[Pigments and Binders]
[Pigments and Binders] |
Green Earth (Terre Verte) |
Duccio's
"Annunciation" from the _Maesta Altarpiece_. Tempera on wood panel.
Green earth pigment, also widely known as terre verte , in its
dry, powder form is a green color about the same as this background. It
is considered an organic pigment, composed of the minerals celadonite and
glauconite. However, according to Anne Wall Thomas, "Green earth, or
terre verte, is a mineral but not in the strictest sense an iron oxide pigment.
Consequently, it cannot be classified as a product of one of the four iron
ores. The presence of celadonite or glauconite, minerals of complex silicate
composition, produces colors that vary from cold bluish greens to warmer
yellow and olive hues. The green earths, which contain small amounts of
iron along with manganese and other components, may have origins in oceanic
deposits" 
(Thomas 12-3).
The pigment is collected by mining. The most famous and "best"
deposit of terre verte could be found near Verona, Italy, and this mine
was functional until World War II (Feller 149). Other mines produced
variations on the color and texture of the pigment, however. "Other
sources of green earth were noted: the Tirol, Bohemia, Saxony, Poland, Hungary,
France, Cypress, and the Mendip hills of England" (Feller 149). Today, the green
earth pigment is artificially produced.
Used in/by --
(Thomas
21); (Feller
149); (Feller
143).Binders used --
Structure and Composition
Green earth has a complex chemical structure. Its chemical formula is:
K[(CAl, FeIII), (FeII, Mg)] (AlSi3, Si4) O10(OH)2.
Testing Techniques
To verify the presence of a green earth pigment, several methods may
be used. "Microchemical methods may be employed to identify both the
ferric [Fe III] and ferrous [Fe II] ion in the green earth, provided the
ions are put into solution by treatment with acid. Detection of the presence
of both ferric and ferrous ions would distinguish green earth from other
green pigments with reasonable assurance" (Feller 153). Feller goes on
to explain what tests to make and what reactions to expect: "The organic
base, alpha, alpha'-dipyridyl, reacts with ferrous salts in
acid solutions to give a stable, deep red complex cation." As a test
for ferric ions, potassium ferrocyanide will give a dark blue precipitate
in acid solution (154). An instrumental method for verifying the presence
of green earth pigment is to use "x-ray diffraction and infrared spectrophotometry,
preferably both" (Feller
155).
Colorimetry Data (L*a*b* color space)
L= 64.51 a= -11.86 b= +18.20
Interpretation: more white than black [Lightness greater than 50]; more
green than red ["a" less than zero]; more blue than yellow ["b"
greater than zero].
Tested on paint made with green earth pigment and gum arabic binder
approximately one week after paint was made in laboratory.
Other Characteristics
Earth pigments, and green earth in particular, are often described as
the most permanent colors. "[Earths] are not affected by sunlight
or by atmospheric conditions....They do not react with solvents and have
reasonable tinting strength and covering power. They have the capacity
to screen harmful ultraviolet rays. Further, they are non-toxic and are
invariably the least expensive pigments available" (Thomas 24).
According to Feller, green earth is "partially soluable in acids and
alkalis" (Feller
146). The pigment is easily ground. Since it is a clay, green earth absorbs
oil at a moderate to high rate. The pigment is non-toxic (Feller 147).
Finally, "Particles range in size from 0.5 to 50 micrometers in diameter
in commercial samples, but in samples from paintings they tend to measure
less than 10 micrometers" (Feller
152).
In lab, the paint made with green earth and egg tempera got clumpy before
I could paint much with it. The oil paint dried out fast (partially because
it was spread too thin in the mortar in the first place). The green earth
pigment was often over-powered when mixed with other pigments, especially
by ultramarine, vermillion, and verdigris. The watercolor (gum arabic binder)
worked especially well.
As mentioned above, medieval painters used green earth pigments for flesh undertones. This underpainting of green neutralized the effect of the pinks and reds of the flesh colors. On the color wheel, the hue of green earth is the approximate complement to the pink used by medieval painters. Complementary colors neutralize each other, and this neutralization was important for medieval painters because of the materials they worked with. If they were not working in fresco, medieval painters worked on wood panels. To acheive a clean and smooth surface on which to paint, the painter would cover the wood panel in a layer of gesso, which is usually white. He then laid his paints (made of pigments and almost exclusively an egg tempera binder) on top of the gesso. To paint the pinks of flesh directly onto the white gesso would a acheive a "sunburn" effect in the flesh of the figures. To neutralize the pink, painters painted a layer of green earth under the pink. The light would pierce the pink layer, then the green, and reach the white. White reflects all light colors, so the light would "bounce" back through the paint. Each color paint absorbed some colors, or wavelengths, of the light. Thus, as the light passed back through the paint to the viewer, the flesh tones appeared neutral. See Duccio's "Annunciation" at the top of this page for an example.
Lonsdale MacFarland Green, 1997.