- Won't catch fire
- Won't leak
- Won't crack or break
Won't Catch Fire
This seems like a pretty obvious rule to follow, but I've seen it happen. Making a candle out of a birch bark bowl, or a coconut shell, or a plastic margarita glass, or anything you can't hold an open flame to is a definite no-no. Another widely reported example of this happening (though I have never had this told to me first hand) is in using flower pots or other porous ceramic containers for candles. The terra cotta, or other porous material, can soak up the wax and become, in effect, a giant wick, causing an enormous flame to rise up along the rim of the flower pot. Is this physically possible? Yes. Is it likely? No. The wax absorbed into the pot would have to get incredibly hot to burst into flames. Nonetheless, it is highly recommended that you coat any flower pots or porous ceramic containers with two heavy coats of Mod-Podge before you make the candle. And again, no matter how pretty it looks, or how sure you are that the flame won't even get close to the edge of the container - don't ever make a candle in a container that can burn.
A leaking container is a problem in two ways. First and foremost, you don't want hot melted wax spilling out onto your counter, coffee table, mantle, shelf or nightstand. A big mess! But secondly, depending on how quickly or how suddenly the wax leaks out, a leak could cause a fire hazard. If the melted wax leaks out of the candle as quickly as its melted, your wick is going to burn higher than it should. You could have a big flame in just a few minutes. The most common leaky containers I see are metal tins that have a seam along the side or bottom. The best way to test a tin to see if it will hold hot melted wax is to fill it with water and let it sit a day or two.
This is perhaps the most common container candle problem, and the one that can cause the biggest problems. Similar to a leaking container, a cracked container is going to spill hot wax all over underneath itself. Also like the leaking container, the wax soaked wick is going to be suddenly left with no pool surrounding it, and the flame can just as suddenly grow to be several inches high - high enough to catch the nearby drapes or cabinet or plant on fire. There are stories and urban legends about exploding gel candles that basically were caused by the gel getting too hot and cracking the fish bowl, martini glass, or other unsafe glassware it was made in.
What to Look for in a Container
So, in addition to being fire, leak and crack resistant, look for containers that are heat resistant. Even if it burns completely properly, the wax pool and flame get hot as it burns. This is why jelly jars have been so popular over the years; They are designed to withstand high heat, and hold wax very reliably. Other glassware is perfectly fine too, as long as it's thick enough to withstand the heat. Ceramic bowls, properly treated flower pots, and metal tins are wonderful...and are often more heat resistant than clear glass. Be sure to take into account the shape of the container as it will burn. If it has a wide mouth at the top and a narrow mouth at the bottom, it's going to get hotter and hotter as it burns and may be prone to cracking.
The important thing to remember is that any container, even one designed to be heat resistant, can crack. The most important safety consideration for container candles - or any candle for that matter - is proper supervision. Be sure that the candle is on a heat-resistant surface, away from anything flammable, and never leave a burning candle unattended.