Dear WW: When a flame coes not have enough oxygen mixed with the fuel to burn it all,
it has a lower temperature, is less bright, and may smoke. Surely you have seen a candle
whose wick has gotten too long and is too straight, so tip is not burning brightly in outside
portion of flame. There is not enough oxygen mixed with the vaporized wax, and it smokes
because the wax is not completely burning. If you hold test tube over the flame, the bottom
gets coated with soot. The bucchero pottery was evidently in second firing had air supply
reduced, producing soot which blackened the pottery. I can't help wondering what the glaze
was that could vitrify under those conditions. Lead could do it, perhaps. And maybe (as a
jest,) that was what caused the downfall of the Etruscans.

Incidentally, going back to the candles, there is a trick to the wick, One of the fibers has to
have more tension than the others, so that when the wax melts, it can contract slightly,
and meake wick end curl and poke out through side of flame where it reaches enough
oxygen to burn with a much brighter yellow than any other part of the flame. When I made
candles, I could not reproduce that effect, and my candles were obnoxiously smoky.
That's my theory about wicks. Maybe other members know more about the secret pf
making the wick curve out of the flame.