What Is Bronze?
Bronze (copper tin alloy)is the earliest alloy in the history of metal smelting and casting. The addition of tin or lead to pure copper (red copper) has special importance and historical significance. Compared with pure copper (red copper), bronze has high strength and low melting point (25%). The melting point of tin smelting bronze will drop to 800°C. The melting point of pure copper (red copper) is 1083°C). Bronze has good castability, wear resistance and stable chemical properties.
Bronze has the characteristics of low melting point, high hardness, strong plasticity, wear resistance, corrosion resistance, and bright color. It is suitable for casting various appliances, mechanical parts, bearings, gears, etc.
There are twelve categories of bronze ware, drinking vessels, water vessels, musical instruments, weapons, carts and horses, agricultural implements and tools, currency, seals and symbols, weights and measuring instruments, bronze mirrors, and miscellaneous objects. Divided into several sub-categories. Among them, food vessels, drinking vessels, water vessels, musical instruments, and weapons are the most important and basic.
Bronze was made as early as 3000 BC, but it was much later for artefacts for general applications. The earliest bronzes appeared in the Mesopotamia Valley of Babylon 6000 years ago. The large bronze sword with the image of a lion during the Sumerian civilization is a representative of early bronzes. In the epic “Iliad”, Homer mentioned that Hephaestus, the Greek god of fire, put copper, tin, silver, and gold into his furnace, which turned into a shield used by Achilles. The ratio of copper to tin varies widely (measured from the remaining artifacts, the copper content is 67% to 95%); but in the Middle Ages, it was known that different ratios can produce different effects. The 11th-century Greek manuscripts in the library of St. Mark’s Basilica in Venice list an alloy of 1 pound of copper and 2 ounces of tin, which is an 8-to-1 ratio, which is similar to the gun bronze used later.
Bronze is harder than copper, has a lower melting point, and is easier to melt and cast; bronze is also harder than pure iron, and bronzes with different alloy compositions are suitable for making gun barrels and machine bearings. Among tools and weapons, historically replacing bronze with iron is not because iron itself has any special advantages, but because iron is more abundant than copper and tin. The characteristic of the bell bronze is that it can make a loud sound when struck. Its tin content is relatively high, ranging from 1/4 to 1/7. Sculpture bronze contains as low as 1/10 tin, and sometimes a mixture of zinc and lead is added. Zinc can increase hardness, and bearing alloys usually contain a small amount of zinc. Adding a small amount of phosphorus to bronze can improve its performance and strength; phosphor bronze ingots with phosphorus content can reach 1% to 2%, and the castings contain only trace amounts; it has high strength and is especially suitable for pump plungers, valves and sleeves. . Manganese bronze is also used in the machinery industry. It contains a small amount of tin or even no tin, but contains a large amount of zinc and manganese. In addition to being used as tools and weapons, bronze is also widely used to make coins; many copper coins are actually cast in bronze, with a typical composition of 4% tin and 1% zinc. Bronze is a substance that expands and contracts like water.
The advancement of bronze craftsmanship during the Spring and Autumn Period and Warring States Period is highlighted by the use of the following two technologies: one is the gold-silver cooperating technique. The so-called gold-silver cooperating technique is to inlay gold and silver wires on the surface of the bronze to make patterns or texts. This technology appeared in the middle of the Spring and Autumn Period, when the weapons of Chu, Song and other countries had wrong gold art characters. At the beginning of the Warring States period, large pieces of gold and silver patterns appeared on copper ritual objects. In the middle of the Warring States period, this technique was not only used on weapons and ritual objects, but also on talisman, seal, vehicle, bronze mirror, copper with hooks and lacquerware. buckle. Second, after the middle of the Warring States period, the engraving technique developed. This technique is to make hair-like engraved portrait pictures on relatively thin pots, cemeteries, mirrors, and husks. Generally, they are mostly water and land warfare, hunting, and hunting. Pictures of banquet etiquette. These pictures were carved into the shape of a vessel and then carved with steel knives.
The status of bronze ware in social life has gradually declined, and most of the artifacts are daily-used, but specific to some bronze ware, there are still many exquisite works. For example, the two-carriage copper carriage and horse unearthed in the Mausoleum of the First Emperor Qin in Lintong, Shaanxi. The first rider drove four horses, with a shed on the carriage, and the king was seated. Both carriages and horses are made of bronze castings, and the size is in proportion to the actual size and is extremely exquisite. There are also many gold and silver ornaments on the car, which are painted all over. The second riding horse, 3.17 meters long and 1.06 meters high, can be said to be the most complex and huge bronze ware discovered so far.
By the end of the Eastern Han Dynasty, pottery had developed greatly and played an increasingly important role in social life, which further squeezed daily bronze ware from life. As for weapons, tools, etc., iron weapons already dominated by this time. The bronzes in the Sui and Tang dynasties are mainly various exquisite bronze mirrors, usually with various inscriptions.
Bronze Ware was called “Gold” or “Jijin” in ancient times. It is an alloy of red copper and other chemical elements such as tin and lead, and its patina is blue-green.
The use of bronzes began in Turkey and Iraq in the late Neolithic period, as well as copper beads unearthed at the ancient TellRamad site in Syria. Chinese bronzes began from the Majiayao to the Qin and Han dynasties, with the Shang and Zhou dynasties being the most exquisite. It appeared in the early period of Yangshao culture and Majiayao culture in China.
Small tools or accessories first appeared in China. Bronze vessels and weapons began to be available in the Xia Dynasty. In the mid-Shang Dynasty, bronze wares were already rich in varieties, with inscriptions and fine patterns appearing. From the late Shang Dynasty to the early Western Zhou Dynasty, it was the heyday of the development of bronze wares. The types of wares were varied, vigorous and dignified, the inscriptions were gradually lengthened, and the patterns were rich and splendid. Subsequently, the bronze carcass began to thin, and the ornamentation gradually simplified. From the late Spring and Autumn Period to the Warring States Period, copper tools became less and less due to the popularization of ironware. During the Qin and Han dynasties, as pottery and lacquerware entered daily life, the variety of copper containers decreased, the decorations were simple, mostly plain, and the carcass became lighter and thinner.
Chinese bronzes are exquisitely produced and enjoy a high reputation and artistic value among the world’s bronzes. They represent the superb technology and culture of China’s bronze development for more than 5,000 years.
Fan casting method and lost wax method. Fan casting method is earlier and most widely used. Understanding ancient manufacturing methods is helpful to distinguish fake.
Fan casting method
Fan casting method is also called die casting method. The model is first made of clay, sculpted various patterns and inscriptions, dried in the shade and then fired to make it a master model, and then the clay model is made from the mother mold. Melt the alloy, pour the alloy into the pottery fan cavity to form a device, and then clean and polish the finished bronze product after it is removed.
Based on the physical objects excavated from the ruins of ancient bronze workshops, combined with the analysis of the shape of the bronze ware, experts believe that the vast majority of ancient Chinese bronze wares were made using the paradigm casting method
The fan casting process is divided into five steps:
- The first step is to mold, using clay to shape the basic shape of the copperware. Draw the outline of the copper ware decoration on the made clay mold, the concave part is directly carved out of the clay mold, and the convex part is separately made and attached to the surface of the clay mold;
- The second step is to change the model. Use the pre-mixed and even fine soil to press firmly on the surface of the mud mold, and then pat the shape and texture of the mud mold to reverse imprint on the mud sheet;
- The third step is to join the fan, divide the turned mud pieces into several pieces, and then burn them into pottery. Such a fan is hard and not easy to deform, and is called a pottery fan. Combining the pottery fan to form the outer cavity of the vessel is called the outer fan. After the outer model is made, a thin layer of the clay mold used for turning the model is evenly shaved off to make the inner surface of the utensil, which is called the inner model. The inscription of the bronze ware is engraved on the inner model. Combining the inner and outer vanes into one, the gap cut between the inner and outer vanes is the place where the copper liquid remains, and the distance between the two is the thickness of the bronze;
- The fourth step is pouring, inject copper liquid into the pottery fan. After the copper liquid has solidified, break the inner and outer pottery fan into pieces and take out the cast copperware. A set of pottery fan can only cast one bronze ware, so it is impossible to have two identical bronze wares;
- The fifth step is polishing and refurbishing. The freshly cast bronzes have rough surfaces and unclear patterns. They need to be polished and refurbished to become an exquisite piece of bronze.
Lost wax method
The lost wax method is a precision casting method for bronze and other metal objects. The method is to use beeswax to make a model of the casting, and then use other refractory materials to fill the mud core and apply it to the outside. After heating and baking, the wax mold is all melted and lost, making the entire casting model into an empty shell. Then pour the solution into it and cast it into a utensil. The utensils can be exquisitely cut through, and have the effect of hollowing out. The bronze statues and plates unearthed from the tomb of Zenghouyi in Suixian County, Hubei Province are the earliest lost wax castings known in China.
In the late Spring and Autumn Period, the Chinese may have invented the lost wax casting process. The process flow of the lost wax method is divided into three steps. First, a wax mold is made of easy-melting paraffin, and the wax mold is poured with fine mud many times to harden it to form a casting shape. Then, the cast shape is baked and ceramicized. During this process, the paraffin melts and flows out, forming a cavity in the casting. Finally, copper water is poured into the cavity to make a utensil. The lost wax method is usually used to cast bronzes with very complex shapes. The Chu State Copper Forbidden unearthed in Xichuan, Henan and the Zenghou Zun plate unearthed in Suizhou, Hubei, are considered to be cast by the lost wax method.
The casting method in which the utensils are cast into shape at one time is called the muddy casting method. If the shape of the vessel is too large or the shape is too complicated, it is necessary to divide the whole object into several pieces and cast them separately, and finally splice them into a whole. This casting method is called the split casting method. When casting multiple smaller objects, multiple casting molds are stacked together, and copper water is poured from a gate to cast multiple objects at a time. This process is called stack casting. The stack casting method is mostly used to cast coins and other small objects. It appeared in the Spring and Autumn Period and gradually became popular in the Han Dynasty.
Repair and maintenance
Repair and protection of bronzes. Because the repair and protection technology of metal cultural relics is basically the same, the repair and protection technology of bronzes is more comprehensive in metal cultural relics, and bronzes account for the highest proportion of metal cultural relics. Therefore, this article focuses on the restoration and protection of bronzes as a comprehensive system. Introduce the restoration and protection technology of bronze, you can learn from the restoration and protection methods of other cultural relics. There has been a long period of bronze age in human history, which is a stage of human material civilization development characterized by bronze manufacturing tools, utensils and weapons. The so-called bronze is an alloy made of copper and tin or lead in a certain proportion. It is mainly copper with a blue color, hence the name bronze.
For an unearthed incomplete bronze artifact, the first step is to weld the bronze ware into a completed artifact. Welding is an important part of the traditional repair technology, and it is the main method for repairing broken bronzes and restoring the shape of the artifact. It is necessary to adopt different welding methods, namely “big welding” and “small welding” according to the nature, brokenness and corrosion of the bronze. First, file the solder joints with a stubble, and then use an electric soldering iron to melt the tin and pour it into the bronze to be repaired.
Replenishment is an important technique for restoring incomplete parts of traditional bronze repair techniques. Re-matching means that the broken bronze ware is incomplete. According to the type, shape, and defective part of the bronze ware, it is made and matched with casting. Complementary matching should be based on the decoration of the bronze ware first, and then use the decoration to make the complementary shape, and then put together a finished bronze artifact.
Bronze ware buried in the ground was compressed and deformed and cracks appeared due to the collapse of the tomb and the change of the stratum. The unearthed bronzes often need to be reshaped. Bronze shaping methods include hammering, molding, sawing, heating shaping and physical shaping. The selection method is based on the degree of deformation of the utensils and the texture of the copperware.
Corrosion mechanism of cultural relics
To repair and protect bronzes, the corrosion mechanism of bronzes must be explored in order to take correct and effective protective measures.
The corrosion mechanism of bronzes has been continuously developed with the development of science, and various theories and opinions continue to emerge, but now the views are more consistent:
When the utensils are buried in the ground, they come into contact with chloride. Because of the small radius of chloride ions, it is easy to penetrate the water film and react with copper to form cuprous chloride: Cu 十Cl=→CuCl 十e Cuprous chloride reacts with water to form cuprous oxide And hydrochloric acid: 2CuCl 10 H20→Cu20 10 2HCl When cuprous oxide meets oxygen, water and carbon dioxide, basic copper carbonate can be formed; Cu20 10-02 10 H20 10 C02→CuC03·Cu(OH) 2 Cuprous oxide meets water and oxygen , Plus hydrochloric acid can be converted into basic copper chloride: 2Cu20 ten 2H20 ten 02 ten 2HCl→CuCl2·3Cu(0H)2.
Therefore, the corrosion products formed by bronzes under the influence of the external environment are CuCl and Cu20 from the inside to the outside, and then CuC03·3Cu(OH)2 or CuCl2·3Cu(OH)2, or both. This result has been confirmed by X-ray diffraction analysis.
Since the conversion product of the cuprous oxide layer, basic copper chloride, is loose and swollen, it is powdery, usually called powdery rust, and oxygen and water can still enter it, so that the cuprous chloride layer is converted into basic chlorination. Copper: 4CuCl 10 02 10 4H20 → CuCl2·3Cu(0H) 2 10 2HCl This creates the condition for the internal formation of powdery rust; the hydrochloric acid produced meets the eutectoid structure, and the copper is converted into cuprous chloride: 4Cu 10 The cuprous chloride formed by 4HCl + 02→4CuCl + 2H20 reacts with the oxygen and water immersed inside to generate basic copper chloride. This is repeated, so that the corrosion products of the bronze ware continue to expand and deepen until the ware is ulcerated and perforated, which is called “bronze disease”.
Some data on the corrosion mechanism of bronzes believe that it is also closely related to the tin and lead in the bronze. This sulfide is converted into hydrogen sulfide produced by microorganisms reducing sulfate under the action of aerobic bacteria.
The theories about the relationship between organisms and the corrosion of bronzes still need more in-depth research.
Rust removal and protection
Most of the bronzes were once buried underground, and were therefore corroded to varying degrees. As the corrosive medium, the capillaries and pores of the soil are filled with air, water and electrolyte. Bronze ware is buried underground. Under the action of air, water, and electrolyte, various corrosive coatings of different colors are naturally formed, including black copper oxide (CuO), red cuprous oxide (Cu2O), and indigo copper sulfate (CuSO4), blue copper sulfate (CuSO4·5H2O), green basic copper sulfate (CuSO4+3Ca(OH)2), white cuprous chloride (CuCl), white tin oxide (SnO2), etc. Different colors.
Most of them are corrosion products, which not only did not damage the ancient works of art, but added the artistic effect of bronze ware. The corroded layer of patina has become a symbol of the solemnity, simplicity and age of bronzes. The rust layer generally does not change the shape of the bronze objects, and the nature of the bronze rust is relatively stable, so that it will not cause damage to the objects. Therefore, this type of corrosion layer should be retained. However, since most of the unearthed bronzes are basically covered with soil and rust, the rust must be removed if the background color, patterns, patterns, and inscriptions are to be exposed. But rust removal can’t damage the copper tyre, and the good rust color should be retained.
The difference from basic rust removal is the removal of “powder rust”. The main mechanism of bronze rust is that the presence of chloride ions has the greatest impact on the rust of bronzes, which is the main reason for the destruction of bronze wares due to “powder rust”. To protect the bronze ware, the key lies in how to deal with the chloride ions, how to remove the chloride ions from the inner layer of the utensils, or to seal and stabilize the chloride ions inside the utensils to isolate them from oxygen and moisture, and protect them from the external environment. The influence of factors. There are many methods to remove excess copper rust and “powder rust”. The method used depends on the specific circumstances of each cultural relic. However, there is a general principle that the original appearance of the artifacts must be maintained, and the inscriptions, patterns and patterns of the artifacts must not be harmed. Ancient spots.
Rust removal method
There are three main treatment methods: mechanical method, chemical method and electrochemical reduction method. The three types of methods are used in conjunction with each other.
1. Mechanical method: divided into manual operation and mechanical operation.
- Manual operation: mostly used for powdery rust that has been exposed on the surface of bronze. You can use various tools, such as stainless steel needles, hammer carving knives, chisels, chisel, stainless steel scalpels, multi-functional engraving pens, dental scalers, etc., to operate directly on the utensils to carefully remove powdery rust. After the powdery rust is removed, a very thin layer of copper is often found. This is not the copper body of the bronze, but the copper produced during the hydrolysis of copper chloride. It often hides a lot of off-white cuprous chloride underneath. Therefore, after piercing the thin layer of copper with a steel needle, it is found that the chloride can be removed until the copper body is seen.
- Mechanical methods include: picking, cutting, scraping, sawing, sweeping, blowing, and polishing.
- Mechanical operations include: Sandblasting machine: It can be used to remove rust and corrosion on the metal surface. Its principle of rust removal is to spray metal particles by air pressure, and the rust will be quickly removed. This method is fast and convenient, and the rust removal area can be large or small. This method has greater advantages than laser rust removal and ultrasonic rust removal. Fourth, the rust in the depths of some cavities can also be removed.
- Laser rust removal: the use of laser to remove chloride from deep lesions in the shape of holes in bronzes has the characteristics of accuracy and ease. It mainly utilizes the huge light energy excited to act on the surface rust layer instantaneously, causing the surface temperature to rise rapidly, and the optical effects such as photothermal, photochemical, and light pressure generated when the laser beam interacts with the substance. Because the structure of the rust layer is loose and the energy absorption capacity is strong, the rust layer is quickly melted and vaporized and separated from the body. It can quickly, efficiently, and pollution-free remove the green harmful powdery rust on the surface of the bronze, so as to achieve the extension The purpose of the life of bronzes and the effective protection of cultural relics. This method is not suitable for the removal of large areas of harmful rust.
- Ultrasonic rust removal method: Ultrasonic cleaners use ultrasonic micro-mechanical oscillating waves, which can propagate in the form of waves in solid or gaseous media. The mechanism: the liquid is subjected to high-frequency shock and vibration due to the effect of cavitation. When the liquid is subjected to negative pressure at a certain time in a cycle of the ultrasonic wave, the liquid is drawn away at the liquid-solid interface to make it a vacuum there, resulting in cavitation bubbles. In another period, the process of cavitation formation to rupture due to the positive pressure is repeated with high frequency, and the dirt on the cleaned items is periodically and strongly impacted, so that it is separated from the item, and the surface of the dirt item is cavitation The violent oscillation effect of the air bubbles further promotes the peeling of dirt from the article, so ultrasonic waves can achieve an excellent cleaning effect. It is also possible to add sodium sesquicarbonate solution to soak and accelerate the reaction by ultrasonic, so as to achieve a long-term soaking effect in a short time.
- In addition, you can also use an ultrasonic scaler, engraving pen, etc.
2. The chemical method uses chemical reagents to prepare rust removal liquid, and there are many formulas for rust removal liquid.
- 1. With 5%-10% citric acid, 5%-10% ammonium hydroxide, alkaline potassium sodium tartrate, the bronze can be directly soaked in the rust removal solution, or you can use absorbent cotton dipped in the rust removal solution, and then apply it to Rusty parts.
- 2. Sodium sesquicarbonate method: Sodium sesquicarbonate is also known as alkaline bath immersion method. The chemical agents used are sodium carbonate and sodium bicarbonate, which are prepared into a sodium bicarbonate solution, and the bronze vessel containing chloride is immersed in 1% or 5% Soak in the sodium sesquicarbonate (Na2C03·NaHC03·2H20) solution. It is best to heat it during soaking to keep the liquid temperature at about 40°C during the day. Cool down by itself at night. In the solution, until there is no chloride ion in the immersion solution. Then rinse the utensils with distilled water, put the rusted bronze into the solution and start to change it once a week. After a few weeks, it can be changed for half a month or longer. The soaking should be at least three months until the chloride ion concentration is below 4PPm. So far, this is a method that has been used for a long time, and the disadvantage is that it is extremely time-consuming. This method, by immersing the corrosion products and sodium sesquicarbonate into the solution, allows chloride ions to enter the solution, which is beneficial to preserving the green patina. When inscriptions, patterns and ancient spots need to be preserved, this method is more suitable, so until It is still widely adopted today. But from the perspective of the efficiency of removing chloride ions, it is not particularly good. This is because the corrosion layer on the surface of the bronze is affected by many factors and is a dynamic process controlled by diffusion. Only by changing the soaking solution several times can the chloride ions continue to diffuse out. In order to improve the effect of rust removal, it is necessary to extend the soaking time. If the concentration of sodium sesquicarbonate is 5%. Not only releases more chloride ions, and the speed is also fast, but the consumption of copper also increases accordingly, so it is not appropriate to use too concentrated sodium sesquicarbonate solution.
- 3. Benzotriazole (BTA) method: The BTA method is a very effective bronze corrosion inhibitor commonly used to protect copper and copper alloys at home and abroad. It is used for the protection of ancient bronzes and has achieved good results. Benzotriazole is a white to cream-colored powder crystal that can be dissolved in organic solvents such as ethanol and benzene. There are two main mechanisms for BTA to inhibit copper corrosion, namely adsorption theory and film formation theory. The adsorption theory believes that after BTA is adsorbed on the surface of the copper vessel, the interface structure between the metal and the solution is changed, and the activation energy of the anode reaction is significantly increased, thereby reducing the reactivity of the copper itself. The film formation theory believes that the protection of copper by BTA is related to the existence of the Cu20 film, which can form a Cu(I)-BTA complex protective film, and it can also form a Cu(I)-BTA complex protective film on the Cu0 surface. This kind of film has good covering performance; it is close to the outside of the metal and separates the metal surface from the corrosive medium. It is a transparent covering film that is insoluble or insoluble in water and some organic solvents. The resulting film is relatively strong, so that the degree of dissolution or ionization of the metal is greatly increased. Lowering, played a role in protecting the metal. For example, the mixing of BTA and benzylamine not only speeds up the film formation speed, but also improves the corrosion inhibition ability. When BTA is mixed with molybdate, its corrosion inhibition effect is doubled.
- 4. Hydrogen peroxide method: Use hydrogen peroxide as an oxidant to oxidize and remove chloride ions. The concentration used depends on the rust. The remaining hydrogen peroxide can be completely decomposed by heating it slightly without any influence on the utensils. This method is compared with the sodium sesquicarbonate soaking method: the treatment time is short and the chlorine ion removal is more thorough. Compared with the partial electrolytic corrosion method and the silver oxide sealing method, the hydrogen peroxide method can remove powdery rust with different areas and different depths. It has a wide application area and is relatively easy to handle.
- 5. Acetonitrile method: add water to 100% with 50%, 5% acetonitrile, 5% ethanol. Acetonitrile and cuprous ions in this solution form stable basic copper chloride. This solution has poor effect and cannot function in a short time. The disadvantage of this method is that a long soaking time will cause the green patina to turn black, and because the acetonitrile vapor has a moderate degree of toxicity, a good ventilation environment or sealing measures are required when soaking.
- 6. Silver oxide protection method: This method is applicable to utensils that are locally corroded by spotted “powder rust”. It uses silver oxide and cuprous chloride to form a corner silver film under the action of water vapor in the air to seal the exposed surface of the cuprous chloride to achieve the purpose of controlling the corrosion of the bronze. First, mechanically remove the off-white waxy cuprous chloride, the source of “powder rust”, until fresh copper is seen, wipe the corroded area with acetone, and then use ethanol to make the silver oxide into a paste. Fill the rejected part to make the unremoved cuprous chloride contact with silver oxide to react, forming a corner silver film to prevent the role of chloride ions, and stabilize the copperware. However, brown spots are formed on the surface of the filled pits by this method, and the color matching treatment is required.
- 7. Deionized water method: For the cleaning of general bronzes, deionized water or distilled water at 40℃-60℃ can be used to rinse the corroded bronzes repeatedly, which can wash away the chloride ions without changing the green rust of the bronzes.
- 8. Citric acid and thiourea mixed solution method: 5% citric acid, 1% thiourea aqueous solution (PH=0.95) to remove local harmful rust, and then use 1% NaHco3 aqueous solution to neutralize the remaining reagents. This method has a very good effect on large bronze cultural relics, especially when it is necessary to reveal the surface inscriptions and patterns.
- 9. Alkaline sodium dithionite method: soak the utensils with 5% sodium dithionite aqueous solution for 24 hours, and then use alkaline sodium dithionite solution to remove sulfate. When using alkaline sodium dithionite solution to remove chloride, care should be taken to control the pH of the solution below 13. After treatment with this method, it should be washed in distilled water for 48 hours to remove the remaining corrosive solution. Sodium dithionite has a strong pungent odor, and the treatment must be carried out in a closed container.