Improper handling of highly flammable fragrance oils, including essential oils, during the addition to wax can lead to flashovers on the candle's surface. In addition, pooling of fragrance oils inside the candle can create pockets of oil that may combust during consumer use.
Not all glass is suitable for use as a candle container. Glass that is not designed to withstand high temperatures can break when exposed to the heat of a burning wick. To ensure safety, candle glass should undergo proper annealing, a process that relieves internal stresses introduced during manufacturing. The current trend is to recycle candle containers for other purposes in the home after they have been fully burned.