Harriet F. Beaubien
with Tania E. Collas, Catherine E. Magee, Susan B. Peschken, Ellen F. Rosenthal, Marie E. Svoboda, and C. Mei-An Tsu, and additional contributions by Joanne M. Boyer, Stephanie E. Hornbeck, and Elizabeth C. Robertson
Abstract
Background: On the basis of a pilot study and research design initiated during the 1994 field season*, a long-term study was implemented during the 1995 season at Harappa, Pakistan, to evaluate the damage caused by residual soluble salts in porous, low-fired archaeological ceramics. Since soil and water salinity are very high, and the volume of excavated material requiring desalination is enormous, this process places great strain on equipment, time, water and electricity, resources which are in short-supply. Thus, the initial question was whether the desalination process could be terminated leaving a “safe” amount of soluble salts, i.e., at levels that would not compromise the stability of these collections under standard storage conditions at the site. Substantial savings in resources might be realized if the process need not go as far as “zero”. Results from the long-term study and from related studies carried out at SCMRE since 1995 would be used to evaluate and potentially modify the desalination protocols followed at Harappa’s field conservation laboratory.
The long-term study at Harappa: Damage trends after four years of “real-time” aging will be discussed for samples of terra-cotta bangle bracelets, desalinated by daily change of distilled water baths in a 1:2 g:ml ratio, to various salinity levels as measured by the conductivity of the terminal bath. The initial study, using levels from “zero’ to 400 pS/cm (at increments of 100) and undesalinated controls, was expanded in 1996 to include levels from 500 to 2000pS/cm (at increments of 500), based upon results of an accelerated aging study on samples carried out at SCRME.
Related studies at SCRME: Salt trends. Salts collected from a series of desalination baths were analyzed to investigate their pattern of release, as a possible guide to determining an appropriate desalination termination point. Water usage. Variations in g:ml proportions and the frequency of bath change (daily vs. equilibrium) were investigated to determine the most water-efficient method for reaching target conductivity levels. Conductivity correlations. Tests were carried out to investigate the issues of using conductivity measurements to describe solution concentration of different salt species.
Results of these studies and implications for further desalination will be discussed.
*Holbrow, K. A., E. Kaplan, and H. F. Beaubien. 1995. Desalination parameters for Harappan ceramics. Objects Specialty Group Postprints. American Institute for Conservation 23rd Annual Meeting, St. Paul. Washington DC: AIC. 70-76.