Why did the ancients cut candles? The "wick effect" behind romance

The line "When will we cut the candle in the west window together and talk about the night rain in Bashan" in Li Shangyin's "Night Rain Sent to the North" makes people sigh at the delicate emotional description of candles by ancient poets. Candles are not only a lighting tool, but also a symbol of warmth and longing. In the eyes of the ancients, candles are weak and lasting flames that warm the lonely soul. But why did Li Shangyin specifically mention "cutting candles"? Why do candles need to be cut? This question not only reflects on the poetic meaning, but also triggers thinking about the principle of candle combustion.

01Candle burning and wick effect

The burning of a candle is a complex physical process, and the principle behind it is what we call the "wick effect" today. The candle attracts wax through the wick and releases heat through combustion. This process can be simply explained as capillary action: the wax continues to rise through the capillary action of the wick, and when it reaches the flame, the wax evaporates and burns, forming a stable flame.

However, the wick is not just a simple channel, it plays a vital role in the burning process of the candle. The existence of the wick can prevent too much heat from being lost through the bottom of the candle, maintaining the stability of the burning. If the wick is too long or not suitable for burning, it may cause the flame to be unstable, produce a lot of smoke, or even fail to burn completely, thus affecting the effect of the candle.

Zhen Huan is cutting candles in the TV series "The Legend of Zhen Huan" (Image source: Screenshot of the TV series video)

02Why do we need to cut the candles?

When using candles, we often see the flame jump or the flame grows larger. At this time, shortening the wick becomes a necessary means to maintain stable burning. This is not only to prevent the flame from jumping, but also to prevent the candle from burning incompletely, thereby reducing smoke and wasting wax. Especially when the wick is too long, the fuel supply is too much, the flame tends to grow larger, resulting in the flame outside the wick no longer contacting the fuel, causing waste and unstable flame.

Trimming a candle is not only about controlling the size of the flame, but also about ensuring that the heat is released evenly for a more efficient burn. When the flame is stable, the heat from the wick is more evenly distributed, and the flame stays at a comfortable size without jumping or producing excessive smoke.

When copying Tang Dynasty murals, please note that the wax candle wicks of the ancients are different from those of today.

03Wick Effect and Fuel Characteristics

The reason behind the wick effect is actually closely related to the fuel characteristics of the candle. The fuel of traditional candles is usually beeswax or paraffin, which is solid at room temperature, but melts into liquid under the high temperature of the flame and is transported to the combustion area through the wick. The candle fuel is like a special substance that solidifies at low temperatures and melts at high temperatures, which makes it less likely to produce excessive smoke or volatile substances during the combustion process.

However, modern candles use synthetic wax, which has a lower melting point, burns cleaner and more stably, and is less likely to produce odor or pollute the air, which is one of the reasons why candles are now widely used in homes, decorations, and as emotional sustenance.

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04Modern candles: no more candle snuffing

Since the French invented the three-cotton wick in 1820, candle technology has made great progress. Most modern candles use a wick containing metal wire, which can automatically bend when burning, ensuring that the flame no longer jumps and keeps the flame in the best burning zone without manual shortening.

The wick design of modern candles no longer requires frequent trimming. Instead, this special design ensures a more stable burning process, extending the use time of the candle while reducing smoke and heat waste.

05Candle Wick and Fire Fighting

From candles in daily life to fire handling in firefighting, the principle of candle wicks can also provide some inspiration. In a fire, porous media such as human fat and cotton clothes can play a "candle wick effect", making the fire more difficult to extinguish. The candle wick principle reminds us that whether it is a small candle in daily life or the behavior of flames in a fire, there are complex thermodynamic and physical processes at work behind it.

Conclusion: The meaning and science of candles

Li Shangyin's poems give people more emotional resonance with candles, and scientific analysis allows us to have a deeper understanding of the mechanism of candle burning. From the burning principle of candles to the wick effect, candles, an ancient lighting tool, have occupied an indispensable position in history and modern life. Every action of cutting a candle is not only the control of the flame, but also the attention and perception of the details of life.

Although candles are tiny, their combustion principle is a true reflection of the complex thermodynamics and physical laws of nature. Understanding these phenomena can not only help us use candles better, but also allow us to experience the wonderful beauty of the interweaving of nature and humanity in our daily lives.

Author: Ma Tingguang, Associate Professor, School of Urban Construction and Safety Engineering, Shanghai University of Technology

Produced by: Science Popularization China