Saturday, June 25, 2011

Slumping Ceramics

Gordion archaeologists are not excavating this season which means that we can devote more of our time to taking care of the collection. Our job as conservators does not end once objects are cleaned and stabilized after they are excavated. A continuing challenge is our slumping ceramics. Most of our pots, plates, and other ceramic vessels were carefully assembled from many fragments because they rarely come out of the ground unbroken. In the earlier days of Gordion, a PVA resin adhesive was used to glue the ceramics back together. Over time, these earlier ceramics have started to sag, slump, and come apart.

This Phrygian jug, treated in 2009, slumped over the course of one year
and could no longer stand on its own.

The adhesive slowly starts to soften and the joins begin to pull apart exposing strings of adhesive. One reason this happens is that the adhesive's glass transition temperature, or Tg, is too low. The Tg refers to the temperature where a material like an adhesive goes from a hard, brittle material to a much softer, rubber-like material. Our storage areas have no air conditioning and the temperatures can get pretty hot! The older adhesive used here has a Tg that is in the same range as the high temperatures in storage so the adhesive starts to soften and pull apart. 

Adhesive strings in joins indicate that the vessel is unstable
and could begin to slump and fall apart. 

We keep an eye on our ceramics each year to determine if there are any emergency cases that should be treated. The only solution is to remove all of the adhesive and put them back together again with an adhesive that has a higher glass transition temperature. 

A collapsed pot that is being treated this year. 

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