Three-dimensional digital documentation
To monitor the conservation treatment and structural deformation of the kiln site, it is very important to precisely digitize and record the detailed present condition of the surface of the kiln, including its spatial arrangement. Therefore, a 3D scan was performed before and after the conservation treatment of the interior and exterior of the kiln, including the protecting shelter of the kiln site. First, a mid-range scan was performed for a total of 56 points considering the height and overlap ratio so that the point density was uniformly distributed throughout the kiln site. The point density was set to 5 × 5 mm/10 m considering the scan range and resolution. Then, 3D modeling of individually scanned point cloud data was completed through registering, filtering, merging, and texture mapping as reported in many previous studies [5, 6].
The spatial information model of the completed kiln site basically has a relative coordinate system (Fig. 2a). Therefore, to link the coordinates with the real-world coordinates, a georeferencing process that can convert the relative coordinate system into an absolute coordinate system is required. Currently, in South Korea, the planar positions of national and city reference points are determined based on the GNSS. For surveying the ground control points of the kiln site, five random points that had no obstacles around them and uninterrupted GPS signals were selected. From the information of each control point, it is seen that the kiln site is located in a terrain with a slight slope rather than on a horizontal terrain in the longitudinal direction; it has an altitude of 51.2–52.7 m. This ground control point information was used to correct the absolute position of the 3D model built based on mid-range scanning data (Fig. 2b).
Meanwhile, in this study, precision recording was performed using a handheld scanner, considering the size and accessibility of the kiln site. Precision scanning was conducted for the working space and combustion chamber, which are the regions where most shape changes are predicted to occur during kiln site conservation treatment. The 3D scan was performed at a rate of about 16 frames per second in an environment without direct sunlight because it was necessary to distinguish the structured light scanned on the kiln. The scan data acquired in the field basically undergo almost the same process as the mid-range scanning data, but they are different in that the former data were obtained in the form of a polygon mesh rather than as point cloud data [7]. In particular, the mid-precision scanning results were of excellent quality in terms of both the polygon mesh and RGB texture mapping results. Further, as the data had millions of high-resolution polygons, the surface texture, manufacturing technique, and cross-sectional structure of the kiln working space, including the entrance of the combustion chamber, were clearly revealed (Fig. 3).
The overall shape was actually surveyed based on the 3D scanning model of the kiln site. The bottom of the kiln and the combustion chamber were actually measured to be about 6.7 and 1.1 m long, respectively. In addition, the working space extended about 3.4 m in the longitudinal direction. The furnace on the left side of the combustion chamber was about 0.5 m deep. The entrance of the combustion chamber was 0.7 m wide. By analyzing the left and right longitudinal sections, the slope of the bottom of the kiln was determined to be about 16.7° (Fig. 4).
Conservation treatment of the kiln site
The Gangjin Kiln site was hardened and dismantled after excavation in 1982 for relocation. It was restored at its current location in 1987. The restoration was performed by placing the combustion chamber and the left and right kiln walls according to the slope of the kiln on a reinforced concrete foundation. Then, the outer wall of the kiln and the damaged part were reinforced with synthetic resin. Since then, because of deterioration over 20 years, aesthetic problems arose, as parts of the kiln walls cracked and fell off. Subsequently, in 2007, an effort was made to preserve the original shape through primary conservation treatment. However, soil disintegration and durability degradation occurred continuously over the last 13 years since 2007, and thus, it was decided to undertake secondary conservation treatment in 2020 (Fig. 5a).
The secondary conservation treatment was largely performed for recovering the damaged parts, structural reinforcement, and partial restoration in principle. First, powdered soil was removed from the entire kiln, and the outer wall of the kiln was fixed with bolts and wires. In addition, a material was prepared by mixing red clay, Dobak (Pachymeniopsis elliptica YAMADA) glue, and kneading straw, and the inner and outer walls of the kiln were restored using this material. Some detached pieces were joined using synthetic resin. Thereafter, using mixed materials such as red clay + quicklime and decomposed granite soil + synthetic resin, the inner floor of the kiln, flue, and working space and its periphery were reinforced. Finally, foreign substances were removed by dry and wet cleaning, and the heterogeneity of the restoration was reduced to the maximum extent possible via coloring (Fig. 5b, 5c).
Analysis of shape change before and after conservation treatment
In statistics, a deviation is the difference between an observed value and the mean. It is expressed as a positive or negative number, indicating that the observed value is greater or smaller, respectively, than the mean. The magnitude indicates how far the observation is from the mean. A deviation can be referred to as an error or a residual and is the observed value minus the mean or median. The variance is obtained by squaring the difference between the observed value and mean, adding the squares for all observations, and dividing this sum by the total number of observations. The standard deviation is the square root of the variance, and it transforms the variance with an enlarged value back to its original magnitude.
The root-mean-square (RMS) value is a representative value that is indicative of the characteristic or tendency of a certain group, such as the mean, median, or mode. The RMS value is obtained by taking the square root of the mean value of squared deviations. RMS is commonly used when dealing with the difference between an estimated or a predicted value and a value observed in the real world. It is generally suitable for expressing precision. Each difference value is also called a residual, and the RMS deviation aggregates the residuals into one scale. The lower the RMS value, the more accurate is the prediction. A perfect prediction model has an RMS value of zero. This value is easy to interpret because it expresses the difference between the model predicted value and the actual value as a single number, and it is widely used for image analysis because each numerical value can be visualized [8–10].
In this study, RMS analyses were performed using 3D point cloud and mesh data before and after the conservation treatment of the kiln site. The big data of deviations were visualized. For this purpose, the kiln site before the treatment was set as a reference model, and the kiln site after the treatment was set as an analysis model. Thereafter, the two models were registered with the iterative closest point algorithm based on the points that did not experience any shape change because of the conservation treatment (Fig. 6). The deviations between the two shapes were analyzed using the matched 3D model, and based on the results, the statistical values such as the minimum, maximum, mean, RMS, standard deviation, (+) mean, and (−) mean were calculated.
For analyzing the deviations of the kiln site between before and after the treatment, a mid-range scanning model was used to represent the overall shape, and a precise-scanning model was used to indicate the minute changes in the shapes of the working spaces, including the combustion chamber. First, the changes in the shape of the entire kiln were studied by comparing the shapes after treatment with the mid-range scanning point cloud data. The results showed that the conservation treatment focused on the floor inside the working space, bottom of the kiln, left side of the flue, and upper wall. In addition, there were notable changes on the left side wall of the combustion chamber. In the deviation visualization image, the most noticeable change after conservation treatment was determined to be around 15 mm, and many changes of 40 mm or more were also identified (Fig. 7).
The changes in the conservation treatment of the working spaces, including the combustion chamber, were analyzed using the precision-scanning polygon model. The results showed that though the shape of the stone works as a whole did not change remarkably, positive (+) values were remarkably visualized in the filling material between the stone works. This indicates that reinforcement was performed to prevent deformation of the stone during the conservation treatment. The greatest shape changes in the analysis area were observed in the case of the floor and the interior of the combustion chamber. It was seen that the floor had been reinforced and flattened to a thickness of at least 40 mm. The interior of the combustion chamber was also visualized to be reinforced by at least 50 mm. When the numerical analysis results before and after the conservation treatment of the entire working space, including the combustion chamber, were taken together, the RMS was calculated to be 14.1 mm, which is the average shape change due to the conservation treatment. Further, the average deviation value was 10.2 mm in the (+) direction and 4.3 mm in the (−) direction, indicating that the dominant conservation treatment centered on the reinforcement (Fig. 8).