One thing you should know about Candle Cappers
Candle Cappers were made for jar candles, these types of jar candles have a fairly narrow neck. The main idea was hey would shield the flame from drafts, and allegedly result in “a cleaner, smooth burning candle”.
We tested them and were not impressed. Because air flow is restricted, the cappers may reduce the air flow around the wick, which is already a problem for the apothecary “Yankee Candle” style jars. It causes the candle to puff or cough which then creates black soot. The caper can also limit amount of fragrance that gets diffused; however, the reduction is generally not dramatic for most scents.
If you have a good burning candle, and you keep it away from drafts, then the Candle Capper is superfluous. We have found that wide mouthed jars with a perfectly round opening yield the best burn results.