Detective Work: Investigating art

Why?? to help direct restorationto help conservationto aid interpretationjust to “know”

“Is a painting ever done?” Ball, Time as Painter, Chapter 11

Methods:1.spectroscopy (“quantify interaction with light”)2. chemical behavior ( “does it react?”)3. microscopy (“visual with a microscope”)

light source sample detector

The Basics of Spectroscopy:

Transmission

Reflection

lightcolor “thrown away(observed)

Transmission and Reflection effectively do the same thing: throw away unabsorbed colors

Absorption

Absorption

“Light” is a term that refers to a electromagnetic radiation.

And electromagnetic radiation are waves of different energies that extend over a broad range:

If the electromagnetic spectrum were a piano keyboard…..

cosmic gamma X-rays UV Vis IR Radio induction power

wavelength, nm 10 -8 10 -6 10 -4 10 -2 1 10 2 10 4 10 6 10 8 10 10 10 12 10 14 10 16 nm

frequency, Hz 10 26 10 24 10 22 10 20 10 18 10 16 10 14 10 12 10 10 10 8 10 6 10 4 10 2 Hz

the visible spectral region would be just one key!

X-rays UV Vis IR

light source sample detector

interacts with light;“removes” (absorbs)some light components

Interaction type: electronic vibration

Observed results: none none color heat

Chromate, CrO4 2-

“chromophore” in chrome yellow

X-rays UV Vis IR

Interaction type: electronic vibration

Observed results: none none color heat

X-rayaffectsinnerElectrons:Higher energy

UV/visaffectsouterelectrons:Lower energy

X-rays UV Vis IR

Interaction type: electronic vibration

Observed results: none none color heat

where in painting: penetrates all surface below paint

wavelength selected to interact most with black of underdrawing

X-rays interactionstrength dependson number of electrons:

Pb >> Zn, Ti

Lead white scatters X-raysmore than zinc white or titanium whiteor most other pigments (except HgS or Ba-pigments)

Infrared SpectroscopyInstrumentation

An infrared spectrophotometer is composed of:1) an IR light source, 2) a sample container, 3) a prism to separate light by wavelength, 4) a detector, and a recorder (which produces the infrared spectrum).

http://www.chemistry.ccsu.edu/glagovich/teaching/472/ir/instrumentation.html

SEE:AlizarinCarmineIndigo

Infrared Spectroscopy – compare two red dyes

Alizarin

Carmine

Carmine

Cochineal

begin with historical context:•when did artist/work occur•workshop/atelier context and influences?•location•information on commission/motivation for work

Feast of the Gods

An analysis by WebExhibits

Microscopy: What can you observe? Crystals? Shape: Are there faces or regular shapes or blobs?

Large or small? Homogeneous or a mixture?

Size? Pigment particles have characteristic size depending on how produced.Ground minerals are larger (and usually retain crystal morphology)Precipitated manufacture pigments are smaller (fine)High temperature processes (smalt) makes larger chunks,

 Color? There’s the obvious…. What color is it?

Also, does reflected light have the same color as transmitted light?Or, is it pleichroic? (appears a different color in different orientations)

 Chemical reaction?

Does it dissolve?Does it react / change with an acid? With a base?

Refractive Index: relative measure of light velocity as it passes through pigment? 

Isotropy: isotropic? Anisotropic? Axial? Biaxial?