Casting Origin | Why Did Bronze Make Casting?

It was casting, the essential foundation of all civilization, that unlocked the future and placed man on the path to conquering his environment. Based on the existing archaeological findings, this paper combs out the origin of casting. It is almost certain that the emergence of casting is inevitable for the development of human civilization. Before the emergence of casting, human technological accumulation in stone polishing, quarrying, metallurgy, pottery making and well sinking paved the way for casting to finally reach the stage of history. Today, we are going to see the history and development of casting.

 

History of Bronze

Discovery of ores according to archaeological findings, about 2.5 million years ago, the first human beings appeared in East Africa. One of the characteristics of these people is that they began to manufacture and use stone tools, and mankind entered the Paleolithic Age. At about 10000 BC, human beings began to manufacture and use grinded stone tools and entered the Neolithic Age.

 

In the early Stone Age, stone was obtained “locally”, and whatever was nearby was used. In the process of using stone tools for a long time, people have gradually discovered the differences between stones. Although it was impossible to quantify the composition of stone materials with the technical conditions at that time, they had a considerable understanding of the nature of stone materials in the course of tens of thousands of years of use and contact. They knew how to distinguish the soft, hard, tough and brittle stones, and could use different stones to make different stone tools according to their needs. The stone needed to make stone tools should be hard, brittle and uniform in texture, such as siliceous rock and obsidian, while the stone needed to make stone tools should not be too hard, and the particles should not be too thick, so as not to fall off, such as sandstone. In this way, “local materials” can not meet the actual needs. Gradually, people began purposeful quarrying activities, that is, looking for stones that can meet the requirements of specific stone tools in the mountains, and this activity brought another unexpected discovery.

 

In the process of quarrying, human beings discovered pure metal. In high school chemistry, we have learned the activity list of metals, including potassium, calcium, sodium, magnesium, aluminum, zinc, iron, tin, lead, hydrogen, copper, mercury, silver and platinum. The activity of metal decreases from left to right. Because gold, silver and copper are relatively difficult to oxidize, their pure metal form can be found in nature. The earliest metal discovered and used by human beings may be pure gold, because the luster of pure gold has the same fascination for human beings at any time, and the pure gold with stone can not be ignored. Silver and copper are considered to be the next metals to be recognized and utilized. The initial processing method for these metals is mainly forging, that is, heating to a certain temperature to make them soft, and then hammering them into the required shape. The earliest pure copper forging found in archaeology can be traced back to 9000BC. In the early stage, the forged products were mainly small ornaments. In the later stage, with the increase of pure metal found, some tools were also forged, mainly copper tools. However, in that era, stone tools were still the dominant production tools, and the amount of pure metal tools forged was very small. In any case, the activity of forging natural metals has enriched human understanding of metals.

 

With the continuous development of quarrying activities, in addition to the pure metal that is very easy to find, humans have also found some different “stones”. These “stones” are obviously different in appearance from stones that can be used as stone tools. For example, the main components of flint and obsidian, which are commonly used in stone making, are SiO2. The flint is mostly gray and obsidian is black. The newly discovered “stone” is bright and bright in color and is very suitable for decoration. Now we know that these “stones” are actually some copper ores, that is, the existing state of copper elements in the form of oxides, sulfides or salt and alkali compounds. For example, malachite is a copper ore containing more than 50% copper. Because of its unique color, the ancients used it as decoration. In addition to copper ore, tin ore and lead ore were later discovered. It is this series of discoveries made by humans in quarrying activities that even some scholars believe that the earliest science born in humans should be geologists!

 

Tao cast Kunwu flowers bloom in two, each showing one branch. In addition to polishing stone tools, humans in the Stone Age also developed another skill: pottery making. The history of pottery is also very long. Archaeology has found that human beings have begun to manufacture pottery as early as the Paleolithic Age. Pottery kilns produced with pottery manufacturing have been able to burn to temperatures above 900 ℃ as early as 6000BC. The ceramic kiln can not only control the high temperature, but also find the reducing atmosphere through long-term practice. Because wood is the main fuel used by early humans, and wood can be made into carbon after smouldering. Carbon will generate gaseous CO under the condition of insufficient oxidation. The control of the reducing atmosphere by the ancients can be proved by the higher proportion of black pottery found in the later human sites. Because in the process of pottery firing, CO can reduce the red iron oxide (Fe2O3) in clay to black iron oxide (Fe3O4).

 

CO is a highly reducing gas, and its reduction capacity is even higher than that of hydrogen. The ancients should have inadvertently discovered that if some stones were put into a high-temperature ceramic kiln with a reducing atmosphere, pure metal could be “transformed”, and metallurgical technology would emerge. Of course, the actual metallurgy is a more complex process. Before the reduction reaction, some still need to be heated and decomposed. For example, to smelt copper from malachite, it is necessary to first heat malachite to decompose it from Cu2 (CO3) (OH) 2 to CuO, and then conduct reduction reaction. The discovery of metallurgy is a long process. It may have taken at least five or six thousand years for mankind to smelt the first copper from the use of ore as decoration. Archaeologically, it is believed that the earliest metallurgy of mankind began at about 6500BC. This technology of “turning stone into gold” is simply too useful, because the stock of metal compound minerals in nature is much larger than that of pure metals. The development of metallurgical technology has greatly increased the amount of pure metal that can be used by human beings.

 

At this time, the ancients had developed well sinking technology for drinking groundwater. As stone, minerals can generally be found in two places: rocky mountains and underground rocks. The technology of shaft sinking endows human beings with the ability to mine underground minerals. The development of metallurgical technology has also greatly enhanced the enthusiasm of human beings to search for ore in the mountains and valleys.

 

The contribution of ceramic technology to casting is not only the development of metallurgical technology. For molding the wall of pottery, the ancient pottery mainly used the clay bar and molding method, while the modern common wheel method was developed later. The molding method is the main reference method for the early casting mold (mold for casting). Why do you say that? Because the shape of early castings is almost the same as that of pottery, that is to say, the earliest mold used is the mold for molding pottery. Of course, with the further development and maturity of casting technology, casting also began to be used in the manufacture of various tools.

 

Bronze Casting

The essence of bronze casting is to heat and melt the metal and solidify the mold into the required structure, which is not a phenomenon that is difficult to find. The three major elements needed for casting production: metal (such as copper found naturally or smelted), high temperature of molten metal (ceramic kiln) and casting mold (mold used for molding) have all appeared. Some objects cast in pure copper are also found in some archaeological relics. At present, the earliest casting found by humans is a 3200BC copper cast frog, found in the Mesopotamian plain. However, there are two major problems with pure copper as a casting material for production tools. The melting point of copper is 1085 degrees. In order to improve the fluidity and castability of molten metal during casting, the actual heating temperature is generally much higher than the melting point. It would be difficult to achieve under the conditions of ceramic kiln at that time. Second, the hardness of pure copper is still low, and it is not practical as a tool material. It was not until the invention of bronze that the casting technology was widely used.

 

Bronze is mainly an alloy of copper and tin (which can also contain lead). As mentioned above, human beings began to use natural copper very early, and then gradually learned to smelt copper. Why did bronze appear much later? This is because tin, the main component of bronze, was discovered relatively late. There are many reasons for this. On the one hand, according to the proven abundance of various elements on the earth, the content of tin is only about one tenth of that of copper. Due to its activity, pure tin metal hardly exists in nature, and the main tin-bearing stone produced by Mother Earth is tin-pyrite (Cu2FeSnS4). Tin pyrite is generally buried deep underground. Only when it is brought to the ground by geological movement and oxidized into tin ore (SnO2) can it become the tin ore that human want. This leads to the fact that tin ore is not as many as copper ore, and is also very scattered. What is more difficult is that many of these cassiterite are embedded in hard marble, which makes mining very difficult. On the other hand, cassiterite is also difficult to find. As the main raw material of tin ore, cassiterite is not as colorful as the stone of copper ore, so it is difficult to be noticed. It is precisely for these reasons that human beings understand and use tin much later than copper.

 

According to textual research, bronze was first discovered by Sumerians. During 5500~4000BC, Sumer civilization, considered as the earliest human civilization, rose in Mesopotamia Plain. Although historiography has not yet figured out how Sumerian civilization originated, it must be related to the domestication of animals and plants in the region by about 7000BC, the transition of life style from hunting and gathering to agricultural production, and large-scale settlement. The technological level of Sumerian people is far ahead of any other people in the same era. Like a torch in the dark, it illuminates the origin of human civilization. Sumerians have invented many technologies, such as wheels, glass, bread, wine, etc., and bronze is also one of their important contributions to all mankind. The Sumerians were probably the first to invent the smelting of metallic tin. About the middle of 4000BC, Sumerian Somehow found that the melting point of the alloy could be reduced to 800 degrees after adding some tin into pure copper, so it was easy to melt the bronze for casting with mature ceramic kilns. As mentioned above, the earliest copper forging found so far is 9000BC. It has developed from copper to bronze for nearly 5000 years, which shows how difficult it is. At present, there is no definite conclusion about the origin of bronze in China, but there are more scholars who prefer to be introduced into China from the Mesopotamian Plain through the later “Silk Road”.

 

Why Did Bronze Make Casting?

There are many reasons:

-The main material of bronze alloy is copper, which is the third metal used by humans besides gold and silver. Bronze is obtained by adding a certain amount of tin to copper. This progressive innovation based on the improvement of raw materials is easier to achieve than the subversive innovation based on the complete use of new metal materials.

-The melting point of bronze can be reduced to about 800 degrees, which makes it easy to melt bronze with mature ceramic kilns.

-The curing shrinkage of bronze is small. The volume of molten metal will change due to phase change during solidification. Most metals will shrink in volume during the curing process, for example, the curing shrinkage of zinc materials commonly used in modern casting is more than 4%, and that of aluminum is more than 7%. The more severe the curing shrinkage is, the easier the casting is to produce defects. However, the volume shrinkage of copper-tin alloy during curing is very small, and the linear shrinkage rate is only 1.45%~1.5%. It is easy to make complex castings with accurate size requirements and handicrafts with clear patterns.

-The chemical properties of bronze are stable and wear-resistant. According to the different tin content, the hardness of bronze is several times higher than that of pure copper, thus meeting the hardness requirements as a tool material. In addition, the reason why there is only the Stone Age, the Bronze Age and the Iron Age but not the Ceramic Age is that the age is judged by the material of production tools. Pottery has only existed as a container in history.The natural color of bronze is not the oxidized cyan now seen, but golden, which is very “rich” visually.