Historical Painting Techniques, Materials, and Studio Practice

Gilding. Gold leaf was applied for a small number of particular features such
as halos and the decorative borders of drapery. Two samples were taken to
establish the gilding technique; in both cases, analysis by FTIR indicated that
a resinous mordant was used to adhere the gold leaf. The supporting layer
consists of calcium carbonate combined with a protein, probably glue. At 5
jJ.m, the gold leaf is exceptionally thick, more than twice that found at Idensen
(1-2 jJ.m) where it was applied over a "bole" of lead white and carbon
black bound with prepolymerized linseed oil (15).
Fading oj the lake pigment. Although the susceptibility of red lakes to fa ding
was known in the Middle Ages, they were nonetheless often used in wall
paintings (16). The key factor in the fading of lake pigments is exposure to
ultraviolet radiation, and, indeed, a darker pink is apparent where fresh losses
have occurred in linear details overlaying Nicodemus's drapery. Lake pigment
mixed with white, applied over a white ground and unprotected by a glaze
(i.e., the Holy Sepulchre Chapel paintings), is particularly vulnerable to internal
reflection (17).
Alteration oj vivianite. Initial identification of the pigment by polarized light
microscopy, in which the pigment is typified by blue-yellow pleochroism
every 90°, was confirmed by X-ray diffraction (18, 19). In recent years, vivianite
has been identified in medieval painting in Germany, as on the Romanesque
lectern at Freudenstadter (ca. 1150) where it was applied over a
gray ground of lead white combined with carbon black (20). In the context
of English medieval polychromy, a preliminary identification of the mineral
has been made on an Anglo-Saxon stone sculpture from York (21).
Vivianite occurs naturally in two discrete environments. It is fo und in the
oxidized upper layers of some metalliferous ore deposits, as at St. Agnes in
Cornwall, where it generally appears as dark indigo, blue-black, or green
crystals (22). It is also fo und in organic, phosphate-rich environments, and is
frequently associated with bones, decaying wood, and other organic remains.
Vivianite is generally stable and dark blue or green in color, though the
mineral may be colorless when initially exposed (23).
It seems likely that in the medieval period vivianite was used only where the
mineral was locally available; thus, mineral deposits are well known in Germany.
Good crystalline deposits of the mineral have been found at Whale
Chine on the Isle of Wight, and in its earthy form at Fordingbridge in Hampshire,
both close to Winchester (24). Current work on samples from the
chapel includes an analysis of trace elements and examination of the crystalline
structure to determine whether a mined or peaty alluvial deposit was
the source of the mineral.
Vivianite is known to be generally stable in its blue fo rm but at Winchester
some of the particles have altered to a yellow color, giving an overall green
effect. The mineral's color change from colorless to blue on initial exposure
is due to increased ferric ion concentrations, and it has been established that
mechanical grinding of the colorless crystals, heating in air, storage in a vacuum,
or chemical treatment of samples can produce a more rapid conversion
from ferrous to ferric ions, and so to a blue color (25, 26). It is therefore
interesting to speculate whether grinding of the blue mineral to produce a
particle size suitable fo r use as a pigment may have contributed to an additional
increase in ferric ion concentration and ultimately to a further color
alteration from blue to yellow.
Technique of the thirteenth-century paintings
Invasive conservation interventions-including waxing, facing with glue for
detachment, thinning of the original plaster support, consolidation, and cleaning-have
compromised the results of the technical examination of these
paintings. Nevertheless, certain conclusions can be drawn.
Plaster. Following the keying of the twelfth-century painting, a single layer
of plaster approximately 5-8 mm thick was applied to the surface. This
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