Materials
Ancient carbonized samples were unearthed from Balingshan Tomb (403 B.C.–221 B.C.), in Jingzhou, Hubei Province, China, denoted as JZ (Figure 1). Modern silks were supplied by Guanghua Silk Co. Ltd. Superoxide dismutase (BR, activity 1750U/mg) was manufactured by Worthington Ltd.
Preparation of artificially carbonized samples
According to references [11],[12], modern silks were heated under 230°C for 40 h to obtain artificially carbonized samples with exceedingly low strength, denoted as 40hTH. JZ was heated under 230°C for 6 h (6hJZ) to mimic the generation of fresh carbon radicals in storage.
Determination of experimental conditions
The SOD solutions with concentrations of 0.01%, 0.05%, 0.10%, 0.20% (w/v) were prepared by dissolving SOD into distilled water. The scavenger solutions were sprayed onto 40hTH (the sample area to mixture volume ratio was 1 cm2: 10 μL) at room-temperature for 24 h and 48 h respectively. Variance analysis of double factors cross classification without repetition was performed to obtain the optimal concentration and scavenging time.
Evaluation methodology for the scavenging effects
The scavenging effects were evaluated by analysis of samples before and after scavenging with electron paramagnetic resonance. For EPR measurements, each sample was weighed and placed into standard quartz EPR tubes. The EPR spectra were performed with a JEOL JES-FA200 EPR spectrometer at room temperature. The microwave frequency was 9.06 GHz, the microwave power was 0.998 mW, and the modulation amplitude was 0.35 mT. All signal intensities were normalized to be equated with sample weighed 1 mg.
The clearance rate of carbon radicals through the change of EPR signal intensity was calculated as follows.
The clearance rate of carbon radicals on 40hTH:
where I
o
and I' represent the EPR signal intensity of the sample before and after applying scavenger respectively, m
1
is the clearance rate of carbon radicals.
The clearance rate of newly generated carbon radicals on 6hJZ:
(2)
where I 0 represents the EPR signal intensity of the ancient fabric before artificial carbonization. I represents the EPR signal intensity of the ancient fabric after artificial carbonization. I' represents the EPR signal intensity of the ancient fabric which was artificially carbonized and applied with SOD.
Aging resistance assessment
40hTH before and after being treated with 0.05% SOD solution was artificially aged at 60°C and relative humidity of 80% for 72 h and cut into strips of 1.0 cm × 5.0 cm. Then the tensile strength of these samples was examined using using a Dynamic Mechanical Thermal Analyzer Q800 (TA instruments Inc., USA). Five replicates were analyzed for each sample.
Fibre morphology observation
SEM images of the ancient carbonized silk fabrics fibres and artificially carbonized silk samples before and after treatment by the superoxide Dismutase (SOD) were observed with a JEOL JSM-6700 F scanning electron microscope at 5.0 keV accelerating voltage. Samples were mounted on an aluminum stub and sputtered with aurum before observed.