What is a plumbers candle?

Many years ago, plumbers used the plumbers candle to join pipes together. Wax from the burning candle would be dripped onto a "wiping cloth" to form a protective coating on the cloth. Molten lead would then be poured onto the cloth and wiped onto the joint of the pipes to seal the joint. The candle wax prevented the lead from sticking to the cloth. This process of joining pipes together was called "lead wiping."
It is likely that plumbers needed a candle that would give off a lot of light while they worked in dark confined spaces. They may have asked candlemakers to create a candle that provided more light than a normal taper candle. So, another theory for the origin of the plumbers candle is that the plumbers candle was made specifically to provide light for plumbers as they worked. In the days before paraffin wax, tallow (animal fat or oil) was used to make candles. Plumbers candles used to be made from spermaceti, the oil from the head of the sperm whale. Spermaceti candles were renowned as the brightest burning candles of their times. The plumbers candle was made a bit thicker with a larger wick, which made it burn even brighter. Due to overhunting, the sperm whale become an endangered species, and use of the spermaceti became illegal. Paraffin wax quickly became a replacement for spermaceti and burns almost as brightly.
ALSO: Plumbers' candles contain more stearic acid, which means they burn longer than cheap ones which contain less.