Historical Painting Techniques, Materials, and Studio Practice

plaster consists of lime combined with an aggregate of subangular quartz in
a ratio of 1:3. It is generally more yellow and less well prepared than the
plaster for the earlier decoration, with large lumps of unmixed calcium carbonate
clearly visible.
It is now difficult to establish the sequence of application of the plaster, since
surface features have been completely flattened by the detachment process
(Figs. 3, 4) . Examination under raking light of the surface of the east wall
paintings (now transferred to the west end) does, however, indicate that plaster
joins may originally have run along the upper edge of the central border and
the lower horizontal of the cross in the upper register. This indicates that
generally the plaster may have been applied in broad horizontal bands, roughly
corresponding to scaffold lifts. However in the east recess of the south wall
there appears to be a vertical join between the Harrowing of Hell and Noli me
Tangere, suggesting that in this area at least a fu rther division was considered
necessary.
Preparatory techniques. A preparatory sketch in red iron earth was used to set
out the main features of the composition. Unlike the twelfth-century painting
wherein both a sinopia and then fu rther preparatory drawing on the final
plaster layer exist, in the thirteenth-century scheme the preparatory drawing
is confined to the final plaster layer.
Direct incisions into the plaster are also clearly visible, such as the fine, sharp
incisions that outline the curls in the hair of the figure at far right in the
Entombment and the hair of St. John in the Deposition.
Pentimenti. An interesting alteration discovered during the detachment process-and
presumably visible on the reverse of the thinned plaster layer-was
that above the horizontal arm of the cross in the Deposition was a sketch fo r
the sun and moon, which are symbols more usually associated with the Crucifixion
(27).
Pigments. The present research has established that the original palette included:
natural ultramarine (3Na20·3Al203·6Si02·2Na2S); vivianite (Fe3 +
2[P04L'8H20); copper chloride green; cinnabar (HgS); hematite (Fe203); yellow
iron oxide (Fe203'H20); lead white (2PbC03·Pb[OHl2); lime white
(CaC03); charcoal black (C).
Binding media. The inclusion of lead pigments in the palette makes highly
likely the incorporation of organic binding media as part of the original
technique but, owing to the previous conservation interventions, it was not
possible to confirm their presence.
Application. Preliminary drawing in red ochre was followed by blocking in of
the basic background colors. Oakeshott considered that the powerful black
outlines, which are such a major feature of the scheme, were also painted at
this stage and were thus more firmly bound by the carbonation of the plaster
(28). The stratigraphy is remarkably simple, with one or at most two paint
layers applied directly to the plaster (Tables 2, 3, 4, 5). In some cases the final
paint layer was applied in thick impasto, and it seems likely that here an
additional binding medium was used.
Pigment alteration. Although the background colors of the scheme are now
red and green, it seemed much more likely on the basis of comparisons with
other paintings of the period that red and blue would originally have been
used. Analysis indicated that this was indeed the case: the present green color
is due to the alteration of some of the original blue pigment (vivianite) to
form a yellow alteration product.
The alteration of a lead pigment is evident in the fo rm of black spots on the
paint surface throughout the scheme, and analysis confirmed the presence of
a dark red/brown lead-based material. In wall paintings elsewhere where this
type of alteration occurs, the alteration product has been found to be plattnerite
(lead dioxide) (29). In the Holy Sepulchre Chapel however, analysis by
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Historical Painting Techniques, Materials, and Studio Practice