Bronze (English spelling)

It has two meanings. One is a copper alloy with tin added as the main alloying element to copper. The other is a synonym for copper alloy in general. The latter is because bronze is the oldest copper alloy used by humans, with a history of several thousand years, and was the only copper alloy for 2000 to 3000 years until the appearance of brass. The English word bronze is also used in the same sense. Aluminum bronze is a good example, a copper-aluminum alloy that does not contain any tin. This term was used until the first half of the 20th century, but after World War II, the last of these, beryllium bronze (an alloy of copper and beryllium, also without tin), was renamed beryllium copper, or in English, beryllium copper. Since then, new copper alloys have been called by adding copper to the main alloying element, such as titanium copper, zirconium copper, and chromium copper.

The original bronze, as used in the first way, was obtained in ancient times as a natural alloy in forest fires in mountains where copper and tin ores coexisted, where the two metals were simultaneously reduced by heat and carbon produced by burning forests, and it is thought to have been used for tools, arrowheads, and blades because it is more corrosion-resistant than iron and has a certain degree of hardness. Eventually, humans discovered that this natural alloy was made of copper and tin, and mixed the copper and tin bullion obtained by reducing each ore separately to create products with various tin percentages depending on the purpose. The "Six Equalities of Gold" of the Zhou Dynasty in China was a system in which the relationship between this purpose and composition was divided into six types and displayed.

When tin is gradually added to copper, the reddish color of copper remains up to 3%, but gradually turns yellow, and when it exceeds 20%, it turns gray-blue. It is thought that the blue of bronze came from this. The maximum amount of tin that can dissolve in copper is about 14%, and the strength is highest at 17-18% tin, but the extension of deformation capacity is greatest at 3-4% tin. Alloys with 12-32% tin undergo a eutectoid transformation, so they can be significantly strengthened by heat treatment such as quenching and tempering. In the past, cannons were made from bronze castings with about 10% tin, so it was also known as gun metal, and was also used for the goosenecks and mouthpieces of smoking kiseru cigarettes. When a small amount of zinc is added to 3-8% tin, it has a high deformation force and is easy to engrave, so it is used for bronze coins.

Among the bronze-based alloys that are made by adding other elements to copper-tin alloys in the true sense of the word, phosphorus bronze is a typical example, and is used for packing and leaf spring materials. Bronze for bearings contains over 10% tin to create a hard γ (gamma) phase, while bronze for artistic use contains zinc and lead to improve castability.

[Ryōtsugu Mishima]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

二義があり、一つは銅にスズを主要合金元素として加えた銅合金のこと。一つは広く銅合金の代名詞である。後者は人類の使った銅合金のなかでは青銅が最古のもので、数千年の歴史をもち、次の黄銅が現れるまでの2000年から3000年の間、これが唯一の銅合金であったためで、英語のbronzeも同じ意味に使われる。たとえばアルミ青銅aluminium bronzeがその好例であり、銅‐アルミ合金でスズはまったく入っていない。この呼称法は20世紀前半まで使われたが、その最後であったベリリウム青銅（銅とベリリウムの合金で、やはりスズは普通加えない）を第二次世界大戦後ベリリウム銅、英語でもberyllium copperといいかえたのを契機に、その後の新しい銅合金はチタン銅、ジルコニウム銅、クロム銅のように主要合金元素名に銅をつけてよぶようになっている。

　第一の使い方の本来の青銅は、太古には銅とスズの鉱石の混在している山の山火事で、熱と森林の燃えてできた炭素によって同時還元された両金属が合金化した自然合金として得られ、鉄より耐食性がよく、ある程度硬いために工具や鏃(やじり)や刃物に使われたと思われる。やがて人類は、この自然合金が銅とスズからなることを知り、それぞれの鉱石を別々に還元して得た銅とスズの地金を配合して、用途に応じて種々のスズ％のものをつくった。この用途と組成の関係を6種に分けて表示したのが中国周代の「金の六斉」である。

　銅に順次スズを加えていくと、3％までは銅の赤みが残るが順次黄色くなり、20％を超すと灰青色になる。青銅の青はこれからきたと思われる。銅中にスズの溶解する最大量は約14％で、強さはスズ17～18％で最高となるが、変形能力の伸びはスズ3～4％がもっとも大きい。12～32％のスズの合金は共析変態があるので、焼入れ・焼戻しの熱処理により相当強化する。昔は10％スズ程度の青銅の鋳物で大砲をつくったので、砲金gun metalという別名があり、喫煙用のきせるの雁首(がんくび)や口金にも使われた。スズの3～8％に少量の亜鉛を加えると変形力に富んで刻印しやすいので青銅貨に使われる。

　銅‐スズ合金にさらに別元素を加えた本来の意味の青銅系合金のなかではリン青銅が代表的なもので、パッキング、板ばね材に使われる。軸受用の青銅は十数％スズを加えて硬いγ(ガンマ)相を出し、美術用は亜鉛、鉛を加えて鋳造性をよくしている。

［三島良續］

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