My Jar Supplier Changed the Glass Thickness but Not the Advertised Dimensions — Should I Repeat Heat and Burn Testing?

Why glass thickness can change a candle when the outside size does not

A supplier telling you the glass is now thicker or thinner, while the advertised height and width stay the same, is useful information, not a reason to panic. It is still a change to one of the three physical parts of your approved candle, alongside wax and wick, and the burn results you trust were produced in the old glass. Whether you need a full retest or a short targeted one depends on what the change does to the inside of the jar and to how the jar handles heat.

Glass thickness affects a container candle in three separate ways. Each has its own check, and separating them stops you either over-testing or waving the change through.

1. The inside gets smaller or larger

If the outer size is fixed, thicker walls take their extra glass from the inside. The internal diameter shrinks, so the wick has less surface to melt and may burn hotter than before. Thinner walls do the opposite and widen the inside, which can leave wax on the wall. Advertised dimensions on jar listings, including CSI's, are outer and approximate, so they will not reveal this.

2. Heat is absorbed and released differently

Thicker glass takes longer to warm through and holds heat longer once it is warm. Thinner glass warms and cools faster. That can change how quickly the melt pool reaches the wall on the first burn, how the wax surface sets after extinguishing, and how the glass feels near the end of the candle's life.

3. The glass responds to temperature differences in its own way

As a general principle of glass, uneven heating creates stress, and a thicker wall can develop a larger temperature difference between its inner and outer surface than a thin one. Thicker is not automatically safer. Glass quality, annealing and freedom from defects matter as much as thickness, which is why you inspect every jar for chips and cracks regardless of the wall.

First, confirm what has actually changed

Do not rely on the supplier's description alone, and do not assume the change was uniform across the shipment. Measure it. CSI does not publish glass thickness or jar weight for any container, so your own readings are the only baseline you will have.

Ten from each batch is a suggested starting sample, spread across cartons. If you have no old jars left, measure a finished candle from the old batch and record whatever you can reach, such as the rim wall and weight, and note what you could not compare.

Look at the four rim readings on each jar as well as the averages. A wall that reads 3.5 mm on one side and 5 mm on the other is uneven glass, and unevenness matters more than the average thickness because it means one side of the jar heats and cools differently from the other. Jars like that deserve their own note in the record and a place among the test candles.

Worked example: thicker walls on a 150 g jar

Suppose the outer diameter stays at about 70 mm, but the rim wall measures 4.5 mm on the new jars against 3.0 mm on the old ones. These are your own illustrative readings. With 1.5 mm more glass on each side, the inside shrinks from roughly 64 mm to roughly 61 mm.

The melt surface ratio is (61 ÷ 64)², and 61 ÷ 64 = 0.953, which squared is 0.908. The wick now has about 9% less wax surface to melt. That is enough to tip a wick that was already on the strong side into a taller flame and hotter glass, particularly towards the end of the candle.

The test that answers the question is a matched comparison of six candles: three in retained old jars and three in new jars, same wax, same oil, same load, same wick, same pour temperature, same cure and same room.

Under a thousand rupees of material to find out whether a change of glass has shifted your burn is a sensible trade. Skipping it and discovering a sooting flame after a few hundred candles have shipped is not.

What the targeted heat and burn test should cover

You do not need to repeat your whole original development programme. You need to repeat the parts that glass thickness can influence, and you need to watch the end of the candle, where the glass is working hardest.

Making the goods-in call

Once the six candles have burned down, the decision is straightforward to write. Treat the rules below as a suggested starting point you validate against your own product range.

Be as strict with good news as with bad. If the thicker jars actually perform better, it is still a change to your product, and your records should show when it happened and why you accepted it. And if the comparison fails, the shipment waits in quarantine whoever supplied it, including us; ask the supplier on WhatsApp (+91 7397976926 for CSI) whether the previous glass is still available before you commit to reworking your wick.

Where this page fits

CSI already has guides that cover parts of this question. This page covers the goods-in check for the question above; these go deeper on the rest:

Three things to buy for this job

Frequently asked questions