Preparing Samples

Bulk reference samples were prepared from each biological source and historical recipes were used when available. When unavailable, modern laboratory practice, based on aqueous extractions and fermentations only, was used to prepare the samples. Dyeing of wool and silk was performed with or without preliminary mordanting with alum. Pigments were prepared on a hydrated aluminum oxide base. Paints were prepared by suspending pigments in a diluted animal glue medium or by concentrating the dye extract and adding glue.

All preparations were documented in designed data sheets to ensure reproducibility and later large-scale production. Initial small-scale production allowed for refinement of procedures and confirmation of the biological source by Liquid Chromatography-Photodiode Array-Mass Spectrometry before large-scale productions were prepared. All preparations and reference samples were cataloged in a database.

The Dunhuang Academy and the Getty Conservation Institute studied the wall paintings in Cave 85 at the Mogao Grottoes for nearly two decades. The compositions of the plaster walls, ground layers, and inorganic paint layers are well researched and understood. The Dunhuang Academy prepared 150 plaster wall painting mock-ups using clay and other materials available on site. Each coupon has a traditional ground layer applied to half of its surface and seven strips of common inorganic paints used in the grottoes.

For shipment of the coupons to the Institute laboratories, the Getty Museum Preparations Department constructed a specially designed crate to protect the fragile coupons during road and air transit. The crate was shipped from the Getty to the Dunhuang Academy in January 2007. Under the supervision of the Dunhuang Academy, the coupons were loaded into the crate and sent back to the Institute, arriving in March 2007.

At the Institute, organic paint prepared with the reference pigments was applied to the mock-ups. The plaster, ground, inorganic paints, and lake pigments provided crucial combinations to evaluate an analytical scheme. From each biological source, five mock-ups were produced: one for investigating the best possible analytical procedure(s), one for natural aging in the grottoes, one for accelerated aging at the Institute, and two for the Institute’s reference collection for future research.