My 7.5 cm Soy Candle Has a 1 cm Deep Melt Pool After Four Hours — How Do I Determine Whether the Wick Is Generating Too Much Heat?

Why 1 cm at four hours is the edge, not the verdict

The 0.5–1 cm figure is a rule of thumb makers use to describe a healthy pool on a container candle: deep enough to hold fragrance at temperature, shallow enough that the wick is not flooding the jar with heat. It is not a CSI figure and not a safety limit. A reading of 1 cm sits on the line, and on its own it can mean three different things.

It can mean the pool reached the glass and kept melting downwards, as pools do. Once a pool meets the wall it cannot grow wider, so every extra minute of burning goes into depth. A very rough rule of thumb is about one hour of burning per 2.5 cm of jar width. On a 6.8 cm inside that is about 2.7 hours, so a four-hour session runs about 1.3 hours past full pool; even on a true 7.5 cm inside it runs 1.0 hours past. Some depth gain in those hours is expected. The question is how much.

It can mean the wick is supplying more heat than the jar can spread. Then depth races ahead early, the flame is tall, and the curve is still climbing when you blow it out. That is the over-heat pattern, and the fix is less wick.

Or it can mean the session was simply long. If your label will tell customers to burn for two to three hours, a four-hour reading describes a candle nobody is supposed to burn. That does not make it irrelevant (people do burn longer, and the label-policy post owns what to print), but it changes which depth you judge.

So "1 cm at four hours" is a question with three missing parts: the depth at the earlier hours, the flame at each of them, and the session length you will print. The skewer dip collects the first two in one burn. You choose the third.

What a 7.5 cm jar means in surface and hours

The title gives 7.5 cm without saying inside or outside, so measure: steel ruler inside edge to inside edge across the rim, twice at right angles, and again at mid-height with a strip of card curled inside (length ÷ 3.14). The full method is the external-versus-internal diameter post. If 7.5 cm is the outside and the wall is 0.35 cm (our labelled assumption), the inside is 6.8 cm.

The column height is an estimate. A common candle-maker rule of thumb puts wax at roughly 0.86–0.9 times the weight of water in the same space, so 150 g of soy takes up about 170 cm³, which in a 6.8 cm inside stands roughly 4.6–4.8 cm tall. A 1 cm pool is then about 21%–22% of the candle's height, and about 36.3 cm³ of liquid across the surface. That is why depth is worth measuring in millimetres, not by eye: a fifth of the candle is molten at once.

Why 150 g? It is this series' standard for 7–7.5 cm jars: 150 g at 8% of total weight is 12 g of oil and 138 g of wax. If your fill differs, weigh water to your fill line and scale.

The skewer dip: depth at 1, 2, 3 and 4 hours

Use a candle that has already had its first burn. The first burn starts from cold wax and cold glass and is judged elsewhere (the first-pool rules are in the 7.5 cm catch-up hub). Burn 2 or 3, at the top of the jar, is the cleanest place to read heat. Keep a twin from the same pour lit beside it, undisturbed, so you can see whether the dipping changes anything.

This is a top-of-jar reading. How depth grows as the candle burns down (the same log after every burn, set against the wax left) is a different test with its own chart: the deep-pool-near-the-bottom post. Do the dip once near the top; repeat it in the lower third only if you are already watching that zone.

Three curves that all end at 1 cm

Here are three worked-example logs from a 6.8 cm-inside jar, each ending at 10 mm at four hours. They are illustrative, not CSI data. Each is a pattern you might see.

Curve A: the one most readers will see

The pool deepens quickly while it spreads, reaches the glass at about three hours (close to the 2.7 h rule of thumb), then adds only 0.5 mm in the last hour. The flame is steady. This is a wick delivering heat at about the rate the jar spreads and loses it. At a four-hour session, 1 cm is where it settles. No heat verdict. Continue the candle through its full life and keep watching the final third, where heat commonly builds (the lower-third flame post separates stored heat from wick size there).

Curve B: calm for three hours, then climbing

Depth creeps for three hours, then jumps 3.5 mm in the fourth while the flame grows from 3.5 to 5.5 cm. Between hours two and three it rose 2 mm; in hour four it rose more than that. Something changed late in the session: the glass and wax have warmed through, the exposed wick has lengthened, or carbon is gathering. If your label says three hours, curve B passes inside it. If it says four, you are testing past the calm. It is a warning, not yet a verdict. Repeat on the twin; if hour four climbs again with a taller flame, treat it as heat for that session length.

Curve C: deep by hour two

The pool is 7 mm deep in an hour and full by about an hour and a half, well inside the rule-of-thumb 2.7 hours, and the flame runs at about 5.5 cm from the start. The depth stops at 10 mm only because the session is long enough for the pool to settle. Fast pool plus tall flame is the common over-wick pattern. A fast pool alone is not proof (a warm room or a relit candle can do it; the 45-minute pool post lists the confounders), but with the flame beside it, this is the curve that justifies a step-down pair.

The combined heat rule: depth, flame and session length

This is our judgement, set out so it can be applied the same way every time. The thresholds use two common rules of thumb (pool about 0.5–1 cm; flame about 2.5–5 cm) plus the slope you just measured. Read everything at the longest session you will print on your label, not at an arbitrary hour.

Two things this rule does not do. It does not approve a wick: approval needs the full life, because heat commonly builds as the wax level falls (acceptance gates by depth are in how far down to burn a test candle). And it does not decide between cutting the wick and cutting the load. A 10% candle with a deep pool has a second lever, and the route between the two is the load-first or wick-first comparison.

If the rule says heat: the step-down pair, costed

Hold everything but the wick: the same wax bag, oil and load, jar batch, 150 g fill, pour temperature, cure and room. Cell 1 is your current wick (the control). Cell 2 is one rung down. On CSI's range that is C2 → C1. If you are on Cotton Braided, note its page describes it for candles "exceeding 8 cm", which a 7.5 cm jar is not; step to C2. If you are on C1, CSI lists no smaller sized, tabbed wick. Say so to yourself honestly and ask us, or test another supplier's smaller wick as a labelled arm.

Per candle that is ₹127.88 on C1 and ₹131.42 on C2 before the jar; the C2 candle costs ₹3.54 more. Costs are worked examples and exclude jars, labour, freight, packaging and GST. If you need fresh twins for the first dip log as well, two more C2 candles use 24 g of oil and 276 g of wax (₹262.84 before jars); the six candles together need 72 g of Gardenia (one 100 g bottle covers it, with 28 g left) and 828 g of wax (172 g left in a 1 kg bag).

Reading the pair

Run the same dip log on all four, burn 2 or 3, same room. Then:

Kit for the dip test, and what to skip

Five items, in the order you need them, chosen on page statements, price and stock on 29 September 2026. CSI publishes no wick-performance or depth data, so nothing is ranked on burn results. Oil is counted at 12 g per 150 g candle.

Frequently asked questions