Why you should not touch a halogen capsule with bare fingers

Halogen lamp boxes often say do not touch the glass. How a fingerprint damages the capsule, how to fit the lamp properly and what to do if you touch it by accident.

Anyone changing a G9 or G4 halogen capsule, or a long R7s lamp for a floodlight, will find an odd warning on the box: keep skin off the quartz envelope. It is often dismissed as excessive caution. But the advice has a real physical reason, and ignoring it can shorten the lamp's life and, in some cases, make the capsule crack.

How a halogen capsule works

A halogen lamp is an improved type of incandescent lamp. It has a tungsten filament, but the bulb is very small and filled with a halogen gas mixture. The gas creates a chemical cycle that returns tungsten atoms evaporating from the filament back onto it. For the cycle to work, the bulb wall has to be very hot.

That is why the capsule is made not of ordinary glass but of quartz, which withstands high temperatures. In operation its surface reaches temperatures measured in hundreds of degrees, conditions under which ordinary glass would soften.

Why a fingerprint does harm

Human skin always carries a thin layer of oil, sweat and salts. Touching quartz with a finger leaves an invisible mark of this layer on the glass.

When the lamp is switched on, the marked area heats differently from the surrounding surface. The oil burns and leaves a residue that absorbs heat and light differently, creating a local hot spot on the capsule. In addition, salts from the skin, particularly sodium compounds, can alter the structure of quartz at high temperature: the glass turns cloudy at that point, begins to crystallise and loses strength.

This is not visible straight away. The lamp works normally for a while, but the cloudy patch gradually spreads. Eventually the capsule fails early, bulges or, in rare cases, cracks. Because the gas inside is under pressure, a crack can scatter small fragments of glass, which is one of the reasons many halogen fittings have a protective glass.

How to fit the lamp correctly

When changing a halogen capsule, follow a few simple rules:

• Switch off the light and wait until the old lamp has cooled completely. Straight after use a capsule can cause a serious burn.
• Do not take the new lamp fully out of its packaging. Capsules often come in a plastic or paper sleeve, and you can hold the lamp by the sleeve while pushing it into the holder.
• If there is no sleeve, use a clean dry cloth, a paper towel or a thin cotton glove.
• Push a G9 capsule straight into the holder without forcing it; seat an R7s lamp against the spring-loaded end first, then the other end. Pushing at an angle can damage the contacts or break the capsule.
• If the fitting has a protective glass, do not switch the lamp on until the glass is back in place.

If you touched it by accident

If you have touched the capsule with your fingers, there is no need to throw the lamp away. Cleaning the mark before switching on is enough.

Dampen a soft, lint-free cloth or cotton pad with isopropyl alcohol and gently wipe the capsule. Then wait for the alcohol to evaporate completely. Do not use ordinary glass cleaner, soapy water or scented wet wipes, as they can also leave a residue.

If the lamp has already been used and there is a cloudy, whitish or brown patch on the capsule, damage has occurred. It is safer to replace such a lamp, especially in a fitting without a protective glass.

Which lamps the rule applies to

The warning applies mainly to halogen lamps with bare quartz capsules: G9 and G4 capsules, R7s linear lamps, car headlamp bulbs, and floodlight and stage lamps. In MR16 and GU10 halogen reflector spots the capsule is usually enclosed by a front glass, and the part you handle is the glass reflector; the risk there is much lower, though keeping the front glass clean is still worthwhile.

LED lamps are not subject to this rule. LED replacements with G9 or R7s caps do not get hot and fingerprints do them no harm. If you are tired of replacing halogen capsules, switching to LED removes both this problem and the risks associated with high temperatures; just check that the LED lamp is small enough to fit the fitting.

Zaferoğlu Elektrik sells an E40 JTT halogen lamp, and for anyone leaving halogen behind it also offers GU10 and G5.3 LED spots. Whichever you choose, handle any new halogen capsule with a cloth or glove.