Conservation at Sardis (2020)
by The Sardis Conservation Team
Metals Rehousing Project
Metal Artifacts at Sardis
Excavations at Sardis have produced a collection of over five thousand copper-alloy and iron artifacts, which have been stored in our on-site depots since their excavation. As early as 1970, reports from the Sardis laboratory reference instances of bronze disease and the spalling and fragmentation of iron finds. For decades our treatment of freshly excavated metals has been aimed at cleaning for study purposes and stabilization. However, despite our best efforts over many years, a sizable proportion of excavated copper and iron finds show signs of ongoing chloride-related deterioration.
Given the size of the collection, a complete survey of all metal finds stored on-site at Sardis was conducted over the 2016 and 2017 seasons. A total of 5,609 copper alloy and iron objects were surveyed, and each item’s storage depot location, excavation metadata, lab treatment record number, and brief condition assessment were recorded. A visual glossary was created to aid in controlling vocabulary throughout the survey, conducted by a team of conservators with varying levels of experience inidentifying archaeological metal corrosion phenomena. The survey found that roughly 40% of all metals (37% of copper-alloy and 43% of iron finds) exhibited some sign of ongoing deterioration. Year-round environmental monitoring data collected over several seasons also determined that the relative humidity (RH) inside our storage depots is often well above 50% RH in the spring and fall, and reaches between 80% and 100% RH for many weeks in the damp, winter months.
Existing standard treatment protocols were clearly not enough to stabilize all freshly excavated metals or older finds with recurring instability. While we did identify a trend of increased stability in copper alloy finds after the introduction of BTA passivation and coating with Paraloid resins in the 1980s, a sizable fraction of treated objects with residual chlorides nevertheless remain unstable in these oxygen-rich and high humidity storage conditions. We determined that more interventive, aqueous chloride extraction methods—including sodium carbonate/sesquicarbonate soaking of copper alloys and deoxygenated alkaline soaking of iron finds—were not practical treatment options due to the time and costs involved for such a large collection. Following on the research conducted by McPhail et al. (2003), Brown (2010) and Paterakis and Mariano (2013), as well as our own tests storing objects in vapor barrier film enclosures over several years, we concluded that the most cost-effective and efficient way to halt chloride-induced deterioration across the entire collection of metal finds would be through a systematic rehousing of every unstable find in low-RH or anoxic, heat-sealed Escal enclosures.
The Metals Rehousing Project
The Metals Rehousing Project was carried out over the 2016, 2017, and 2018 seasons (Figs. 1, 2, 3, 4). For the vast majority of iron and copper-alloy objects, sachets of desiccated, regular-density silica gel were sufficient to create low-RH (i.e. 10% RH) internal environments capable of inhibiting chloride-induced deterioration (Fig. 5). For a minority of metal finds with organic remains (e.g. textile, wood, or bone) that cannot be stored at low RH, an oxygen scavenger system (i.e. RP System or Ageless) was included (Fig. 6). Depending on the organic material type and its fragility, conditioned silica gel was added to ensure that the interior RH did not vary with temperature fluctuation.
Most literature and anecdotal reports indicate that a properly sealed Escal enclosure can maintain a low-RH environment for at least ten years. We found that a small percentage of enclosures fail due to human error in sealing, however this becomes obvious through indicator color change within a few weeks of sealing and can be easily remedied by resealing. Although these enclosures may require maintenance each decade, the benefits of this approach cannot be overemphasized.
The implementation of our Escal Protocol marks significant progress for the conservation of metal finds at Sardis and has already paused the deterioration of all rehoused metal finds. While at Kaman Kalehöyük they have abandoned traditional treatment methods in favor of housing all metal finds in individual, anoxic Escal enclosures, we are still maintaining our standard treatment of BTA passivation and coating of newly excavated metal finds, and reserve the use of Escal enclosures for objects known to be unstable after storage for one year in normal conditions. However, with further research and tests this protocol may change.