Research and Research Methods
Fingerprints from Antiquity
In cooperation with the Department of Fingerprints Research of the Forensic Science Laboratory of the Netherlands Ministery of Justice in Rijswijk, research has been carried out on fingerprints of ancient coroplasts. Hellenistic terracottas in particular have yielded large numbers of fingerprints. These have been left by the coroplasts pressing the clay into moulds and have survived the firing process.
The research has resulted in a database accompanied by digitalized photographs. It is intended to examine dactyloscopical a large body of terracottas from a single site, such as Tarentum in southern Italy, which has already been arranged into stylistic groups. Creating a catalogue of coroplast fingerprints will help to date the terracottas and ascribe them to individual coroplasts.
Scanning of Pottery
It has been common practice to make profile drawings of pots and potsherds in order to compare their shapes. This is a time consuming practice. The results are often not very precise. Besides, in the case of 'closed' pottery-shapes with a small mouth, it used to be impossible to measure the inside and to make profile drawings at all.
After conducting a number of tests with a medical CT-scanner at the Radio diagnostic Department of the Academic Medical Centre, it has now become possible to produce perfect profiles, even of closed potshapes. By further computer tomography, which is based on the principle that an object is reconstructed by a computer after having been X-rayed from many different directions, it has become possible to attain pot profiles in both horizontal and vertical planes.
The profiles made by the scanner show a large variation of grey tones. This is due to different chemical cells of baked clay absorbing a different amount of X-rays. The shades of grey may be assigned to distinct elements. In this way, a histogram can be produced indicating the chemical composition of the clay. Every single profile has its own histogram: by comparing these, differences in the compound of the clay can be shown. In cooperation with the Computer Centrum Letteren, this method will be further developed.
Chemical ageing: conservation of pottery
It is generally assumed, that pottery lasts forever, because baked clay can withstand fire, water and rotting. However, microscopical research at the Museum, in cooperation with Schooling Restorers Amsterdam and F. Drijfhout en Zns B.V., has proven that pottery is affected by chemical ageing. Large parts of the surface show minuscule traces of damage, caused by crystals, which exist in the clay and are of regular shape.
During a series of XRD-analyses, these crystals appeared to consist of aluminium-silicon and complex chloride-combines. Clay contains four kinds of crystals: total water-soluble, hardly water-soluble, non water-soluble and water-absorbing crystals.
The common method of purifying pottery is washing out water-soluble crystals with distilled or demineralised water. As has become clear now, the water-absorbing crystals have to be removed without the use of water to stop the process of damaging. If this method is not applied, the aluminium-silicon-combines will absorb the water, which causes the crystals to increase in volume three times.
An alternative method of purifying has been developed, by which all damaging elements are removed. By the use of a thermal laser, the crystals get heated locally. This is done with the pot placed in a argon-filled space, which causes the crystals at the surface to evaporate. At the same time, the holes left behind are closed by glazing the surface. The crystals that yet remain beneath the surface can cause no more damage, because they cannot contact humidity.
Hair cracks in Silverware
In antiquity, metal dinner-sets were considered the most valuable. These lent the owner prestige, especially when made of gold or silver. The metal vessels were 'embossed', which means that the shape was hammered from a metal plate. In order to do this, the plate was heated and subsequently hardened by quickly cooling it down with the help of water; this made the material more elastic.
Further research with the help of X-rays revealed that this silverware often shows a network of cracks. These cracks are the result of the ageing process and the practices of the silversmith. It became apparent that the hardening was done in the wrong way, which has resulted in a lack of elasticity. The tension within the object, resulting from this stiffness, has caused a network of cracks on the surface of the silverware. This process reinforces itself, which leads to the cracks becoming visible. The crack surfaces corrode, which causes new internal residual stress.
In cooperation with the National Aerospace Laboratory the Museum investigated this phenomenon. A special treatment has been devised. If a metal vessel is rinsed with demineralised water and ethanol, and if all water and outgas is removed by means of a high-vacuum chamber, the process can be delayed. Finally, the object has to be impregnated with a colourless acrylic or polyurethane coating.
Mummies under the scanner
On the initiative of voluntary cooperator H. Koens, the mummies and mummy parts of the Museum were scanned in the Academical Medical Centre in Amsterdam. The most spectacular subject was the only complete human mummy in the Museum, a permanent loan from the National Museum of Antiquities in Leiden.
The preliminary results can be summarized as follows:
-It concerns a male person, between 18 and 40 years old at the time of his death.
-Contrary to the well-preserved general condition of the body, the bones of the thorax appeared to be heavily damaged: the ribs were fragmented and dislocated. A possible cause of death ?
-All internal organs were replaced by packs of linen and sand (?). The only exception is formed by the heart, which was replaced by a so-called heart scarab. The brain was removed as well. A striking feature is the relative good preservation of the eyeballs with muscles and all.
-Apart from the already mentioned heart scarab three more amulets were identified: two Eyes of Horus and a smaller scarab.
