39.2 Chemistry and research: Agar-Art: we gained a better understanding

29/08/14

Some years ago, when we added agar-agar to our catalogue, there was a lack of knowledge about the full range of its properties. After testing a multitude of grades available on the market, we finally could identify and choose the best grade of agar-agar that perfectly matches with our goals; that is “effective in cleaning and easy to be removed”: Agar-Art was born!

Nowadays scientific tests and findings validate and support a large variety of agar-agar applications around the world. We are referring to the first study project carried out by the Institute for Conservation and Valorization of Cultural Heritage, ICVBC, CNR, (Milan), in cooperation with the Dept. of Materials Science, University of Milano-Bicocca, (Milan) and National Institute of Optics, INO, (Sesto Fiorentino), based on the experiments performed by Anzani M. and Rabbolini A., the restoration experts of Aconerre Conservation Studio[1]. They tested Agar for cleaning a capital made of Angera stone, a porous dolomitic stone (porosity 16-21%), commonly used in the architecture in the North Italy. Agar-Art at 1% w/w was applied to that surface in three different ways; firstly as in the form of a warm (t 50-60°C) fluid solution and let it to gel on the surface, then as a preformed rigid gel and finally as in the form of homogeneous sized particles, grounded inside a mixer. After that, they tested the preformed gel at different concentrations (from 1 to 4% w/w). The amount of removed salts was measured by Ionic Chromatography (CI) and Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES). These two techniques detected relevant quantities of sulphate salts that were generated by the negative effect of air pollution, particularly severe in Milan urban area, the place where the stone capital comes from. Concerning the different cleaning conditions they came to the conclusion that both the warm solution and the grounded gel work better than the preformed rigid gel. With regard to the composition of the solution, the higher the concentration the greater the power to remove undesired deposits like sulphates of calcium and sodium and magnesium (considering that capital is made of Dolomitic stone). Extraction of salts is not high when the gel is used at 1%; but that capability increases very quickly when using the product at 2% and 3%, up to 4% w/w. When concentration is higher than 5% w/w, Agar gel becomes difficult to be managed and removed, therefore they did not consider going further.


Another important study research, published on Heritage Science [2], was carried out by Politecnico di Milano and by Anzani M. and Rabbolini A. (Aconerre Conservation Studio), the 'pioneers' of Agar agar cleaning methodologies. In that case study the evaluation of the agar gel cleaning procedure was conducted on a sculptured group (so-called “Tornacoro”) behind the main altar of the Duomo di Milano (pic1). A multi-analytical approach was carried out during the set-up phase to compare the efficacy of different cleaning conditions in terms of agar concentration (from 1% to 4% w/w), presence of additives(Tween 20, EDTA, ammonium citrate and sodium polyaspartate) and application time (starting from 15 min. up to 120 min.). Spectrophotometric measurements (colorimetry, FTIR, XRD) were adopted to monitor the aesthetic features of the sculpted surfaces before and after the cleaning in order to confirm the absence of any damage to the stone substrate. Those marbles show more and less smooth finish areas and different grades of overlapped deposits, also because of the orientation and slope of the surfaces. On the surfaces having a vertical orientation (partially protected by dust deposition), a dark-brown to yellow colour was observed, that was due to the deposits of former wax treatments, the grey appearance mostly characterized the semi-horizontal area, covered by irregular layer of particulate matter and dust, made of gypsum, nitrates and sulphates.
Is is interesting to note that sulphur was detected just on this external layer that means no sulphating reaction is in progress, contrary to what occurs to the external marble features of the Duomo.FTIR analyses revealed a nearly total removal of gypsum and nitrates; after the gel cleaning no more spectra peaks were detected, on the contrary the application of a deionized water could not produce the same good performance. In fact the cotton swab usually leaves a higher amount of salts still deposited on the substrate and does not enable a good draining. The soil dust particles are partially absorbed into the gel and part of them tend to be bonded at the agar-stone interface. The few remains of deposit material, after the gel removal, have no residual adherence to the stone substrate and can be removed after the final mild sponging of the surface. Additives do not quite affect the grade of cleaning. The better cleaning methodology has been defined as it follows: apply 3% agar gel to the surface for 1 hour after nebulizing deionized water mixed with 1% Tween 20. Tween 20 has proved to ease the final removal of the indissoluble deposits left on the surface. The application of this methodology on a large scale has required to evaluate many parameters (density, thickness of the gel layer, time) in relation to the morphological characteristics of the sculpted stone and its state of preservation.

References:

1)Sansonetti A., Casati M., Striova J., Canevali C., Anzani M., Rabbolini A.; “A cleaning method based on the use of agar gels: new tests and perspectives” 12th International congress on the deterioration and conservation of stone. New York; 2012. 
2)Gulotta D., Savielllo D., Gherardi F., Toniolo l., Anzani M., Rabbolini A., Goidanch S.; “Setup of a sustainable indoor cleaning methodology for the sculpted stone surfaces of the Duomo of Milan”, Heritage Science, 2014, 2:6.
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