In ancient China, different places had different styles. During the Warring States period, due to frequent wars, countries needed a lot of bronze raw materials to make weapons. The bronze mirror can only be used by some dignitaries, and because the carving technology at that time was still in the primary stage, the back decoration of the mirror in the Warring States period did not fluctuate, and the geometry of its cross section was basically flat except for the edges and buttons. After the Western Han Dynasty, politics was relatively stable, and the use of bronze mirrors gradually spread to the homes of ordinary people. The back pattern art of bronze mirrors quickly developed from bronze blooming art in Warring States mirror to geometric patterns easily made by ordinary mirror-casting craftsmen, such as Zhao, Sunlight Mirror and Xingyun Mirror in the Western Han Dynasty, and then to smooth line carving, such as some breast-nailed animal mirrors and four-god mirror. With the continuous improvement of folk sculpture art, in the process of the gradual development of mirror back art, bronze model casting technology must develop synchronously with it. If we stay in the mirror casting technology in the Warring States period, there will be no mirror species after the Han Dynasty. In the past, few people paid attention to the relationship between mirror back decoration and mirror cross-section geometry and alloy ratio. Through the research on the casting technology of ancient bronze mirror models and long-term repair and reproduction practice, experts found that the alloy ratio has a very close restrictive relationship with various geometric bronze mirrors. If the cross-section geometry of the unearthed ancient mirror is slightly changed, the imitation bronze mirror will easily cause waste; In the process of copying bronze mirrors, if we only copy the bronze mirrors of all ages according to an alloy ratio, the result is that only one bronze mirror suitable for its alloy ratio can be cast. If there are half mirrors of the Warring States and the Eastern Han Dynasty in a small and medium-sized kiln, if the bronze is melted and cast according to the alloy ratio of the half mirrors of the Warring States, although all the casting blanks are intact, no casting defects can be seen, most of them are intact when the mirror surface is polished and reflected, and the mirror center of the animal mirror is opposite to the button, so there will be shrinkage, and the mirror surface can be seen macroscopically. If the alloy ratio of small and medium-sized mirrors is used to cast large bronze mirrors, for example, the alloy ratio of four gods mirrors is used to cast the Tang Dynasty's sea animal grape mirrors, which will easily cause local shrinkage and cracking of the sea animal grape mirror blank. The direct cause of the above casting defects is that the lead content in the alloy does not adapt to the size and geometry of the cast copper mirror. Ancient bronze mirrors were made of ternary alloys of copper, tin and lead. If only copper is used to cast the mirror surface, the mirror surface will be red, and the reflection effect will be blurred when used. With the increase of tin content, the color of the alloy will change from red to yellow, and then gradually turn white. When the tin content increases to about 24%, the cast copper mirror will have the same reflection effect as today's glass mirror. The early Qijia cultural mirror to Yinxu cultural mirror belongs to the primary stage of bronze mirror casting technology in China, and the alloy ratio of bronze mirrors in this period has not yet formed a standard. After entering the mid-Western Zhou Dynasty, the alloy proportion of bronze mirrors in China gradually became regular and mature. From the Spring and Autumn Period to the Warring States Period, the alloy proportion and model casting technology of bronze mirrors in China have been fully mature. After the Han Dynasty, the Three Kingdoms, the Six Dynasties and the Sui and Tang Dynasties, the high-tin bronze technology was maintained until the late Tang Dynasty. After the Song Dynasty, a large amount of lead was added to the bronze mirror alloy, and a certain amount of zinc was added to the bronze mirror alloy in the Ming Dynasty, which no longer belongs to high tin bronze. Therefore, this paper mainly discusses the relationship between the alloy composition of high-tin bronze mirrors and the cross-sectional geometry of bronze mirrors from the Warring States to the late Tang Dynasty. A workshop for casting mirrors in batches will always produce a lot of recycled materials, such as the nozzle and riser of castings, the copper material leaked from the bottom during casting, the flash material, the residual material that cannot be poured in, and so on. The weight of these recycled materials usually exceeds the weight of the casting itself. Therefore, if new metal materials and recycled materials are divided in half when bronze is smelted, the amount of recycled materials will gradually increase. It is recorded in Kao Lu: "concave mirror, a daily kindling instrument, is listed as an alloy in Kao Lu. It can be seen that its casting process is exactly the same, with decorative patterns and buttons on the back of the mirror and the back of the neck. The difference is that the mirror is flat or convex, while the neck is all concave, and often the workshop that casts the mirror also doubles as the neck. For example, on July 7, 1966,/kloc-0, a six-year rearranged animal mirror was unearthed in Ezhou, and its inscription reads: "Huang Wu was born on November 7, the sixth year, and Ding Sishuo was the teacher and lighting engineer of Yin Shan Bao in Huiji. I should be a descendant, raise my age, be rich and old. My family is in Wuchang first, and I don't think much about the world. My Wang Gan yesterday is now in the Hubei Provincial Museum. Through the inscription on this mirror, we know that this mirror casting artist named "Tang Bao" not only casts mirrors, but also casts the year of Yang, and has posted advertisements about the year of Yang on the bronze mirror. Therefore, in a workshop of batch casting cabinet mirrors, new and old materials must be half-filled and half-refined before they can be refined into bronze. Because the recycled material of bronze flint and bronze mirror has already become an alloy, its melting point is lower than that of copper, and it is easier to melt than copper. When melting, if the old and new materials are half, the melting time can be shortened and the chances of oxidation and gas absorption during alloy melting can be reduced. After the new blue copper material is proportioned, at least more than 1.5% of tin and more than 2% of lead must be burned during smelting. When preparing new burden, it is necessary to consider increasing the burning loss of tin and lead during smelting. However, it is difficult to accurately judge the tin content and lead content of the return burden after countless smelting; This requires that the mirror casters of past dynasties must have the ability to look at the alloy section, and judge the tin content and lead content according to the color of the section, so as to determine the tin content and lead content that should be added when the return burden is remelted. There was no such advanced chemical composition analysis equipment in ancient times, so the percentage of tin content and lead content in ancient bronze mirrors was somewhat different. However, under the original workshop conditions, the tin content of most bronze mirrors can be kept at about 23% ~ 25%, which shows that it is very valuable for the ancient mirror casting teacher to judge the tin content and lead content of the alloy from its cross section, so as to determine the tin content and lead content to be increased during remelting. From the Warring States Period to the Late Tang Dynasty, about 1300 years, although the back lines of China bronze mirrors were ever-changing, the average tin content in its alloy remained within the numerical range of 23.8%. Usually, the metallographic structure of bronze mirror alloy consists of α solid solution, (α+δ) eutectoid and granular lead. α phase is a Cu-Sn solid solution with face-centered cubic lattice, and its tin content can only reach 16%. δ phase is an intermetallic compound, the chemical composition is Cu3 1Sn8, and the highest tin content is 32.6%. Both are cubic lattices. The optimum tin content of ancient bronze mirror alloy should be between 23 ~ 25%. In practice, when the content of tin in the alloy reaches about 23%, its cross section is silvery white, fine and smooth, and the surface of galvanized iron sheet can be seen with eyes like modern ice lines. These irregular ice grains sparkle in the sun. These ice grains are the grain boundaries of dendritic crystals of high tin copper liquid, which are completely consistent with the cross section of unearthed bronze mirrors. If the tin content in the alloy is higher than the value of 23% tin and 4% lead, the relative amount of (α+δ) eutectoid in the copper mirror matrix will increase, and the relative amount of α phase will inevitably decrease. δ phase is brittle, which often leads to the fracture of castings in the mold cavity during solidification and shrinkage. There is no broken lens blank in the mold cavity, and it is easy to be broken in the grinding process after casting. If the tin content in the alloy is less than the normal value, it will inevitably lead to the increase of melting temperature and the extension of melting time. Higher than the normal melting temperature will accelerate the burning loss of tin and lead, and prolonging the melting time will increase the chances of metal oxidation and gas absorption. The temperature of copper liquid during pouring must also be higher than the temperature of normal tin content. The higher the pouring temperature, the more serious the adhesion between the surfaces, which will increase the roughness of the casting surface. Polished copper mirrors are more prone to yellowing in use than copper mirrors with normal tin content. It is generally believed that adding lead to the alloy can improve the filling ability during pouring, so that fine and delicate patterns can be cast. The filling ability of high tin blue copper alloy containing only about 24% Sn without lead has already far exceeded that of pure copper. When the molds are poured together, even if the surfaces of the molds are tightly closed, arcing often occurs. The alloy thickness of arcing is sometimes only about 0. 1 ~ 0.2 mm, which is enough to miss more than half or all of the copper liquid that has been poured into the mold cavity. In ancient bronze mirror alloys, lead played an unnecessary role in improving the stamping ability of Fan Tao. There are two reasons for porosity in bronze mirror castings. One is that the melting temperature of the alloy is too high and the melting time is too long, so that the metal liquid absorbs hydrogen from water in the air at high temperature; Second, when pouring, because the ceramic mold did not completely decompose the crystal water in the clay mold in the kiln, or the ceramic mold baked before pouring absorbed moisture, the mold cavity was gassed due to the high temperature of copper liquid during pouring. In the past, the casting defects caused by hydrogen-absorbing copper liquid were all under the casting skin, and the casting surface was intact. When the blank layer is worn off, the shiny pores are exposed on the whole mirror body, and the pores are all on the cross section of the mirror body. Because this kind of blowhole is produced in the smelting process, the molten metal absorbs hydrogen (H2) instead of oxygen (O2) at high temperature, so the blowhole under the skin or on the cross section of the casting is bright and has no oxidation. The latter is that the cavity is gassed during pouring, and the pores are mostly on the surface of the casting. Because the gas occupies a certain cavity space, the surface decoration of the casting is vague, but there are few pores on the cross section of the copper mirror substrate. Because oxygen (O2) exists in the gas released by heating in the cavity during pouring, and the oxygen reacts with the high-temperature copper liquid, almost all the pores generated by the gas generated in the cavity are outside the skin, and their colors are equal to or lighter than those of the casting surface. Lead did play two roles in the ancient high tin blue bronze mirror alloy. Firstly, because the δ phase in high tin blue copper alloy is relatively large and brittle, it is easy to be broken in the process of scraping and polishing the casting mirror. After adding lead, it is precisely because lead is not dissolved and combined with the alloy, but dispersed in the alloy in a spherical or irregular shape, which creates conditions for processing after casting. In the process of long-term large-scale processing of imitation bronze mirrors, the author is sensitive to the lead content in the mirror body. If the lead content is low, the mirror body is easy to be broken, but it is easy to be worn out. If the lead content is high, the mirror body is not fragile, but it is not easy to be ground. Secondly, the addition of lead can delay the solidification time of the mirror itself. The high-tin-blue copper alloy grows by three-dimensional crystallization along the fan wall to the center, and lead is dispersed in the alloy in a spherical shape, which objectively stops the crystallization speed of the alloy, thus prolonging the feeding time of the mirror at the water inlet. This characteristic of lead played a vital role in bronze mirrors after the Warring States Period, especially after the Eastern Han Dynasty. Usually, when casting the mirror, the alloy is in a high-temperature melting state of about 1000℃. Because metals usually expand when heated and contract when cooled, high tin blue copper is no exception. Therefore, the copper liquid poured into the mold cavity belongs to the copper liquid whose volume has expanded, and the mirror body filled with the mold cavity will inevitably shrink during the cooling process. The contraction of the mirror body needs copper liquid outside the mirror body to make up for it. The nozzle pressed above the mirror edge in Figure 1 and Figure 2 is called nozzle when pouring, and becomes a riser after the pouring process is completed. It is this riser that plays the role of feeding copper liquid outside the mirror. During the Warring States period, the thickest part of the mirror was near the mirror. Although the mirror buttons in the center of the mirror back are more decorated than the mirror back, most of the mirror buttons in the Warring States period are chord buttons or bridge buttons, which are relatively small and thin. When curing, it can basically shrink and cure before or at the same time as the mirror body, and there is no problem of insufficient feeding. The feed gap between the mirror edge and the mirror body can be obtained directly from the riser, thus maintaining the shrinkage of the mirror body during the Warring States period. Therefore, the lead content of Warring States mirror is generally not large. After the Western Han Dynasty, the geometric shapes of mirror body back lines and mirror body sections have changed greatly compared with the unified geometric shapes in the Warring States period. The mirror edge of the Western Han Dynasty changed from the thin concave arc edge of the Warring States mirror to the generous flat edge. The chord buttons of the Warring States mirror have also been changed to semi-circular buttons. Due to the change of the cross-section geometry of the mirror body in the Western Han Dynasty, the shrinkage and solidification period of the mirror body in the Han Dynasty after casting is uneven. It is impossible to shrink and solidify at the same time as the mirror in the Warring States period, but it is carried out according to the natural law of thinning first and then thickening. As a result, in the mirror body, the shrinkage and solidification after casting are separated one after another, which relatively causes the increase of shrinkage; If the alloy proportion of Warring States mirror remains unchanged at this time, there will be obvious casting defects. Because increasing the feeding amount of the mirror body can not be solved by increasing or heightening the riser, the solidification time of the mirror body is limited, and the mirror surface is thinner than the mirror button and the mirror edge, so naturally it shrinks and solidifies first, and the mirror edge can be directly fed by pressing on it with the riser, but the feeding mode of the mirror button in the center of the mirror back is limited by the surrounding pre-solidified mirror surface. In this case, casting defects will inevitably appear in the button-centered part, and a shrinkage cavity will appear in the center of the ground mirror in the case of weak light. The only way to solve this contradiction is to adjust the lead content in the alloy. The melting point of metallic tin is 232℃, that of metallic lead is 327℃, and that of lead is 95℃ higher than that of tin. However, when people prepare solder, because passive tin is not easy to melt, 40% lead is often added to pure tin to become solder, but it is easier to melt than pure tin. The reason is that lead and tin form a solid solution with low melting point. When lead is added to tin bronze, this characteristic of lead has not changed. With the increase of lead content, the solidification speed of bronze liquid will slow down. When the lead-containing alloy copper liquid fills the mold cavity, there will be a time difference between the solidification speed of the thin part and the thick part of the mirror body, which decreases with the increase of lead content, and the higher the lead content, the smaller the solidification time difference. The increase of lead content in bronze mirror alloy after the Eastern Han Dynasty is mainly to solve the time difference between the mirror buckle and the mirror body during solidification. However, if the lead content is too large, the mirror surface will turn black, which forces the mirror casters in the Eastern Han Dynasty to solve the casting defects of more lead and less shrinkage by increasing and thickening the mirror edge, which is the best way to solve this contradiction. During the Eastern Han Dynasty, some mirror circles with triangular cross sections appeared, such as Panlong Mirror, Dragon Tiger Mirror and Portrait Mirror. The lens buttons of these mirrors are relatively large, and the heightening and thickening of the mirror ring plays a role in prolonging the contraction and solidification time of the lens buttons, so that the contraction and solidification process of the lens buttons can be carried out in advance. This cross-sectional geometry maintains the balance between shrinkage and compensation of the mirror body. Judging from the chemical composition of bronze mirrors in Han dynasty, small and medium-sized semi-circular button mirrors, such as Zhao, Jing and Lian Jing, etc. Generally, there is an outer diameter of 12 ~ 16cm, and few of them are more than 20cm. The buttons of these mirrors have not developed as big as those in the late Eastern Han Dynasty, so the lead content of these small and medium-sized mirrors is generally acceptable. The lead content in the alloy of all mirrors with big buttons, such as semi-circular square animal mirror, triangular animal mirror and portrait mirror, is generally above 6%, among which the lead content of portrait mirror is above 7%. Because the animal mirror has no outer edge as high and thick as portrait mirror, the lead content is more than that of portrait mirror. Unearthed historical remains prove that the mirror-casting craftsmen in Han dynasty realized these characteristics of lead and made full use of them, so that the bronze mirror castings met the design quality requirements. With the development of history, especially after the mid-Western Han Dynasty, after various geometric patterns were put on the back of the mirror and the semi-circular button was invented, the mirror caster in the Han Dynasty began a difficult process of exploring the alloy ratio. In this process, the newly designed mirror geometry should conform to the sequence of shrinkage, feeding and solidification of high tin blue copper. In the trial casting process, unreasonable cross-section geometry and unreasonable alloy ratio are continuously improved, so that new mirrors can be gradually manufactured. For each new kind of mirror, the bronze mirror made of bronze alloy with tin content and lead content greater or less than the corresponding values will have one kind of defect or another. In the process of exploring the alloy proportion, ancestors will inevitably find that the higher the tin content, the whiter the alloy, but the easier it is to break in the processing after casting; The lower the tin content, the less likely it is to be broken, but it is not white and easy to yellow; The content of lead must be adapted to the size of the mirror body and the geometry of its cross section, otherwise, the bronze mirror will either turn black and yellow, or shrink loose and shrink holes. Through long-term large-scale casting and post-casting processing of bronze mirrors, our ancestors obtained the correct alloy proportion value of each mirror. This technique of determining alloy proportional parameters according to the geometry of mirror section continued from the Warring States period to the late Tang Dynasty. After the Tang Dynasty, due to various reasons, the alloy proportion of bronze mirrors changed, and the tin content gradually decreased. After the Ming Dynasty, zinc smelting was invented, and the content of zinc in bronze mirror alloy gradually increased, which no longer belongs to high tin blue copper alloy. Therefore, there is no large button mirror in the bronze mirror varieties after the Tang Dynasty, and the mirror body naturally does not need tall and thick mirror edges or triangular mirror edges.