The problem with salts
Breakage, chipped edges, flaked off glaze or slip, detached handles etc. are common and very obvious types of damage that we often find in museum ceramic collections. But there are also more hidden risks for archaeological ceramics that have been buried in the ground for centuries.
Soluble salts that have travelled into the ceramic from the surrounding soil can pose a huge problem. The salts are dissolved within the ceramic fabric at high humidity, but they start to crystallise as soon as the environment gets drier. This process is reversed again when the humidity increases.
Salt crystals take up much more space than the dissolved salts and the crystals exert pressure on the ceramic fabric, causing flaking of the surface and disintegration of the pottery.
If the object is kept in an unstable environment the damage continues in a cycle as the salts dissolve and re-crystallise.
To preserve ceramics that show signs of salt contamination it is important to desalinate them. This normally requires a washing process where the salts dissolve into the wash water and can then be removed with the water.
It’s not just a pot in water
Placing the ceramic into a water bath can cause problems for the conservator as some ceramics are already quite fragile and can be damaged further by being immersed in water. In this case the long term benefit of desalinating the ceramic has to be balanced against the damage that could potentially be caused by placing the ceramic into water. This is a decision that has to be made by the conservator every time desalination is considered.
If the ceramic is too fragile, i. e. very flaky or very low fired, desalination can not be carried out and the crystallisation of the salts in the ceramic fabric has to be avoided by controlling the temperature and humidity in which the ceramic is stored.
To carry out desalination the ceramic is placed in a deep container and then de-ionised water added to the container very slowly. It is important to add the water to the ceramic and not the ceramic to the water as this is gentler for the object.
As the water gets saturated with more and more salts the water has to be changed regularly. The intervals are variable as they mainly depend on how quickly the water is saturated. For example, a heavily contaminated ceramic might need a bath change within a few hours.
The saturation of the water is measured by its conductivity. The more salts are dissolved in the water the higher the conductivity will be.
Early baths for badly affected ceramics can give readings which run into 1000’s of μS/cm. We usually aim to change the water baths until the readings stabilise at somewhere below 100 μS/cm. Once the desalination is complete the ceramic is allowed to dry in a controlled way to avoid cracking and further loss of fragile surfaces.