The compositions and pore structures of Benshanzhu Zisha ceramic

Yixing Zisha ceramics are popular traditional tea sets with a long history in China but there are seldom studies to explore their compositions and microstructures. In this work, Benshanzhu-Zisha, a typical representation of Zisha from Yixing City of China, is investigated. The XRD results demonstrate that the Benshan Zhu clay is the natural mineral compound, which contains the high hematite content. After sintering, in comparison with pottery and porcelain, Benshanzhu-Zisha has the highest hematite (11.5 wt%) and amorphous phase (71.2 wt%) and the lowest mullite (10.7 wt%). Moreover, there are many dispersed and thin pores in Benshanzhu-Zisha ranging from nanometer to micron, and there are large numbers of hematite particles in the pores. It is the unique pore structures that make Benshanzhu-Zisha be perfect brewing tea vessel, impermeable to water but breathable to gas.


Introduction
Yixing Zisha ceramic is the most representative and in uential traditional tea sets in ancient and contemporary China, especially for tea brewing [1]. They have passed through a thousand years of history and the technological process of Yixing Zisha ceramic has also been listed as one of the rst national intangible cultural heritages of China. Until now, the unique charm of Zisha has attracted the attentions of some scholars. Some reports focus on the artistry and humanity history of Zisha [2][3][4]. Furthermore, several reports point that the high porosity is the reason of its perfect tea brewing effect [5,6]. Recently, Dicaoqing-Zisha ceramic is reported by scienti cally research [7]. However, Zisha ceramic actually, is a very big family containing many members. Each member has its particular features. It is worth exploring the compositions, microstructures and the relationship of each member for Zisha ceramics.
Yixing Zisha ceramics are made from different Zisha clays. And conventionally, the clays' names are also used to name the corresponding Zisha products. Different raw clays lead to great differences in the properties of the Zisha products. Red clay Zisha is one of the three broad categories of Yixing Zisha, the two others are Purple clay Zisha and Duan clay Zisha [8]. The Zisha clays from Huanglong Mountain mining district, Yixing City, Jiangsu Province, China are collectively called the "Benshan Clay". Zhu clay belongs to Red clay. Its ores are distributed between the Red clay layer or shelf soil and tender clay ore layer, closer to the land surface. Since the Zhu clay is weathered by the in ltration of surface rainwater into the tender clay layer with different degrees, the thickness of Zhu clay ore layer is also different, the thick could be several meters, and the thin could be only a few centimeters. Yield of Red clay is less than Purple clay and Duan clay, further, Zhu clay is a rare class of Red clay. Zhu clay from Huanglong Mountain mine called Benshan Zhu clay, i.e. the material of Benshan-Zhu clay Zisha, labeled as Benshanzhu-Zisha.
In this work, we focus on the compositions and microstructures of Benshanzhu-Zisha. The mechanisms about the formation of the compositions and pore structures of Benshanzhu-Zisha are demonstrated by relating to the minerals of Benshanzhu clay. First, the Benshanzhu clay is the natural mineral compound which contains illite of 24.1 wt%, kaolinite of 15.7 wt%, montmorillonite of 9.2 wt%, hematite of 9.3 wt% and goethite of 10.1 wt%. Second, comparing with porcelain and pottery, the Benshanzhu-Zisha has the highest contents of hematite (11.5 wt%) and amorphous phase (71.2 wt%) due to its low content of Al 2 O 3 but extremely high content of Fe 2 O 3 of its clay. And the content of mullite in Benshanzhu-Zisha (10.7 wt%) is much lower than porcelain and pottery. What's more, there are many dispersed and thin pores on the surface of Benshanzhu-Zisha, which have a wide pore size distribution ranging from micro to nano scale. Moreover, there are large numbers of hematite particles in the pores. Benshanzhu-Zisha has unique pore structures make the Zisha impermeable to water but breathable to gas, i.e. a perfect tea vessel.

Materials
Benshanzhu raw material was obtained from Dingshu town, Yixing city, Jiangsu province, China. The formula porcelain material in this work was purchased from Jingdezhen, China. As far as the pottery, it is an earthen pot was purchased from a commodity market.

Samples preparation
Before the Benshanzhu raw material becoming green body, it was processed by selecting, grinding sifting, and then aged by natural weathering for a few months. And then the aged materials were processed by continuously beating and crafting to green body. The sintering process of Benshanzhu-Zisha is as follows: Benshanzhu green bodies were dried in a vacuum oven at 80°C for 24 h. Then the dried samples were red in a Mu e Furnace, they were sintered from room temperature to 300°C with a rate of 3°C/min and held for 30 min, and then the temperature was raised from 300°C to 700°C with a rate of 7°C/min and held for 30 min.
Finally, the samples were sintered from 700°C to 1180°C with a rate of 5°C/min and held for 30 min.
The green body of porcelain, cut from the commercial formula porcelain material, was rst red from room temperature to 100°C at a rate of 2°C/min and held for 30 min, then the temperature was increased to 200°C at a rate of 2°C/min and then increased to 300°C at a rate of 2°C/min, both the two temperatures were held for 30 min. After that, the temperature was raised from 300°C to 600°C at a rate of 8°C/min and then raised to 1000°C at the same rate; both of the two temperatures were held for 30 min. Finally, the porcelain sample was red from 1000°C to 1320°C by 8°C/min and held for 30 minutes.

Characterization
The crystalline phase composition of samples were tested by using X-ray diffractometer with Cu Kα radiation

Results And Discussion
The whole body of teapot which is made of Benshanzhu clay has a bright red color and the Benshanzhu raw material is a yellow mineral (Fig. 1). We characterized the mineral phases of green body of Benshanzhu-Zisha by X-ray diffraction. As illustrated in Fig. 2 takes the second place, with minimum percentage of montmorillonite (9.2 wt%) in the clay minerals. Moreover, the green body has much amount of hematite (9.3 wt%) and goethite (10.1 wt%).  Figure 3 is the TG-DSC analysis of Benshanzhu-Zisha green body, and corresponding differential results (DTG and DDSC curves). The TG analysis of the Benshanzhu-Zisha green body shows an overall weight loss of 9%.
Benshanzhu-Zisha green body has two clear endothermic valleys at about 245℃ and 450℃, accompanied by obvious continuous mass loss, which results from the features of illite and montmorillonite [9]. The removal of absorbed water in clay minerals leads to the endothermic valley at 245℃, while the removal of structural hydroxyl groups leads to the endothermic valley at 450℃ [10][11][12]. There is an exothermic peak at 1043℃, which is ascribable to the new formation of crystalline phases during ring [13][14][15]. This result should be resulted from its low total mass percentages of clay minerals, which, further, consist of high content of illite but low content of kaolinite. The total contents of alkali metal oxide and alkali earth metal oxides (K 2 O + Na 2 O + MgO + CaO) of Benshanzhu-Zisha green body (3.35 wt%) are also lower than that in porcelain (4.83 wt%) and pottery (7.35 wt%). But it is worth noting that Benshanzhu-Zisha green body has much higher content of Fe 2 O 3 than others thanks to the rich hematite and goethite. The XRD patterns and quantitative analysis results of the Rietveld re nement for red porcelain, Benshangzhu-Zisha and pottery are also investigated (Table 3 and Fig. 4). The pottery is comprised of amorphous phase (52.9 wt%), mullite (22.0 wt%), quartz (16.6 wt%), anorthite (7.6 wt%) and few cristobalite (0.5 wt%) and rutile (0.4 wt%). Pottery is usually red at a low temperature (below 1000℃). Therefore, its reactions during ring are not su cient. Hence, we mainly compared the chemical compositions and crystalline phases of Benhsanzhu-Zisha with that of porcelain. As shown in Table 3, the content of mullite in Benshanzhu-Zisha (10.7 wt%) is much lower than porcelain (24.7 wt%) and pottery (22.0 wt%), but Benshanzhu-Zisha has the highest contents of hematite (11.5 wt%) and amorphous phase (71.2 wt%) among the three ceramics. The low content of mullite of Benshanzhu-Zisha can be ascribed to the insu cient Al 2 O 3 .
And the signi cantly high content of hematite and amorphous phase should be contributed to the very rich  As shown in Fig. 5, the microstructures and pore size distributions of pottery, Benshanzhu-Zisha and porcelain were also tested. The microstructure of pottery maintains plenty of large and irregular pores with sharp corners, and the pore size distribution of pottery is mostly concentrated on 1 µm. Benshanzhu-Zisha contains many dispersed and thin pores; it shows a wide pore size distribution ranging from micro to nano scale. The pores of porcelain are large and round in shape. The pores in porcelain mainly focus on around 0.2 µm and dozens of microns, and the distribution is relatively sparse.  Table 4 shows the porosity characterizations of porcelain, Benshanzhu-Zisha, and pottery. The porosity characterizations are tested via both mercury porosimetry and BET measurement. When measured by mercury porosimetry, the Benshanzhu-Zisha mercury has the lowest porosity, pore volume and speci c surface area compared with porcelain and pottery, however there is a large improvement of the data when measured by BET. The pore volume and speci c surface area of Benshanzhu-Zisha is still lower than pottery, but higher than porcelain. It is known that the BET measurement depends on the nitrogen absorption and desorption in the pore while the mercury porosimetry depends on the lling of mercury into the pore. Compared with porcelain, it's di cult for mercury to ll into the thin pores of Benshanzhu-Zisha, while gas lling is not affected. No matter which method is chosen, the pottery shows better pore volume and speci c surface area than porcelain and Benshanzhu-Zisha, which could be attributed to its plenty of large pores.
Moreover, more speci c microstructures of Benshanzhu-Zisha were investigated. The morphologies of two typical pores could be seen in Figs. 6(a) and 6(b). We analyzed the EDS results of 5 spots of the two typical pores to further discover their features, spots 1-3 for pore 1 in Fig. 6 (a) and spots 4 and 5 for the pore 2 in Fig. 6  can narrow the pore and form an uneven inner wall, which is di cult to be lled by mercury but permits the gas passing through. This result is accord with the porosity characters of Benshanzhu-Zisha shown in Table 4, which also account for the good breathability but no water seepage of Zisha ceramics.

Conclusions
The   The EDS results of 5 spots in two typical pores for Benshanzhu-Zisha.