Production of copies

Time and again the restorers at the Department of Prehistory make casts of objects from the collection. Museums located near the sites where fibulae, daggers, statuettes and other precious objects were found often request a copy of the original item housed in the Natural History Museum. Copies are also needed when a very fragile object would be at risk of being damaged by environmental conditions if placed on display in the exhibition.

a) A copper wire is placed around the mould. - b) The mould is coated with electrically conductive graphite powder. - c) The mould is dipped in an electrolyte bath under DC voltage. - d) A pure copper coating 0.5 mm thick forms. - e) The coating is separated from the mould. - f) The copper wire with the clips is still clearly visible. - g) The clips are removed and the blank is roughly polished. - h) Superfluous material is removed and a fine polish is applied.

The restorers also make casts so that visitors can touch and hold exhibits – something which is not possible with the original items. A 1:1 scale copy, which sometimes imitates the actual weight of the original, gives visitors a good impression of the real thing.

Casts such as these must be clearly recorded as replicas. A small "c" is added after the inventory number, as with other copies made by the restoration team.

The Department of Prehistory’s restorers usually use dental silicone to make a mould of the original as this guarantees a high degree of accuracy. The object to be moulded is covered on one side with plasticine. Then the opposite side is covered with silicone. The material is then left to harden for some time before the object can be turned over. After removing the plasticine, restorers can start work on the other side.

Removing the silicone coatings is quite easy – as the material hardens, heat is generated. This melts the wax in the silicone. This wax comes to the surface and settles between the object and the mould, like a protective film.

The two halves of the mould are then filled with polyurethane compound and put together. By adding different substances this polyurethane mass can be matched to the original object in terms of colour, weight and also load-bearing capacity.

Electroplating is a special process for making copies. Here, too, the first step is to produce a silicone mould. This is then made electrically conductive using graphite powder. It is given a negative electrical pole and placed in an electrolyte solution. An electrical current at low DC voltage is then passed through the solution. This results in a metallic coating being deposited in the mould, the thickness of which can be precisely controlled. Electroplating makes it possible to produce copies which can hardly be distinguished from the original.

The Experimental Archaeology Department in Hallstatt occasionally requests wax casts for archaeologists wishing to produce replicas using the lost-wax method. In the experiments, these wax casts are first wrapped in a mass of clay and plant fibres and then dried at room temperature. The mould is then heated slightly, causing the wax to melt. It is removed from the mould and the metal – bronze, for example – is poured into the resulting cavity. This makes it possible for archaeologists to produce objects they need for their experiments.

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