A certain museum has collected the hemp colored paintings of Chinese Liao and Jin era, and the samples are taken from the fragments of the rotten hemp colored paintings. The reinforced hemp nets were prepared as follow: the same kind of fiber compatible reinforcement materials was selected. The hemp net with a mesh of 2 mm × 2 mm is woven by the hemp wire with a diameter of about 75 μm. Then the 5% mass fraction of the PVA-217 aqueous solution was prepared as the adhesive, and a spray gun was used to evenly spray the adhesive on the hemp net. The distance between the gun head and the cotton net is 3 cm, the spray pressure of the spray gun is 0.5 MPa~0.8 MPa, and the coating thickness of the adhesive is 15~20 μm. Finally, the sprayed hemp net was dried naturally, and the hemp net was cut out from the weaving frame to get the reinforcing hemp net.
2.1 Aging process of reinforced hemp net
Three aging methods were employed to complete the aging process of reinforced hemp net, and three parallel experiments were carried out for each group of aging methods. Each sample has been respectively aged for 3 days, 5 days, and 7 days. One aging method is dry heat aging, the prepared hemp net and PVA film were put in a blast drying aging box at 105 °C. The second aging method is wet heat aging, the prepared hemp net and PVA film were put in a constant damp heat aging test box at a temperature of 80 °C and a relative humidity of 65%. The third aging method is ultraviolet light aging, an ultraviolet weather resistant test box was applied to age the hemp net and PVA film, the temperature of the aging box is set at 25 °C, the power of the ultraviolet lamp is 60 W, and the vertical distance between the sample and the ultraviolet lamp is 5 cm.
2.2 Characterization of material
2.2.1 Characterization of hemp
Preparation of Herzberg coloring agent and samples was reported [19, 20]. The KH-7700 ultra-depth-of-field microscope was used to observe the fiber morphology and dyeing state, and take the fiber pictures for comparison with the standard fiber atlas. The SU3500 tungsten filament scanning electron microscope (SEM) was utilized to characterize the hemp painting, spray the gold 80s on the sample surface with an ion sputtering instrument (30 mA), observe and analyze the sample surface under an acceleration voltage of 5 kV. Meanwhile, the SEM was applied to perform the X-ray energy spectrum analysis on the sample under the condition of an acceleration voltage of 15 kV, and make a preliminary guess on the filler of the hemp painting based on the elemental analysis results. EDX-7000 energy type X-ray fluorescence spectroscopy (XRF) was used to test the element composition of the hemp painting surface with test conditions as follow: Rh target, air atmosphere, tube voltage with Na~Sc, 15 KeV and Al~U, 50 KeV, the X-ray spot diameter of the sample is 1mm. Gas chromatography-mass spectrometer (GC-MS) was adopted to analyze the rubber in the hemp painting, 6 mol/L hydrochloric acid solution helped the hydrolysis of the protein in the rubber compound , and then MTBSTFA reagent was employed to derivatize the hydrolysate. The GC-MS analysis started after the derivatization is completed. The test conditions are as follow: helium as carrier gas, the sample inlet temperature is 280 °C, the flow rate is 1.5mL/min, the initial temperature is set at 100 °C, and then the temperature is increased to 280 °C at a rate of 6 °C/min, and it is maintained until all the sample flows out .
2.2.2 Durability characterization of these reinforcement materials
The QT-1136 universal material testing machine was adopted to test the tensile strength of the hemp net before and after aging for 5 days. The samples were cut into a long strip of about 15mm×5mm, then a universal material testing machine was used to measure the load when the hemp net is broken, and each sample was measured 10 times to get the average value of the load. The tensile strength is calculated according to the formula: S=F/Lw, where F is the load at break and Lw is the width of the sample. The X-RiteVS-450 spectrophotometer was applied to test the chromatic aberration ∆E* of the reinforcement materials before and after aging for 3 days, 5 days, and 7 days, and each sample was measured 5 times to get the average value. The four layers of hemp net were superimposed to perform the test. The larger the value of ∆E* is, the greater the color change of the sample is.
2.3 Restoration process
The Chinese Liao and Jin era dates back more than a thousand years, the colored paintings of hemp fabrics buried underground for so many years have been severely damaged, which seriously affected their viewing and exhibition in museums. Therefore, it is necessary to conduct the restoration treatment of the colored paintings of the damaged hemp fabrics, such as preliminary splicing, dust and stain cleaning, colored painting pigment pre-reinforcement, fine splicing, restoration of fabric textures, back reinforced hemp net, and restoration of defects with hemp net as the skeleton, to improve the mechanical strength of hemp fabrics. For this damaged hemp sample in the article, the restoration procedure was described as follows: flatten, pre-reinforcement of colored painting, dust and stain cleaning, fine splicing, reinforcement of the hemp net and filling holes.