Your oven light lasts months, then gradually dims until the glass looks like it has been smoked. You replace it, and the cycle repeats. Most people conclude the oven is faulty. It is not — the lamp was chosen wrong, and the failure pattern tells you exactly how.
First, look at where the blackening is. Internal blackening — a grey film on the inside of the glass — is tungsten evaporation. The filament runs hotter than its design temperature, tungsten atoms escape the wire and deposit on the glass. The bulb is not broken; it is aging fast because it is working outside its design envelope. This is the classic "wrong temperature rating" failure: a G9 capsule rated for room temperature installed in a 300°C oven environment.
External blackening — soot or discoloration on the outside of the glass — is a completely different problem: contamination. Grease, food residue, or the extreme heat of an oven’s self-cleaning cycle carbonize onto the glass. No lamp survives this; cleaning the housing and switching to a lamp with better thermal isolation is the fix, not a different bulb.
If the blackening is internal, three causes cover almost all cases. Cause one: the capsule is a decorative G9 lamp, not an oven-rated one. Decorative lamps state no operating ambient temperature; oven-rated capsules state 300°C or 500°C. If the spec sheet lists only voltage and wattage, it was never designed for an oven. Cause two: wattage too high. Filament temperature rises faster than wattage, so a 60W capsule evaporates tungsten dramatically faster than a 33W one at the same ambient temperature. Oven lamps are for seeing, not heating — 25W or 33W is correct for most ovens. Cause three: loose filament mounting. Cheap capsules spot-weld the filament support; thermal cycling loosens the weld, the filament develops a hot spot, and a bright patch appears on the glass before rapid blackening. If you see an initial bright spot, that is the loose-mount signature.
One more check before you blame the bulb: the lamp socket. A corroded or weakened socket creates arcing at the contacts, and arc heat is far hotter than the oven itself. Burnt or blackened contact points on the socket mean the socket — not the lamp — is the problem, and replacing lamps will not fix it.
A five-point checklist for the next purchase: require an operating ambient temperature rating (300°C minimum); insist on quartz glass; stay at 25–33W for domestic ovens; buy two from the same batch and compare after a month to verify consistency; and inspect the socket before blaming the lamp. If a supplier cannot state the temperature rating in writing, the product is not rated for the job. Oven-rated G9 capsules with documented 300°C operation and 2,000-hour life are exactly what our G9 series is built for — request the spec sheet and check it against this list.