The chimney of oil lamp cracks when touched with the blade of cold knife.
Why ?

It’s very frustrating, cracks in the glass chimney within minutes after lighting an oil lamp. Main reason? The (unintentional) usage of a wrong type of glass chimney.

Always use the appropriate chimney for each specific burner. A burner with a flame spreader, such as the 20’’’ Ideal and 15’’’ Odin Burner, requires a strong bulged chimney. Round burners (Kosmos) need a pinched chimney instead. For more detailed information check out this spare parts guide.

The function of the chimney is to direct the flames up by draft. Relatively cool air enters the burner basket, travels around and inside the burner mechanism and up the chimney. It is designed in such a way, that not all the air is consumed by the flames, but part of it also travels up the chimney, sucked in by the draft. This flow of cool air, travels alongside the chimney and prevents the flames from touching it and cooling the chimney at the same time. The strong bulged chimney produces a flame that looks like a tulip while the pinched chimney of a round burner gives a nice candle-like flame.

If cracks still appear when you are using the appropriate chimney, it can be caused by the gallery. The gallery is the ring of clamps on the burner that hold the chimney in place. These springy clamps are not only meant to keep the chimney in place but also to bend a little as the chimney expands by the heat. Re-bending the gallery has to be done very precisely. Make sure all clamps are bend a little so the chimney fits sufficiently, but not too tight.

Sometimes, the oil lamp is lit without taking enough time to do it correctly. It needs about 5 minutes to heat up. This creates draft in the chimney, allowing the rising air to protect the chimney. If the flame is turned up high immediately, there is not enough draft to protect the glass. The glass touched by the flames expands so rapidly that thermal stress causes it to break or crack. Our advice is to light the oil lamp, turn the flame down low and put the chimney back on. After a few minutes the chimney is hot enough and a draft is present so the flame can be adjusted to the desired height.

At last a cracking chimney can be caused by water drops on the chimney. These drops cause temperature differences on the surface which result in thermal stress, causing it to break.

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