In 1966, within the Conservation Institute of Croatia, the Laboratory for Conservation Research was established. The Laboratory was set up in response to the need for an exploratory department, where chemical, mineralogical and physical analyses would be employed to investigate the condition of monuments and the causes of their deterioration. In addition to the diagnostics of monument condition, the Laboratory is tasked with quality control of repair procedures and exploring the application of new materials and conservation-restoration methods on immovable cultural heritage. In 1998, after the founding of the Croatian Conservation Institute, the Laboratory for Conservation Research changed its name to the Natural Science Laboratory.
In respect of analytical methods, nowadays the Natural Science Laboratory performs microscopic, microchemical and X-ray fluorescence analyses of pigments of painted coats; chemical and IR spectroscopic analyses of binders of painted coats; chemical, physical and microscopic analyses of plaster composition; mineralogical analyses of stone; chemical analyses of water-soluble salts; microscopic and microchemical analyses of textiles; X-ray fluorescence analyses of metals; and microscopic analyses of wood. In addition to these analytical methods, the Laboratory measures humidity in constructional elements of monuments, and microclimatic parameters relating to the cycle of salt crystallization, primarily in wall paintings.
The Laboratory has not always been capable of providing all types of expertise, because some analyses have fallen outside the scope of regular activities, due to shortage of special equipment and specialized staff. In such cases, experts have been engaged, coming from specialized institutes in Croatia and abroad. We have established successful cooperation with Zagreb University, with the Faculty of Chemical Engineering and Technology, Faculty of Forestry, Faculty of Science, Faculty of Mechanical Engineering and Naval Architecture, and the Ruđer Bošković Institute. Over the last decade, cooperation with the Institute of Mineralogy and Petrology of the Department of Geology within the Faculty of Science of the University of Zagreb has been particularly important, as well as with the Laboratory for Ion Beam Interactions within the Division for Experimental Physics of the Ruđer Bošković Institute.
Samples for microsections are covered with resin. Once firm, they are ground and polished vertically, to obtain layers. The microsections are then analysed stratigraphically, and photographs are taken with a polarized fluorescence microscope.
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Pigments of painted coats are analysed by X-ray fluorescence and microscope analysis, and they are compared to the standards.
XRF fluorescence spectroscopy is based on irradiating the sample with X rays, causing the so-called photoelectric effect, resulting in characteristic X rays for each element of the sample. These new X rays are called X-ray fluorescence or XRF, and their detection and analysis can be used to establish the presence of elements between potassium and uranium.
Analysis of Binders
Organic binders of painted coats, fillings and varnishes are analysed by microchemical analysis, thin-layer chromatography and IR spectroscopy.
Microchemical analysis includes testing the solubility of the sample with a range of solvents. The tests are based on microchemical quality analyses. The solubility of the sample allows us to place the binder within a certain group, or, if the sample quantity is sufficient, to establish the exact type of binder.
Thin-layer chromatography is a chromatography method in which various components of the mixture (sample) are separated, depending on the speed with which the mobile phase carries them through the stationary phase.
As the mobile phase (developer) 'climbs' the stationary phase (silica-gel plate) due to capillary forces, the mixture’s components separate, depending on the differences in their solubility in the mobile phase. A mixture component that is better dissolved in the mobile phase 'travels' faster than a component which is less well dissolved. The positions of spots on the plate deriving from the sample are then compared to the positions of spots deriving from a range of well-known clean mixtures (standards).
Infrared (IR) spectroscopy is a measuring method which analyses the interaction between infrared radiation and a substance. IR spectroscopy is based on the fact that molecules and atomic groups vibrate at specific frequencies within the range of the IR part of the electromagnetic spectrum (~ 400-4000 nm). The measurement result is a spectrum which shows the dependence of the transmittance percentage (proportion of incoming light of a certain wavelength passing through the sample) on the wavelength of the IR radiation. The chemical structure of each substance is different and results in a unique IR spectrum. The spectrum analysis encompasses reading of the tapes (frequencies), comparing them to the literature and comparing them to the spectra of standards (clean compounds), to identify the binder, and other substances present.
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Plasters and mortars are composite materials, i.e. mixtures of granules, sand, binder (lime, gypsum, cement) and water. Macroscopic and microscopic analyses of a plaster sample can establish the colour of the binder, the type of the aggregate and the size of the aggregate granule. Chemical analysis, consisting of dissolution of a plaster sample in an acid (HCl; CH3COOH), separates the binder and the aggregate, thus providing the ratio of the carbon binder and the non-carbon aggregate in the plaster sample.
The granulometric analysis of the separated aggregate (the distribution of the granule size in the plaster) follows the rinsing of the aggregate and its sieving through screens of various sizes.
Identification of Textile Fibres
Textile fibres are analysed under polarized transient and reflected light, and they are dissolved in various solvents.
Determination of Tree Species
A tree species is determined by the microscopic analysis of cross-, tangential and radial sections of wood samples under transient light.
In a paper sample, certain types of fibres can be identified (e.g. cotton, linen…), and the presence of lignin can also be established. The presence of lignin would indicate that pulp was used to produce the paper. The fibres are identified by microscopic analysis under transient polarized light passing through preparations, while the presence of lignin is established by microchemical analysis, with a fluoroglucinol reagent that dies lignin red or purple.
Diagnosis of Condition of Immovable Monuments
In addition to establishing the composition of materials and painted coats on immovable monuments, the laboratory also identifies causes of deterioration of construction materials, with a view to issuing recommendations and supervising the repair work.
For this purpose, the presence of damaging water-soluble salts in materials is established by spectroscopic and chemical methods. Furthermore, the quantity of moisture in the material is established, together with microclimatic conditions in the environment, hygroscopicity and water absorption of the material, necessary for the desalting process or salt neutralization.
Domagoj Mudronja, PhD, Head of the Natural Science Laboratory
Phone: +385 (0)1 4683 515