A good burning candle should:

• Have a good stability without any tendency of deformation at room temperature.
• Burn with a slightly bent wick out of the low temperature zone, where due to the exposure of oxygen, a better and cleaner combustion takes place.
• Have an even and steady burning
• Have a clean and dry pool
• Burn without noticeable soot and smoke.
• Leave no carbon deposits at the tip of the wick.

The candle diameter is probably the most important factor in the selection of the correct wick size. If the wick selection is too small, the wick would not be able to absorb all the available liquid wax. Consequently, the burning pool will fill up and eventually overflow. Commonly known as "dipping".
If the wick is too large, it would lead to sooting and smoking.

 

The right selection depends also in great part on the wax formula. For pure paraffin candles and/or paraffin/stearic acid composition dipped candles (up to 25% stearic acid), flat braided wicks are usually used. However, it will also be possible to use square braided wicks. If square braided wicks are being used, the direction of the wick is important. By looking at the flat side of a square braided wick it should show a "V" direction. It the "V" is upside down it means that the wick is placed into the candle in the wrong direction.
Only flat braided wicks are being for used for 100% stearic acid candles and only square braided wicks are used for beeswax candles.
Additives etc. can lead to clogging up the wick and thereby reducing the capillary attraction. Special wicks treatment can lead to burning improvements.

 

A good testing method is:

Determine a wick which meets your requirements and expectations according to the candle diameter and manufacturing process and use this wick in your pure wax formula.

After you have found the correct wick for this formula, start adding the required additives (colour, fragrance etc.) and make a burn test after every addition of an additive. If the burning quality gets worse, you will know which additive caused this. If the problem cannot be corrected by a different wick size or type, you should use a substitute for the "bad" additive.

If the wick is too thick
The flame is too big in proportion to the candle diameter and will tend to smoke.

If the wick is too thin
The flame will melt more wax than it can combust (absorb). The candle bowl fills up with liquid wax which eventually will overflow and dip.

If the wick is too short
Due to impurities in the candle wax formula, the wick will clog up. The flame shrinks, the bowl fill up with liquid wax and the candle will drip under extreme conditions. – The flame may even extinguish. This is caused by impurities in the wax formula of inorganic nature: hard wax, synthetic waxes, dyes (especially pigments) and fragrance oils.

If the wick is too strongly bent
This can be caused by weak/soft paraffin (high penetration) or insufficiently treated wicks. A specially treated wick will stay upright better in weak/soft paraffin than a regularly treated wick. This is often the case with pressed candles. Regularly treated wicks used in pressed candles bend too much. It is therefore recommended that a specially treated and braided wick is used for pressed candles.

If the wick forms like a corkscrew
This is either caused by an uneven tension in the yarn bundle during the braiding process or by tow yarn bundles with a different number of threads. It may also be caused by twisted wicks strands before inserting into the candles.

If the wick is falling over
The wick is too loose in the candle. This particularly occurs when candles are pressed and the wick guide is not properly adjusted. This may also occur with moulded candles if the wicks have too much slack.

If the wick is buckled
This is due to a braiding fault, e.g. a non-braiding spot in the wick.

If the wick is too straight
This "mushrooming" is probably the most common and annoying burning problem. The wick does not bend so that the tip of the wick does not reach the outside of the flame, where the highest temperature is registered ensuring complete carbon combustion.
Causes for this can be cores such as lead, zinc, paper or impurities such as hard wax, additives, dyes, fragrances.

If the wick smokes
This type of smoking is caused by drafts. Candles should be burned in a draft free environment.

If the wick forms a residue of ashes
The formation of ashes is caused by impurities in the wax formula. The non-combustible materials concentrate at the end of the wick in the shape of thin threads and can even lead to a double wick. Special wick treatments can completely combust a limited amount of impurities.