The glass is the part owners break first and understand least. A chimney is a working component, not a decoration, and it fails for reasons that are entirely predictable once you know what heat does to thin glass.
Thermal shock in one paragraph
Glass expands when it is hot. If one part of a chimney is at burning temperature and another is suddenly cooled, the two parts pull against each other and the glass splits, usually with a soft tick rather than a dramatic shatter. Cold water, a cold worktop, a draught from an open window and a wet cloth are the four classic triggers.
The cooling rule
After extinguishing a lamp, leave the chimney where it is until it is properly cool to the back of the hand, which takes considerably longer than people expect. Do not lift it onto a stone or metal surface, do not carry it to the sink, and do not stand it on a windowsill. Patience here costs nothing and replacements are a recurring expense.
Matching a replacement
Chimneys are not interchangeable. The base diameter, overall height and bulge shape all affect the draught, and a mismatched one will make a previously good lamp smoke. Measure the gallery that holds it and the height of your original before ordering; a specialist range such as https://lamplight.us.org/ lists chimneys by base size, which is far more reliable than matching by eye.
Cleaning without scratching
Warm water with a little washing-up liquid removes ordinary carbon film. Use a soft cloth wrapped around a wooden spoon handle to reach inside a narrow chimney, rinse in water of a similar temperature, and dry with a lint-free cloth. Abrasive pads and scouring powder leave micro-scratches that trap soot and make the next clean harder.
Stubborn baked-on carbon
When the film has been burnt on repeatedly it goes brown and resists soap. A paste of bicarbonate of soda on a damp cloth, worked gently, usually lifts it. If it will not come off at all, the chimney has probably been used with a badly set flame for a long time, and the flame is what you should fix next.
Shades and the heat they sit in
Decorative shades add glare control and character but also sit in the rising hot air. Keep them clear of the chimney top, clean them cold, and check the shade ring is not distorted. A shade resting directly on hot glass will discolour and can crack both pieces.
Storage between seasons
- Clean and dry the glass fully before putting anything away.
- Wrap chimneys individually in paper rather than stacking them.
- Store upright in a box, not loose in a drawer with tools.
- Keep one spare chimney with the emergency kit, wrapped separately.
- Check for hairline cracks before the first light of the season.
When a crack is not cosmetic
A cracked chimney disturbs the airflow that keeps the flame clean, so the lamp starts to smoke, and the crack can widen as the glass heats. It is also a sharp edge above an open flame on a table where hands reach. Retire it and fit a new one rather than getting another few evenings out of it.