Barbara H. Berrie

Unusual pigments found in a painting by Giotto (c. 1266-1337) reveal diversity of materials used by medieval artists

BackgroundThe important trecento Florentine artist Giotto (c. 1266-1337) is renowned for his naturalistic and realistic works in tempera and fresco. His innovative paintng style involved painting expressive, emotive faces and use of pictorial devices for depicting space. This report focuses on the analysis of the materials and methods used in a panel in the collection of the National Gallery of Art, Madonna and Child (1310/1315).ResultsGiotto used inky washes under thin layers of egg tempera paint. Yellow iron earth and lead tin yellow are present in the paint used to depict the lining of the Virgin’s mantle. SEM-EDX of one of the yellow pigments confirmed it is lead tin yellow type II, PbSn1-xSixO3. The ratio of colorant to the glassy phase indicates this material was produced for use as a pigment rather than as a glass. Ultramarine was not used in this painting, azurite is the blue pigment. The azurite used here does not contain elemental impurities, however malachite and the rare green–blue mineral mixite, BiCu6(OH)6(AsO4)3(H2O)3, are found in the blue paint.ConclusionsWe do not know if Giotto intentionally used mixite as a green–blue pigment or if it was adventitious with the azurite, though given the rarity of the mineral, the latter is more probabale. Nevertheless, the artist’s choice of color was deliberate and the presence of mixite and the clear-hued lead tin yellow pigment attest to his coloristic decisions, in this case opting for cool hues rather than the warm blue of ultramarine which we typically associate with paintings of the Madonna from this time period.Graphical abstractA microscopic sample from Giotto’s Madonna and Child (c. 1310/15) shows the paint layer structure and range of pigment colors. SEM-EDX reveals the chemical diversity of the phases, and BSED proves the presence of the rare mineral mixite, BiCu6(OH)6(AsO4)3H2O

Partially hydrolyzed poly(vinyl acetate)-borax-based gel-like materials for conservation of art: Characterization and applications

Abstract A versatile gel-like system for the treatment of art has been prepared from partially hydrolyzed poly(vinyl acetate), borax, and large fractions of ethanol, isopropyl alcohol, n-propyl alcohol, and acetone. Variables such as the concentrations of the two gelating components, the degree of hydrolysis and molecular weight of the polymer, and the type of liquid gelated were investigated to establish formulations of gels with physical and chemical properties that are best suited for specific applications. The gels were designed to have an elastic character that allows them to conform to the topography of complex surfaces and be removed with ease by being lifted from the surface. Results from fluorescence studies demonstrated that the solvent is constrained within the area of the gel, allowing for localized treatment. Polymer and boron residues were not detected after cleaning tests on acrylic and dammar test paint-outs, and on two oil paintings with degraded surface coatings. The efficacy of the cleaning systems was determined visually. Studies of the materials removed during treatments showed that the gels appear to act by softening the coating surface while typically a pass with a solvent-dampened swab after gel treatment removes the softened coating. Two case studies and notes on other applications of the gels are described; recipes and preparation procedures are included.

Reaction of Pb(II) and Zn(II) with Ethyl Linoleate To Form Structured Hybrid Inorganic–Organic Complexes: A Model for Degradation in Historic Paint Films

To investigate soap formation in drying oils in historic paints, the reaction between metal acetates (K+, Zn2+, Pb2+) and ethyl linoleate (EL) was studied using optical microscopy, X-ray powder diffraction, and electron microscopy. Pb(II) and Zn(II) react rapidly with EL to form highly structured, spherulitic, luminescent crystallites that aggregate. Evidence from Fourier transform infrared (FTIR) and scanning electron microscopy/energy dispersive X-ray analysis and high-resolution synchrotron powder X-ray diffraction indicates that these are organic–inorganic hybrid complexes or coordination polymers. FTIR absorbance peaks at ca. 1540 cm–1 for Pb(II) and ca. 1580 cm–1 for Zn(II) are consistent with the formation of carboxylate complexes. The complexes formed offer insight into the degradation processes observed in oil paint films, suggesting that soap formation is rapid when metal ions are solubilized and can occur with unsaturated fatty acids that are present in fresh oils. These complexes may account for the atypical luminescence observed in lead-containing cured oil paint films.

Change is permanent: thoughts on the fading of cochineal-based watercolor pigments

BackgroundColor change in artworks has been commented on for centuries. Fading of watercolor pigments is a notable alteration. Pigments based on carminic acid are among those particularly prone to color loss, but the mechanism and factors are not well understood.ResultsWe painted out three pigments prepared from the aqueous extract of carminic acid (CA) from Dactylopius coccus: the uncomplexed, and aluminium- and tin-complexed lakes. These were applied in a 2% gum Arabic solution to papers that were acidic, neutral or alkaline pH and exposed to accelerated light aging in a weatherometer. The comparative rate of fading was dependent on the complexation and on the pH of the substrate. On alkaline paper, the Al complex was least light stable, on neutral paper the three colorants responded similarly, and on acidic paper the tin complex was the least light stable. This is discussed in light of the published information on reactions and mechanism of color loss of CA.Graphical abstractCochineal-based pigments fade on exposure to light. In watercolor paints, the rate depends not only on the complexation of the ligand but also the pH of the paper substrate

Soft, Peelable Organogels from Partially Hydrolyzed Poly(vinyl acetate) and Benzene-1,4-diboronic Acid: Applications to Clean Works of Art

We have developed soft, peelable organogels from 40% hydrolyzed poly(vinyl acetate) (40PVAc) and benzene-1,4-diboronic acid (BDBA). The organic liquids gelated include dimethyl sulfoxide, dimethylformamide, tetrahydrofuran, 2-ethoxyethanol, and methanol. The rheology of these soft materials can be tuned by altering the concentration of the polymer and/or crosslinker. Insights into the mechanisms leading to gelation were obtained from 1H NMR experiments, fluorescence measurements, and studies comparing properties of materials made from BDBA and phenylboronic acid, a molecule incapable of forming covalent crosslinks between the polymer chains. These organogels can be easily peeled off a surface, leaving no residue detectable by UV-vis spectroscopy. They are demonstrated to be effective at softening and removing deteriorated coatings from water-sensitive works of art and delicate surfaces. They have the needed characteristics to clean topographically complex surfaces: good contact with the surface, easy removal, and little to no residue after removal. A 2-ethoxyethanol organogel was used to remove oxidized varnish from a 16th century reliquary decorated with painted gold leaf, and an ethanol gel was used to remove solvent-resistant coatings from 16th and 18th century oil paintings.

Colloidal Properties of Aqueous Poly(vinyl acetate)-Borate Dispersions with Short-Chain Glycol Ethers.

We report the influence of adding five short-chain glycol ethers (SCGEs) on the structure, stability, and viscoelastic properties of aqueous dispersions of partially hydrolyzed poly(vinyl acetate) and borax. The properties of these gel-like materials have been investigated as a function of the structure of the added SCGE both below and above the critical aggregation (or micellar) concentrations using (11) B and (13) C NMR, rheology, and small-angle neutron scattering. The results indicate that the SCGE aggregation behavior is not affected by incorporation into the gel-like network. However, changes in the viscoelasticity and structural properties of the dispersions were detected that can be correlated to the nature of the solvent system. Also, the ability of these materials to clean an unvarnished acrylic paint surface coated with synthetic soil has been evaluated using colorimetery, and the surface of the dispersion after cleaning was visualized with scanning electron microscopy.

THE EFFECT OF pH OF THE PAPER SUPPORT ON THE FADING OF CARMINE WATERCOLOR

The colored extract from the female scale insect (Dactylopills coccus) has been a major source for red pigments and dyes called cochineal or carmine. The colorant is carminic acid [I]. The preparation of carmine pigments from the extract from the cochineal insect varies: it can be modified by the addition of potassium tartrate or potassium oxalate, either alone or combined with calcium, aluminum or tin salts. Other salts have been employed but appear to have been used much less frequently. The cations, such as aluminum or tin, form complexes with the carminic acid. Tin complexes of cochineal are considered to produce the brightest and finest reds. However, it is known empirically and experimentally that pigments based on preparations containing tin are the least stable in the presence of light [2]. The chemistry of the fading reactions of uncomplexed cochineal has been studied. The loss of color is due to photooxidation. The fully deprotonated form of the colorant has been found to be the least stable. Visible light (436nm) was shown to be almost as effective as ultraviolet in fading cochineal in pork luncheon meat [3]. The effect of pH on the fading of cochineal used to color meat products suggests that a similar pH effect might be found for the stability of cochineal used in watercolor paints. The authors are investigating this hypothesis by preparing pigments based on cochineal according to late nineteenth-century recipes and measuring their stability on paper substrates treated to have different pH [4]. We are studying the stability of three variants of the pigment: the pure colorant; the aluminum lake; and the aluminum/tin lake. Paint was prepared by mixing the pigments with deionized water and it was applied to paper using a paint spray gun until a reflectance of 35-45% was reached [5]. The papers used in this study are buffered acid-free tissue and Whatman qualitative filter paper (No.3). To provide various pH, Whatman paper substrates are used untreated; washed in calcium hydroxide solution to give an alkaline paper; and washed in alum/gelatin solution to simulate a traditional watercolor paper with a pH below 7. The samples are undergoing light aging in an Atlas weatheromctcr equipped to approximate sunlight that has passed through a plate glass window. Color changes in the paint are monitored using a Hunter Lab spectrophotometer. Preliminary data (light aging to complete disappearance of color) suggest that carmine on a neutral Whatman paper support fades more readily than carmine on a prepared watercolor paper. Quantifying the effect of pH on the stability of these cochineal pigments will provide useful information for specifying archival housing, exhibition conditions and aqueous treatment of watercolor paintings on paper. Yoonjoo Strumfels is a paper conservator and Barbara H. Berrie is senior conservation scientist at the National Gal/elY of Art, Washington, DC 20565, USA.