Updated: Jun 6, 2020
1. Trim Your Wick.
Every time you want to burn your candle, start by trimming the wick to between 1/8 and 1/4 inches long. You can use scissors, nail clippers, or a specialized wick trimmer, but no matter how, always trim.
Why is this important? Firstly, a freshly trimmed wicks will give you a cleaner, brighter flame. Untrimmed wicks are a lot more likely to fray and expand and in turn into something that’s more likely to dull and obscure the flame.
Secondly, excessively long wicks are a top cause of those black smoky stains that end up on your glass jar candles. Trimming the wick keeps the flame in control and prevents those marks from forming in the first place.
Thirdly, the closer your (newly trimmed) wick is to the wax, the easier you will be able to achieve the full melt. Our next point.
2. Burn Long Enough To Form An Even Melt Pool
Once your candle's lit, don't blow it out until the top layer of wax has melted all the way across. This might take several hours — so don't set out to burn a candle at all unless you've got time to kill.
Whenever you fail to achieve full melt, you're contributing to a process called tunnelling. The wick starts to sink lower and lower, like a tunnel is forming right through the center of the candle.
Eventually, the tunnel will grow so deep that it'll be tough to light the wick at all. More importantly, all that unmelted wax on the sides represents hours of lovely fragrance and burn time you bought but won't ever get to utilize.
It takes patience, but if you melt the wax all the way across every time you burn, the surface of the candle will stay flat and the sides of the jar will stay clean, all the way down until the candle is spent.
3. Buy Multi-Wick Candles
I know from experience that it's hard to find enough time for a proper burn. The solution? Buy a candle with two or three wicks. More flames means more heat — which leads to a quicker melt.
On that note: Be wary of extra-wide candles that only have one wick. If the candle surface has just one wick and a diameter of more than 3 or 4 inches, don't buy it. One wick will never produce enough heat to melt it all the way across.
4. Keep Away From Draughts.
Do your best to keep your burning candle away from fans, air conditioners, open windows, or heavily trafficked areas where people walk back and forth a lot. Moving air can disturb the flame, leading to even more unsightly black marks on the glass.
5. Aluminium Can Fix Your Candle Sins.
If you’ve already committed one of the aforementioned sins and are contending with uneven wax piled up on the outer edges of your candle jar, it’s not too late. Grab a big sheet of tin foil and fold it over onto itself lengthwise three or four times to get a long rectangular shape that you can wrap around the outside of your candle jar. Leave it there, tightly wrapped, while the candle burns and VOILÀ! You’ll have a perfectly even candle in no time.