A method for identifying and protecting components of oil finish and color painting of wood truss rafter heads of ancient buildings

Abstract

This invention provides a method for identifying and protecting the components of painted decorations on the rafter ends of ancient wooden structures. Through SEM-EDS and 532nm Raman spectroscopy, Pb·Lα and Cr·Kα co-occurred at 848±10 cm⁻¹. ‑1 554±10cm ‑1 Characteristic peaks and enrichment of the 0-80 μm surface layer were used to determine the PbCrO4-sensitive repainting layer. The layer was cleaned with a pH 5.5-6.8 citrate-sodium citrate buffer solution, stabilized with 0.01-0.03 mol / L phosphate or zinc salt solution, and protected with a 2-4 wt% B-72 transition layer and a 5-8 wt% B-72 surface layer gradient, thus achieving specific identification and protection of the PbCrO4-sensitive repainting layer.

Description

Technical Field

[0001] This invention belongs to the field of ancient building protection technology, and in particular relates to a method for identifying and protecting the components of painted decorations on the rafters of ancient wooden structures. Background Technology

[0002] The painted decorations on the rafters of ancient wooden structures are not only a precious cultural heritage integrating history, art, and craftsmanship, but also an important material carrier reflecting the evolution of ancient architectural techniques and aesthetics. However, because the rafter ends are directly exposed to the outdoor environment, they are subjected to strong ultraviolet radiation, drastic temperature and humidity fluctuations, and biological erosion over a long period of time. As a result, the painted decorations commonly suffer from defects such as pigment fading, powdering, cracking, and blistering and peeling of the ground layer. Among these defects, the problem of the "repainting layer" containing specific sensitive pigments is particularly troublesome. The PbCrO4 (chrome yellow) sensitive repainting layer is a typical example.

[0003] The so-called PbCrO4 sensitive repainting layer refers to the painted layer formed by the industrial synthetic pigment chrome yellow (PbCrO4) introduced during restoration activities. From a layering perspective, this sensitive layer is usually concentrated on the outermost layer of the painted surface and is typically applied to the surface of ancient buildings using an oil-based binder. However, PbCrO4 is a chemically unstable compound, highly sensitive to changes in pH, light, and redox environments. Under alkaline conditions or the action of cleaning agents containing calcium ions, it readily undergoes chemical transformation, leading to the dissolution of hexavalent chromium or the formation of lead oxide / chromium hydroxide on the surface, manifesting as irreversible blackening and darkening of the artifact's surface.

[0004] Currently, the industry mainly follows a protection approach that combines traditional experience-based processes with general-purpose chemical materials. In the component identification and current status assessment stages, existing technologies largely rely on visual observation, historical literature review, or methods such as Raman spectroscopy and X-ray fluorescence spectroscopy (XRF) for component qualitative analysis. For example, Raman spectroscopy detected 848 cm⁻¹... -1 With 554cm -1 The presence of characteristic peaks indicates the presence of chrome yellow (PbCrO4) in the region. However, existing identification methods suffer from a logical flaw: they can only identify "what kind of pigment it is," but cannot determine "what layer environment the pigment is in." Inspectors often cannot accurately determine whether PbCrO4 is located in the outermost repainted layer or a deeper historical remnant, let alone its spatial relationship with the underlying oily binder. This "only seeing the composition, not the layer" identification model prevents conservationists from accurately identifying the true PbCrO4-sensitive repainted layer, thus failing to provide targeted decision-making basis for subsequent conservation efforts.

[0005] In the conservation process, due to ambiguity in the identification stage, existing technologies have fallen into the trap of generalization. For identified PbCrO4 areas, existing technologies typically employ the same general methods used for protecting ordinary iron oxide red and azurite pigments. For example, film-forming materials such as B-72 and trimethylolpropene resin are directly applied to the painted surface. While this method can provide some physical isolation, for a highly chemically reactive and sensitive layer like PbCrO4, directly covering an untreated surface not only fails to address the chemical degradation of the pigment itself, but the film-forming material may also be incompatible with aging binders, leading to flaking and peeling. Furthermore, for cleaning the painted surface, existing technologies often use deionized water or weakly alkaline cleaning agents (such as calcium carbonate slurry, ammonia, or lime water). However, this cleaning method often promotes the dissolution of hexavalent chromium or triggers surface redox reactions, causing irreversible blackening and damage to the artifact during the conservation process.

[0006] Therefore, there is an urgent need for an improved method for identifying and protecting the components of painted decorations on the rafters of ancient wooden structures. Summary of the Invention

[0007] To address the shortcomings of existing technologies, this invention provides a method for identifying and protecting the composition of painted decorations on the rafters of ancient wooden structures, achieving specific identification and protection of PbCrO4-sensitive repainting layers.

[0008] To achieve the above objectives, the main technical solutions adopted by the present invention include:

[0009] This invention provides a method for identifying and protecting the components of painted decorations on the rafter ends of ancient wooden structures, comprising the following steps:

[0010] S1. Take a depth-cut sample in the non-main viewing area of ​​the rafter head, with a sample mass ≤3mg;

[0011] S2. Perform SEM-EDS detection and Raman spectroscopy detection under 532 nm excitation on the sample;

[0012] S3. When the detection result shows the simultaneous presence of Pb and Cr elements, and the Raman spectrum is at 848±10 cm⁻¹ -1 554±10cm -1 When characteristic peaks appear at a certain location, and Pb and Cr elements are enriched in the surface layer (0-80 μm) and co-located with the C / O oil background, the sample area is determined to be a PbCrO4 sensitive repainting layer. The following steps are then used to protect this area:

[0013] S31. Use a citric acid-sodium citrate buffer solution with a pH of 5.5-6.8 to clean the oil-painted artwork by applying it to the surface, and clean it 1-3 times.

[0014] S32. Use 0.01-0.03 mol / L phosphate buffer or 0.01-0.03 mol / L zinc salt solution as stabilizing solution, and control the pH to 6.0-7.0. Stabilize by applying the solution for 60-300 seconds.

[0015] S33. First, apply a 2wt%-4wt% B-72 solution once as a transition layer. After drying, apply a 5wt%-8wt% B-72 solution once or twice as a top layer. Dry between layers to complete the protection.

[0016] Optionally, in S1, the depth cutoff is achieved by changing the feed resistance or by setting a feed depth ≤150μm. After sampling, in-situ repair is carried out using backfill putty. The backfill putty is a mixture of iron oxide red, bone char, boiled tung oil and calcium carbonate.

[0017] Optionally, in S3, if mercury (Hg) is detected and the Raman spectrum is within 252 ± 10 cm⁻¹ -1 344±10 cm -1 If a characteristic peak appears at a certain location, the sample area is excluded as a PbCrO4 sensitive redraw layer.

[0018] Optionally, in the citrate-sodium citrate buffer solution, the molar ratio of citric acid to sodium citrate is 1:1.4-1:1.8, and the total concentration of citric acid and sodium citrate is 0.08-0.15 mol / L.

[0019] Optionally, a nonionic surfactant is also added to the citrate-sodium citrate buffer, wherein the amount of nonionic surfactant added is 0.3wt%-0.9wt% of the total mass of the citrate-sodium citrate buffer; the nonionic surfactant is selected from one or more of tea saponin extract, Tween-20 and alkyl glycosides.

[0020] Optionally, in S31, after each cleaning, the surface residual liquid is immediately washed away by adsorption with a buffer rinse solution.

[0021] The buffer rinse solution is a dilute solution of citric acid and sodium citrate with a pH of 6.0-6.8, an ionic strength ≤10 mmol / L, and a total concentration of citric acid and sodium citrate ≤0.01 mol / L, and does not contain nonionic surfactants.

[0022] Optionally, the phosphate buffer solution is a mixture of NaH2PO4 and Na2HPO4, with a molar ratio of NaH2PO4 to Na2HPO4 of 1:1.7-1:2.4, and a total concentration of 0.01-0.03 mol / L; the zinc salt solution is a zinc acetate solution or a zinc nitrate solution.

[0023] Optionally, a UV absorber is added to the phosphate buffer or zinc salt solution, and the amount of UV absorber added is 0.02wt%-0.04wt% of the total mass of the phosphate buffer or zinc salt solution; wherein the UV absorber is a benzotriazole or triazine derivative.

[0024] Optionally, S2 also includes: determining the saponification value of the sample;

[0025] S3 also includes: if the saponification value of the sample is lower than a preset threshold, the binder is judged to be heavily aged. Before S31, modified tung oil with a viscosity range of 50 mPa·s-200 mPa·s is used at a concentration of 0.1-0.3 mL / m 2 Use the appropriate amount for capillary infusion pre-reinforcement, allow it to stand for 12-24 hours to allow it to seep in, and then execute S31-S33.

[0026] Optionally, nano-silica aerogel powder is added to the 2wt%-4wt% B-72 solution, with the added mass of nano-silica aerogel powder being 0.05wt%-0.15wt% of the total mass of the B-72 solution; the solvent in the B-72 solution is a mixed solvent of acetone and anhydrous ethanol, and the volume ratio of acetone to anhydrous ethanol is 7:3-5:5.

[0027] The beneficial effects of this invention are:

[0028] This invention provides a method for identifying and protecting the components of painted decorations on the rafters of ancient wooden structures. By combining SEM-EDS with Raman spectroscopy, the sensitive PbCrO4 repainting layer is accurately identified, and a specific protection method is proposed. This method utilizes a citric acid system with a pH of 5.5-6.8 to clean and activate the surface. A lead phosphate passivation film is then constructed in situ in a near-neutral environment using an extremely low concentration of phosphate (0.01-0.03 mol / L) to block photochemical reduction. Finally, a gradient B-72 penetration reinforcement (2-4 wt%) and a weather-resistant surface layer (5-8 wt%) are applied. This method ensures that the original color difference ΔE of the painted decoration is maintained at <1.5, while solving the technical problems of blackening caused by traditional alkaline cleaning, poor adhesion of general sealants, and photo-darkening of PbCrO4. This achieves long-term, non-destructive protection for the fragile and sensitive painted layers of ancient buildings. Attached Figure Description

[0029] Figure 1 This is a flowchart illustrating the method for identifying and protecting the components of painted decorations on the rafters of ancient wooden structures, according to a specific implementation method.

[0030] Figure 2 This is a schematic diagram of Raman spectroscopy detection of a PbCrO4 sensitive redrawing layer sample according to a specific embodiment.

[0031] Figure 3This is a schematic diagram of the color difference boundary in the cross section of the painted decoration on the wooden rafters according to a specific embodiment. Detailed Implementation

[0032] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0033] Unless otherwise defined, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains; the terminology used in the specification of this application is for the purpose of describing particular embodiments only and is not intended to limit the invention; the terms "comprising" and "having," and any variations thereof, in the specification, claims, and foregoing drawings are intended to cover non-exclusive inclusion. The terms "first," "second," etc., in the specification, claims, or foregoing drawings are used to distinguish different objects, not to describe a particular order or hierarchy.

[0034] In this invention, the reference to "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of the invention. The appearance of this phrase in various places in the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments.

[0035] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," and "attachment" should be interpreted broadly. For example, they can refer to direct connection or indirect connection through an intermediate medium. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0036] In this invention, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. Additionally, in this invention, the character " / " generally indicates that the preceding and following related objects have an "or" relationship.

[0037] In the embodiments of the present invention, the same reference numerals denote the same components, and for the sake of brevity, detailed descriptions of the same components are omitted in different embodiments. It should be understood that the thickness, length, width, and other dimensions of various components in the embodiments of the present invention shown in the accompanying drawings, as well as the overall thickness, length, width, and other dimensions of the integrated device, are merely illustrative and should not constitute any limitation on the present invention.

[0038] In this invention, "multiple" refers to two or more, including two.

[0039] This invention provides a method for identifying and protecting the components of painted decorations on the rafter ends of ancient wooden structures, comprising the following steps:

[0040] Step S1: Take a depth-cut sample from the non-main viewing area of ​​the rafter head, with a sample mass ≤3mg.

[0041] This step aims to obtain a small sample size sufficient for subsequent testing while minimizing interference with the artifact.

[0042] Specifically, the non-primary viewing area of ​​the rafter head includes the inner corner of the rafter head side edge and the warped, powdery edge. A micro rotary file or a micro-drill bit is used as the sampling tool, with a preferred tip diameter of φ = 0.6-1.2 mm.

[0043] Specifically, depth cutoff sampling is achieved through one or a combination of the following two control mechanisms:

[0044] 1. Feed Resistance Jump Sensing: The system monitors feed resistance in real time through equipment feedback. When the drill bit breaks through the hard pigment layer and contacts the relatively soft "seam filler" layer or the wooden rafter itself, causing a sudden change in hardness, the feed is stopped immediately. This prevents the drill bit from continuing to penetrate and causing irreversible damage.

[0045] 2. Preset feed depth: Based on the estimated thickness of the pigment layer from the previous cross-sectional microscopic observation, the preset feed depth is ≤150μm (micrometers) to ensure that only the surface aging or diseased paint material is removed, without touching the ground layer and wood base layer.

[0046] To ensure the integrity of the original appearance of the cultural relics, in-situ repairs were carried out using backfill putty after sampling. The backfill putty was a mixture of iron oxide red, bone charcoal, boiled tung oil, and calcium carbonate; the proportions of each component needed to be fine-tuned according to the original color differences around the sampling point to ensure visual consistency after repair. Iron oxide red provided the base color, bone charcoal was used to deepen the tone to match the somber feel of the old painted decorations, boiled tung oil served as a binder to provide adhesion and gloss, and calcium carbonate served as a filler to increase the putty volume and adjust its hardness.

[0047] Step S2: Perform SEM-EDS detection and Raman spectroscopy detection under 532 nm excitation on the sample.

[0048] This step aims to accurately identify the material composition of the painting using modern spectroscopic analysis methods, and to define its layer environment based on the elemental distribution characteristics, thereby distinguishing between ordinary pigment layers and PbCrO4 sensitive repainting layers that require special protection.

[0049] Specifically, the sample obtained in step S1 is placed in the sample chamber of a scanning electron microscope (SEM) for backscattered electron imaging and energy dispersive spectroscopy (EDS) analysis. Characteristic X-rays are collected to obtain the elemental types and content distribution within the micro-region. The Pb (lead) Lα line and Cr (chromium) Kα line in the periodic table are monitored. If coexistence of Pb and Cr elemental signals is detected, it is preliminarily determined that the region contains lead chromate-based pigments. Simultaneously, elemental surface mapping (surface scanning) technology confirms that the Fe (iron) elemental signal is mainly enriched in the lower red oil band or ground layer, rather than the outermost layer, thus eliminating interference from Fe element on the surface pigment protection.

[0050] Subsequently, Raman spectroscopy was performed on the same sample (e.g. Figure 1 (As shown), to further verify the molecular structure at the level of assignment. 532 nm was chosen as the excitation wavelength to avoid fluorescence interference and ensure high sensitivity to inorganic mineral pigments. During the spectral scan, the following characteristic peaks were observed:

[0051] 1. Main characteristic peak: If it is at 848±10 cm -1 554±10 cm -1 The presence of a distinct Raman shift peak confirms it as a characteristic fingerprint peak of lead chromate (PbCrO4), corresponding to the symmetric stretching and bending vibration modes of the Cr-O bond.

[0052] 2. Auxiliary characteristic peaks: If necessary, supplement with low-frequency lattice vibration peaks at 272±10 cm⁻¹ to enhance the accuracy of the determination.

[0053] 3. Interference Elimination: If a mercury (Hg) element signal is detected, it is necessary to check whether a 252±10 cm² area exists. -1 344±10cm -1 If the characteristic peak is present, it is identified as cinnabar (HgS), and it must be excluded as a PbCrO4 sensitive redrawing layer to avoid misuse in subsequent protection processes.

[0054] Furthermore, this step also includes: determining the saponification value of the sample.

[0055] The aging degree of the oil-based binder is quantified by determining the content of free fatty acids in the sample using micro-titration. If the measured saponification value is lower than a preset threshold (e.g., calibrated to 150 mg KOH / g based on extensive experimental data), the binder is considered severely aged, characterized by significant oil loss and brittle fracture. If the value is higher than this threshold, the binder is considered lightly aged, retaining some toughness. This evaluation result will directly serve as the basis for selecting the protection process in subsequent step S3.

[0056] Finally, combining the cross-sectional color difference boundary of the pigment layer (such as...) Figure 2 (As shown) Layer assignment is performed using elemental surface distribution mapping data: If Pb and Cr are enriched in the outermost layer at a depth of 0-80 μm, and their spatial distribution is highly co-located with the C / O grease background signal, then it is confirmed as a typical structure of pigment particles suspended in grease binder. In this case, the area is determined to be a PbCrO4 sensitive repainting layer, and the process proceeds to step S3 for a dedicated protection process; conversely, if Fe is enriched and located in a continuous red grease band, it is determined to be a Fe2O3-based traditional grease layer, and the conventional protection process is performed.

[0057] Step S3: When the detection result shows the simultaneous presence of Pb and Cr elements, and the Raman spectrum is at 848±10 cm⁻¹ -1 554±10 cm -1 When characteristic peaks appear at a certain location, and Pb and Cr elements are enriched in the surface layer (0-80 μm) and co-located with the C / O oil background, the sample area is determined to be a PbCrO4 sensitive repainting layer. The following steps are then used to protect this area:

[0058] Based on the saponification value determination results in step S2, this step adopts a differentiated protection strategy:

[0059] If the binder is determined to be severely aged (saponification value below the preset threshold), capillary infusion pre-reinforcement must be performed before proceeding with the S31 cleaning step below. The specific procedure is as follows: Use modified tung oil with a viscosity range of 50 mPa·s-200 mPa·s, applying a trace amount of 0.1-0.3 mL / m², and infuse it into cracks and hollow areas using capillary action. This viscosity of modified tung oil allows it to penetrate the loose pores smoothly without being too thin and causing runoff, or too thick and preventing penetration. After infusion, allow it to stand for 12-24 hours to fully swell and fill the gaps left by the aged binder, restoring interlayer adhesion, before proceeding with the subsequent S31-S33 steps.

[0060] If the adhesive is determined to be only slightly aged, no pre-reinforcement is required, and the S31-S33 process can be performed directly.

[0061] Step S31: Clean the oil painting by applying a citric acid-sodium citrate buffer solution with a pH of 5.5-6.8. Clean 1-3 times, with each cleaning session lasting ≤60 seconds.

[0062] This step aims to remove harmful biofilms, smoke and aging degradation products from the painted surface, while strictly avoiding damage to the PbCrO4 sensitive layer caused by alkaline environment and mechanical friction.

[0063] Specifically, the citrate-sodium citrate buffer solution is prepared according to the following ratio: the molar ratio of citric acid to sodium citrate is 1:1.4-1:1.8, and the total concentration of both is 0.08-0.15 mol / L. This ratio range can maintain the pH value between 5.5 and 6.8, providing the necessary acidic environment to activate the surface while avoiding an overly acidic environment that could damage the PbCrO4 crystal structure or cause excessive hydrolysis of the oil binder.

[0064] The choice of buffer system for cleaning PbCrO4-sensitive repainting layers was a specific decision made by the inventors, as detailed below: Cleaning PbCrO4-sensitive repainting layers is subject to three chemical constraints: ① It must be maintained in a weakly acidic range of 5.5–6.8 to avoid alkali-sensitive blackening of PbCrO4, but it cannot be too acidic, which would exacerbate Cr dissolution or hydrolyze the oily binder; ② The anion of the buffer pair must not be a strong Pb. 2+ Complexing agents (such as oxalate, EDTA) must be used, otherwise the PbCrO4 bulk will be dissolved; ③ Ca must not be introduced. 2+ / Mg 2 + (Easily reacts with CrO4) 2- (Formation of sparingly soluble salts leading to surface bloom) or excessively strong buffered anions (such as phosphate, which preferentially react with Pb) 2+ The precipitate is Pb3(PO4)2, which interferes with the interface. Under this constraint, although the pH of the acetate-sodium acetate system is adjustable, its buffering capacity is weak, and its wetting compatibility with aged greases is not as good as that of the citric acid-nonionic surfactant system; although the pH of the phosphate-disodium hydrogen phosphate system is suitable, the phosphate ions at the washing concentration will react with Pb. 2+ The formation of Pb3(PO4)2 precipitate will preemptively occupy the PbCrO4 interface, damaging the pigment layer; while oxalic acid-sodium oxalate, due to the oxalate ions' attraction to Pb... 2+ The strong complexation of citrate and sodium citrate can dissolve PbCrO4 in bulk. However, the citrate-sodium citrate ratio, adjustable to a working pH range of 5.5-6.8 via a 1:1.4-1:1.8 molar ratio, provides a suitable buffering capacity; citrate ions also have a good effect on Pb... 2+ The complexation constant is much lower than that of oxalic acid / EDTA, so it will not dissolve in bulk; and the citrate ion can weakly complex trace amounts of Ca that may seep out from the soil. 2+ It inhibits CaCrO4 salting out.

[0065] To further reduce the surface tension of the solution and achieve efficient encapsulation and removal of contaminants within microcracks without damaging the aged adhesive layer, a nonionic surfactant is added to the citrate-sodium citrate buffer solution at a concentration of 0.3wt%-0.9wt% of the total buffer mass. In this embodiment, the nonionic surfactant is preferably one or more of tea saponin extract, Tween-20, and alkyl glycosides (APG). These nonionic surfactants effectively emulsify oil stains and are easily removed during subsequent rinsing, with no risk of residue.

[0066] The cleaning process should be performed using a dressing application method; brushing is strictly prohibited. Specifically, soak a cotton pad or hydrogel patch in the buffer solution described above, then gently apply it to the painted surface. Each contact time should be at least 60 seconds. Depending on the severity of the contamination, the cleaning process can be repeated 1-3 times.

[0067] Preferably, after each cleaning, a buffer rinse solution is immediately used to remove residual liquid from the surface by adsorption. The buffer rinse solution is a dilute citric acid-sodium citrate solution with a pH of 6.0-6.8, an ionic strength ≤10 mmol / L, and a total concentration of citric acid and sodium citrate ≤0.01 mol / L, and does not contain nonionic surfactants. Using a low-ionic-strength rinse solution, instead of deionized water, is to prevent osmotic pressure shocks or salting-out phenomena caused by sudden changes in ion concentration in the surface micro-areas, ensuring that the PbCrO4 pigment is in a stable chemical environment. The adsorption operation involves gently absorbing the liquid with a non-woven fabric or cotton swab until the surface feels no longer noticeably wet, then proceeding to the next step.

[0068] Step S32: Use 0.01-0.03 mol / L phosphate buffer or 0.01-0.03 mol / L zinc salt solution as stabilizing solution, and control the pH to 6.0-7.0, and perform stabilization treatment by application.

[0069] This step aims to chemically modify the exposed PbCrO4 pigment layer after the cleaning process. By generating a passivation film of insoluble substances and an ultraviolet shielding layer, the photochemical reduction pathway is blocked, thus solving the problem of continued blackening after protection, which cannot be solved by traditional processes.

[0070] Specifically, based on their different mechanisms of action, stabilizing liquids are divided into the following two preferred options:

[0071] 1. A mixture of NaH₂PO₄ and Na₂HPO₄ is used as the phosphate buffer. The molar ratio of NaH₂PO₄ to Na₂HPO₄ is 1:1.7-1:2.4, and the total concentration of the mixture is 0.01-0.03 mol / L. This ratio can control the pH value between 6.0 and 7.0. Under this weakly acidic to neutral environment, trace amounts of phosphate ions (PO₄²⁻) are released. 3- Pb that will dissociate from the surface of PbCrO42+ The reaction results in the in-situ growth of an extremely thin and dense lead phosphate (Pb3(PO4)2) passivation film on the surface of the pigment particles. This film is not only chemically stable but also transparent and colorless, without altering the appearance of the painting. It effectively blocks direct contact between water molecules, oxygen, and acidic contaminants and PbCrO4, thereby inhibiting the growth of hexavalent chromium (Cr). 6+ Dissolution and reduction.

[0072] It should be noted that the inventors discovered that extremely low concentrations of phosphate (0.01-0.03 mol / L) react with PbCrO4 under near-neutral conditions in a limited manner to form a passivating protective film, rather than a destructive precipitation. This application utilizes this property to use phosphate buffer solution as a stabilizing agent.

[0073] 2. Use zinc acetate or zinc nitrate solution as the zinc salt solution, with a solute concentration of 0.01-0.03 mol / L. Zinc ions (Zn) 2+ It possesses moderate complexing ability, enabling it to bind to active sites on the PbCrO4 surface to form a more stable Zn-Cr-O complex. Since the electrode potential of zinc differs from that of lead, this process eliminates the micro-cell effect on the pigment surface, slowing down the electrochemical corrosion rate.

[0074] Both the phosphate and zinc salt methods involve adding a UV absorber to the solution at a concentration of 0.02 wt%–0.04 wt% of the total solution mass. The UV absorber is preferably a benzotriazole or triazine derivative. These absorbers selectively absorb ultraviolet light in the 290–400 nm range, converting it into heat energy, thereby interrupting the photoinduced reduction reaction chain of PbCrO4 under UV excitation and delaying pigment blackening at its source.

[0075] The stabilization treatment is applied by plastering; brushing is strictly prohibited. Gently apply a cotton sheet soaked in the stabilizing solution to the painted surface for 60–300 seconds. Too short a time will result in incomplete film formation, while too long a time may lead to surface salt crystallization.

[0076] Step S33: First, apply a 2wt%-4wt% B-72 solution once as a transition layer. After drying, apply a 5wt%-8wt% B-72 solution once or twice as a top layer. Dry between layers to complete the protection.

[0077] This step aims to construct a gradient protection system for the cleaned and stabilized PbCrO4 sensitive redraw layer, which combines breathability, chemical inertness, and high adhesion, to solve the problems of "hard outside and soft inside" or "interface peeling" caused by traditional single-concentration sealing.

[0078] Specifically, B-72 solution refers to a solution in which the solute is an ethyl methacrylate-methyl methacrylate copolymer (trade name Paraaloid B-72), and the solvent is a mixture of acetone and anhydrous ethanol. This copolymer is a thermoplastic acrylic resin widely used in the field of cultural relic preservation, possessing excellent light transmittance (without affecting the color of the artifact), reversibility (it can be removed with solvent if necessary), and weather resistance. The volume ratio of acetone to anhydrous ethanol is 7:3-5:5. This ratio ensures good solubility of B-72 while providing a suitable evaporation rate to ensure penetration depth. Pure acetone evaporates too quickly, easily causing "instant drying" and closing pores, preventing deep penetration, while pure ethanol evaporates too slowly and has slightly weaker dissolving power.

[0079] A low-concentration B-72 solution of 2wt%-4wt% is selected. The low-solids resin solution has lower surface tension, which allows it to penetrate deeper into the aged grease binder with opened micropores after acid washing and stabilization treatment, thereby enhancing interlayer adhesion.

[0080] To further address the common powdering and flaking issues in PbCrO4 repainting layers, nano-silica aerogel powder was added to a 2wt%-4wt% B-72 solution. The added mass of nano-silica aerogel powder was 0.05wt%-0.15wt% of the total mass of the B-72 solution. This aerogel possesses an extremely high specific surface area and a nanoporous structure. After ultrasonic dispersion, it can physically fill the micro-gaps between pigment particles and form a micro-nano-scale anchoring effect after drying, significantly improving the cohesive strength of the transition layer without altering the optical transparency of B-72.

[0081] After the transition layer dries, apply 1-2 coats of a 5wt%-8wt% B-72 solution. The higher concentration of the topcoat is intended to provide physical abrasion resistance and UV aging resistance, serving as the first line of defense against the external environment.

[0082] A metering coating method is used to control the coating amount per unit area to be 0.5-1.2 mL / m². 2 Avoid applying too much resin, which can cause it to drip or create a heavy, plastic sheen; maintain the original matte texture of the ancient architectural paintings. After each coat, ensure good ventilation and allow at least 30 minutes for the layers to dry completely, allowing the solvent to evaporate completely. This prevents subsequent coatings from turning white or swelling with the underlying layer due to solvent residue.

[0083] By combining low-concentration penetration reinforcement (including nano-aerogel) with high-concentration weather-resistant sealing, the defects of B-72 in existing technologies, such as poor compatibility with aged grease binders, easy brittleness, and insufficient adhesion, are effectively overcome, achieving long-term protection for PbCrO4 sensitive redrawing layers.

[0084] Example 1

[0085] A sample was taken from the non-main viewing area at the side corner of a late Qing Dynasty ancient building. A φ=1.0mm micro-drill bit was used, with a preset feed depth of 100μm, and the sample mass was approximately 2.5mg.

[0086] SEM-EDS analysis revealed the co-occurrence of Pb·Lα and Cr·Kα; the 532 nm Raman spectrum was observed at 848 cm⁻¹. -1 With 554 cm -1 A characteristic peak appears; mapping shows that Pb / Cr is enriched in the surface layer (0-60 μm) and co-located with the C / O signal. This is determined to be a PbCrO4 sensitive remapped layer.

[0087] Micro-saponification titration showed that the saponification value of the binder in this area was 120 mg KOH / g (lower than the preset threshold of 150 mg KOH / g), and it was determined to be heavily aged.

[0088] Before cleaning, modified tung oil with a viscosity of approximately 100 mPa·s was used for capillary infusion, with the dosage controlled at 0.2 mL / m. 2 Allow to stand and permeate for 18 hours to allow the oil to fully swell.

[0089] A citrate-sodium citrate buffer solution (molar ratio 1:1.6, total concentration 0.12 mol / L, pH≈6.2) with 0.6 wt% tea saponin added was used. The solution was cleaned with absorbent cotton pads, with each contact lasting 60 seconds, for a total of two washes. Each wash was followed by adsorption and rinsing with a diluted buffer solution at pH 6.5 (ionic strength 8 mmol / L) between washes.

[0090] A mixture of NaH₂PO₄ and Na₂HPO₄ (molar ratio 1:2.0, total concentration 0.02 mol / L, pH≈6.5) was used, with 0.03 wt% of a benzotriazole UV absorber added. The dressing was applied for 180 seconds.

[0091] Transition layer: 2wt% B-72 + 0.1wt% nano-SiO2 aerogel (acetone:ethanol = 6:4), coated once. Topcoat: 6wt% B-72 (acetone:ethanol = 6:4), metered coating at 0.8 mL / m². 2 , 1 pass. Interlayer drying for 45 minutes.

[0092] After protection, the painted surface exhibits uniform color and no blooming. Testing revealed the following: Color difference ΔE: The protected area and the original area showed ΔE = 1.1 (<1.5), indicating no visual color difference. Adhesion: The cross-cut adhesion test achieved Grade 1 (ASTM D3359), with no flaking or peeling. Cr(VI) leaching: After accelerated aging testing, the Cr(VI) leaching amount decreased by 65% ​​compared to the untreated area.

[0093] Example 2

[0094] A sample was taken from the non-main viewing area at the side corner of a late Qing Dynasty ancient building. A φ=1.0mm micro-drill bit was used, with a preset feed depth of 100μm, and the sample mass was approximately 1.8mg.

[0095] SEM-EDS analysis revealed the co-occurrence of Pb·Lα and Cr·Kα; the 532 nm Raman spectrum was observed at 848 cm⁻¹. -1 With 554 cm -1 A characteristic peak appears; mapping shows that Pb / Cr is enriched in the surface layer (0-60 μm) and co-located with the C / O signal. This is determined to be a PbCrO4 sensitive remapped layer.

[0096] Micro-saponification titration revealed that the saponification value of the binder in this area was 180 mg KOH / g (higher than the preset threshold of 150 mg KOH / g), indicating mild aging. The modified tung oil pre-consolidation step was omitted, and cleaning was performed directly.

[0097] A citrate-sodium citrate buffer solution (molar ratio 1:1.6, total concentration 0.12 mol / L, pH≈6.2) with 0.6 wt% tea saponin added was used. The solution was cleaned with absorbent cotton pads, with each contact lasting 60 seconds, for a total of two washes. Each wash was followed by adsorption and rinsing with a diluted buffer solution at pH 6.5 (ionic strength 8 mmol / L) between washes.

[0098] A mixture of NaH₂PO₄ and Na₂HPO₄ (molar ratio 1:2.0, total concentration 0.02 mol / L, pH≈6.5) was used, with 0.03 wt% of a benzotriazole UV absorber added. The dressing was applied for 180 seconds.

[0099] Transition layer: 3 wt% B-72 + 0.1 wt% nano-SiO2 aerogel (acetone:ethanol = 7:3), coated once. Topcoat: 5 wt% B-72 (acetone:ethanol = 7:3), metered coating at 0.8 mL / m². 2 , 1 pass. Interlayer drying for 45 minutes.

[0100] After protection, the painted colors are uniform and there is no blooming. Testing showed: Color difference ΔE: ΔE between the protected area and the original area = 1.0 (<1.5), with no visual color difference. Adhesion: The cross-cut adhesion test achieved Grade 1 (ASTM D3359), with no flaking or peeling. Cr(VI) leaching: After accelerated aging testing, the Cr(VI) leaching amount decreased by 72% compared to the untreated area.

[0101] The above are merely preferred embodiments of this application and are not intended to limit this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A method for identifying and protecting the components of painted decorations on the rafter ends of ancient wooden structures, characterized in that... Includes the following steps: S1. Take a depth-cut sample in the non-main viewing area of ​​the rafter head, with a sample mass ≤3mg; S2. Perform SEM-EDS detection and Raman spectroscopy detection under 532 nm excitation on the sample; S3. When the detection result shows the simultaneous presence of Pb and Cr elements, and the Raman spectrum is at 848±10 cm⁻¹ -1 554±10 cm -1 When characteristic peaks appear at a certain location, and Pb and Cr elements are enriched in the surface layer (0-80 μm) and co-located with the C / O oil background, the sample area is determined to be a PbCrO4 sensitive repainting layer. The following steps are then used to protect this area: S31. Use a citric acid-sodium citrate buffer solution with a pH of 5.5-6.8 to clean the oil-painted artwork by applying it to the surface, and clean it 1-3 times. S32. Use 0.01-0.03 mol / L phosphate buffer or 0.01-0.03 mol / L zinc salt solution as stabilizing solution, and control the pH to 6.0-7.

0. Stabilize by applying the solution for 60-300 seconds. S33. First, apply a 2wt%-4wt% B-72 solution once as a transition layer. After drying, apply a 5wt%-8wt% B-72 solution once or twice as a top layer. Dry between layers to complete the protection.

2. The method for identifying and protecting the components of painted decorations on the rafter ends of ancient wooden structures according to claim 1, characterized in that, In S1, the depth cutoff is achieved by changing the feed resistance or by setting the feed depth to ≤150μm. After sampling, in-situ repair is carried out using backfill putty. The backfill putty is a mixture of iron oxide red, bone char, boiled tung oil and calcium carbonate.

3. The method for identifying and protecting the components of painted decorations on the rafter ends of ancient wooden structures according to claim 1, characterized in that, In S3, if mercury (Hg) is detected and the Raman spectrum is within 252 ± 10 cm⁻¹ -1 344±10 cm -1 If a characteristic peak appears at a certain location, the sample area is excluded as a PbCrO4 sensitive redraw layer.

4. The method for identifying and protecting the components of painted decorations on the rafter ends of ancient wooden structures according to claim 1, characterized in that, In the citric acid-sodium citrate buffer solution, the molar ratio of citric acid to sodium citrate is 1:1.4-1:1.8, and the total concentration of citric acid and sodium citrate is 0.08-0.15 mol / L.

5. The method for identifying and protecting the components of painted decorations on the rafter ends of ancient wooden structures according to claim 1 or 4, characterized in that, The citrate-sodium citrate buffer also contains a nonionic surfactant, which is added at a rate of 0.3wt%-0.9wt% of the total mass of the citrate-sodium citrate buffer. The nonionic surfactant is selected from one or more of tea saponin extract, Tween-20, and alkyl glycosides.

6. The method for identifying and protecting the components of painted decorations on the rafter ends of ancient wooden structures according to claim 1, characterized in that, In S31, after each cleaning, the surface residual liquid is immediately washed away by the adsorption method with a buffer rinse solution. The buffer rinse solution is a dilute solution of citric acid and sodium citrate with a pH of 6.0-6.8, an ionic strength ≤10 mmol / L, and a total concentration of citric acid and sodium citrate ≤0.01 mol / L, and does not contain nonionic surfactants.

7. The method for identifying and protecting the components of painted decorations on the rafter ends of ancient wooden structures according to claim 1, characterized in that, The phosphate buffer solution is a mixture of NaH2PO4 and Na2HPO4 with a molar ratio of 1:1.7 to 1:2.4 and a total concentration of 0.01-0.03 mol / L; the zinc salt solution is a zinc acetate solution or a zinc nitrate solution.

8. The method for identifying and protecting the components of painted decorations on the rafter ends of ancient wooden structures according to claim 7, characterized in that, The phosphate buffer or zinc salt solution contains a UV absorber, the amount of which is 0.02wt%-0.04wt% of the total mass of the phosphate buffer or zinc salt solution; wherein the UV absorber is a benzotriazole or triazine derivative.

9. The method for identifying and protecting the components of painted decorations on the rafter ends of ancient wooden structures according to claim 7, characterized in that, S2 also includes: determining the saponification value of the sample; S3 also includes: if the saponification value of the sample is lower than a preset threshold, the binder is judged to be heavily aged. Before S31, modified tung oil with a viscosity range of 50 mPa·s-200 mPa·s is used at a concentration of 0.1-0.3 mL / m 2 Use the appropriate amount for capillary infusion pre-reinforcement, allow it to stand for 12-24 hours to allow it to seep in, and then execute S31-S33.

10. The method for identifying and protecting the components of painted decorations on the rafter ends of ancient wooden structures according to claim 1, characterized in that, The 2wt%-4wt% B-72 solution also contains nano-silica aerogel powder, with the added mass of nano-silica aerogel powder being 0.05wt%-0.15wt% of the total mass of the B-72 solution; the solvent in the B-72 solution is a mixed solvent of acetone and anhydrous ethanol, and the volume ratio of acetone to anhydrous ethanol is 7:3-5:5.

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