Why Does a Candle Get Smaller as it Burns?

Do you ever find yourself wondering where the wax actually goes as your candle burns lower? It's a solid, right? We obviously know it melts, but what happens to it after that?

Candle-Making is a Science

Part of what I love so much about making candles is the science behind them. Despite what many might think, it isn't as simple as melting some wax, throwing in some fragrance, and pouring it into a vessel. In truth, producing a safe and effective candle is a very precise art that relies very much on math and chemistry. Accurate measurement and proper timing are essential for the final product to burn properly.

So, why is this? Doesn't the flame just use the wick? 

How a Candle Works

Even though the fire is on the wick, this isn't what your candle is using to keep its flame going. There is actually a really cool multi-step chemical process that happens to accomplish that task. This is how it breaks down:

1. Begin: You light your candle, providing the initial fire
2. Solid to Liquid: Heat from the flame radiates down onto the wax, creating a melted wax pool
3. Capillary Movement: The absorbent wick pulls the melted wax into small spaces between its strings, causing the liquid to travel up toward the flame
4. Liquid to Gas: The melted wax is heated further by the flame, transforming into a gas
5. Chemical Reaction: Combustion occurs as the hot wax vapor reacts with oxygen in the surrounding air, releasing the fragrance and fueling the fire

The Importance of Candle Wick Sizing and Care

Remember the capillary action we talked about in the burning process? If a wick is too large for the vessel, it will pull too much fuel into the fire creating an unstable and potentially dangerous flame and increase the chance that your vessel may shatter (if it is glass). This is easily identified by the height of your melted wax pool. 

Conversely, if your wick is too small, your melt pool will never reach the edges of your vessel after a 4-hour burn period and your flame might extinguish because there is simply not enough fuel being pulled up into the fire.

Candle care on the consumer's end is equally important. When you do not trim your wick before each burn, the initial flame will be taller and thus radiate more heat, use more wax for fuel, and create an unstable flame.  This reduces the life of your candle and increases the risk of flame transfer to things we don't want catching fire.

Why Candle Testing Matters

We actually use two different wick sizes for our candle tins depending on the fragrance blends used for that particular product.  Even though the vessels hold the same volume and are made of the same material, not all oils burn the same.

I like to do what's called "power burning" to test our candles. This means I do exactly what I tell all of you never to do: I let them burn for at least 12 hours before extinguishing the flame. 

By observing the flame and the melt pool, I can determine two important things: 1) If the wick is the correct size for the scent, and 2) How the vessel will handle the amount of heat produced if a customer were to forget they lit a candle several hours earlier or to trim the wick before lighting. We offer comfort and relaxation here, not house fires.

Why a Candle Gets Smaller Over Time

The main takeaway I want you to understand here is that your candle gets smaller throughout its use because the wax is what is burning. The process isn't as simple as lighting a wick and the cotton fuels the flame. If the wick were the only fuel, the candle wouldn't release any fragrance into the air and its fire would extinguish once the wick reached the melted wax. 

I hope this piece has answered your questions and helps you enjoy your candles even more knowing now just how cool and complex they really are.