What Actually Happens Inside an Oil Lamp When You Light It

Most people who own an oil lamp have never been told how it works, which is why so many of them smoke or crack their glass within a fortnight. Every part does one specific job, and the flame punishes you the moment one of those jobs is not being done.

The fuel does not burn as a liquid

Liquid paraffin sitting in the fount is not what burns. The heat of the flame vaporises fuel at the very tip of the wick, and it is that vapour which combusts. The wick is a delivery system, not a candle, which is why the fount stays cool whilst the burner does not.

Capillary action and why the wick must fit

Flat cotton wick draws fuel upwards by capillary action, the same way a paper towel pulls water up its edge. A wick too narrow for the burner slot leaves gaps and burns hot at the corners; one jammed in too tightly is compressed and starves. Suppliers who sell lamps and consumables together list wick widths against burner sizes, and a catalogue such as https://lamplight.us.org/ is a quick way to check what your burner takes before you cut anything.

The chimney is not decoration

The glass chimney creates a draught. Hot gases rise inside it, pulling fresh air in through the burner’s air holes and across the flame. Remove the chimney and the same lamp gives a fraction of the light and a great deal of soot, because the flame is no longer getting the air it needs.

What a correct flame looks like

A well set flame is low, steady, roughly leaf-shaped, mostly clear yellow with a small blue base, and it sits well below the top of the chimney. If it is licking upwards or throwing a black plume, the wick is too high and you are burning fuel without enough oxygen.

Signs to read while it burns

• Black smudging inside the glass means the wick is up too far.
• A sputtering, uneven flame usually means a ragged or badly trimmed wick.
• A flame dimming after twenty minutes often means the fount is low.
• A sharp chemical smell means the wrong fuel is in the lamp: put it out.

The fuel rule has no exceptions

Only lamp oil or liquid paraffin sold for indoor lamps goes into the fount. Petrol, methylated spirit, garden torch fuel and citronella all have different flash points, and indoors they give heavy soot, unpleasant fumes and, with petrol, a fire you cannot control.

Heat, glass and the things that break it

The chimney runs hot, and cold air hitting hot glass is what cracks it. Keep the lamp away from open windows and draughts, and never wash or set down a chimney until it has cooled fully. A crack is not cosmetic; it disrupts the draught and makes the lamp smoke.

Refuelling and attention

Never top up a lamp that is lit or still warm. Put it out, let it cool, fill to the marked level rather than the brim, and wipe any spill from the fount. A burning lamp is an open flame on a table: it needs a stable non-combustible surface, clearance from curtains and paper, and a person in the room. It is never left running overnight.