METAL CORROSION IN THE LAZDININKAI (KALNALAUKIS) CEMETERY. PARTICULARITIES IN THE FIELD AND LABORATORY CONSERVATION

 

Dr. Audrone Bliujiene, Milda Jankuniene*

Lithuanian Art Museum P.Gudynas Restoration Centre

Rudninku str. 8,

2001 Vilnius, Lithuania

Donatas Butkus

Kretinga Museum

Viniaus str. 20,

5700 Kretinga, Lithuania

 

The Baltic Nordic Conference on Conserved and Restored Works of Art 6-9 October 1999, Taliinn, Estonia. The Conservator as Investigator. Tallinn, Conservation Centre Kanut, 2000. P. 151-152.

 

* Author to whom correspondence should be addressed

 

The Lazdininkai (Kalnalaukis) cemetery in the Kretinga district, the western part of Lithuania, was excavated for a long time, starting from 1940. Until 1999 over 435 inhumations and cremations were found. Burials in this cemetery date back to the early Roman Age. The last graves are dated from the first half of the 13th century.

The Lazdininkai (Kalnalaukis ) cemetery is regarded as one of the most famous sites in Lithuania, with a history of more than a thousand years. During this period cultural traditions changed several times: from the culture of coastal circle stone in the 2nd - 5th centuries to the Curonian culture of the 8th-12th centuries. The opinion prevails in Lithuania that the Curonian culture formed during the 6th century. The Lazdininkai (Kalnalaukis) cemetery site covers grave treasures of all these periods. Probably the site was always the local administrative cultural centre.

Near the Lazdininkai (Kalnalaukis) cemetery on the hill two huge cattle farms were built during the Soviet era. The third one was built in 1976-1977. It should be said that from the beginning of the 20-th century the cemetery was used as arable land. Later some of the land was reclaimed which changed essentially the surroundings of the cemetery. Most excavated burials were heavily disturbed and contained numerous isolated finds. Even now the site as a whole is not under the protection of the law. One of the above-mentioned farms is still operating.

Absolutely all the bronze and iron artefacts, not depending on their date and location in the cemetery, were corroded and as a rule without any metal core or with a thick layer of rust formed on metal strips. We are sure that during the Roman and Migration Period, as well as in the Viking Age, copper alloys were imported from different sources and for this reason the quality of bronze should differ. However, the artefacts found by us during the excavations are in a very poor state to support this fact. It seems that the differences in the quality of the bronzes have no influence on the metal corrosion in the soil. Presently, the archaeologists and the conservators have the opinion that factors like constantly wet land, ploughing and the high acidity of the soil increased because of the excess of fertilisers applied to the lands under cultivation and account for the low quality of the metal.

Five years ago archaeologists in Lithuania started to use field conservation during excavations. Co-operation with conservators proved to be a success. Before the expedition, the archaeologists to whom the conditions of the cemetery surroundings and degree of deterioration of metal finds were well known discussed the situation with the conservators and selected chemicals to be used in field conservation. If the chemicals are selected incorrectly for field conservation, their later elimination may be dangerous for the already deteriorated find. Therefore, the archaeologists should use chemicals compatible to those used in the laboratory and should along with the metrics of the find also report the chemicals used for field conservation.

The metal finds from the Lazdininkai (Kalnalaukis) cemetery were sent to the conservation department after applying a protective surface coating. Sometimes the whole pack of soil together with highly deteriorated finds was bonded with the help of Polybutylmetacrylate (PBMA). The box collar was received namely as a whole pack of soil bonded with PBMA. Based on this example with the box collar, the main details of conservation are provided.

To start the laboratory conservation without injuring the parts of the find present in soil an X-radiograph of the whole pack was made. Supported with the radiographs the restorer may judge the number of finds in the pack of soil, their state and degree of deterioration. In our case the X-radiograph showed some parts of a collar which were invisible to the unaided eye and which had to be found and saved. Then on the basis of X-radiograph and with the help of a small needle, scalpel and a small brush the fragments of the find were cleaned of soil. Polymers used for soil bonding was softened with acetone. Very brittle and flaky parts of the collar were then additionally bonded with the help of 5% PBMA solution in xylene. Gradually, layer by layer, all the details of the collar, present in the pack of soil, were taken out by using the processes of cleaning and bonding.

Preliminary bonded fragments of the collar were further cleaned in small areas one after another. Softening the surface layer sustained by the products of corrosion only, the process of cleaning was particularly complicated and called for great care using a mixture of acetone and xylene (1:1). The whole process was observed through the microscope. In this case it was needed to save the remaining elements of decoration of the collar, therefore, the cleaning of the box was continuously followed by the bonding procedure. Further conservation was carried out in routine order: full consolidation of the find, gluing of the fragments, covering of the object with the protective wax coating.

Usually a vacuum is used for the consolidation of the finds. In this case it was not used, since the form of the find was sustained only by the products of corrosion which after immersion into the solution might simply disperse. For the same reason the procedure of removal of the active chlorides was rejected. When metal is almost or fully mineralised, the procedure of active chloride ion removal makes no sense.

A similar sequence of conservation processes may be applied for fully mineralized silver, tin and iron finds. For iron finds mechanical cleaning may be rougher (by ultrasound, drill and sandstream), since the layer of rust usually is very firm and thick on them.

Field conservation must be performed for saving highly corroded metal finds. Once again it should be mentioned that the use of the same chemicals both by the archaeologist in field conservation and the conservator in the laboratory is of utmost importance.

 

Materials

Polybutylmetacrylate (PBMA)

Nischegorodskij Gosudarstvennyj

Universitet im. N. I. Lobachevskogo. 603600

Nischnij Novgorod

prospekt Gagarina 23

Russia

 

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