My Jar Has a Thick Glass Base — Does That Affect How the Candle Burns Near the Bottom and Therefore My Wick Selection?

What a thick base changes, and what it doesn't

Makers often expect a heavy-bottomed jar to need a different wick, and are surprised when the first burns look exactly like a thinner jar's. That is because the first burns happen near the top, where the base is 5 or 6 cm away. The base matters at the other end. Split the question in two and it becomes easy to test.

Four of the six rows are about how much candle you get and where it ends, not about wick size. Only the last row can send you back to the wick, and only a test tells you whether it does.

Measuring a base the listing doesn't state

Our listings give an outside height, sometimes a base diameter, never the thickness of the glass under the wax. Neither do most suppliers'. It takes two minutes per jar to find it.

The skewer method (any jar, clear or coloured)

Stand the empty jar on a flat table. Push a wooden skewer straight down until it touches the inside floor at the centre, where the wick will go. Lay a ruler across the rim and mark the skewer where it meets the ruler's edge. The length from the tip to the mark is the inside depth. Measure the outside height the same way, ruler across the rim to the table. Base thickness = outside height − inside depth. Repeat near the wall: if the depth there is different, the inside floor is domed or dished, and you have learnt something the listing would never tell you.

Clear glass: look from the side

With clear jars you can see the inside floor through the wall. Hold a ruler against the outside and read where the floor starts. It is a quick check that the skewer reading is sensible. Do it at eye level, because glass bends the view.

Measure more than one

Bases can vary from piece to piece, so measure every jar in your first pack and record the thickest, the thinnest and any with a visibly uneven floor. Those are the jars to burn. Mass-produced glass varies between batches too; how that affects a wick you have already approved is in My Glass Jars Are Cheap but Their Dimensions Vary Slightly Between Batches — How Can That Affect Wick Testing and Production Consistency?.

Worked example: one outside, three bases

This uses the series' reference geometry for the 8 × 7 cm, 200 ml jar family: 200 ml over a 7 cm inside depth, straight sides, about 28.6 ml per centimetre of height, and the 0.86 to 0.9 rule-of-thumb factor for wax. We then change only the base: 0.6 cm, 1.0 cm (the reference case) and 1.5 cm. Every figure is a worked example.

Three things follow. The whole candle drops by about 22 to 24 g between the thinnest and thickest base, with the same outside jar, so a fill weight set on one jar type cannot be copied to a lookalike. The burnable column drops from 5.4 to 4.5 cm, 16.7% less; with the same wick burning at the same rate, burn time falls roughly in step. And the roughly 25 to 26 g you leave below the stop line is the same in all three, so it becomes a larger share of a thick-based candle: 18.2% against 15.6%.

Put a price on that reserve. In a worked example candle in the Clear 150 g jar (138 g of CSI 464 at the 10 kg price, 12 g of Sandalwood at the 1 kg price, a C1 wick and a sticker, all at 8%), the whole candle costs ₹233.14 in materials, excluding labour, freight, packaging and GST. About 25 g of it stays in the jar at the stop line, worth ₹21.72. That is not waste; it is the safety margin you are selling. But on a thick-based jar it is a bigger slice of what the customer pays for, which is one more reason to know the base before you set a price or a burn-time claim.

The last 2 cm: heat, floor shape and the wick tab

Heat near the base

As the candle burns down, the flame gets closer to the glass under it and the melt pool sits on a slab of glass that is being heated from above and cooled from below. Glass conducts heat slowly, so a thick slab can be much hotter on its top face than its underside for a while. Uneven thickness and sudden temperature differences are commonly cited causes of thermal stress in glass, and heavier glass is not automatically safer; see Is Heavier Glass Automatically Safer for Candle Making or Can Thick Glass Still Fail From Thermal Stress?. We cannot tell you how any particular jar behaves, and CSI publishes no burn-down or thermal-shock data for its jars. What you can do is watch it: the final third of a thick-based jar is where you look hardest.

Floor shape: domes, dishes and rounded corners

Many thick-based jars have an inside floor that is not flat: a slight dome in the middle, a dish, or a curve where the floor meets the wall. A dome lifts the wick tab above the lowest wax. A curved corner narrows the pool in the last few millimetres. If that curve is smaller than your 1 cm stop line, the stop line protects you: you never burn into it. If the jar is bowl-shaped for the last few centimetres, the pool shrinks while the flame stays the same size, and that is a shape problem covered in My Jar Narrows Toward the Bottom — Should I Select the Wick Based on the Top, Middle or Bottom Internal Diameter?.

The wick tab sits on the inside floor, not the table

Every measurement about the end of the burn starts from the inside floor. The tab and its crimp stand a little above it, so the flame reaches the stop line with the tab still below. A tab that is not stuck flat on a domed floor can lean, and a leaning wick puts the flame closer to one wall just when the glass is hottest. Stick tabs down with a wick sticker and hold the top centred while the wax sets.

So does it change your wick? A decision table

Notice what is missing: "size up the wick". A thick base never asks for a bigger wick. If your first burns tunnel, the problem is at the top, and it belongs to the diameter questions in My Candle Jar Holds 200 ml but Has a 7 cm Internal Diameter — Which Measurement Should I Use When Choosing the Wick?.

The final-third test for thick-based jars

This assumes you have already chosen a wick at the top of the jar (say C2 in the Clear 150 g, as a worked example) and want to know whether it survives the bottom. Never leave a burning candle unattended, burn on a heat-proof surface away from draughts, and keep test candles away from children and pets.

Four 150 g candles at 8% need 48 g of oil. The 50 g Sandalwood would leave 2 g for pitcher loss, which is too tight, and it is dearer per gram (₹5.43 against ₹5.03, 8.0% more at the 50 g size), so the 100 g is the better test size. The Sandalwood page states "Recommended load up to 10% for soy, up to 6% for paraffin", so 8% in CSI 464 sits inside both ceilings. If you already own the jars, the kit is ₹1,263.48.

Jars and wicks for the test

Chosen on what the pages state, price and stock on 26 September 2026. We have no burn data for any of these jars; the thick-walled wording is the pages' own, and it is the reason to test, not a result.

CSI's thick-walled listings, read carefully

Five families of CSI container listings, thirteen listings in all, describe thick glass or thick walls. None states a base thickness, and three of the families carry a heat figure, all the same 85°C hot-pour wording. Here is what each page says, quoted, and what you still have to find out.

Two cautions. "Even heat distribution" and "retain and diffuse heat evenly" are the pages' claims, not measurements; a thick base is exactly where heat can be uneven for a while, so test rather than rely on either. And the 85°C figure is a pour-temperature statement on those listings only: it says nothing about how the glass behaves under a flame in the final third, and it does not transfer to any other jar. If you are weighing a thick base against thin side walls, that combination has its own watch-list in My Jar Has a Very Thick Base but Thin Side Walls — What Should I Watch for During Candle Burn Testing?; a jar that survives early burns and cracks near the bottom is examined in Can a Candle Jar Survive Early Burn Tests but Still Crack When the Candle Burns Closer to the Bottom?.

Frequently asked questions