We often talk about the unique characteristics of purple clay teapots, such as their ability to retain fragrance when brewing Tea and lack of cooked soup aroma. While the claim that they remain fresh even in summer might be an exaggeration, purple clay teapots do indeed have a distinctive “porosity” not found in other types of Teaware.
▲ Brewing tea with a purple clay pot
The porosity of purple clay teapots comes from their unique “double-pore structure,” which refers to the sand particles and their agglomerate structure within the clay material itself.
▲ Cross-section particles
Purple clay materials used for making teapots vary in coarseness, indicated by different mesh sizes, leading many to wonder if the porosity of a purple clay teapot is related to its mesh size.
Mesh Size
The mesh size refers to the size of the purple clay particles. When we grind and sieve purple clay ore, we use a screen to determine the mesh size, with common sizes being 24, 32, 40, 60, 80, and 100 mesh.
The smaller the mesh size, the coarser the clay particles; conversely, the larger the mesh size, the finer the clay particles.
▲ Cross-section particles
The mesh size also affects the texture of the finished surface after firing. Teapots made from lower mesh purple clay will have a coarser texture and feel less smooth, with the sand particles more prominent.
Making teapots with either very coarse or very fine sand can be challenging. Commonly used mesh sizes are 40 and 60, resulting in both a fine texture and a noticeable sandiness, high production rates, and good gloss after firing and Seasoning.
Porosity
As mentioned earlier, the porosity of purple clay is primarily due to the sand particles, which is an inherent property of the raw material.
▲ Sieved and washed purple clay
When the difference in mesh size is small, its impact on porosity is minimal. For example, two types of clay with 60 and 80 mesh sizes would have similar porosities.
However, grinding the material too finely disrupts its physical structure, affecting its porosity. A comparison between 60 mesh and 180 mesh materials would show that the latter has virtually no porosity.
▲ 200 mesh slurry
The porosity of purple clay also depends on the number of sand particles in the material. Mesh size alone cannot determine the quality of porosity.
Summary
When discussing the relationship between mesh size and porosity, we can understand it this way:
The size of the mesh does not determine the quality of porosity;
A very large mesh size, close to 180 or 200 mesh, results in almost no porosity and is typically used for slip casting.
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