Secondary ignition
Updated
One of the properties ASTM F2417 addresses, and the one makers are least likely to have thought about.
What it means
Something other than the wick catching fire: a label, a decorative element, a dried botanical, debris in the melt pool, or the wax itself burning at the surface.
It is distinct from a large flame. A candle can pass every flame height observation and still have something in it that ignites at hour three.
Where it comes from
Embedded decoration. Dried flowers, herbs, glitter, wood, crystals. They look beautiful in photographs and they are fuel sitting in a pool of liquid wax next to a flame.
Wick debris. A carbon ball that falls in and continues to glow.
A trimmed wick end left in the pool.
A label or a lid liner that reaches too close to the top.
A second wick too close to the first, merging the flames.
Why it is a design question
This is the failure that most often comes from a decision made for appearance rather than for burning.
Botanicals on the surface of a candle are a strong social-media product and a genuine hazard, and there is no wick size that makes them safe. The honest positions are to keep them well outside the melt pool, or not to use them.
Which is worth deciding before a range is photographed rather than after.
Avoiding it
Nothing combustible within the melt pool, ever. Test to find out where the pool actually reaches rather than assuming.
Labels below the level the wax reaches.
Trim debris out rather than letting it fall in, and put that instruction on the care card.
And watch the whole burn. Secondary ignition by definition happens late, which is exactly the part of the test people stop watching.