What is the black substance on the wick?

The black substance on a wick is typically carbon buildup from incomplete combustion of the wick material or the fuel it burns. This residue can accumulate over time if the wick is not trimmed properly or if the fuel quality is poor. In candles, this can lead to soot production and reduced burning efficiency. Regular maintenance, such as trimming the wick and using high-quality fuel, can help minimize this buildup.