4.1. Documentation of Treatment Stage using Dome RTI
Virtual relighting and filters were applied to visualize the status of each coin. Yellowish brown dust piled up near and between the characters were clearly observed in all three coins. Removal of dirt from coin 1 revealed a pit which also had a different surface specularity as seen in figure 3. The dirt was very reflective compared to the surrounding regions in figure 3(a) and then was observed as a dark empty hole in figure 3(b). Also, the wide rim surrounding the characters show much deeper scratches after cleaning. After cleaning, coin 1 and coin 2 revealed minor scratches and chipped marks hidden under the foreign residue. The removal of the thick patina on coin 3 was assisted using the RTI image to detect the strokes of inscriptions. RTI image effectively shows the dynamic change of surface morphology in the reverse side of coin 3. The change in surface normal are visible in figure 4. Through the process of cleaning, the overall surface became more complexed as in figure 4(b) where more RGB colors are blended to represent the X, Y, and Z components of normal values on the surface.
The nature of sandcasting methods left an uneven surface with shallow pits made by sand grain, where abrasives for cleaning resided after polishing the coins. These areas were checked under the microscope and visualized using RTI diffuse gain filters. (Figure 5) Diffuse gain filters increase the representation of height depth where areas quickly change on the surface. In figure 5(b) dark lines visualize the difference in height depth around the areas where white alumina powder is pitted. This area was then analyzed with a scanning electron microscope - energy dispersive spectroscopy (SEM-EDS) to confirm the white residue powder as alumina. The analyzed data detected alumina (Al) as well as copper (Cu), tin (Sn) and lead (Pb). (Figure 5c, 5d) The After cleaning stage was recorded with RTI before stabilization stage to make sure that no residue was left behind, and the abrasive was removed before moving on to the next stage.
The final stage was also captured using RTI. Each stages were visualized using specular enhacement filter and normal vector map filter as in table 2. Normal vector map especially shows the change in surface pixel-by-pixel by creating a false color rendering. X, Y, and Z components of the normal are represented as red, green, and blue, for each pixel respectively. Specular enhancement visualizes different reflective properties between the proceeding stages. The original state of all three coins have a relatively matte surface due to dust and patina covering its shiny surface. Between the processing stages, the change was most dynamic in the after cleaning stage. Sharp specular regions are revealed on the characters and the outer surrounding rim of the coin and the normal vector map visualized the surface of excluded corrosive substances. The consolidation stage only shows minor difference in images, appearing dark and blurry compared to the after cleaning stage. The specularity diminishes due to the coating layer of Incralac.
4.2. Detailed Visualiztion using Microscopic RTI
Detailed images were attained using microscopic RTI with a magnification of 20 and 30 for coin 1 and 3. The letter ‘常’ in coin 1 was acquired to examine the process of excluding corrosive substances near the upper area of the inscription. In figure 6, image (a) and (b) of the specular enhancement filter, the removed area of the corrosion is clearly visual. The area revealed was originally a negative area of the relief where rust eventually took place. Also, the removal of foreign matters is visualized in image (a) and (b) of the normal unsharp masking filter. Yellow dust was thoroughly removed from the surface. The change of the morphological feature can be seen by comparing image (a), (b), (c) in normal vector map. Shallow scratches and dents from everyday usage can be seen most clearly in (b) however less clear in (c) where the solution is coated on the surface resulting minor gaps to be filled. The change between each stage is also clearly visible in specular enhancement images where the surface of the inscription seems rather even and flat; then showing numerous scratches; lastly filling dents. However, in the surface normal image, in the case of deep scratches they are more apparent due to the filter effect.
The character ‘元’ was revealed during the process of treatment. In figure 7 areas of corrosive substances were visualized in the normal vector map (a) where patina is seen as red. The result of the removal of cupric rust can be compared in the specular enhancement image. Horizontal strokes of the character are dramatically visualized by shading different directions of light. Specular enhancement image (b) reveals a chipped area of the lower left stroke which was not easily detected in any of the original microscopic image.