Physicochemical Colorimetric and Mineralogical Characterization of a Plaster

Physico-chemical, Colorimetric and Mineralogical Characterization of a Plaster Sample Coming from "Palazzo Governi", Cagliari (Italy) C. Genova 1, G. Visco 1, M. P. Sammartino 1, G. Cau 2 1 Dept. of Chemistry, University of Rome “La Sapienza”, P. le Aldo Moro 5, I-00185 Rome, Italy 2 Architect responsible for the maintenance, Ittiri (SS), Italy AIM OF THE WORK The Aim of this work is the physico-chemical, colorimetric characterization of the plaster of “Palazzo Governi”, an historical building located in Cagliari (Italy). As a fact the plaster shows a bad conservation state and such characterization has been requested as a useful support for its maintenance. To replicate the original color, it was measured by a colorimeter and by a “panel test” using the comparison with a color atlas so obtaining a comparison between its subjective perception and objective measure. Macrophotos (fig. 4) and thin sections (not shown) as well as the soluble salts content (fig. 3) were considered in order to indivituate the degradation causes. CASE OF STUDY “Palazzo Governi” is an historical building (about 1880), located in the Cagliari old town, district Stamapace; it stands on the ruins of an ancient church and during the second world war, bombing provoked several damages, particularly on the main façade that together with other destructive events led the need of several restorations. Its architectural style respects the neoclassical canons: it bases on a strong axial symmetry with a slight alternating rhythmic pattern; it consists of four orders, marked by cornices modestly jutting from the line of the facade, and finished at the top by a wide frame. Fig 2: colorimetric results. The innermost circle reports the results of the colorimeter, the other one reports the results of the panel test realized on five samples (CA 3, CA 4, CA 5, CA 6, CA 7) THE PROBLEM OF THE COLOUR Fig. 1 shows the damage on the main facade and the evident loss of colour of the plaster. So, our first analysis pointed to study its original color in order to reproduce it in the restoration. The colorimetry is the science used to quantify and describe the human color perception otherwise not objectively determinable. The study aims to compare the subjective perception obtained using a panel test (see Materials and Methods section) with the objective colorimetric measure. Fig. 2 evidences the significant difference of the results obtained by the two methods even if the Dominant wavelength differ very slightly. This highlights the uneasy task of determining the “real color”, duty is reserved to the responsible of the restoration that can choose its favorite result from the serie made available. THE POBLEM OF THE CONSERVATION The second question to solve concerns the causes of the plaster deterioration. IC data (fig. 3), according to some European guidelines show a medium risk from sulphate and a low one from all the other salts. Salts content of our sample results similar to the “Pharma” one data obtained for the “Church” sample resulted no significant due to the scarce amount of sample (really it resulted to be a gypsum small fragment. Fig 1: the main facade of the building. The picture highlights the bad conservation stauts of the plaster, mainly on the highest part of the wall MATERIALS AND METHODS 716 compact IC ion cromatograph (by Metrohom, Switzerland), colorimeter CM 2006 -D (by Minolta, Japan), The panel test was carried out by a group of students on five different samples of the plaster, 2 analyzed such as and the other 3 as filtrate of the solutions prepared for the salt content analysis (CA 1, CA 2, CA 3 in Fig 3 ). Through the Sikkens colour palette (Akzo. Nobel Coatings, Italy) code the L*a*b* coordinates and the dominant wavelength were obtained. IC results were compared with two other samples coming from a frescoe (Church in Fig 3) and from an external wall of a building inside the Sapienza University (Pharma in Fig 3) Fig 4: Macrophoto of a polished section of a plaster sample Fig 3: Concentration of soluble salts found by IC in the five analyzed samples. CONCLUSIONS With regard to the color to be used in the restoration intervention, basing on the "panel test" we have defined the original hue and let the architect and Sovrintendance the choice of the saturation. The degradation state must be mainly imputed to the lean plaster while the sulphate content correspond to a moderate risk of damage. Surely more samples must be analyzed in order to better understand if the plaster was originally lean or, as an example, the lime has undergone a sulfation and subsequent leaching. THE LACK OF BINDER MATERIAL Considering the fact that the cause of deterioration is not attributable to the presence of soluble salts, through others kinds of investigations we demonstrate that it is attributable to the lack of Ca. CO 3, the binder material of plasters. The sample was attacked by CH 3 COOH (acetic acid) that dissolves the Ca. CO 3. Through simple mathematic calculations, it was possible to have the percent of Ca. CO 3, that is less than 25% of the total. This not respect the ordinary concentration of binder material. CMA 4 CH 2014, Mediterraneum Meeting, Employ of Multivariate Analysis and Chemometrics in Cultural Heritage and Environment Fields, 5 th ed. , Rome, Italy, Europe, 14 -17 December 2014