A reader left a comment on an earlier blog post about oil lamps as follows: "Do you have any advice on stopping my lamp from smoking so bad? The chimney gets black quickly and then light cannot shine through. Thanks ma'am!"
I don't purport to be an expert on All Things Oil Lamp, but I believe I can help in this case. It probably has to do with the height of the wick.
The wick determines the shape and quality of the flame. Wicks themselves don't burn; what burns is the oil (or kerosene) that is absorbed (or "wicked") by the wick. By the way, when using a brand-new wick, install the wick in the cap and let the wick dangle in the oil or kerosene for a few minutes so it can fully absorb the fuel. Otherwise you'll just burn the wick itself. I've used the same wicks for years and years and years.
The shape of the wick is important. A new wick has a square end. It is advisable to trim the wick to an oval shape at the tip; this will result in the classic "flame"-shaped flame. If a wick is left square, the flame has kind of a funny "turreted" shape. For a new wick, you might have to light the flame and then blow it out and trim a bit more, then light the flame and then blow it out and trim a bit more, until you're satisfied by the shape of the flame.
What a lot of people don't understand about wicks is they should NOT be visible while the lamp is lit. For example, let's say you light the lamp with the wick visible, as below:
The flame is WAY too high (and, not incidentally, is burning too much kerosene). A flame of this height will instantly soot up the chimney.
Instead, the wick should be just below the mouth of the cap, not really visible.
Some people prefer to light the lamp with the wick high, then adjust it downward (this works fine). Me, I prefer to light the lamp when the wick is already lowered. To do this...
...I hold the match horizontal to the cap, rather than pointing it downward.
This lights the flame with the wick already in the correct position, so I usually don't have to make any adjustments at all.
However, if I do have to adjust the wick up or down in order to regulate the flame, that's what the little knob is for, on the right. Don't hesitate to raise or lower the wick as necessary.
Besides protecting the flame, the chimney helps regulate the air flow to the lamp and will stabilize the light. The flame will "fatten" and keep still once the chimney is in place.
No matter how well trimmed your wick, some soot is expected on the chimney, but it shouldn't accumulate very fast. In our oil lamp experiment we did last week, we had the lamp burning for 33 hours almost nonstop, and this is what the chimney looked like at the end of it. A little bit of soot, but not much.
Nonetheless, once in awhile you'll have to clean the chimneys. I don't recommend washing them directly in the sink, as they're easy to break if the glass bumps the sink walls. Instead I wash them in my plastic dish tub.
I also recommend stocking up on chimneys. There are online sources for chimneys; or, if you frequent thrift stores, just keep an eye out for chimneys in the household glass section. I snag them whenever I find them and tuck them into empty liquor boxes, which have built-in cardboard dividers. I probably have fourteen or fifteen spare chimneys. (My thought is, I never know when it may be necessary to provide an oil lamp to someone left in the dark after an emergency. Because I can assemble up to twenty lamps from caps that fit on canning jars, I'd like to have enough chimneys to match.)
With a little practice, oil lamps can provide a soft, beautiful, clear light to fill a darkened room. It's not the best light for reading (unless the lamp is right next to the book) but is fine for just puttering around. Oil lamps should be part of everyone's non-electric lighting repertoire, unless health reasons don't permit their use.
And stock up on matches!