1.8 ka old glass from the Roman ship Julia Felix: glass-water oxygen isotope exchange

A set of glass samples from the sunken Roman ship Julia Felix, off the town of Grado (Gorizia, Italy) around the second half of the 2nd century AD has been studied along with other glass samples from almost coeval levels of archaeological excavations in the cities of Vicenza (northern Italy) and Pozzuoli (Naples, southern Italy) and in the town of Grado (7th–9th century AD). The glass samples are fairly homogeneous in their chemical composition with typical silicic–sodic–calcic compositions. Superficial lamellar layers of various thickness (from about 200 μm to less than thousand μm) were formed by the glass–water alteration process. The oxygen isotopic composition of the alteration patinae was measured along with that of the apparently unaltered internal section of glass. The oxygen isotopic composition of the Northern Adriatic seawater is known from previous studies along with the mean oxygen isotopic composition of local precipitations at Vicenza, Pozzuoli and Grado. The mean annual temperatures at the sea bottom are also known from previous studies. The knowledge of the age of the ship and of these variables allows the calculation of the diffusion rate of water molecules through the glass and the glass/water fractionation factor at low temperature. The calculated α-values for the ship glasses are very close to one another and consistent among different samples. One of the glass samples from archaeological excavations (Grado) shows an α-value smaller than those calculated for the ship samples. This suggests partial isotope exchange probably related to the reduced exchange time with water (~50%) and, perhaps, to the small amount of water available to this sample. From the results obtained, it is possible to conclude that glass– water oxygen isotope exchange occurs even at relatively low temperatures, isotopic equilibrium being reached in less than 1.8 ka when large amounts of water are available.