A candle is a clever little stand-alone combustion engine. The wax provides the fuel, the wick allows the fuel to be vaporised and burned giving you light, heat and fragrance (if used).
For what is termed as a 'clean-burn' the combination of wick and wax is important. If the wick is too small it will simply melt a hole straight down the middle of the candle, too large and it will melt more wax than it is capable of using and simply drown in it's own melt pool.A 'clean-burning' candle will have a good size flame, produce very little soot and will create a melt pool that extends to the edges of the candle.
Container candles are designed to be burned in the container they were made in. The melt pool should extend to the edges of the container after a 3 hour burn period, but the molten pool shouldn't be more than approx 1/2" deep. The flame should be steady and strong, with no whisps of smoke or soot visible. The wick should stay in a central position throughout the burn of the candle.
The candle should be discarded once the wax is down to the last 1/2" in the bottom of the container to stop the wax from getting too hot and possibly igniting. Sustainers with long necks are often used for safety reasons in container candles, this means the wick is kept out of the last little bit of wax to stop it from over-heating.
Votive candles need to be used in a tight fitting candle holder. The wax will melt and spread as the candle burns, but the wick is designed to suit a candle with the original diameter. If a votive is used in a holder that's too large for it, the wax will spread to fill the container, but the wick won't be able to use it all and you'll get a much shorter burn-time for your money and oodles of unused wax! A correctly burning votive will have a melt pool that extends to the edges of the holder, will burn with a bright flame that doesn't soot and the wick will remain central throughout the burn. There should be very little left-over wax at the end of it.
Tea light candles are self-contained little units in either metal or polycarbonate holders. A well burning tea light will burn with a small, but steady flame. The wick will remain central throughout the burn and the melt pool should extend to the edges of the container. The flame shouldn't soot or smoke and shouldn't drown in it's own melted wax.
Pillar or Free-Standing Candle
Any free standing candle needs to keep it's shape during the whole burn. The molten wax shouldn't burn a hole throught he side and cascaded down the length of the pillar (guttering), the wick shouldn't burn only a central 'core' down the centre and the wick shouldn't smoke or soot during burning. Any free-standing candle needs to be placed on a heat-proof plate to protect surfaces from the heat and to catch any wax that might escape.
To get the best from any candle, they should be left alight for a least a 2 hour stretch to allow the melt pool to become effective, but shouldn't be burned for more than a 4 hour period to prevent the melt pool from becoming too hot. The wick should be trimmed back to approx 1/4" above the wax surface the first time you light it. It should be trimmed regularly to prevent a sooty carbon build-up on the top of the wick and to prevent the flame from getting too large.
Candles should never be left alight close to soft furnishings, left in a breeze which will cause the candle to burn unevenly and never left unattended whilst it's burning.