Discharge Tube Glass

At the operating temperature of the lamp sodium is chemically very active. Ordinary glasses, even quartz, are rapidly stained brown which blocks out light. The stained areas also have a different coefficient of expansion from the unstained material, and differences in expansion may then cause the glass to crack. In 1920, a special sodium resistant glass - borate glass - was developed by Arthur H. Compton. However, pure borate glass is unsatisfactory in lamp manufacture. It is known as a 'short glass', meaning that it has a short working temperature range. On heating, the glass changes from the rigid condition to fluidity very quickly, thus making it rather difficult to process. The earliest sodium lamps could only be manufactured with the assistance of the most skilled master glassblowers who were adept at handling this glass.

The problem was overcome by using a 2-ply glass tube. A very thin layer (about 0.02mm) of barium aluminoborate glass is blown onto the inside of an ordinary soda-lime tube. The soda glass can be worked relatively easily on automatic equipment, and acts as a support for the borate lining which is so thin as to be almost insignificant. This allows the sodium-resistant ply tubing to be formed using standard glassworking methods. Figure S13 shows the general arrangement.

The coefficients of thermal expansion of the borate and soda-lime glasses are not perfectly matched, and this can cause problems of cracking through the introduction of permanent thermal stresses. It is necessary to provide longer annealing schedules for ply tubing than for other glass types. In addition, it is important to maintain uniformity of the borate coating thickness around the tube circumference, so that radial stress differences are minimised. The most critical operation is bending the tube. During this process the glass wall thickness naturally decreases on the outside of the bend, and increases on the inside. This effect must be minimised by correct location of the melt in the U-shaped mould and by adjustment of the glass wall thickness before moulding.

If there is any unevenness in the thickness of the borate layer, sodium corrosion will be more of a problem in the thin areas. This will cause early lamp failure by excessive light absorption arising from stained glass, or cracking. Even if the glass is made well, good lampmaking practice is also required because the borate coating can be easily damaged. This glass type is highly sensitive to moisture and is readily attacked by the same. For this reason, borate glass is generally delivered to a lamp factory in heated trucks, and stored in a heated room to prevent moisture condensing in the glass. The temperature must also be kept constant, since it can otherwise result in expansion and contraction of the air inside the tubes, with a risk that moist air may be drawn in during cooling.

A major drawback of borate glasses is the fact that they tend to clean up argon, a gas which is essential for easy lamp starting and long life. Many decades of development have produced a modern glass which has good sodium resistance and a relatively low argon cleanup rate. Although the glass is not yet perfect, it has been improved to the state that the rate of argon cleanup is no longer a life-limiting mechanism. Further information on argon clean-up can be found on the previous page.

The wetting and adhesion of liquid sodium to borate glasses is another matter of considerable importance to the sodium lamp engineer. This was not a great problem until 1955 when a new glass type was developed by Philips that showed a considerable improvement in its resistance to sodium. The new glass did not stain so rapidly over lamp life, and delivered a significant boost in lumen maintenance and lamp lifeteime. One significant drawback however, was the fact that liquid sodium did not adhere so well to its surface. If the lamps were not operated perfectly level, the sodium would tend to run down to one end of the lamp forming large light-blocking mirrors, and leaving the other end depleted. The problem was partly overcome with the introduction of the so-called Bamboo lamp, whose discharge tube is rilled-in at frequent points along its length. These constructions were sufficient to limit the movement of sodium around the discharge tube. A typical Bamboo lamp can be seen here. It was a relatively short-lived lamp that was manufactured only by Philips, and was superseded by 1958 once improved glass types had been developed that showed good chemical resistance to sodium while also causing better adhesion of the liquid metal.