Cored wicks, in other words wicks where a cotton outer braid surrounds an inner filament of cotton, zinc, nylon or paper, are used mainly with container and votive candles, but sometimes with pillar candles as well.
The presence of a core gives cored wicks a certain amount of rigidity, needed for the wick to remain upright and not fall over and drown in the potentially large and deep melting pool present in container candles. This quality is also appreciated when making gel candles.
Of the four aforementioned materials, zinc offers the best rigidity, followed by nylon, paper and cotton.
But those different materials also produce different levels of heat. On this subject, know that cotton produces the hottest flame, followed by paper and zinc. Nylon-cored wicks are relatively recent and not widely used; I do not have enough information about them at this time to be able to tell you how they behave in comparison to the other three core materials.
Once again, you will have to test and test again to find which type of cored wick is best suited to your particular situation.
Because they will often be used in container or votive candles, cored wicks can commonly be bought pre-coated and pre-tabbed (the wick is coated with wax and attached to a small, round piece of metal called a wick sustainer tab). Of course, you can also buy cored wicks in their original form.
Manufacturers have a tendency to give cored wicks barbaric names... but some retailers will just label them small, medium and large or specify for what type of wax and candle diameter they are best suited.
Did I have you at "barbaric name"? Let's have a look at it then. A cored wick for votive candles can be labeled 44-20-18C. Whaaaat?
The first number is an indicator of the wick size, or thickness: the higher the number, the thicker the wick.
The second number indicates the speed at which the wick went through the braiding machine. The higher the number, the faster the speed and the tighter the braid is.
If there is a third number, it usually means the wick has been wax coated and this number is a code for the type of wax used for the coating. This number varies according to the two previous number. A final letter hints at what material has been used for the core: Z for zinc, C for cotton, P for paper.
Besides paper, zinc and cotton, lead has long been used as a material for the wick core.
Everybody knows (or should know) today that lead is not exactly the most inoffensive material where our health is concerned. Experiments have shown that using lead-cored wicks can create, in the room where the candle is burned, lead levels in excess of 2,200 micrograms per hour, five times the rate that could lead to elevated levels of lead in children.
In the United States, Canada and Europe, the manufacture and the sale of lead cored wicks are strictly forbidden and they have not been manufactured or sold since the end of the nineteen nineties. But if, for some strange reason, you buy your wicks from an asian or south-american supplier, where the laws on that matter are unclear at best, beware!
A simple test will tell you if your cored wicks do contain lead: rub a piece of white paper against the wick (of course, the wick must never have been lit). If it leaves a light grey mark on the paper, the wick contains lead. Dispose of it immediatly and do not use it in your candles under any circumstances.