3.1 Static resistance stress-strain tests on model samples
Figure 4 provides the variation of strains and temperature with respect to operation time. It appears that the strain gauge on the top of glass I sample (where melted menthol is applied) provides negative values indicating compression. Meanwhile, the other strain gauge on the bottom of glass provides positive values which suggest tensile deformation. It is known that solidification of menthol melt is accompanied by volumetric shrinkage24, which leads to concave bending of the glass I sample in the present experiment. This bending results in compression and stretching, respectively, in the upper and lower portion of glass, in consistent with the measurements of strain gauges. Figure 4 also shows that the two strain curves provided by upper and bottom strain gauges are roughly symmetrical about the horizontal axis which indicates zero strain.
Combined with temperature curve, more information can be read from Figure 4. When menthol melt is just applied, i.e. before 500 seconds, low level strain noises were observed due to thermal impacts. After that, from 500 seconds to 1000 seconds is an early stage of solidification. In this stage, temperature decreases below the melting point of menthol and both compressive and tensile strains start to increase, indicating that the volumetric shrinkage of menthol upon solidification starts to lead to bending of glass substrate. From 1000 seconds to about 2000 seconds is the steady stage of solidification. In this stage, temperature rises again due to exothermic solidification process. When the thermal balance is established between internal heat generation and diffusion to outside ambience, the temperature becomes stable, indicating a stable solidification stage. In this stage, the strains increase with nearly constant gradients, implying that menthol solidifies in a constant rate. Completion of solidification is found around 2000 seconds, from when the temperature drops again due to termination of exothermic solidification. At the same time, both strains reach peak values and then stop increasing. This verifies the completion of solidification from mechanical view, i.e. menthol stops to shrinkage and, therefore, is unable to cause further bending of glass. The period after 2000 seconds is a relaxation stage. In this stage, temperature gradually lowers down showing that no solidification reaction takes place. The strains decrease slightly, probably due to structural relaxation/rearrangement which is often observed in imperfectly crystallized polymers42.
Figure 4 provides an important insight to the application of menthol for temporary conservation. In real applications, although white waxy solid can be observed just a few minutes after menthol melt is applied, it is still difficult for people to tell whether solidification is completed. Our experiment shows that the solidification of menthol takes much long time than we expected and the solidification achieves peak mechanical strength in about 2000 seconds. When melted menthol is applied in archaeological excavations, enough time should be given to enable the menthol solidifies completely for a higher strength.
Figures 5a and 5b are prepared to discuss the impacts of menthol quantities and melt temperatures on solidification, respectively. Figure 5a shows that application of more menthol leads to longer solidification time as well as higher strains. This indicates that stronger stress effects would be induced in applications of large quantities of menthol. Figure 5b illustrates that application of menthol melts of higher temperature would enhance the bending deformation of glass substrate and lead to larger strains.
3.2 Static resistance stress-strain tests on simulated samples
Previous studies have characterized the solidification process of menthol using a model sample. In this part we will focus on samples closer to real cases in conservation of cultural relics. Three simulated samples are investigated, i.e. glass II, Yungang sandstone and rice paper, representing rigid non-porous, rigid porous and soft materials, respectively. These are all common materials encountered during excavations. The resistance strain gauges need to be glued on the samples firmly. Thus, for the convenience and practicality of the research work, non-aged simulated samples with relative strong mechanical strength are used. Based on the basic physical understanding of this phenomenon, the occurrence of inner stress should not be related to whether samples are aged or fragile. The outcomes of inner stress are closely related to the status of the samples. So, the results obtained should be applicable to aged or fragile artifacts as found in archaeological excavations.
As demonstrated in Figures 6-8, two strain gauges are pasted on one side of each sample. In order to investigate the comprehensive stress profile induced due to menthol application, one gauge denoted by “O” is in the region where menthol melt is applied, the other one denoted by “A” is away from melt application. Precautions have been taken to prevent outflow of menthol melt.
The results on rigid non-porous glass II samples are shown in Figure 6. The stain curves are divided into 2 regions. The first region, from 0 to about 400 second, is an invalid phase as the signals are mainly induced by thermal impact of high temperature melts. After 400 seconds, increasing strain at position O is observed, indicating solidification of menthol melt. The negative values indicate compression, in consistent with model sample (glass I). The strain reaches maximum value at around 1000 seconds, much faster than it does in glass I sample. This is because in glass II case, the melt is cooled down much faster. After 1000 seconds, the strain starts to gradually decrease due to some degree of structural rearrangement before it stabilizes42. For position A, strain just oscillates in a small range around zero strain except the first invalid stage. It indicates that the stress in the region outside menthol application is basically unaffected by the solidification of menthol.
Yungang sandstone, representative of rigid porous material, is also investigated. In Figure 7a, when menthol melt is applied on position O, strain at the O changes rapidly due to thermal impact. After that, when the melt starts to solidify, strain at position O switches quickly from negative to positive side, indicating that solidification of menthol melt finally leads to facial stretching of sandstone. This phenomenon may be attributed to the porous microstructure of sandstone. When applied, melted menthol might permeate into the porous sandstone. It is speculated that solidification of menthol inside the micro pores would generates expansion actions on the pore walls, resulting in stretching of sample. The strain of position A is around -10, indicating weak compressive stress. This is reasonable as the stretching of the small core area has to be balanced by the compression of surrounding area.
In order to verify the proposed mechanism above, two more sets of experiments are carried out on sandstones. As shown in Figure 6b, four strain gauges, referred as 1 (top), 2 (top), 3 (bottom), and 4 (bottom), are glued on both sides of the sandstone. 2mL of menthol melt is applied. It can be seen from the figure that both gauge 1 and 2 provide positive values indicating tensile strain in the upper portion, while gauge 3 and 4 show negative values indicating compressive deformation in the lower portion.
In another test shown in Figure 7c, much less menthol melt is applied. In this case, variation of strain is a bit similar to that applied on glass. The strain at position O is always negative. This suggests that the little menthol mainly solidifies on the surface of sandstone instead of infiltrating into the pores due to fast cooling. The surface deformation is therefore contraction.
Rice paper, representative of soft material, is also tested, as seen in Figure 8. As paper does not conduct heat well, thermal influence on the strain at position A is insignificant. It is seen that the strain at position O stabilizes at, which is larger than that of glass II and sandstone. This is because paper is too soft to withstand the contraction of menthol. Unlike rigid samples, the strain at position A of rice paper is also compressive, the same as position O. It suggests that the contraction of the menthol area may lead to contraction in larger neighboring area in soft substrate.
3.3 Stress evaluations of solidification of menthol melts on simulated samples
The information revealed by the static resistance stress-strain tests is the strain of certain media upon solidification of menthol melt. It is quite straight forward according to elastic constitutive that stress strain relation is:
σ (stress)= E (Elastic Modulus)× ε(strain) eq.2
The elastic modulus of glass II sample is acquired to be 71.52 GPa by dynamic elastic modulus measurement method. The elastic moduli of Yungang sandstone and rice paper are 3.08 and 1.12 GPa, which are acquired by compression or tensile method on a universal mechanical testing machine respectively. The maximum strain values of point O (position where menthol melt is applied) in each figure are chosen to calculate the stress. The internal stress induced by menthol melts on glass (Figure 6), sandstone (Figure 7a) and rice paper (Figure 8) is calculated to be 0.71, 0.11 and 0.39 MPa, respectively. Although these data might not be very accurate, the order of the magnitudes is acceptable. It shows that all the stresses are quite small. Application of menthol as temporary conservation material is safe to cultural heritages from mechanical viewpoint.