Red lead

A reddish-orange pigment whose main component is lead oxide Pb ₃ O ₄ . It is also called red lead. It is used for paints along with red iron oxide, but because of the slight alkalinity of lead oxide and the chemical activity of lead monoxide PbO, it has long been used as a basic rust-preventive pigment for steel or as a primer pigment. The manufacturing method is to heat metallic lead to about 600°C, melt it, and then oxidize it in air to produce lead monoxide. This lead monoxide is separated from unreacted lead, and heated again to 400-500°C to fully oxidize it to produce red lead oxide (red lead). There is also a method to produce red lead by simultaneously grinding and oxidizing metallic lead in a ball mill that can be heated. It is a vivid yellowish red, and when left in the air, it becomes partially white due to the formation of lead carbonate. The more lead oxide it contains, the redder it becomes, and conversely, the more lead monoxide it contains, the more yellow it becomes. According to the JIS (Japanese Industrial Standards), there are four types of lead oxide: 97% or more is called special, 96% or more is called No. 1, 93% or more is called No. 2, and 80% or more is called No. 3. The higher the amount of lead monoxide, the greater the reactivity with drying oil and the easier it is to produce lead soap, and the coating film becomes flexible and adheres well and is resistant to weathering, but its storage properties decrease. The best rust prevention properties are said to be with about 80% lead oxide. When the amount of lead monoxide, an effective rust-preventing component of red lead, is extremely high, it is called lead suboxide, which is a mixture of lead and lead monoxide.

[Jun Otsuka]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

四三酸化鉛Pb₃O₄を主成分とする赤橙(せきとう)色顔料(がんりょう)。光明丹ともいう。べんがらとともに塗料用にあてられているが、四三酸化鉛の示す微アルカリ性と、このほかに含まれている一酸化鉛PbOの化学的活性のため、鋼材の塩基性防錆(ぼうせい)顔料あるいは下塗り用顔料として、古くから使用されている。製法は、金属鉛を約600℃に加熱、溶融後、空気酸化して一酸化鉛とし、この一酸化鉛を未反応の鉛と分離し、ふたたび400～500℃で加熱、十分に酸化して、赤色の四三酸化鉛（鉛丹）とする。このほか、金属鉛を、加熱できるボールミル中で、粉砕および酸化を同時に行い、鉛丹をつくる方法もある。鮮明な黄みの赤色で、空気中に放置すると、炭酸鉛の生成により、部分的に白色を示す。四三酸化鉛の含有量が多くなると赤みを増し、逆に一酸化鉛の多いものほど黄みが強くなる。JIS(ジス)（日本工業規格）により四三酸化鉛97％以上を特号、96％以上を1号、93％以上を2号、80％以上を3号とよぶ4種があるが、一酸化鉛の量が多くなるほど、乾性油との反応性が大きく、鉛せっけんをつくりやすくなり、塗膜は柔軟性、密着性がよく、風化に耐えるが、貯蔵性は低下する。防錆性は四三酸化鉛80％ぐらいがもっともよいとされている。この鉛丹の有効な防錆成分である一酸化鉛を極端に多くしたものが、亜酸化鉛とよばれるもので、鉛と一酸化鉛との混合物である。

［大塚　淳］

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