Candle Jars: Why Thick Glass Matters and How to Spot a Cheap One Before You Buy

Most people choose a candle by its scent. Edmundo Benzaquen chooses one by picking it up.

“The jar tells you more about a candle than the label ever will,” says Edmundo, owner of US glass packaging supplier The Bottle Depot, which supplies jars to candle makers. “Pick up two candle jars of the same size and one feels like a paperweight, while the other feels like a plastic-thin tumbler from a festival bar. That difference isn’t only about looking luxurious. It’s about how the glass copes with a flame burning a few centimetres away for hours at a time.”

And we’re burning more of them than ever. The UK candle market was worth US$828.6 million in 2025, according to Grand View Research, and it’s forecast to grow by 9.4% a year through to 2033.

That’s a lot of glass sitting on mantelpieces, bathroom shelves and bedside tables.

So before your next candle haul, here’s what’s worth knowing: what actually happens to glass when a candle burns, why thick and even glass is worth paying for, and six quick checks that’ll help you spot a cheap jar before it comes home with you.

What Actually Happens to Glass When a Candle Burns

Glass heats unevenly. The part nearest the flame and the melted wax gets hot, while the rest of the jar stays relatively cool. That temperature difference creates stress inside the glass, and glass doesn’t enjoy being stressed.

The British Candle Makers Federation puts it plainly in its advice to the trade: heat from the flame can set up thermal stresses in the glass that cause it to shatter, and pieces can travel a metre or more. The same guidance notes that there are around 1,000 candle-related fires a year in the UK. Most of those come down to how candles are used rather than faulty products. But a jar that cracks mid-burn can turn a small lapse, like leaving a candle going while you pop into the kitchen, into something much worse.

“Here’s the part most people miss,” Edmundo explains. “Glass rarely fails just because it’s thin. It fails because it’s uneven.”

Why Thick (and Even) Glass Is Worth Paying For

Thickness buys you time.

Thicker walls absorb and spread heat more slowly. That gives you a margin for the everyday mistakes we all make: a wick that drifts towards the side, a draught from an open window, a burn that runs longer than planned. Thin glass has no margin at all.

Evenness matters more than you’d think

When a jar is thick on one side and thin on the other, the thin patch heats up faster than its neighbour, and the stress piles up exactly where the two meet.

“Cheaply made jars often have exactly this problem, because the molten glass hasn’t settled evenly in the mould,” says Edmundo. “You can’t see it in a product photo, but you can often spot it in person.”

Annealing is the invisible bit

After a jar is formed, good glass is cooled slowly and carefully. This step is called annealing, and it relaxes the stress that gets locked in when hot glass cools too quickly. Poorly annealed glass looks perfectly fine on the shelf. It just behaves badly once it’s heated.

In the US, there’s a dedicated standard for this, ASTM F2179, which covers annealed glass containers made for candles. It requires glass manufacturers to test every production batch using a polariscope (which shows hidden stress in the glass) and a thermal shock test, with zero failures allowed. The standard was updated again in 2026. Here in the UK, the British Candle Makers Federation advises makers to ask their glass supplier for an annealing certificate and proof of thermal shock testing.

Edmundo’s advice for shoppers is simple: “If I were buying from a small independent maker, I’d happily ask whether their glass comes with that paperwork. The good ones won’t mind the question. Some of them will be delighted you asked.”

And if you make candles yourself, that certificate is the first thing to ask for when you’re choosing jars for candle making.

How to Spot a Cheap Candle Jar Before You Buy

You don’t need a lab. You need your hands, a window and about thirty seconds.

• Weigh it in your hand. A decent candle jar feels heavier than it looks. If it feels like the glass you’d pour squash into, it probably is.
• Hold it up to the light and turn it slowly. Look at the walls. Do they thin out on one side? Is there a ripple or wobble in the glass? Even thickness all the way round is what you want.
• Check the base. It should be thick, flat and sit level on the table without rocking. A thin or domed base puts the hottest part of the burn, right at the end, on the weakest part of the jar.
• Look for bubbles and specks. A tiny bubble in hand-blown glass is part of its charm. Clusters of bubbles or dark specks near the base of a machine-made jar aren’t charming. They’re weak points.
• Run a finger round the rim and the seams. Sharp, raised mould seams or a rough rim usually mean the jar was rushed through production. If corners were cut where you can feel them, they were probably cut where you can’t.
• Check where the wick sits. It should be dead centre. The British Candle Makers Federation notes that wicks drifting against the glass are a common cause of breakage, so a wick that already leans towards the side is a warning sign.

Buying a set? Edmundo has one more trick. “Say you’re buying four matching jars from a market stall or a gift shop. Line them up side by side. Identical jars should look identical. If one has a visibly thicker base or sits differently in your hand, the batch wasn’t tightly controlled, and that’s the one I’d leave behind.”

Burning Smarter, Whatever Jar You Have

Even the best jar can’t make up for bad habits. A few rules worth following:
● On the first burn, let the melted wax reach the edges. As a rough guide, that’s about an hour for every 2.5cm of the jar’s width.
● Trim the wick to around 5mm before every burn. A long wick means a tall, hot flame.
● Don’t burn for more than four hours at a time.
● Stop when there’s about a centimetre of wax left. The base gets hottest at the end, and that’s when weak glass is most likely to give up.
● Never set a hot jar on a cold or wet surface, and never pour water into it to put it out. That sudden temperature change is exactly what makes glass crack.

The Jar Is the Bit You Keep

“I’d rather pay a few pounds more for a candle in glass I’ll still be using in five years,” says Edmundo. “The scent is gone in a month or two. The jar isn’t.”

Once the wax is gone, a good jar makes a lovely pen pot, a cotton wool holder in the bathroom or a spot for your makeup brushes. To clean one out, pop it in the freezer for a few hours. The leftover wax shrinks and usually lifts out in one piece. A thick, well-made jar handles that without a fuss. A thin, cheap one may not.

More makers are starting to offer refills for their jars too, which only makes sense if the jar was worth keeping in the first place. So next time you’re choosing a candle, give it the smell test, then give it the pick-up test. Your mantelpiece will thank you.