a group of painted figures surround a black horse whose tail hair is being cut off. Above and below the image is script

Transforming Discolored Pigments in a 16th-Century Persian Watercolor

a group of painted figures surround a black horse whose tail hair is being cut off.

fig. 1: Pre-treatment image of Tail and Ears of Khusrau Parwiz’s Horse Are Docked (detail), from a Book of Kings (Shahnama) manuscript (ca. 1540), taken in normal light

A close visual examination of this watercolor from the Shahnama, or Book of Kings, a chronicle of the rulers of greater Iran from the creation of the world to the Arab conquest in 651 C.E., revealed that the painting’s original colors had altered over time (fig. 1). Gallery conservators worked with conservation scientists at Yale’s Institute for the Preservation of Cultural Heritage (IPCH) to look deeper into the painting’s support and pigments in order to treat the discoloration.

Produced in southern Iran by an unknown artist and calligrapher, the manuscript dates from the Safavid period in the 16th century. Abu’l Qasim Firdausi authored the text in the 11th century, collecting Iranian stories and setting them to verse—more than 50,000 rhyming couplets in all, amounting to the longest poem ever written by a single author. With a blend of history and lore, the Shahnama presented models of leadership and conduct that guided generations of rulers.

This miniature depicts a court official cutting the tail of Prince Khusrau Parwiz’s horse, the prince’s punishment for letting his horse escape its stall and eat some of a local farmer’s crops. Since the prince’s father had just declared harsh penalties for unruly behavior, the depicted scene of the son’s punishment speaks to his fair leadership. The disgruntled farmer, visible to the left, watches closely to see this carried out, while two court officials stand by on horseback beyond the hill. Some areas of the painting remain unfinished, including the hill and two of the saddles.

A image of a horse black horse surround by figures. A scientific analysis image, the color palette is comprised on black, yellow and white

fig. 2: Overlay of XRF maps revealing the location of lead (white) and arsenic (yellow) in the watercolor

With time, many of the skin tones in the watercolor had become darkened and discolored, as had the hillside. The conservators and conservation scientists studied the painting’s colors in depth. Persian artists used colorants derived from a variety of plants, animals, and minerals, their palettes featuring both natural and man-made pigments. The artist would likely have mixed the colorants with a solution of gum arabic in water to bind them to the paper. Anikó Bezur, the Wallace S. Wilson Director of Scientific Research at the IPCH Technical Studies Lab, oversaw an analysis of the watercolor using scanning X-ray fluorescence (XRF) spectroscopy, a 14-hour process that gathered more than one million points of information about the painting’s elemental composition. The XRF analysis revealed the presence of pigments such as vermilion, white and red lead, ultramarine blue, realgar, orpiment, verdigris, brown iron oxide, carbon black, gold, and silver. 

Conservators considered how these pigments may have interacted over time, arriving at the hypothesis that the discoloration resulted from reactions between incompatible pigments: lead carbonate (or lead white) and arsenic trisulfide, known as orpiment, which is a beautiful golden-yellow. Orpiment contains sulfur in a reduced form, and lead pigments are notoriously unstable in the presence of sulfides. When mixed together or even juxtaposed in an artwork, these pigments—often with a little help from atmospheric pollutants such as hydrogen sulfide—react to form black lead sulfide. This reaction is evident in the darkening or graying of several areas of the watercolor. The XRF lead map revealed the location of lead white in the painting and the arsenic map the location of orpiment (the main source of sulfur). An overlay of the two maps (fig. 2) showed areas where the two pigments may have even been mixed. 

a group of painted figures surround a black horse whose tail hair is being cut off. Above and below the image is script

fig 3: Post-treatment image taken in normal light

The darkening of lead white does not generally threaten the longevity or stability of the paint, but in many small artworks, like this one, the original power and clarity of the figures and their expressions are often lost. Fortunately, the discoloration is sometimes chemically reversible. When the watercolor was requested for exhibition, Denise Leidy, the Ruth and Bruce Dayton Curator of Asian Art, asked that the discoloration be reduced if possible. After testing small areas to ensure that the method would work safely, conservators exposed the lead paint to an oxidizing agent, a dilute mixture of hydrogen peroxide in diethyl ether. Hydrogen peroxide can be applied in gel, liquid, or vapor form, and it is crucial to select a method that causes the least disruption to the paint or paper surface while delivering the greatest possible color reversion. For the treatment of the Shahnama miniature, very small quantities of the liquid mixture were applied carefully with a soft brush; the hydrogen peroxide instantaneously volatized off with the ether vapor. The reagent oxidized the lead sulfide to lead sulfate, which is white, more stable, and visually almost identical to the original artist’s material, basic lead carbonate. 

Following the treatment, the skin tones and hillside appear much brighter and more natural in their coloration (fig. 3). With its vibrancy restored, the composition and story come to life, and the painting can now be studied and appreciated in a state that is much closer to how it would have looked originally. This treatment can be repeated in the future, should the converted lead sulfate discolor, and storing the work in sulfur-free matting and Solander boxes can help prevent further discoloration.

Sarah Schlick
Conservation Pre-Program Intern

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