THE RECOVERY AND DRYING OF TEXTILES FROM A DEEP OCEAN HISTORIC SHIPWRECK
KATHRYN A. JAKES, & JOHN C. MITCHELL
4 DRYING EXPERIMENTS
In drying waterlogged textiles, consideration must be given to the possibility that the fibers could collapse on removal of the water they contain. An ideal drying method would preserve the fiber and fabric structure while allowing the textile to be stored in ambient conditions in a museum. To determine the best drying method, samples of wet trunk lining were treated in the following ways:
- Air dried. A sample of trunk lining approximately 1 × .6 cm was allowed to dry in a desiccator in the laboratory.
- Ethanol dehydration. A sample of trunk lining approximately 1 × .6 cm was treated by immersing it in gradually increasing concentrations of ethanol/distilled water. The sample was placed in baths of 50/50, 60/40, and 70/30 ethanol/water for 30 minutes each, then in baths of 80/20, 90/10, and 95/5 ethanol/water for 60 minutes, and finally in two 100% ethanol rinses. After the second 100% ethanol rinse, the sample was allowed to dry in ambient conditions in the laboratory, while being protected from dust, and then was placed into a desiccator. The goal of ethanol dehydration was to dry the waterlogged material by replacing the water with ethanol and then evaporating the ethanol. In this manner, the collapse of fibers on removal of the fluids they contain could be eliminated.
- Critical-point drying. A sample of trunk lining approximately .6 × .6 cm was treated in the same steps as the ethanol dehydration, except that on immersion in the second 100% ethanol rinse, the ethanol-soaked sample was placed in a Denton Vacuum Critical Point Dryer, Model DCP-1. Critical-point drying was accomplished using liquid carbon dioxide as an exchange fluid. After drying, the sample was placed in a desiccator. The critical point-drying technique allows a wet sample to be dried completely with minimal disruption of structure caused by the surface tension effect of water. The critical temperature—the temperature above which a gas cannot be liquified by additional pressure—and the critical pressure—the pressure under which a substance may exist in equilibrium as a gas with a liquid at critical temperature—are both met in critical-point drying. As a result, there is no phase boundary between the liquid and gas phases. Since water has such a high critical temperature (approximately 375°C) and critical pressure (3,212 psi), it is replaced with a more convenient fluid, CO2, which has a critical temperature of 31°C and critical pressure of 1,072 psi. However, since CO2 and water are not miscible, the water is replaced with ethanol by introducing the sample into ethanol/water mixtures containing increasing concentrations of ethanol and decreasing concentrations of water, ending in 100% ethanol. Ethanol is miscible with both water and CO2, so the ethanol-wetted sample is placed in a pressure chamber and flushed with liquid carbon dioxide to replace all of the ethanol. The temperature is then raised to just above 31°C, the critical point, and the gaseous carbon dioxide is slowly vented to avoid distorting the sample.
- Vacuum freeze drying. A trunk lining sample approximately .6 × .6 cm was placed in a rapid freezer (−19°F) while still wet. The frozen sample piece was then placed in a Labconco Freeze Dry 5 Model 75050 freeze dryer unit and processed at a pressure of 0.1 μm (1 × 10−4 Torr [mm of Hg]). The dried sample was then placed in a desiccator. Freeze drying is often used in specimen preparation. To avoid the surface tension distortion effects of liquid water, the sample is frozen and a vacuum is applied. Water is removed by sublimation, avoiding the liquid phase.
- Frozen and slowly dried. A sample approximately .6 × .6 cm in size was removed from a larger section of trunk lining that had been kept at −19°F since the salvage operation in November 1990. With the approximate 5 mph air current in the freezer, the effective temperature was approximately −26°F. The trunk lining was dry when it was removed in January 1991. Due to the low humidity of the freezer, the ice contained in the fabric had sublimed. No method had been established for measuring the drying rate in the freezer; the fabric is labeled “slowly dried” because it dried more slowly than the vacuum-dehydrated sample.
A man's handkerchief that had been slowly dried in the freezer was also selected for examination by scanning electron microscopy. Two samples less than 1 cm square were studied.
Scanning electron microscopy and energy-dispersive x-ray analysis (EDS) were carried out on each of the test specimens, using a Jeol JSM-820 scanning electron microscope with Tracor Northern TN-5501 EDS unit. The sample was mounted with colloidal graphite on a spectroscopically pure carbon planchette. The entire area of the sample was examined to assess its overall condition. Scanning electron micrographs of representative areas of each of the samples were taken at 25, 250, and 500 x magnification. No statistical analysis was undertaken; the results of the drying treatments were evaluated visually.