One of the criteria for judging a Purple Clay Teapot is its airtightness. Traditionally, people believe that if you fill the Teapot with water and press the air hole on the lid, no water will come out of the spout; if you invert the teapot while pressing the spout and the lid doesn't fall off, it indicates good airtightness. If not, it suggests poor airtightness.

In fact, this judgment is a misconception. After conducting thousands of experiments, we have finally explained the principle of airtightness and combined our knowledge of physics to propose a new viewpoint regarding the airtightness of purple clay Teapots.

Let's use the Shi Piao and Xi Shi as examples, which are the most common and popular styles in the purple clay teapot market. First, here are the data from our experiments: 100 Shi Piao teapots and 100 Xi Shi teapots, all priced between 200 and 3,000 yuan (we'll exclude poorly made ones from our data). When we pressed the air hole, 97 out of 100 Xi Shi teapots did not leak from the spout. When we filled them with water and inverted them while pressing the spout, none of the lids fell off. For the Shi Piao teapots, we could not press the air hole, so we only conducted the spout-pressing experiment. The result was that all 100 Shi Piao lids fell off when the teapot was inverted after being filled with water and the spout was pressed.

This experiment led to three conclusions:

1. The lids of Shi Piao teapots almost always fall off.

2. The lids of Xi Shi teapots generally do not fall off, and the number of teapots whose lids do not fall off does not equal the number whose spouts do not leak water. In other words, there are cases where the spout still leaks water when the air hole is pressed, but the lid does not fall off when the teapot is inverted.

3. When the air hole is pressed, no water comes out of the spout of well-made Xi Shi teapots.

This experiment raises some questions. Normally, when buying a purple clay teapot, one might say, “If the lid doesn't fall off when the teapot is inverted, it means the workmanship is good; if the lid falls off, it means the workmanship is poor.” How much weight does this judgment carry? If it's a matter of workmanship, why do all Shi Piao lids fall off, while all Xi Shi lids stay on? It turns out that the essential difference between Shi Piao and Xi Shi teapots lies in the depth of the air hole on the lid. The air hole depth of Shi Piao teapots is only 2 to 3 millimeters, while that of Xi Shi teapots is 10 to 15 millimeters. Could this significant difference in air hole height explain everything? Thinking along these lines, I immediately gathered 50 teapots, including various styles such as Antique, Du Qiu, Du Zhi, Rong Tian, Shui Bian, and Han Duo. Their characteristic is that their air hole depths are all greater than 10 millimeters. The experimental results confirmed this idea – none of the lids fell off. However, the lids of Shi Piao and Man Sheng Jing Lan teapots, which use bridge-shaped spouts, all fell off because their air hole depths were less than 3 millimeters.

Now, let's use physics to explain this phenomenon, hoping that after reading this article, you will gain a new understanding of the airtightness of purple clay teapots. When a purple clay teapot is full of water and the air hole is pressed and the teapot is inverted, water molecules spread evenly across the plane between the teapot lid and rim, creating a completely sealed state. Then, the water inside the teapot flows to the lowest point of the air hole, forming a water column within it. Taking the Xi Shi teapot as an example, the water column is approximately a cylinder with a diameter of 1.5 millimeters and a length of 10 millimeters, generating two forces. The first force is due to the pressure difference: the pressure at the bottom of the air hole is greater than the pressure on the inner surface of the lid, resulting in a force caused by this pressure difference. The second force occurs because some water enters the air hole, causing the internal air pressure of the teapot to be less than the external air pressure, creating a pressure difference that generates another force. These two forces combined are enough to keep the lid from falling off. Therefore, as long as the plane between the lid and rim is absolutely flat, even if there is a slight gap, the lid can still remain in place.

On the contrary, if the air hole depth is insufficient, the force generated by the pressure difference is very small. Additionally, since less water enters the air hole, the pressure difference between the internal and external air pressures of the teapot is minimal, making the force due to the pressure difference also very small. These two forces combined are not enough to support the lid, which is why the lid of a Shi Piao teapot inevitably falls off. The phenomenon of stopping the flow of water by pressing the air hole is also interesting. I specifically used an extremely well-made Jin Nang teapot – Ju Lei, and indeed, pressing the air hole did not stop the water flow. However, when inverted according to the depth of the air hole, the lid remained firmly in place without falling off. This is because when pouring water in the forward direction, the gap between the Jin Nang lid and the rim is quite noticeable, and pressing the air hole tightly cannot seal the gap through which air can pass. Thus, the water flow does not stop. When inverted, the water fills the entire rim and covers the plane between the lid and rim, isolating it from the outside air. Simultaneously, based on the pressure difference and pressure, the lid does not fall off.

Conclusion on Airtightness of Purple Clay Teapots:

The quality of a purple clay teapot's airtightness is determined by whether the water flow stops when the air hole is pressed, indicating good airtightness if the water flow stops, and slightly inferior airtightness if it does not. As for whether the lid stays on when the teapot is inverted, it can only detect whether the plane between the lid and rim is flat. For example, if the lid falls off when a Xi Shi teapot is inverted, then the water flow cannot be stopped by pressing the air hole, as a flat rim is a basic condition for good airtightness. However, even if the rim is flat, the airtightness may not be good.

Formula for the Lid of Purple Clay Teapots:

If the water flow stops when the air hole is pressed (good airtightness), the lid will certainly not fall off when inverted (flat rim) for deep-air-hole teapots like Xi Shi.

If the lid does not fall off when inverted (flat rim), it does not necessarily mean that the water flow will stop when the air hole is pressed (good airtightness) for deep-air-hole teapots like Xi Shi.

For Shi Piao-style teapots, since the air hole cannot be pressed and the lid cannot be kept from falling off, conventional methods cannot be used to test their airtightness. However, generally speaking, ordinary round-mouthed purple clay teapots have very good airtightness. Airtightness is just one criterion for a purple clay teapot, a physical phenomenon, and not the sole attribute of a good teapot. Therefore, after understanding the essence, we should rationally treat the airtightness of purple clay teapots.

Note that the main issue is that Xi Shi lids are light, while Shi Piao lids are heavy, and the size of the air hole has little to do with this. If you hang a 20-gram weight under the Xi Shi lid, it will fall off. This is indeed a problem of atmospheric pressure. Physically, the calculation should be that the weight of the lid plus the weight of the water above the lid produces a pressure less than atmospheric pressure, meaning the lid will not fall off. If the pressure is greater than atmospheric pressure, the lid will fall off.

Three factors affect whether the lid of a purple clay teapot will fall off:

1. Airtightness: Poor airtightness definitely leads to the lid falling off, but good airtightness does not guarantee that the lid won't fall off.

2. The weight of the lid divided by the