Oil lamps are far more common than oil lanterns. Oil lanterns are also more expensive. The nice thing about oil an lantern is that it is contained internally and use a wick and a metal pump to increase the pressure in the oil well and force the oil up into the wick system. This means there are internal mechanisms and because of the small parts and difficulty in manufacture, these items are rare and highly sought after. They are usually only owned by people who spend a lot of time underground and have somehow managed to acquire them.

 

Tinkers and gear-smiths work together to make them, which is one of the reasons they're so rare. They also require an expert smith to make some of the small plumbing pieces for the pump. And the wicks are also specialized, and wear out after time. Therefore they have to be replaced as well, and are partially composed of glass made by a glazier who uses wax, fabric, ground glass, and thin glass fibers to compose them.

 

All in all, these items are extremely difficult to build with the existing technology, and few people even know the full instructions. Therefore you won't find one costing less than a good suit of armor. And even then, you'll have to convince the owner to sell it.

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