Priming a wick is a simple but often misunderstood and underestimated operation, yet it is essential to ensure that your candles will burn as they are intended.
Because this is well worth an individual article, let's take a look at the "why" and the "how".
For a detailed explanation, please read How do candles work?
In most cases, a wick is made of braided cotton threads.
As tight as the braid is, inevitably there will be some amount of air trapped inside the wick.
The article titled How do candles work? shows that it's the vaporized paraffin that keeps the flame burning and not the wick itself. However, the first time a candle is lit, it can take some time before the flame reaches the top of the candle and some more time for the heat to start melting and vaporize enough paraffin to feed of it.
"Some time" and "some more time" put together can result in a relatively long time. If you add to this the chemical treatment a wick receives, aimed at slowing down its combustion, you'll understand that before enough paraffin can be vaporized to feed the flame, the latter has major chances dying out.
Most specialized wicks have either already been primed at the factory (HTP, CD, ECO, RRD, TCR) or have a «wax» jacket (LX). In this case, it neither necessary nor advised to repeat the operation.
You're not entirely sure? Cut a short (about 4 inches) section of wick and expose it to the hot blow of your heatgun. If it "sweats", it's likely been primed already.
To promote and ensure an optimal combustion, a simple operation is required: before you install it in your mold or container, let the wick soak for a couple of minutes in melted paraffin wax. In other words, just throw it at the bottom of your melting pot
After a few seconds, you should start noticing a fair amount of tiny air bubbles escaping from the wick and floating towards the surface of the paraffin. And that's exactly what you want: to replace all the air trapped inside the wick with paraffin. Thanks to this, the flame will find enough "fuel" inside the wick and is less likely to die out before it reaches the top of the candle.
After a couple of minutes (or more, that's not an issue), fish the wick out of the paraffin with an old fork or your wicking needle, let it drip above your melting pot for a few seconds, then take on end in each hand and pull to straighten the wick. Let it rest on a piece of waxed paper ad allow to cool off completely. A freshly primed wick should always be straightened before it is laid to cool off; it's almost impossible to do afterwards without giving it a bath of hot paraffin again.
Priming your wicks has another advantage : once the air has been replaced by paraffin and because paraffin returns to a solid state, you will find it much easier to insert your wick through the mold's wick hole. And keeping the wick straight while pouring should be a breeze too.
Always try and prime your wicks in the same type of wax you plan to use it in. For instance, do not prime a wick you're planning to use in a soywax candle in paraffin.
Also, when you prime, try and do so in a wax with a high melt point. If you're making container candles with a paraffin-based wax blend, prime your wick in staright paraffin rather than in the container blend.
If you're feeling creative, you can try and prime your wicks in colored paraffin wax.
No one ever said a candle wick must be white!
A snow white candle looks gorgeous with a red wick! Of course, don't over do it with candle dye: your wick will always have a pastel shade. You'll never get a wick of a deep black (or whatever color you're trying to get).
And don't think you can anyway: all you will do is clog the wick and end up with a poorly burning candle that once had a red wick....