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Introduction to the most common glass materials in the light bulb industry

The most commonly used glass in the light bulb industry is sodium-lime silicate glass. By slightly changing the composition of the charge, the glass shells of various ordinary incandescent lamps and fluorescent lamps can be made. Large furnaces can melt about 50 tons of low-cost glass per day in continuous production.

The glass tubes of fluorescent lamps can be continuously drawn and formed in similar furnaces, generally using the Danner process.

The internal glass parts of ordinary light bulbs and fluorescent tubes are made of lead-alkali silicate glass. The method of making this glass is the same as that of sodium-lime silicate glass, but the production scale is smaller. Its advantage is that it has a high resistivity and can prevent electrolysis in the pinch seal. Lead-alkali silicate glass is easy to seal with sodium-lime silicate glass shells, has a lower softening point than sodium-lime silicate glass, and has a wider operating temperature range, all of which are conducive to the production of light bulbs.

The operating temperature of spotlights and high-pressure discharge lamps is too high, so sodium-lime silicate glass should not be used, but borosilicate glass should be used. In addition to being able to withstand high operating temperatures, borosilicate glass also has the characteristics of a low thermal expansion coefficient and the ability to withstand large temperature changes. Therefore, it is suitable for manufacturing press-sealed floodlights and other special bulbs that must withstand sudden temperature changes. If the temperature is higher, aluminosilicate glass must be used. This is the most heat-resistant glass in the bulb industry, and is generally produced in small batches using traditional manual processes.

Silica Since ordinary glass is not suitable for small, high-power light sources, transparent silica has to be used as the material for producing bulbs. This material is basically pure silicon dioxide, with only 1% impurities (Table 7-1), commonly known as quartz glass. Although it is transparent, it is not a crystal.

The main advantages of quartz glass are transparency, heat shock resistance, and high operating temperature (up to 800°C).

Vycor is a material produced by Corning Glass Works. It has been increasingly widely used to replace quartz glass in the bulb industry, and is particularly suitable for producing bulbs with high optical quality and accurate geometric dimensions.

There is a kind of glass in the bulb industry that is not widely used but is quite special, that is, sodium-resistant glass. Because the hot alkali vapor in the glass shell has a strong reducing property, it can cause ordinary silicate glass to quickly turn black due to the reduction effect. To overcome this shortcoming, the glass used can only contain very little or no silicon dioxide, as well as other easily reduced oxides.

Although aluminum-boron is resistant to chemical corrosion, it will slowly develop spots throughout its life; the improved type by adding barium oxide will not discolor. However, both types of glass are easily corroded by moisture in the air, and are expensive and difficult to process. In order to make a durable and economical bulb, a thin layer of corrosion-resistant glass (50 microns thick) can be coated on the inner wall of a glass tube made of standard soda-lime silicate glass. This glass shell is called a "ply-tubing".

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