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  From Roger

6th Jan 07

Removing lime deposits from archaeological ceramics.

Archaeological ceramics are often contaminated by water soluble salts during burial, and post excavation will may exhibit a white surface layer of crystallised salt, much of which will probably be calcium carbonate. This may coat parts the surface with a thin bloom but in extreme cases can totally obscure the surface with a thick accretion of calcium and other salts.

 

Lime, or calcium is an alkaline, or base material, which can be broken down by acid solutions, however, the product of the reaction between an acid and base is a salt, and most salts are soluble in water.

 

Removal of surface accretions with dilute Hydrochloric acid is very effective, since it breaks down the calcium but does not affect the fired ceramic, and doesn't present the inherent risk of damage to the object, associated with mechanical removal of the encrustation by scalpels etc.

 

However, with acid cleaning there is a real risk of re-introducing these salts into the porous fabric of the pottery. The frothy sludge produced as the acid dissolves the lime deposit, contains all the contaminants in dissolved form which are being removed from the pottery. If the object is dry when this takes place the salts removed from the surface are simply absorbed by the body of the pot, and will rise to the surface, or efflouresce, all over again.

 

This problem can be overcome by immersing the ceramic in purified water for twenty four hours before treatment, ( boiled water which has cooled down is as good, and thought by some to be better), the water saturates the pottery and prevents the ingress of the spent acid/dissolved salt sludge. The cleaning process should start as soon as the object is lifted from the water, it is vital that it does not dry out during cleaning. Flood the accretion with dilute acid and wait for the frothy reaction to occurr, soft brushing with a toothbrush or small paint brush often helps at this point. As soon as the frothing dies down, the acid is neutralised and will clean no more, and the sludge should be quickly and thoroughly washed away with clean water.

 

This acid cleaning followed by clean water flushing should be repeated until the surface accretion is removed, at which stage the pottery would benefit from immersion in changes of purified water again, to reduce, if not remove entirely, any resuidual salts. This should be followed by slow drying. A white bloom appearing on the surface during the drying process indicates the presence of salts, and the pottery should be steeped in clean water again for twenty four hours, in order to remove the residual salts by osmosis.

 

The above is good for undecorated ceramics in good condition. Care should be taken with delicate, damaged or poorly fired ceramic, since it may be the very salt crust which is holding the object together.

 

Similarly painted pottery, decorated with low fired mineral pigments may be harmed by this process, and pottery with a post firing decorative layer should not undergo this treatment at all.

 

The damaging effects of water soluble salts can be nuetralised by keeping a contaminated object in an environment with a constant relative humidity, a museum case being ideal. However, storage or display in a fluctuating environment will leave a ceramic object containing soluble salts prone to long term, insideous decay. Each time the humidity drops below the crystallisation point of a salt, small crystals will begin to form. These crystals exert considerable pressure as the grow, sufficient to displace particles of ceramic from a low fired pot or flakes of poorly attached pigment or glaze. As the Humidity rises, these newly formed salt crystals go into solution, releasing their grip on the displaced ceramic material, which drops away as a fine powder. In an unregulated environment such as a house, this will probably happen on a daily basis, with no thought given as to why there is always dust around the base of a prized antiquity.

 

Thanks Roger! :o)