WORKSHOP Fiber Optic Reflectance Spectrometry for pigments identification in paintings Marco Gargano Department of Physics
Light and Colors Primary source of color Incandescence luminescence Scattering Diffraction and interference Interference physical object Secondary source of color Absorbtion Absorbtion and emission chemical
Color for scattering sky is blue sunset is red
Color for interference
Color for diffraction and interfence
Color for refraction
Color for polarization
Color for reflection and absorbtion We have selective reflection and absorbtion of certain wavelengths of light.
reflected light Absorbed light We just see the reflected light!
Interaction light-object LIGHT EMISSION This radiation has interacted with the object CARRIES COLOR INFORMATION DIFFUSE REFLECTION Lambertian diffusion LIGHT SOURCE SPECULAR REFLECTION This radiation didn t interact with the object CARRIES ILLUMINANT (SOURCE) INFORMATION
Diffuse reflection Reflection of light with no directional dependence from the incident ray
Specular reflection A ray from a single incoming direction is reflected into a single outgoing direction. Normal Surface The direction of incident ray and the direction of reflected ray make the same angle with respect to the surface normal (Snell s law) Mirror Metal smooth surface Smooth surface in general
SPECTRAL REFLECTANCE REFLECTANCE IS A PHYSICAL PROPERTY OF THE OBJECT Amount of back scattered radiaction from a surface depending on its wavelenght. reflectance reflected intensity R(λ)= incident intensity absorbance
reflectance absorbance Reflectance spectra
Color and pigments know the color does not mean knowing the pigment Color Pigment Different colors can be obtained from the same pigment and different pigments can give the same color
widespread pigments GREEN CHLOROPHYLLE ABSORBTION ORANGE CAROTENE RED OXYHEMOGLOBINE
Organic and Inorganic pigments Organic pigments are made of living substances or substances that were once part of living things like animals, plants or synthetic treatment of plants. Inorganic are derived from earth deposit, minerals, metallic compounds, oxides, sulfure, sulfate
Organic pigments Cochineal Rubia s roots purple Indigo plant
Inorganic pigments Malachite basic copper carbonate Ultramarine-lapislazuli Verdigris-copper acetate Lead white-biacca-basic lead carbonate
Preparing color Pigment s grinder
Preparing color + Egg tempera } + Oil
Interaction between light and pictorial layers varnish Reflection Rifraction Absorbtion Pictorial layer Priming layer
Why using spectrophotometry? In the field of conservation we can know the condition of the pigment in terms of colorimetrics information that can be used for proper cleanings or restorations controls of alteration of pigments In historical and artistic field Identification of pigments in the surface layers (in combination with other techniques) to define palette of an artist or a school to help also restorers in restorations to have an indirect dating of the materials
Identifiability of pigments Pigment identification is based on finding one or more spectral characteristics that are fixed and different from those of other pigments. This is true for most of the pigments There is a need of large database
pros Portable Non-invasive Easy to perform cons S-shaped pigment (ambiguos interpretation), dark pigments, blackened pigments Varnish Yellowing Mixture of pigments Often need complementary analyses: X-Ray Fluorescence, Scanning electron microscope, Raman, etc
Example of integration of different analyses to obtain complementary information 1400 1200 Cu 1000 Counts (150 s) 800 600 400 200 0 Ca Fe Cu 0 2 4 6 8 10 12 14 16 18 20 Pb Energy [KeV] XRF (elemental analysis) Pb Pb Spectral reflectance (%) FORS Wavelenght (nm) Hans Memling, Crucifixion (1468-70) Vicenza, Musei civici
Color: blu, pigment:azurite
Color: blue; pigment: ceruelan
Color: green, pigment: copper resinate
Color: yellow, pigment: naples yellow
Color: red, pigment: chrome red
Color:yellow, pigment: ochre
Color and pigment: violet
What to do in the (20 min) session?
the instrumentations
1 2 3 4 5 6 7 8 Connector Slit Filter Collimating Mirror Grating Focusing Mirror Detector Collection Lens CCD Detector
artwork to be investigated with unknown pigments Database table to compare the unknown pigments
15.10 15.30 15.30 15.50 15.50 16.10 16.10 16.30 Workshop I upstairs 16.40 17.00 17.00 17.20