The possibility of determining the age of ceramic materials using thermoluminescence (TL = thermally stimulated luminescence) is due to the fact that some minerals contained in ceramics can store energy. This energy comes from the decay of durable natural radioactive materials (uranium, thorium and potassium-40), which are, in small amounts, present in all natural minerals.
By heating the energy-storing minerals to sufficiently high temperatures, the stored energy is released and the "TL Clock" is set to zero. This occurs during firing, but also during any other sufficiently high heating of the ceramic.
To determine the age, the amount of energy stored in the minerals since the last heating of the ceramic is measured. This energy is an indicator of the radiation dose acting on the ceramic over time - the ceramic, therefore, acts as a dosimeter for the natural ambient radiation. During the TL examination, a sample is heated and the energy released in the form of visible light is measured. The higher the light intensity, the higher the radiation dose. To calculate the age of the ceramic it is still necessary to know the dose rate (dose per time unit, e.g. per year).
Thermoluminescence offers the possibility of absolute dating and authenticity testing of ceramics and cast core material from cast metal objects.
Contact: AProf. Dipl.-Ing. Rudolf Erlach
Depending on the research question, we use a combination of different microscopic techniques for the investigation of samples of historical mortar, generally consisting of optical (stereo and polarizing light) microscopy as well as scanning electron microscopy (SEM) - supported by energy dispersive X-ray spectroscopy (EDS) on a variety of different sample preparations.
The information obtained includes the binder type (lime/hydraulic lime/ pozzolanic lime/natural-cement/Portland-cement/ gypsum or mixtures thereof), the petrographic nature, grain shape and the grain size ranges of the aggregate, the type and frequency of pores and shrinkage cracks, as well as an estimation of the binder/ aggregate ratio. The quantification of the mentioned parameters via quantitative binder analysis with EDS or digital image analysis is possible, but the latter requires considerable additional effort.
Stratigraphy examinations of plaster sequences with or without paint/painting
Our choice of method for the investigation of "plaster renders" does not differ from the mortar analysis described above, but takes into consideration how the chronological sequence of successive layers is to be evaluated. In addition to the comparative characterization of binders, aggregates and microstructural characteristics of different plaster layers, we also examine the interface between layers with respect to corrosion or dirt deposits, which provide indications of long-term visual exposure of a layer surface. In addition the existence or absence of sintered skins can provide meaningful information at layer boundaries.
The coatings or paint layers themselves are examined and documented by optical microscopy and SEM-EDX, with particular attention on pigments, fillers and inorganic binder constituents as well as microstructural features, always considering their substrate. If organic binder components are to be detected, we can carry out micro-chemical or FTIR analyses via our cooperation partners, the results of which are incorporated into the overall interpretation.
Analysis of natural stone
Based on the petrographic analysis of rock thin sections, occationally supported by SEM investigations, we can provide the most important forms of rock diagnostics, with a focus on relevant microstructural features including pore space-related properties and also any weather-related changes.
In some cases, we have X-ray diffractometry (XRF) performed by our cooperation partners.
Corrosion and weathering studies
We use the SEM-EDX to describe microstructural damage or weaknesses caused by weathering, as well the extent and depth of mineral secondary products in the form of crusts or accumulations in the pore space. We prefer to work with small samples of salt-loaded substrates, but can also cope well with powdered specimens or pull-off samples. If water-soluble or water-sensitive components are suspected, we refrain from using water in the production of cross sections of samples that have been imbedded in synthetic resin under vacuum. Easily soluble salts in the pore spaces or on surfaces are preferably analysed using stereomicroscopy and SEM on a fractured surface without embedding and polishing. In case of questions concerning harmful salts when the sample contains crystallites, we first rely on their identification by SEM-EDS, whereby the topographical and causal relationship between salt crystallisation and substrate damage is investigated as far as possible. Quantitative ion analyses (generally using ion chromatography, IC) of drilling dust or compress material are passed on to our cooperation partners, who also carry out the preparatory work steps in accordance with the relevant standard. We then calculate the results of the analysis and interpret them together with any SEM results.
To determine the water vapour absorption behaviour of harmful salts or materials contaminated with salt, we offer gravimetric measurements over the entire relevant humidity range with our climatic chamber. Through our cooperation partners, we can also have very precise sorption isotherms measured on small samples or efflorescence salts, whereby on the one hand the exact value of the equilibrium moisture content of a given salt mixture can be determined, and on the other hand, the specific surface of the substrate can also be calculated using BET. For very small sample quantities in particular, we alternatively suggest microscopic observations of hygroscopic melting point and other visible phase transformations in a specially developed moisture-controlled measuring cell, which is currently being converted to a dynamic system.
Due to many years of project experience, we are regarded as experts for the visualisation of the active mineral ingredients of consolidants in the pore space of samples by means of SEM, whereby parameters such as depth of effect, connection to the substrate components, ability to bridge, tendency to shrink, etc. can be investigated and - if necessary using false colour techniques - visually documented and quantified. In addition to these statements, we can determine the dynamic modulus of elasticity with ultrasound, or, together with our cooperation partners, determine all other mechanical or physical parameters. In individual cases we also carry out complete, tailor-made test series for the evaluation of strengthening agents for porous building materials, which range from laboratory application of the products to their evaluation in the above sense.
Further investigations on questions of conservation and restoration of cultural heritage
In addition to the already mentioned materials, we have experience with ceramics, glass and metal for chemical analyses as well as petrographic and microstructural investigations on small samples using SEM/EDS. Any coatings or corrosion phenomena can also be characterised, provided that this is possible with our equipment.
We carry out physical tests to determine the capillary water absorption coefficient and water vapour permeability in accordance with existing test standards. In order to evaluate climate-related ageing resistance of conservation products, we can develop and carry out tailor-made test protocols, which, however, can only be carried out in exceptional cases because they require the use of harmful gases.
As a small, specialized university institute, we are determined to adapt our examinations as flexibly as possible to the respective problem at hand. Therefore, we often dispense with routine examinations in accordance with applicable testing standards.