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·Conservation and Restoration in Art and Architecture
Determining the origin of the tray for wine and waterZ.Schauperl, B.Runje, S.Mahovic
University of Zagreb, Faculty of Mechanical Engineering and Naval Arhitecture
10000 ZAGREB, CROATIA
|Fig 1: Sacral tray for wine and water|
|Fig 2: Galvanoplastic technology||
Fig 3: Bottom of the tray, various magnifications
2.1. Tray Material Structure Analysis
Macro and microstructure of the tray were analysed. Macrostructure analysis of the bottom of the tray shows two areas: smooth parts on which the tray is supported which can be cleaned and parts in the depressions which are not subjected to external influences. On these parts, "droplets" can be noticed, which are characteristic of galvanoplastic technology and they are visible on the contact surface with the mould, Figures 3 (a and b).
Microstructure analysis of the tray cross-section would provide the safest insight into its manufacturing technology. However, since this item must not be cut, the microstructure was analysed at its edge. Microstructure is presented by means of replicas: the tray edge was ground rough and fine and polished, then attacked by a combination of acids for silver. The obtained microstructure is shown in Figure 4.
The analysis shows a directed microstructure near the upper (A) and bottom surface (B) of the tray, whereas the structure in the middle is non-directed.
This proves that the curvilinear shape of the tray edge was generated by some cold forming (bending, pressing, embossing), whereas the non-directed microstructure in the middle indicates that the tray was produced from a silver plate which was then further treated.
|Fig 4: Microstructure at the tray edge||Fig 5: Measuring surfaces of the tray|
2.2. Measuring the Tray Thickness
The tray thickness was measured at four measuring surfaces as presented in Figure 5. On the measuring surfaces No. 1 and 2, the thickness was measured at 40 points, and the tray thickness at measuring surfaces 3 and 4 was measured at 20 points.
Measurement results are presented in Table 1.
|Measuring surface No.|
|Table 1 : Results of measuring the tray thickness|
In case of results obtained by measuring the tray thickness at measuring surfaces 3 and 4, a difference in thickness on the left side compared to the right side of the tray may be noticed. Left side of the tray was declared to be the one whose bottom of surface 1 shows four tiny dotted damages caused by corrosion (tray sides are marked in Figure 5). The results of thickness measurements on the left and right side of surfaces 3 and 4 are presented in Table 2.
|Measuring surface No.|
|Left side||Right side||Left side||Right side|
|Table 2 : Results of measuring thickness on the left and right side of the surfaces 3 and 4|
2.3. Analysis of Surface Microgeometry
At characteristic surfaces marked in the photo (Figure 5), their roughness was tested. These tests were carried out by contact method using electronic mechanical device with Perthometer S8P probe. Roughness parameters Ra and Rz were measured at each marked surface on the upper side and the bottom of the tray. All measurements were carried out under the same conditions, using Gauss electric filter of boundary value lB = 0.8mm with evaluation length of l = 4mm. On every surface measurements were carried out at five measuring points, and these were used to calculate mean values of roughness parameters. Mean values of roughness parameters with estimated standard deviation for all the tested surfaces on the upper side and the bottom of the tray are given in Table 3.
|Ra in mm||s in mm||Rz in mm||s in mm|
|Table 3 : Results of measuring roughness parameters|
Fig 6a: - Characteristic profile and topography of surface 3 |
Fig 6b: - Characteristic profile and topography of surface 4
2.4. Results Analysis
Based on the tray thickness measurement results shown in Table 1, it can be concluded that the thickness of the tray is greatest in the middle (surfaces 1 and 2). From the middle of the tray towards its ends, the thickness decreases. It can also be concluded from the measurement results shown in Table 2, that the left side of the tray is thinner in relation to the right side.
Results of measuring roughness parameters, shown in Table 3, indicate that the tray surface roughness is not even. By comparing measurement results of roughness parameters of the upper surfaces it may be concluded that surfaces 1 and 2 were more roughly treated compared to surfaces 3 and 4. The same holds also for the bottom surfaces, and the roughest treated surface is No.4. It is very likely that the reason for this is in the higher aesthetic requirements regarding upper surface No. 4, compared to the bottom of the observed tray surface.Such results in measuring thickness and surface microgeometry indicate that:
2.5. Analysis of the Chemical Composition
At the Institute Rudjer Boskovic in Zagreb Pixe analysis was used to perform quantitative analysis of the chemical composition of the tray. The results of analysis are shown in Table 4.
Table 4 :Results of Chemical Composition Analysis
Based on the performed testing it may be concluded with very high probability that the tray was made by cold forming technology, most probably embossing. This indicates that the tray may date back to 17th century, but there are no solid proofs for that. It is namely possible that this technology was used also in the 19th or beginning of 20th century. Unfortunately, this cannot be determined by the available knowledge and laboratory methods.
However, if one takes into consideration that this research is the first of its kind in Croatia, the results are satisfactory to begin with. Since there is a great number of such items whose origin has not yet been determined, such work has to be continued. It is believed that further work, i.e. analyses of a greater number of museum items, contacts with experts from abroad, and objective comparisons of the obtained results, will allow significantly improved accuracy in results to be obtained, and it will be possible to date various museum exhibits, as well as to discover secret technologies used by the ancient masters of art.
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