diode laser were slightly than produced by argon laser. Key words: micropulse/laser; argon laser; retinal photocoagulation ; lesions ; rabbit

Evolutionswege zum Larvalauge der Insekten -Die Stemmata der höheren Dipteren und ihre Abwandlung zum Bolwig-Organ

Evolutionary pathways to the larval eyes of insects. Higher Dipteran stemmata and the evolutionary development of Bolwig's organ 1. A cornrehensive morphological study of the photoreceptors in the so-called hemi — Institut fur Biologie I (Zoologie) der Albert-Ludwigs-Universitat, Albertstr. 21a, D-7800 Freiburg i. Brsg., FRGand acephalic lrvae of Brachycera was undertaken. In Brachyceran larvae the head casule originally is more or less retracted into the thorax. The larval hotoreceptor in Musca and Drosophila, here called Bolwig's organ, is situated on the outer side of the cephalopharyngeal apparatus, well below the surface. The aim of this study is to elucidate the homology of Bolwig's organ, i. e. whether Bolwig's organ originated from typical stemmata during the evolution of Cyclor-rhaphan larval organlsation or whether it represents a unique type of hotoreceptor. Larval photoreceptors and the anatomy of the head capsules of representative Erachycerans were examined by means of light and electron microscopy. In particular, the site and the structure of the photofecetors of Stratiomys (Stratiomyidae, Solva (Solvidae), Atherix ibis (Athericidae), Rhagio (Rhagionidae), Thereva (derevidae), Lonchoptera (Lonchoteridae), Epistrohe balteata, Volu-cella bombylans, Eristalis tenax (Syrphidae), Drosophila (f;rosophilidae), jannia and Musca domestica (Muscidae) were investigated. 2. Brachyceran stemmata are either situated on the ocular plates of the free remnant of the head (Stratiomyidae, Therevidae), or on the inner wall of an epidermal invaination, which includes epithelia of the head as well as of the thorax (Tabaniformia). In adfition, they can be found on the outer side of the tentorial phragmata (Cyclorrhapha). In all taxa studied, stemmata keep contact directly with the epithelium or through short processes of eithelial cells. 3. Ancestral Brachycera have fused stemmata that correspond with typical larval eyes of other holometabolan insects. 4. A cornea is found solely in Stratiomyidae and Xylophagidae, in contrast to all other taxa, where no dioptric apparatus is found. 5. In “Orthorrapha” and in “Aschiza”, rhabdomeric photoreceptors occur, forming a fused, star-shaped or Ktticed rhabdom. In Schizophoran larvae, retinular cells are distinguished by a different type of surface enlargement of the photoreceptor cell membrane. Here, the membrane forms flat ramella. The latter originate from transformed and/or rudimentary rhabdomeric microvilli. 6. In the primitive “Orthorrhapha”, pigment cups can be found, that are composed of retinular pirnent, whereas in derived Brachycera, pigment grains are absent. In Cyclorrhapha, tentorial piramata and their optic deressions operate as external and functional piment cups 7. In Therevidae and in Syrphidae, a tracheal tapetum lucidum can be founfi which la, been evolved independently in both families. The tapetum is always found together with inversely orientated retinular cells. 8. The homology of Brachceran stemmata is shown by a transformation series of stem-mata's site and fine structure. The homology is corroborated by the identical innervation in primitive and derived taxa, the identical site within Cyclorrhaha, and structural similarities of “Orthorrhaphan” and primitive Cylorrhaphan stemmata. Although transformations of the fine structure appear as a sequence or reductions, particular adaptations such as multilication of retinular celfs (Epistrophe), or deepening of the pigment cup (Musca) have evolved separately. Despite the fact that in Brachyceran larvae the head has been reorganized, the eyes still keep contact with the epithelium of the frontal sacks and/or tentorial phragmata. 9. The transformation series given shows that Bolwig's organ represents a highly modified stemma.

μ-XRF/μ-RS vs. SR μ-XRD for pigment identification in illuminated manuscripts

For the non-destructive identification of pigments and colorants in works of art, in archaeological and in forensic materials, a wide range of analytical techniques can be used. Bearing in mind that every method holds particular limitations, two complementary spectroscopic techniques, namely confocal μ-Raman spectroscopy (μ-RS) and μ-X-ray fluorescence spectroscopy (μ-XRF), were joined in one instrument. The combined μ-XRF and μ-RS device, called PRAXIS unites both complementary techniques in one mobile setup, which allows μ- and in situ analysis. μ-XRF allows one to collect elemental and spatially-resolved information in a non-destructive way on major and minor constituents of a variety of materials. However, the main disadvantages of μ-XRF are the penetration depth of the X-rays and the fact that only elements and not specific molecular combinations of elements can be detected. As a result μ-XRF is often not specific enough to identify the pigments within complex mixtures. Confocal Raman microscopy (μ-RS) can offer a surplus as molecular information can be obtained from single pigment grains. However, in some cases the presence of a strong fluorescence background limits the applicability. In this paper, the concrete analytical possibilities of the combined PRAXIS device are evaluated by comparing the results on an illuminated sheet of parchment with the analytical information supplied by synchrotron radiation μ-X-ray diffraction (SR μ-XRD), a highly specific technique.

Synchrotron-Based X-ray Spectromicroscopy Used for the Study of an Atypical Micrometric Pigment in 16th Century Paintings

Grünewald is a famous German painter of the 16th century, whose celebrity is associated with his unique skill in handling colors. This article presents the analysis of materials used to render a metallic aspect in the Isenhein Altarpiece and the Basel's Crucifixion. Such samples are challenging objects for microanalysis due to both chemical and physical complexity. Their study by synchrotron-based X-ray microscopy techniques was made possible thanks to recent developments carried out at the ID21 beam line (European Synchrotron Radiation Facility, ESRF). A submicron X-ray fluorescence probe revealed the main presence of lead, sulfur, antimony, and calcium. The fluorescence-line interferences (in particular K-lines of sulfur with M-lines of lead, and K-lines of calcium with L-lines of antimony) were resolved with the fitting program, PyMCA. 2D-mapping highlighted the presence of micrometer grains of sulfur and antimony into a lead matrix. XANES measurements were performed at both the sulfur K-edge and the antimony L-edge to refine information from an atomic to a molecular level. Beam stability was a key point in this study to selectively probe micrometer pigment grains, dispersed in the lead matrix. They confirm that the grains are made of stibnite (antimony sulfide), a very atypical pigment. Chemical mapping of sulfides is perfectly correlated with antimony mapping and provides a clear visualization of the stibnite pigments, in addition to their identification. Besides its artistic relevancy, this work aims at illustrating developments of synchrotron X-ray microprobe methods for the chemical characterization and observation of complex and micrometer-scale materials.

Determination of pigments in colour layers on walls of some selected historical buildings using optical and scanning electron microscopy

For successful restoration of painted walls and painted coloured finishing coats it is necessary to determine the composition of the original colour layers. Identification of the pigments used in The Cistercian Abbey of Stična and The Manor of Novo Celje was carried out using optical and scanning electron microscopy. Selected samples of wall paintings were inspected by the combined application of an optical microscope and a low-vacuum Scanning Electron Microscope to determine their colour and structural features and to identify the position of individual pigment grains. Energy dispersive spectroscopy was used to determine the elemental distribution on selected surfaces and elemental composition of individual pigments. It was found that the most abundantly used pigments were iron oxide red, cinnabar, green earth, umber, calcium carbonate white, ultramarine, yellow ochre and carbon black. These identifications have allowed us to compare the use of various pigments in buildings from different historical periods.

Structure and putative function of dark- and light-adapted as well as UV-exposed eyes of the food store pest Psyllipsocus ramburi Sélys-longchamps (Insecta: Psocoptera: Psyllipsocidae)

The psocopteran Psyllipsocus ramburi Sélys-Longchamps can render food stuffs unpalatable and may serve as an intermediate host for cestodes. Its two circular compound eyes consist of about 26 facets, capped by strongly convexly curved corneae of 10–18 μm in diameter. Corneal nipples or interommatidial hairs are not developed. Beneath each corneal lens a cluster of four cone cells, enveloped by two primary pigment cells, separates an ommatidial group of eight retinula cells from the inner corneal surface. Membrane specializations of the retinula cells, known as the microvilli, measure 60 nm in diameter, and collectively make up the rhabdom, which is columnar in shape and has a distal diameter of 4 or 5 μm, depending on whether it is day- or night-adapted. Cone cell lengths measure 4.5 μm during the day and 8.5 μm at night and retinula cell screening pigments closely approach the edge of the rhabdom during the day. A 1-h exposure to UV-A ( λ max=351 nm) of ca. 1200 lx causes an almost total destruction of the photoreceptive membranes of the rhabdom and bleached all retinula cell screening pigments, but not the pigment grains of the primary pigment cells. Calculations, based on the anatomical data, suggest that the eyes are adapted to function under dim light levels, but cannot produce sharp images since their best possible acceptance angles are 22° and 28° in light- and dark-adapted states, respectively. Destruction of vision, likely affecting biorhythm and reproduction, by exposing the insects to UV-A may offer an alternative to the use of chemicals in controlling these insects.

An acrylate polymerisation initiated by iron doped titanium dioxide

Fe 3+ ion doped anatase samples with homogenously distributed Fe 3+ ions within the TiO 2 particles were prepared using a sol–gel technique followed by a thermal treatment of the resulting powders. The photocatalytic initiation of the polymerisation of an ethoxylated trisacrylate was investigated as a function of the Fe 3+ content of TiO 2 pigment grains. The polymerisation rate slightly increases with increasing Fe 3+ content of the TiO 2 in the concentration range from 0.1 to 1 mol% Fe 3+. Maximum polymerisation rate r p max is observed at 1 mol% Fe 3+ followed by decreasing rates at increasing Fe 3+ contents. A TiO 2 sample containing 10 mol% Fe 3+ does not show any photocatalytic activity. For the polymerisation the heterogeneous photocatalytic formation of initiating radicals is essential. Thus the results of the photopolymerisation experiments may be explained on the basis of the charge carrier lifetime in the surface/subsurface region and the UV–vis absorption properties of the samples: Photo-EMF investigations reveal that Fe 3+ doping decreases the charge carrier lifetime in the surface region of TiO 2 from about 23 ms for the undoped material down to about 1.5 ms for samples containing 3 mol% or more Fe 3+ indicating a very fast capturing of photochemically produced electrons by Fe 3+. For homogeneously distributed Fe 3+ contents ranging up to 1 mol% this detrimental effect of Fe 3+ doping may be compensated by an improvement of light absorption properties of the TiO 2 crystallites in the visible range. At higher Fe 3+ concentrations XRD amorphous Fe 2O 3- or iron-rich TiO 2-phases are formed at the grain boundaries. These iron-rich phases act as light filters, so reducing the light absorption by the photocatalytically active TiO 2 crystallites. These amorphous phases should not show any photocatalytic activity.

Scientific analysis versus restorer’s expertise for diagnosis prior to a restoration process: the case of Santa Maria Church (Hermo, Asturias, North of Spain)

Raman spectroscopy with portable fiber optics microprobe, FT-IR spectroscopy and ion chromatography have been applied to the study of mortars and wall paintings in the church of Santa Marı́a de Hermo (Asturias, North of Spain) before the restoration works. This scientific analysis was performed to compare with a diagnosis report based on restorer’s expertise. The church showed problems of damp as well as clear evidences of flora, fungus and mould presence. The combination of both vibrational spectroscopic techniques made it possible to determine the chromatic palette of the wall paintings composed of: CaCO 3, carbon black, red ochre, yellow ochre, minium and cinnabar. In addition, some decay products have been determined, such as nitrate salts, and gypsum. From the conclusions of the scientific analysis the supposedly distemper wall paintings from the XVIIth century were confirmed as frescoes and were then dated back to the XVth century by the art historians. The detachment of the pigment grains from the wall paintings is attributed to the loss of the binding power of CaCO 3 by its partial transformation into calcium nitrate due to chemical reactions with nitrates. Clear evidence of nitrate migration from the graveyard behind the northern wall of the church has been determined from the quantification of nitrate salts by ion chromatography.

Analysis of bulk and inorganic degradation products of stones, mortars and wall paintings by portable Raman microprobe spectroscopy.

This work reports the use of a portable Raman microprobe spectrometer for the analysis of bulk and decaying compounds in carbonaceous materials such as stones, mortars and wall paintings. The analysed stones include limestone, dolomite and carbonaceous sandstone, gypsum and calcium oxalate, both mono- and dihydrated, being the main inorganic degradation products detected. Mortars include bulk phases with pure gypsum, calcite and mixtures of both or with sand, soluble salts being the most important degradation products. The pigments detected in several wall paintings include Prussian blue, iron oxide red, iron oxide yellow, vermilion, carbon black and lead white. Three different decaying processes have been characterised in the mortars of the wall paintings: (a) a massive absorption of nitrates that reacted with calcium carbonate and promoted the unbinding of pigment grains, (b) the formation of black crusts in the vault of the presbytery and (c) the thermodecomposition of pigments due to a fire.

Post-embryonic photoreceptor development and dark/light adaptation in the stick insect Carausius morosus (Phasmida, Phasmatidae)

The aims of this paper have been (a) to document postembryonic eye growth in the common laboratory stick insect Carausius morosus and (b) to examine whether the capacity of the eye to adapt to changing light intensities varied with age. We found that number of facets, corneal thickness, ommatidial diameter, widths of cone and retinal layers, and rhabdom volume increased linearly. Interommatidial angles, pigment grain sizes, and microvillar diameters, however, remained approximately the same. On the basis of the morphometric data we calculated that sensitivity in the adult stick insect eye was at least tenfold that of the eye of first instar nymphs and could show that adults would be able to perceive a similar amount of detail at considerably dimmer ambient light than the smaller individuals. In terms of resolving power this means that light- and dark-adapted first instar stick insects possess acceptance angles of 5.3° and 8°, respectively and that the corresponding figures for the adult eye are 4.7° and 7.3°. The bigger (and more light-sensitive) eye of fully grown C. morosus makes protection against radiation damage a more serious issue in the adult individual than the small first instar. The findings explain why smaller (=younger) stick insects are less nocturnal than mature, fully-grown individuals. It is, thus, not surprising to see that dark/light adaptational photomechanical changes affecting pigment position and rhabdom widths, are more pronounced in the eyes of the adults.

The penetration depth and lateral distribution of pigment related to the pigment grain size and the calendering of paper

The interaction of ink and newspaper has been investigated and the specific question of penetration of ink into the paper has been addressed with a nuclear microprobe using particle induced X-ray emission. The penetration depth of the newsprint is a critical factor in terms of increasing the quality of newsprint and minimising the amount of ink used. The objective of the experiment was to relate the penetration depth of pigment with the calendering of the paper. The dependence of the penetration depth on the pigment grain size was also studied. To study the penetration depth of pigment in paper, cyan ink with Cu as a tracer of the coloured pigment was used. For the study of the penetration depth dependence of pigment size, specially grounded Japanese ink with well-defined pigment grain size was used. This was compared to Swedish ink with pigment grains with normal size-distribution. The results show that the calendering of the paper considerably affects the penetration depth of ink.

Differentiation of antique ceramics from the Raman spectra of their coloured glazes and paintings

AbstractTwo palettes, characteristic of the production of the Sèvres Factory (Manufacture Nationale de Sèvres), and representative of ancient glazing techniques, were studied. One was made with coloured glazes/enamels on Bisque (1050 °C) and the other one was composed of Moufle (880 °C) painting colours. Spectra specific to the white (opacifiers), yellow (including Naples yellow), green, pink, ‘brown–red’ and black coatings/paintings were considered. Emphasis was placed on the different Raman peaks arising from coloured coatings specific to ancient artefacts: greens obtained with dissolved Cu ions or from chromium pigments (Victoria green garnets), Naples yellows (Pb2Sb2O7 pyrochlore), green and black spinels and pink sphene. Attention was paid to the effect of pigment grain size on the Raman spectrum. To illustrate the capability of the method, we analysed some old artefacts from the eighteenth and nineteenth centuries. Copyright © 2001 John Wiley & Sons, Ltd.

Ultrastructure of the Testis of the Mexican Fruit Fly (Diptera: Tephritidae)

The testis of Anastrepha ludens (Loew) has a special histological structure, which is different from the basic morphological arrangement. The organ consists of a single saclike follicle containing all the development zones of male sexual cells. The base of the follicle is modified as a seminal vesicle where spermatozoa mature. In the testis wall there is a peritoneal sheath of large cells, closely associated with tracheoles. The main feature of these cells is a cytoplasm full of pigment grains. The peritoneum is located on a continuous layer of muscles, not previously reported in insect morphology. There is an apical follicular epithelium supported by a thick basement membrane, and an epithelium at the base of the testis forming a chamber where the spermatozoa are released from the cysts. Most of the mature spermatozoa are inside deep folds of the basal epithelium cells.

Investigation of pigments in medieval manuscripts by micro raman spectroscopy and total reflection X-ray fluorescence spectrometry

Analytical pigment investigation can reveal important information for art-historians. The use of two micro-analytical techniques, micro Raman-spectroscopy and total reflection X-ray fluorescence, is shown to be very successful. These techniques give complementary information: Raman spectroscopy allows the identification of the different materials the pigment grains consist of, and total reflection X-ray fluorescence gives the average elemental composition of the sample. The latter is helpful to distinguish between the different palettes used by different artists or workshops. Samples can be taken with cotton wool swabs (Q-tips), which cause no visible alteration to the artefact. Such a sample allows both Raman-spectroscopy and total reflection X-ray fluorescence to be applied using less than 1 μg of material. This approach is illustrated with the pigment investigation of seven late-medieval Mercatellis manuscripts, kept in the Ghent University Library.

Spectrochemical characterization by micro-FTIR spectroscopy of blue pigments in different polychrome works of art

Analytical examination of blue paint samples, taken from different artworks dating from the second century AD until 1960, was carried out using FTIR microspectroscopy (both in reflection and transmission mode, the latter with the aid of a diamond cell) supported by micro-Raman spectroscopy and SEM observations as well as EDX spectrometry. Pigments examined included azurite, Egyptian blue and ultramarine blue, either natural or artificial. Characteristic spectra for each pigment are presented and an account for the principal differences observed is given. In particular, the FTIR's capability of probing single pigment grains on the surface of paint cross-sections, when integrated with a microscope, brought to light frequency differences and orientation behaviour of the bands of the various minerals used as painting materials. Indeed, a striking effect of shifting and variation of intensity of some of the bands of azurite and Egyptian blue was noted and related to polarization phenomena occurring when the incident light stroke differently oriented faces of the single pigment crystals. The effects of different mineralogical composition of the semi-precious stone lapis-lazuli (from which ultramarine blue is derived) on the appearance of the spectrum of the pigment are also discussed. Finally, a means of distinguishing the natural from the artificial ultramarine blue pigment, on the basis of their FTIR spectra, is proposed.

Monte Carlo simulation of light scattering on turbid layers with spherical pigment grains

Visible-light scattering on glass spheres in resin is considered as qualitatively representative of the actual situations of visible effects due to coatings or plastic sheets with non-absorbing pigments like mica pigments. Diffuse reflection and transmission are simulated by the Monte Carlo technique. The model consists of glass spheres in acrylic resin with section between 7.5 and 30 µm such that geometrical optics can be applied. Phenomena like shot and iridescent effects are described.

Flow‐induced uptake of particulate matter in permeable sediments

We demonstrate the fast transfer of suspended particles from the boundary layer into the upper strata (z < 4 cm) of permeable sediments with topography‐related interfacial water flows. The transport is driven by pressure gradients (▵ P ≤ 3 Pa) generated when bottom flows (u ≤ 10 cm s−1) are deflected by small surface structures (z < 3 cm) of hydrodynamical or biological origin. Acrylic pigment grains of 1‐ and 10‐µm diameter traced the intrusion of particulate matter into sandy sediment (k > 2 × 10−11 m2) incubated in a laboratory flume. Increased pressure up‐ and downstream of small mounds (z = 2.5 cm) drove water 5.5 cm into the core, carrying suspended particles (1 µm) to 2.2‐cm sediment depth within 10 h. Simultaneously, decreased pressure at the downstream slope of the protrusions drew pore fluid from deeper layers (z ≤ 10 cm) to the surface. In the sediment, friction reduced the velocity of the particulate tracers, resulting in size fractionation and layers of increased particle concentration. Ripple topography (0.8–2.8 cm high) enhanced interfacial particle (1 µm) flux by a factor 2.3 when compared to a level control core. The pathways of the particle and solute tracers below a sediment ripple are explained with a source‐sink model that describes the pore flow velocity field. Our results suggest that bedform‐induced interfacial flows are important for the uptake of particulate organic matter into permeable shelf sediments.

The effect of relative humidity and pigment type on paint films

The Courtauld Institute of Art in London has initiated a project to determine the moisture content of ten differently pigmented paint films [1]. This was carried out by exposing the paint films to varying conditions of relative humidity and weighing the paint films at selected time intervals until they reached equilibrium moisture content. The rationale for this is to provide paintings' conservators with data on the moisture response of paint films. Moisture is used in conservation treatment to remove surface deformations on paintings [2,3]. A measure of the time to reach equilibrium moisture content and the quantity absorbed in terms of weight uptake for different pigment types is of importance. Diffusion coefficients for moisture sorption of these paint films have also been calculated for different relative humidity values and pigment type. The aim of this paper is to use thermogravimetry (TGA) to continuously monitor the increase in mass with moisture uptake as a function of time for a given temperature and relative humidity. The paint films were provided by the Courtauld Institute of Art. They had been prepared in 1991 from mixtures of prussian blue and basic lead carbonate oil-based tube paints in different proportions. The paint films were conditioned at 20°C and at 30%, 60%, 75% and 85% RH. The electrical properties of the same paint films were then studied by low frequency dielectric spectroscopy (LFDS) over the frequency range 10 −4 to 10 5 H 2. LFDS was selected because it is non-destructive and it gives additional information on the structure and behaviour of multicomponent complex systems. One of the measured parameters, the critical frequency, was found to increase with increasing values of relative humidity and moisture content. It was shown that values for both the capacitance and conductance increased with increases in relative humidity. The dielectric responses indicated that adsorbed water layers formed on the interior surfaces of one of the components of the paint films. The observed effect was much greater in the prussian blue than in the basic lead carbonate paint films and so it can be deduced that the adsorbed water layers are in fact formed on the surfaces of the pigment grains. This was further substantiated by direct temperature-resolved pyrolysis mass spectrometry (DTMS) where it has been shown that there is a reduction in the long chain fatty acids (e.g. stearic, palmitic) in the hydrated samples. A model is presented to describe the “charge hopping” process which occurs in the adsorbed water layers and which gives rise to the low frequency dispersion.