Premature Failure

Some of the more common abnormal failure mechanisms of incandescent lamps are detailed below:

The lamp has a milky yellow-white deposit on the inside of the glass. This is tungsten trioxide, and it is formed when the hot filament reacts with air inside the bulb and completely burns up. Cracks in the glass are the usual culprits. Most commonly the crack is in the stem near the pinch, and is due to inadequate stress relief after stem making. If the crack is in the capping cement area, it is usually indicative that too much cement is employed. Only very rarely are bulb cracks seen. These are due to exceptionally poor fire control on the sealex machines, or due to hot glass coming into contact with cold surfaces, with the possibility to cause thermal stress damage.

A very slow air leak is sometimes observed, and this will lead to the formation of the lower tungstic oxide, usually W₂0₅, which has a smoky dark blue coloration. A poor glass-to-metal seal is usually responsible.

Lamp has a grey-black appearance on the inside of the bulb (rare). Blackening or sooting will be especially prevalent on the lead wires and filament support wires. The process responsible for this is the water cycle, in which water removes tungsten from the filament and deposits it on cooler regions.

Filament squirming is normally due to oil contamination inside the lamp. Oil may also be broken down into hydrocarbon vapours which conduct heat away from the filament and lead to excessively dim bulbs.

A tangled or stretched filament indicates that the lamp has been under excessive vibration whilst alight, or has received a direct blow. Shock damage when unlit leads to a clean brittle fracture of the filament.

A heat tarnished cap and charred cement indicate that the lamp has been used in an inadequately ventilated fitting or in abnormally high temperatures. May also lead to glass cracking and air lamps.

Lead wires melted near the filament, or support wires melted at pigtail loop. Fuse wires also destroyed.

A sparkling or crystallised filament is typical of a lamp which has served a normal useful life. If this characteristic is observed in early failure, it is nearly always evidence of over-voltage.

Every lampmakers worst fear, this dangerous failure mechanism occurs when the gas fires on the sealex machines are poorly set, introducing a sharp ring of stress into the glass in the neck area. The defect usually does not manifest itself until some time after the lamp has been shipped from the factory, when at a later time the glass can crack in a ring all around the neck. The bulb becomes separated from the lamp and exposing potentially live internal wires.

This is not strictly a failure mechanism, but is nonetheless troublesome. They are found especially on small mains voltage lamps, such as C35 candle and G45 ball lamps, and only on lamps having a white internal coating of titanium dioxide powder. One or two dark spots up to about a centimetre diameter may form on the bulb side during life. Analysis shows that the position of the spots always coincides directly with the position of one or other of the molybdenum filament support wires. The basic process is that if the coating runs too hot and yields water vapour, or was not adequately outgassed, a chemical transport cycle is established between coating and molybdenum supports. This leads to the formation of a low molybdenum oxide in the coating material, which manifests itself as an unsightly dark coloured spot.