From At Home: A Short History of Private Life by Bill Bryson:
Tallow [candles], made from rendered animal fat, had the great advantage that they could be made at home from the fat of any slaughtered animal. […] Because it melted so swiftly, the candle was constantly guttering and therefore needed trimming up to forty times an hour. Tallow also burned with an uneven light, and stank. And because tallow was really just a shaft of decomposing organic matter, the older a tallow got, the more malodorous it grew.
First thought: I need to do a better job of emphasizing quality of light when I’m describing scenes.
Second thought: Could tallow rendered from magical beasts be possessed of special properties? For example, dragon candles could counter darkness spells due to the potency of the light created. Would the tallow from a basilisk counter petrification, aid it, or even cause it?
For those who could afford it, oil lamps were the most efficient option, but oil was expensive and oil lamps were dirty and needed cleaning daily. Even over the course of an evening, a lamp might lose 40 percent of its illuminating power as its chimney accumulated soot. If not properly attended to, they could be terribly filthy. In At Home: The American Family, 1750-1870, Elisabeth Donaghy Garrett records how one girl who had attended a party in New England where the lamps smoked reported afterward, “Our noses were all black, & our clothes were perfectly gray and … quite ruined.”
In running an urban campaign, I make it a point to keep track of when the heroes have been trekking through sewers and getting sprayed with ichor. But it sounds like even their lamps can give them a rather thorough smirching.
Of course, eventually they start using continual light or continual flame spells, and these have every indication of being even cleaner and safer than a modern light bulb. After all, they don’t actually consume anything, so they can’t giving off great quantities of smoke, right?
But here’s another possibility: Perhaps continual flame spells operate by drawing their fuel from the Ethereal Plane — they are literally burning ether (pun intended). A sheen of scintillant silver marks the greasy reside they leave behind on nearby objects, but this is quite harmless (unless it is allowed to accumulate in great quantities, in which case the nearby casting of spells may cause the residue to ignite and produce unexpected wild magic effects). In addition, the small vortexes produced by continual flame spells on the Ethereal Plane (as they draw raw ether from one plane to the next) are quite easy to detect. Continual flames, therefore, are sometimes used to form navigational beacons on the Ethereal Plane, but they may also attract the attention of the ethereal marauders and the like.