Gao Nianzu, ]ia Kuiguang, and Wang Jinyu
Located in the Mogao grottoes at Dunhuang are a wide variety of floor tiles made during medieval times in China. The floors of nearly fifty caves are lined with more than twenty thousand tiles of at least twenty different designs and patterns. These tiles were produced over the centuries, from the Northern dynasties (420-589 c. e.) to the Yuan dynasty (1271-1368 c. e.). The tiles made since the Tang dynasty (618-906 c. e.) are of the best quality A great variety of tiles were manufactured during the Five dynasties period and during the Song and Western Xia dynasties. Taken together, these ancient floor tiles are an important component of Mogao grotto art.
In 1978, research on the protection of the floor tiles began. Abrasion damage, mainly caused by foot traffic, was identified as a problem. Because of the desire to maintain the design integrity of the ancient floor and to avoid intrusive visual elements, an overlaid walkway or other physical protection methods were not considered. Instead, various consolidants were tested as protective coatings. Results from two stages of experiments suggested that polyurethane coating and infiltration with traditional tung oil can increase the wear resistance of the tiles (Gao, Jia, and Wang 1993).
Selection of Consolidants as Coating Materials
A consolidant that is to be used as protective material for floor tiles should be transparent, of suitable viscosity to penetrate the tiles, and sufficiently abrasion resistant to provide a durable coating. Two kinds of coatings were selected for testing: synthetic resins and raw tung oil.
Synthetic resins: Polyurethane resin was selected for its elasticity, durability, and wear resistance. For comparison, epoxy resin coating was also tested.1
Raw tung oil: This pale yellow oil extracted from the seeds of tung trees is quick drying and provides a strong, nonsticky coating. It has a density of 0.924-0.925 g cm-3 and a melting point of 2-3 °C. Tiles permeated with raw tung oil are resistant to erosion by water and alkali, and to deterioration by light and the atmosphere. The “golden tile” used in many important ancient Chinese buildings was made by permeating tiles, after firing, with raw tung oil.
Preparation of Samples and Testing of Consolidants
The equipment used to test the floor tiles were a road-abrasion testing machine, a high-pressure sandblasting machine, and a freeze-thaw testing machine.
Most of the testing samples were taken from the Meridian Gate of the Beijing Palace Museum during a renovation project. Fired during the Ming dynasty, these tiles are strong, dense, and homogeneous, as demonstrated by the consistency of the testing results. For comparison, the samples were treated with different consolidants in wear-resistance and freeze-thaw tests. Some tile samples taken from the Mogao grottoes were also treated and tested. These tiles were produced during the Tang and Song dynasties.
In preparation for testing, the sample tiles were precisely cut into 15 X 15 X 4 cm square pieces with perpendicular sides and flat surfaces to ensure good contact with the steel balls of the road-abrasion testing machine. The test pieces were oven dried to facilitate penetration of the consolidant and weighed before the consolidant was applied. Results are shown in Table 1 for the fifteen samples tested.
World History
