There is extensive flaking and loss of the paint layer in this 12th century Byzantine manuscript. Image courtesy of the Bodleian Library, The University of Oxford
The most common conservation issue related to the care and preservation of medieval manuscripts—such as the pages from the Winchester Bible, on view in the exhibition The Winchester Bible: A Masterpiece of Medieval Art through March 8—is the loss of cohesion in the paint layer. Most often, flaking paint is due to the dehydration of the binding vehicle used in the original mixing of the paint.
Throughout the Middle Ages and during the fourteenth and fifteenth centuries, lead white was the principle white pigment used in manuscript illustration. Unsurpassed for its opacity and covering power, it was essential to the medieval manuscript palette, and large areas of paintings are composed of pure lead white or mixtures thereof.
Lead white is the fundamental component in creating flesh or skin tones. This thirteenth-century manuscript has extensive losses of white paint, exposing the black ink underdrawing. Manuscript Leaf with the Crucifixion, from a Missal (details), ca. 1270–80. Paris, France. Tempera and gold on parchment; Overall (folio): 8 3/4 x 5 7/8 in. (22.2 x 14.9 cm) Mat: 19 1/4 x 14 3/16 in. (48.9 x 36.1 cm). The Metropolitan Museum of Art, New York, Purchase, Bequest of Thomas W. Lamont, by exchange, 1981 (1981.322)
Lead white is not without its shortcomings, however. It is a complex chemical compound of basic lead carbonate, (PbCO3)2•Pb(OH)2 . Used since antiquity, it is synthetically prepared by exposing metallic lead to acetic acid vapor. Partly due to its method of manufacture, lead white forms a brittle paint film when bound with gum or glair and it is therefore very prone to flaking and tenting (the delamination of the paint into an upward pattern resembling the peaked top of a tent), especially in a bound manuscript where the leaves are subject to considerable flexing. Once a fracture or loss in the paint layer occurs, the surrounding area becomes extremely vulnerable and susceptible to increased damage. It can also cause considerable loss of integrity and permanence to the paint layer itself.
Metallic lead ore (left) and the resulting white pigment particles (right)
To compound the problem, the consolidation and stabilization of flaking lead white can be especially challenging since the thin paint film can be swollen and softened by aqueous (water-based) consolidants (binder). The integrity and permanence of a paint layer is influenced by multiple factors, such as the strength of the binder, each pigment's inherent properties, and mechanical stresses caused from handling. The overall physical stabilization of every painting begins with consolidating the areas of cracking and flaking paint. Using a binocular microscope, a warm solution of isinglass is applied with a fine brush to reestablish cohesion of the compromised media to the substrate.
Conservator Yana van Dyke at a microscope stabilizing paint layers
X100 photomicrograph magnification of the consolidation technique
An ultrasonic consolidation unit and a binocular microscope with a bound manuscript strapped to its 90-degree cradle
Another ubiquitous problem is lead white's proclivity to darkening upon exposure to sulfurous compounds in the environment. The pigment darkens to gray or black, and transforms the white lead carbonate into a black lead sulfide. Paper conservators have methods for converting darkened lead sulfide back to a white lead sulfate through oxidation reactions and carefully employing innovative treatment solutions, returning the original visual clarity to the composition.
This mid-thirteenth century work shows distracting forms of discolored lead white in the flesh tones. Manuscript Leaf with the Deposition, from a Psalter, mid-13th century. German. Tempera, ink, gold, and silver on parchment; Overall (folio): 6 5/16 x 4 11/16 in. (16.1 x 11.9 cm) Illumination: 5 11/16 x 4 1/8 in. (14.5 x 10.5 cm) Mat: 12 x 10 in. (30.5 x 25.4 cm). The Metropolitan Museum of Art, New York, Fletcher Fund, 1925 (25.204.2)
The work before treatment (x100 photomicrograph detail). Manuscript Leaf with the Deposition, from a Psalter (detail), mid-13th century. German. Tempera, ink, gold, and silver on parchment; Overall (folio): 6 5/16 x 4 11/16 in. (16.1 x 11.9 cm) Illumination: 5 11/16 x 4 1/8 in. (14.5 x 10.5 cm) Mat: 12 x 10 in. (30.5 x 25.4 cm). The Metropolitan Museum of Art, New York, Fletcher Fund, 1925 (25.204.2)
The work during treatment. The blackened lead sulfide is being converted to white-form lead sulfate (x100 photomicrograph detail). Manuscript Leaf with the Deposition, from a Psalter (detail), mid-13th century. German. Tempera, ink, gold, and silver on parchment; Overall (folio): 6 5/16 x 4 11/16 in. (16.1 x 11.9 cm) Illumination: 5 11/16 x 4 1/8 in. (14.5 x 10.5 cm) Mat: 12 x 10 in. (30.5 x 25.4 cm). The Metropolitan Museum of Art, New York, Fletcher Fund, 1925 (25.204.2)
Medieval manuscripts, such as the Winchester Bible, are tremendously complex objects. Lead white is merely one of the multifarious problematic elements encountered when turning their glorious pages.
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Conservation and Scientific Research