Material Science Laboratory
In the Materials Laboratory we carry out chemical analyses and various physical methods of measurement. These provide information on the characterization of materials and material properties. The equipment and testing methods of the Materials Centre are provided as services to the research departments of the DBM, the museum itself, and external clients.
In the Materials Laboratory, nearly all inorganic materials can be examined to establish their chemical, structural and physical compositions. Depending on the research question and the constitution of the sample, a suitable preparation and method is chosen. The Materials Laboratory has a variety of equipment which is particularly important for the projects of the DBM’s mining archaeology and archaeometallurgy research departments.
In the Materials Laboratory we also conduct test series for the DBM’s research departments of mining technology and heritage protection/materials science, to enable us to understand processes of damage to stones and metals. For materials which are already damaged, we develop protective agents and measures to protect them from further destruction.
The entire spectrum of analytics carried out in the Materials Centre, as well as our on-site sampling, is also used by external clients. These include universities, non-university research institutions, companies and private individuals.
Ion chromatography (IC)
When evaluating damage to the stone components of structures, it is necessary to obtain information about the accumulated water-soluble cations (Na+, K+, etc.) and anions (Cl-, NO3-, etc.). These are determined in the Materials Laboratory using two ion chromatography systems (ICS 1600) from Dionex.
Wet chemistry laboratory
Standard tests such as the measurement of pH values, conductivity, loss on ignition, carbon, carbonate and sulphur content etc. take place in the acid-free wet chemistry laboratory. Aqueous extractions are also carried out here on stone material for the elution of cations and anions. These, determined by ion chromatography (IC), provide important information in the case of damage to structures.
Mass spectrometry (MS)
For chemical element analysis, the Materials Laboratory uses the Element XR, a high-resolution double-focusing mass spectrometer from Thermo (ICP-MS). This spectrometer, purchased in 2009, is currently one of the most sensitive measuring instruments available, allowing elements to be identified in ultra trace amounts. Optional linking with a laser ablation system (UP213, New Wave) makes it possible to carry out “virtually non-destructive” measurements even on objects that have great value for cultural history.
Since determining the chemical composition of the sample material is not enough, particularly in provenance studies, we also offer high-precision determination of lead isotopes by means of multicollector mass spectrometry (Thermo Neptune), in collaboration with the University of Frankfurt.
Before a sample can be examined – except in the case of non-destructive methods –, corresponding preparations are necessary: rock materials such as ore, slag, glass and pottery are generally first sawn up with diamond cutting discs, then ground up in mills with various grinders made of agate or tungsten carbide, and finally dried to constant weight. With metals, the preparation generally consists of sawing and/or drilling (1 – 1.5 mm) the object. The powders or shavings obtained in this way are then processed as appropriate (cleanroom, wet chemistry laboratory) and subsequently analysed for their elemental or phase composition (ICP-MS, XRD, IC).
In order to carry out microstructural analysis using the optical microscope/SEM, thin or polished sections of the objects must be produced. Diamond cup wheels (MPS 2-120, G&N) are used to make thin sections with a thickness of around 20 µm. Polished sections are prepared using cloth laps (Saphir 555, ATM) or lead laps, depending on requirements.
X-ray diffraction (XRD)
Besides chemical and isotopic composition, questions of manufacturing technology and weathering and corrosion are often important. The X-ray diffraction carried out in the Materials Laboratory offers a valuable tool for the identification of minerals and phase mixtures. The diffractometer (Panalytical Xpert Pro) can be operated either with a 15-position sample changer for powder samples or a universal sample stage. This allows non-destructive analysis even of larger samples (up to 10 cm x 10 cm). The samples analysed include stones, mortars, pigments, salts and the products of weathering and corrosion, ores, metals, slags, rocks, archaeological finds, and all collection objects.
X-ray fluorescence spectrometry (XRF)
The demand for non-destructive analysis is ever increasing. The portfolio of the Materials Laboratory therefore includes a portable energy-dispersive x-ray fluorescence spectrometer (Niton Xl3t GOLDD), which not only allows non-destructive surface analysis, but also makes it possible to take measurements outside the lab, on site. The instrument allows semi-quantitative measurement of metals, and also of rocks, soils, ores and ceramics. Despite the advantages, however, it must be borne in mind that the light elements up to sodium cannot be detected by this method, as the measurements are taken “in air”.
In order to be able to carry out analyses in the ultra trace range, or isotope measurements, a cleanroom laboratory for the preparation of specimens was set up when the high-resolution mass spectrometer was purchased in 2009.
The purpose of this laboratory is to prevent environmental contamination during preparation of the samples as far as this is possible. The cleanroom (ISO 8) and the working areas in it (ISO 3) are classified and certified in line with EN ISO 14644-1:1999.
All acid hydrolysis processes for mass spectrometry analysis are carried out in this laboratory, as is the chromatographic separation of lead for isotope analysis.
Here time-compressed laboratory tests simulate processes of weathering or corrosion on treated and untreated stones and metals, and also on materials that have been subjected to conservation measures. Besides standardized tests, which investigate the impact of a particular weathering factor, we also carry out simulation tests with combinations of factors similar to the natural environment. The material changes produced are examined with analytical techniques and measuring methods. Simulation tests make it possible to evaluate the stones and metals and to predict their weathering behaviour on the structure. Correlating the results of the building inspection and the weathering simulation allows conclusions to be drawn about the weathering and corrosion processes which are occurring, the subsequent progression of damage, and possible steps to prevent further damage.
Scanning electron microscopy (SEM)
The field emission scanning electron microscope SUPRA 40 VP from Zeiss, purchased in 2010, allows measurements into the nanometre range. It is equipped with a variable vacuum mode, mainly used for samples where coating with gold or carbon is not possible. These can be thin or polished sections which have to be examined with a light microscope at a later stage. Museum objects or archaeological objects which may not be coated can also be examined directly in this way. Samples which still have a certain degree of moisture or which can become charged are also examined in this mode.
Gold and carbon sputter coaters are available for specific applications. The large sample chamber has no restricting airlock and thus allows analysis of fairly large samples (13 cm x 13 cm x 4 cm). Additional software makes it possible to create three-dimensional representations of surfaces, and can be used to measure the thickness of layers.
The scanning electron microscope is equipped with an energy-dispersive x-ray spectrometer (Noran System 7, Thermo) with a silicon drift detector (SDD), allowing non-destructive, qualitative and semi-quantitative analysis of all sample components. Measurements can be taken at selected points, over a cross section, or across the whole surface of the sample. Two-dimensional distribution of elements is also possible; this makes it possible to see zonal or local accumulations of elements (“mapping”). Samples from all the areas mentioned can be examined with the electron microscope. Technical products and their corrosion or faulty operation can also be checked. Modern ceramics can be assessed in terms of the bonding of the aggregates in the matrix.
Optical polarization microscopy
Polarization microscopy is used to examine the polished and thin sections prepared in-house. One of its functions is to identify mineral phases, the other is to quantify structure and microstructure in conjunction with image analysis. Image analysis makes it possible not just to take various measurements and determine areas, but to create three-dimensional images of surfaces by recording the different layers of the sample.
Polarization microscopy gives various kinds of information, such as:
- the historical sequence of paint applications or paint residues
- the origin and quality of roofing slate
- the conditions in which slags and metals were formed, or the techniques used to process them
- porosity, particle-size distribution curve, ratio of matrix to aggregate, and cracking of mortar
- aggregates used in pottery (determining type of product)
- particle size and type of particle bonding, and thus the susceptibility of stone to weathering