Wick sizing, annealing & burn behavior
Six things a candle maker should settle before the first pour.
A wick is sized to a diameter. In a straight-sided jar that diameter is the same at the top of the wax as at the bottom, so a wick that reaches the wall on the first burn still reaches it on the last. Every other vessel shape asks the same wick to serve two or three different diameters.
Candle glass is a defined specification
ASTM F2179 covers annealed soda-lime-silicate containers made for candle use, setting annealing and thermal shock requirements and requiring scratch testing so residual stress is brought to a commercially acceptable level. A jar that was designed for cold contents may look identical and carry none of that, which is the distinction worth insisting on when sourcing.
Melt pool to the wall in two to four hours
The working target is a melt pool that reaches the glass within two to four hours of burning. Reach it faster and the flame is probably too large for the diameter; reach it slower and the candle tunnels, leaving a ring of wax and a shrinking hot throw. In a straight jar that timing holds all the way down.
Wick series are families, and sizes only mean something inside one
CD, HTP and LX wicks suit paraffin and harder blends; ECO wicks are made for soy and natural waxes. The number after the family name is a step within that family, so an ECO 10 and a CD 10 are not comparable. Specifying a wick without its family is the most common way a burn test result becomes unreproducible.
Two limits from the fire safety standard shape the design
ASTM F2417 sets a flame height limit of 76 mm, requires that the container does not crack or break during burning, and requires that the wick not lean so the flame touches the container for more than five seconds. Those constraints reach back into vessel diameter, wick choice and how securely the wick is centered.
Surface temperature has a ceiling
Candle fire safety work references a maximum container surface temperature of about 140 °F for glass. That figure is why a narrow jar with an aggressive wick fails even when it burns cleanly: the glass gets too hot to pick up. It has to be measured on the filled, burning candle rather than estimated from the wick's rating.
Change one variable at a time, or learn nothing
Useful wick testing keeps wax, vessel, fragrance load and cure time constant and moves only the wick, usually one size cooler and one hotter than the starting point. Flame height, melt-pool reach, soot and glass temperature are logged on the same schedule. Testing several changes at once produces data that cannot be attributed to anything.
