## 4 Discussion  
To ascertain whether artificial and natural aging can be compared using the same FTIR-derived estimators or if more invasive analyses are required to assess the degradation state of the museum objects results from nondestructive infrared-induced vibration absorption (ATR-FTIR) were compared to the destructive but absolute technique of crystallinity calculation (XRD). Crystallinity estimators obtained from ATR-FTIR (E\({}_{\text{C}}\) and E\({}_{\text{CPOL}}\)) and XRD (E\({}_{\text{BXRD}}\)) were juxtaposed against the aging time (both artificial and natural). As shown in Figure 9, a similarity is observed in the decreasing trend of both Ec Ec\({}_{\text{CPOL}}\) and E\({}_{\text{BXRD}}\) values for both short-term (in situ) and long-term (ex situ) artificial aging. However, for naturally aged historical samples, the dropping trend of E\({}_{\text{BXRD}}\) does not coincide with the estimators derived from FTIR. Unlike artificial aging, the estimators Ec and E\({}_{\text{CPOL}}\) do not exhibit clear trends with aging time or crystallinity content in naturally aged samples. This could be attributed to the overall high oxidation level of the silk sample. The substantial increase in C=O groups due to temperature-induced oxidation may lead to significant changes in the FTIR baseline for the energy regions related to strategic vibrations, causing visible distortions in the derived estimators. Moreover, the number of well-organized regions, as indicated by a dichroic ratio close to 1 and an order parameter close to 0, has significantly diminished in the historical samples. Consequently, the vibrations of these samples might not exhibit strong absorbance in the FTIR spectra. It is reasonable to recommend using FTIR-derived estimators only when the dichroic ratio \(R\) is higher than zero, indicating the presence of antiparallel \(\beta\)-sheet pleats in the silk sample. Further investigation will explore the nature of transformations occurring in \(\beta\)-sheets, \(\alpha\)-helix, and amorphous regions during both inherent and simulated aging processes. Figure 8: Crystalline estimator E\({}_{\text{CPOL}}\) calculated from the pol-ATR-FTIR spectra recorded at polarizer angle 0\({}^{\circ}\) (yellow) juxtaposed with Ec, the one obtained from the ATR-FTIR (orange) for model silk sample (MS), in situ thermo-aged MS (isO\({}_{2}\)), ex situ thermo-aged MS in the air (esO\({}_{2}\)), in the humid closed reactor (esO\({}_{2}\)/VOC/H\({}_{2}\)O) and historic: Inscription Banner (IB); Court Banner (CB); [PERSON]’s Funeral Banner (FB). Unlike the crystallinity estimator, the oxidation and hydrolysis FTIR-derived ones can aid in understanding historical samples' natural degradation and behavior by investigating the artificial aging process. As shown in Figure 10, the oxidation estimator exhibits the most consistent trend across different aging methods and durations and thus can be a potential age-correlated estimator. Its value consistently increases from the first hour of in situ artificial aging to the 14 days of all artificial ex situ aging, even in the naturally aged samples. Oxygen steadily and constantly affects the symmetry of the polypeptide, leading to peptide bond oxidation and scissoring. This oxidation reaction appears minimally influenced by the atmosphere in which the silk has aged. However, it remains unclear whether the proportions of oxidation products (such as ketones, aldehydes, or carboxylic groups in \(\alpha\)-keto-acids and dicarboxylic groups) differ under the three degradation conditions. Further analysis using mass spectrometry is warranted to investigate the mechanism of oxidation processes depending on the aging conditions. The hydrolysis estimator \(\mathrm{E_{COOH}}\) is more susceptible to water exposure than the aging conditions and thus can give a nice insight into the overall degradation condition of the museum silk sample.