3.1. Materials
For the preparation of the carminic acid reference and reconstructions of Winsor & Newton cochineal lake pigments and paints the following reagents of analytical grade were used: carminic acid (229253), alum (aluminium potassium sulphate dodecahydrate, AlK(SO4)2·12H2O), borax (sodium tetraborate, Na2B4O7·10H2O), plaster of Paris (calcium sulphate hemihydrate, CaSO4·½H2O), potassium carbonate (K2CO3), vermilion (mercury(II) sulphide, HgS), and PVAc ([CH2CH(O2CCH3)]n) from Sigma-Aldrich; citric acid monohydrate (C6H8O7·H2O) and zinc sulphate heptahydrate (ZnSO4·7H2O) from Panreac; cream of tartar (potassium hydrogen tartrate, KC4H5O6) from Scharlau; calcium carbonate (CaCO3) from Roig Farma; and ammonium carbonate ((NH4)2CO3) from Alfa Aesar. Cochineal (36040) and gum-arabic from Kremer Pigmente, and fresh milk from Vigor were also used.
For the preparation of the colloidal silver nanoparticles used for the SERS analyses, potassium nitrate (KNO3) from Riedel-de Haën, silver nitrate (AgNO3) from Merck, and sodium citrate tribasic dihydrate (Na3C6H5O7·2H2O) from Sigma-Aldrich were used.
For the preparation of the samples for ICP-AES analyses, nitric acid (HNO3 69%) from Panreac was used.
Millipore water was used on all occasions.
3.2. Apparatus
pH measurements were carried out with a Sartorius Docu-pH Meter. Calibration was performed with pH 4 and 7 buffer solutions from Panreac.
X-ray fluorescence data was obtained with an ArtTAX spectrometer, Intax GmbH, with a molybdenum (Mo) anode, Xflash detector refrigerated by the Peltier effect (Sidrift) and a mobile arm.
The spatial resolution was 70 mm and the following experimental parameters were used: 35 kV of of acquisition time.
The ICP-AES analyses were performed at the REQUIMTE Analysis Laboratory (NOVA University of Lisbon). The determination of the elements Al, Ca, B and Zn was carried out in a ICP (Inductively Coupled Plasma-Atomic Emission Spectrometer) Horiba Jobin-Yvon, France, Ultima, model equipped with a 40.68 MHz RF generator, Czerny-Turner monochromator with 1,00 m (sequential) and autosampler AS500. Samples were analysed in aqueous solution. Circa 3 mg of sample were digested with HNO3 69% in an ultrasonic bath at ca. 70 °C. After complete dissolution, the HNO3 solution was diluted with water to a total volume of 2 mL and 10% concentration (v/v) and put again in the ultrasonic bath. Analyses were only carried out in one sample for each reference due to the high amount of sample required.
SERS and Raman spectra were collected with a Horiba Jobin-Yvon Labram 300 spectrometer equipped with a HeNe laser of 17mW power operating at 632.8 nm. Spectra were recorded as an extended scan. The laser beam was focused with an Olympus 50x objective lens. The laser power at the sample was controlled with the use of a neutral density filter with 0.3 optical density (8.5 mW).
All samples were analysed with a laser exposure time of 15 seconds for 15 scans.
Infrared analyses were carried out using a Nicolet Nexus spectrophotometer coupled to a -A detector cooled by liquid nitrogen. Spectra were obtained in transmission mode, between 4000 and 650 cm-1, with a resolution of 4 cm-1 and 128 or 256 scans. Samples were previously compressed using a Thermo diamond anvil compression cell. Spectra are shown here as acquired, without corrections or any further manipulations, except for the occasional removal of the CO2 absorption at ca. 2300-2400 cm .
Fluorescence excitation and emission spectra were recorded on a Jovin-Yvon SPEX Fluorog 3-2.2 spectrofluorometer coupled to an Olympus BX51 M confocal microscope, with spatial resolution controlled with a multiple-pinhole
spot, with a 50× objective. Standard dichroic filters of 525 and 570 nm were used at 45° to collect the emission and excitation spectra, respectively. Emission spectra were acquired by exciting at 510-515 nm and excitation spectra were carried out collecting the signal at 590-600 nm. Both were
excitation slits = 5/3/0.8 mm. The optimisation of the signal, through mirror alignment in the optic pathway of the microscope, was performed. Spectra were collected after focusing on the sample (eye view) followed by signal intensity optimisation (detector reading). Emission and excitation spectra were acquired in the same spot whenever possible.
FORS measurements were performed using two Zeiss spectroanalysers equipped with optical fibres: a MCS 601 UV/VIS model (with a 1024 Si photodiode array sensor) operating in the 190-1025 nm range, and a MCS 611 NIR 2.2 WR model (with a 256 InGaAs photodiode array detector) operating in the 910-2200 nm range, with a resolution of approximately 0.8 and 5.0 nm/pixel, respectively. A tungsten-halogen lamp (Zeiss model CLH600) was used. The 0°/2x45° reflectance configuration was adopted to avoid specular reflectance. A 99% Spectralon diffuse reflectance standard was used for calibration.
For measuring colour, a portable spectrophotometer colourimetry Data Color International was diffuse illumination from a pulsed Xenon arc lamp over the 8 mm-diameter measuring area, with 0º viewing angle geometry. The calibration was performed with a white bright standard plate and a total black standard.
The absorption spectra in solution were recorded with a UV-Vis-NIR Cary Varian 5000 spectrophotometer and absorbances were corrected to dilution.
Emission spectra were obtained using a Spex Fluorolog 212 I Horiba spectrofluorimeter upon excitation at 420 nm.
3.3 Experimental Section
The colloidal silver nanoparticles used for the SERS analyses were prepared by chemical reduction of the metal salt silver nitrate with sodium citrate following the procedure published by Lee and Meisel 1982. A 5-fold more concentrated colloid was used. The silver nanoparticles were prepared as follows: 100 mL of a 5x10-3 M silver nitrate aqueous solution were brought to gentle reflux under magnetic stirring in a two-neck round-bottom flask wrapped in aluminium foil and equipped with a thermometer and a condenser to prevent evaporation. When boiling, 2 mL of a 5%
w/v aqueous solution of sodium citrate were slowly added dropwise always keeping the boiling and the vigorous magnetic stirring. The colour of the solution turned into greyish green, indicating the reduction of the Ag+ ions. The solution was kept under magnetic stirring and in reflux for approximately one hour. Heating was then discontinued, and the solution was left under magnetic stirring until it slowly cooled down to room temperature. When not in use, the colloid solution (pH 8.1) was kept in the refrigerator in the dark, and it was stable for several months with reproducible results. A 0.5 M potassium nitrate aqueous solution was also prepared and used as the aggregating agent. When not in use, the aggregant solution was kept at room temperature. The SERS spectrum of the silver colloid after the addition of the aggregant in a proportion 5:1 is presented in Figure 49.
It exhibits signals in the 1100-750 cm-1 and 1500-1300 cm-1 regions related to the citrate ion and to by-products of the reduction reaction (Munro et al. 1995; Leona, Stenger, and Ferloni 2006 and reference cited therein).
For carminic acid and the Winsor & Newton cochineal lake pigment reconstructions, in which a
were deposited directly onto each sample in powder form in a lid of an Eppendorf. For the historical
way. For the paint samples from tubes of oil paint, in which a small quantity of sample was available, aggregant solution were deposited directly onto micro-samples of the paints in a lid of an Eppendorf. SERS spectra were collected by focusing the laser
beam onto the aggregates that formed inside the analyte-colloid droplet immediately after the deposition of the silver colloid and aggregant in the lid of the Eppendorf. Several spectra were acquired continuously and the best spectra obtained were selected. More than one sample from each reference was analysed to assure reproducibility. It should also be noted that Raman was tried on the pigments with lasers 632.8 nm and 785 nm and no good signal was obtained. The Raman spectra were featureless and displayed high fluorescence background.
Figure 49. SERS spectrum of the silver colloid after the addition of the aggregant (0.5 M KNO3) in a proportion 5:1.
Carminic acid from Sigma-Aldrich and a reference of carminic acid precipitated with aluminium (designated as CA-Al) were used for supporting the interpretation of molecular data. For preparation of the CA-Al reference, colour was extracted from cochineal insects in an aqueous solution with potassium carbonate, and alum was added until precipitation of a purplish pigment at neutral pH.
The precipitate was centrifugated, washed with water and allowed to air-dry.
Gum-arabic and PVAc paints were prepared by hand-grinding 0.05 g of each pigment with a pestle on an agate mortar and adding 20 drops of a 10% gum-arabic solution and 15 drops of a PVAc solution (20% (w/v) in acetone), respectively. Paints were applied onto glass slides with a For the accelerated light ageing experience, all paints were previously applied on the glass slides with a drawdown applicator.
For the preparation of the reconstructions of Winsor & Newton cochineal lake pigments the following conversion table was used for the extrapolation of the ingredient quantities:
Table 28. Conversion table.
Liquid Measures Abbreviations Metric Equivalents (mL)
1 dram 1 dr. 3.55
1 meg pot 1 mp. 47.333
1 gill 1 gill 142
1 pint 1 pt. 568
1 quart 1 qt. 1136
1 gallon 1 gal. 4540
Weight Measures Abbreviations Metric Equivalents (g)
1 grain 1 gr. 0.0648
1 dram 1 dr. 1.772
1 ounce 1 oz. 28.35
1 pound 1 lb. 453.6
1 hundredweight 1 cwt. 50800
Length Abbreviations Metric Equivalents (cm)
1 inch 1 in. 2.54
1 foot 1 ft. 30.48
Temperature Abbreviations Celsius (°C)
212 Fahrenheit 212 °F 100
The micro-sample of carmine paint used to validate the reconstruction of FOC as an accurate reference of an historical W&N carmine was taken from the painting Untitled (Coty), Amadeo de Souza-Cardoso, c. 1917, oil on canvas, Calouste Gulbenkian Foundation, Figure 50, and is presented in the work of Cristina Montagner (Montagner 2015, p.97). The micro-sampling was carried out in the context of the study of the materials and techniques of Amadeo de Souza-Cardoso.
Figure 50. Untitled (Coty), c. 1917, oil on canvas, Amadeo de Souza-Cardoso. Calouste Gulbenkian Foundation, Lisbon. The micro-sampling area is indicated with (see Montagner 2015, p.194). Photo by Cristina Montagner.