My New Jar Narrows Toward the Bottom — Could a Wick That Is Correct at the Top Become Too Powerful Later?

The same wick, a shrinking job

The worry in your title is precise, and geometry supports it. A wick's size sets roughly how much heat it delivers; the jar sets how much wax that heat has to melt. In a straight jar that balance stays the same from the first burn to the last. In a jar that narrows toward the bottom, the wax surface shrinks as the candle burns down while the wick stays the same, so the balance tips further toward the wick with every burn. Your title gives no dimensions, so this page uses a labelled illustration: a jar 9.0 cm inside at the rim and 6.6 cm at the base, 8 cm deep, holding a 250 g candle. At a labelled 0.88 g/ml, 250 g fills it to 6.32 cm, where the first-burn pool is 8.49 cm across.

The fourth column is the one to hold on to. If the wick is right for the first-burn pool — index 1.00 — it is serving only 0.80 of that surface three centimetres down and 0.60 at the base. Put another way, by the last burn the same wick is working in what is effectively a 6.6 cm straight jar, with the glass 9.5 mm closer to the flame on every side than it was on day one. That does not prove the wick will be too strong; many candles cope. It does mean "correct at the top" is not evidence about the bottom.

What "too powerful later" looks like

No burn result is published by CSI for any wick in any jar, and the size of each effect depends on your wax, oil and wick. The list below is what candle makers commonly describe when a wick outgrows a narrowing pool. Use it to design the scoring sheet: every item is a line you can measure against a control.

Where the wax sits: the thirds of a narrowing candle

Splitting the fill into thirds of its height shows something the depth table hides: a narrowing jar puts less wax in its lower third than its upper third, so the burns spent in the risky zone are fewer — but they are also the last, when the jar has been heated the most times and is least watched.

In the illustration the bottom third holds about 28% of the candle, and the pool index runs from 0.72 down to 0.60 across it. That is the band where the approved wick must prove itself, and it is the band a partial burn test never reaches.

The retest matrix for a jar that narrows

The series' four verdicts — KEEP, CONFIRM, REPEAT, NEW. Compared with the general straight-to-tapered post, this matrix leans harder on the lower band and on the safety lines.

The protocol: bracket down from the start

Because the risk sits at the end of the candle's life, a staged test is slow: you would have to burn the approved wick to the bottom, find a problem, then pour and burn a second set all the way down. Pour the bracket at the start instead. In the worked example the approved wick is Eco Thick C2 and the bracket is Eco Thin C1, the two Eco sizes CSI lists; we do not claim either suits this jar or that they are one step apart for your wax. If your wick range has more sizes, bracket with the next size down in that range.

The priced test kit

Built from listed, in-stock CSI sizes on the 24 September 2026 pull, with Luxury Soy Wax Chunks (rated up to 13%) and Cinnamon Vanilla as the worked example, at our 8% working load and a 5% allowance.

Decision rule: four options when the bottom runs strong

CSI publishes no numeric flame-height or jar-temperature limits. Approval means every band no worse than the control on every safety line, within your jar supplier's guidance and the fire-safety standard you work to (ASTM F2417 is a commonly cited US one). A wick is never kept too strong for the bottom in exchange for a faster pool or more throw at the top.

Record the jar as three diameters plus depth, the chosen wick, the band results and any end-of-burn instruction, with the date and supplier. The reproducibility-record post shows the format; the change-control form post turns it into a sign-off. For the general jar list see the only-the-jar post.

Related jar changes

If your jar tapers the other way — wider at the base — the risk reverses to tunnelling late in life, and the straight-to-tapered post covers both directions. A narrowing jar with a thick base combines two end-of-life changes; add the last-centimetre sheet from the thicker-base post. For the underlying area maths, the diameter hub is the starting point, and the alternative-container post scores how much taper a candidate jar has before you commit to one.

Frequently asked questions