The nature and composition of glaze\x0d\\x0d\When we see a piece of ceramics, the first thing that attracts our attention is not so much its shape, style or body, but rather the covering. Glaze on ceramic surfaces. If there is no glaze on the porcelain, no matter how beautiful the shape or new the style, the charm of the porcelain will be lost. Although we boast that ceramics is an art of fire and produces various changes due to the action of fire, the main thing is that the glaze changes in the fire. \x0d\\x0d\Glaze and blank are both made of rocks or soil. The only difference between glaze and blank is that it is easier to melt in fire. When the power of the fire in the kiln makes the blank semi-melted, the glaze raw materials must be completely melted into a liquid state. When cooled, the liquid solidifies into a glass, which is the glaze. \x0d\\x0d\However, once the glaze is applied to the surface of the ceramic body, it is very different from glass. Because when the glaze melts, it interacts with the body of the product to form an intermediate layer, which gradually transforms from the sintered body to the glassy exterior part of the glaze. Although the thickness of the glaze layer is usually only 1~3% of the thickness of the body, it will strongly change the thermal stability, dielectric strength and chemical stability of the product, as well as many other properties. \x0d\\x0d\There are two ways to make glaze: one is to mix the soil or rock as it is. Another method is to mix earth or rock and melt it with fire, then cool it suddenly to make glass, which is called "frit". The glaze made in this way should be broken into fine powder and mixed with water to turn it into a sticky juice for hanging the base. If the slurry is not viscous enough to adhere easily to the base, dextrin, glycerol or other sticky organic substances, such as kelp paste, can be mixed into the slurry. Some bodies are glazed immediately after drying in the open air, but others are pre-calcinated at a low temperature of 800 to 900°C, which is the so-called biscuit firing and then glazed. \x0d\\x0d\The former method is called "green glaze", which is widely used in our country. Foreign porcelain generally uses the so-called "bisque firing" method. Therefore, to identify a piece of porcelain, first look at whether it is raw or plain-fired, and then you can roughly know whether it is made in China or abroad. However, this can only be used as a more convenient clue, and of course it is well known to people who are proficient in ceramics. Some Japanese imitations of ancient Chinese porcelain are deliberately made to make people think they are raw porcelain at a glance. If you make a decision based solely on this, you will often be easily fooled. \x0d\\x0d\When hanging the glaze, if it is a biscuit-fired blank, it is usually put into the prepared glaze juice quickly and taken out immediately, so that the glaze will be absorbed on the surface of the blank, and it can be used with a brush or pen. Apply evenly. If it's a tea bowl, you have to hold the bowl's legs and quickly put it into the glaze, and hurriedly move it up and down two or three times, which is called the "glaze dipping method." If it is a huge object, you can use a ladle or something like a ladle to hold the glaze and hang it from the surroundings. This is generally called the "glaze pouring method" or the "glaze sliding method". \x0d\\x0d\When the green body is glazed, if it is glazed inside and outside at once, it will easily damage the body. Therefore, the glaze should be injected inside and rotated. After drying, the outside can be dipped or hung again. Brushes and pens are also used for hanging. This method is mostly used for colored glazes. For example, there are often brush marks on the feet of blue and white vessels in the early Ming Dynasty. It can be seen that this method has been used before. In addition, although there is a "spray hanging" method, it is mainly used for extremely large or extremely thin objects. For example, there is no other way to hang the glaze on the thin porcelain that seems to be see-through in the so-called "bodiless ware". This method is: first spray glaze on the inside, then thin the outer body after drying, and then spray glaze on the outside. For example, the works of the famous Jiaotan Kiln in the Song Dynasty have thin bases and thick glazes. The thickness of the glazes is even about one to three times that of the bases. If you look closely at the fragments of these works, it is obvious that they are hung with two to three layers of glaze, so this type of porcelain probably used the spray-hanging method. Another example is the peach blossom red in the Kangxi era, which is different from the Langyao red. According to the letter written by the famous Dante Kerr who lived in Jingdezhen at that time, it was also made by the "spray glaze method". \x0d\\x0d\Most of the porcelain in Jingdezhen has its base cut off after the glaze is applied. In contrast, Japanese porcelain is glazed after it is fully formed.
Therefore, if you look closely at the boundary between the outer glaze and the base of Jiangxi porcelain, you will see that the glaze ends with a sharp tangent line, which is very different from Japanese products. Most of the ancient porcelain products of the Ming Dynasty have traces of shaved feet. However, except for the inferior products made in the Qing Dynasty, most of the shaved feet were wiped with a brush or cloth, so the soles of the feet always have a soft and smooth feeling. There are some Qing Dynasty porcelains that are very similar to the Ming Dynasty products, but I have never seen any imitation of this kind of foot-cutting habit. Although it is sometimes difficult to judge with the naked eye, it can be roughly understood if you use a magnifying glass. When we identify Ming porcelain and Qing porcelain, although we can generally look at their shapes, glaze colors, patterns and other aspects, we sometimes get confused and don't know what they are. At this time, if you can pay attention to this habit of cutting off the feet, it will become a powerful clue for identification. \x0d\\x0d\When a glazed ceramic body is fired in a kiln, the moisture and other volatiles contained in the body and the kiln need to be lost and begin to shrink. At the same time, the body is heated and undergoes thermal expansion. When it reaches a certain temperature, part of the components in the body begin to melt and generate liquid to fill the pores in the body and shrink again. The glaze also undergoes thermal expansion and contraction. When the shrinkage of the glaze is greater than the shrinkage of the base, cracks will appear on the glaze. When the shrinkage of the glaze is smaller than the shrinkage of the base, "deglazing" is likely to occur. Some volatile compounds begin to volatilize only when the temperature is relatively high. In order to prevent gas from escaping and generating bubbles after the glaze melts, the temperature of the kiln should not be increased rapidly before the glaze begins to melt, but should be calcined slowly to wait for the gas to escape. After the gas is exhausted, the temperature is raised again until the glaze is completely melted. If the temperature rises too quickly at this time, green bubbles or glaze bubbles will easily occur. \x0d\\x0d\It is actually not an easy task to choose a glaze that matches the product body very well. Such as the Soviet Union E.N. Academician Orlov (Op Ob) once pointed out that the main difficulties in selecting glaze are as follows: \x0d\\x0d\ (1) The fusibility of the glaze causes it to flow and be absorbed by the body (if the body is in When the glaze still has large porosity during melting). It is this effect that causes firing to be carried out at a melting temperature far exceeding that of the glaze. \x0d\\x0d\(2) If the selected glaze is refractory, it will not form a smooth surface with gloss, but will be covered with small protrusions and depressions, making the glaze dull. If the firing temperature is lower than the requirement, the same result will be obtained even if the glaze is correctly prepared; \x0d\\x0d\ (3) If the glaze has a much larger expansion coefficient than the body, then when cooled , it is compressed more fiercely than the green body, resulting in huge internal stress. This internal stress will cause cracks to form on the glaze layer (such as when it is a thin tire product), or even damage the green body; \x0d\ \x0d\ (4) If the expansion system of the glaze is much smaller than the body, the opposite phenomenon will occur, that is, when cooling, the glaze shrinks slower than the body and peeling occurs. Therefore, when selecting a glaze, one must strive to make the expansion coefficients of the glaze and the body consistent as much as possible, and the melting temperature of the glaze must be compatible with the firing temperature of the body. \x0d\\x0d\If we confuse snow with sugar, in this case, no matter how fine it is, sugar is still sugar and snow is still snow. There is an obvious difference between the two. However, if the snow turns into water by heating and the sugar melts in it, it will become a new substance that is neither snow nor sugar. The glaze of the ceramic happens to be similar to this example. That is to say, no matter how hard you crush the rock into fine powder and mix it, there will be no change. If heated, the ore and rock powder will melt into a whole. This is glass, which is used to hang on ceramic blanks. It is specially named "glaze". However, when snow and sugar are mixed and melted, only the shape changes, while when rocks are mixed and melted, not only the shape but also the properties are completely changed. \x0d\\x0d\When making glaze, silicic acid is the main body of the glaze, and the salt base is used as the flux. The main salt bases are: sodium oxide, potassium oxide, calcium oxide, magnesium oxide, and aluminum oxide. Aluminum oxide is neutral. Depending on the situation, it sometimes acts as an acid and sometimes acts as a base.
\x0d\\x0d\The colorants of glaze include iron, copper, cobalt, manganese, gold, antimony and other metals. In addition, there are also those containing calcium phosphate [Ca3(PO4)2], but this raw material only needs a small amount of If some of it is in the glaze, the luster of the glaze will be somewhat lost. This is mixed into the plant ash and enters the glaze. Plants such as ferns contain a lot of calcium phosphate. What Jingdezhen uses is the ash of a fern plant called "Fernweed". \x0d\\x0d\ Ash contains a large amount of silicic acid (SiO2), so the use of ash will inevitably contain silicic acid. Among the ash, especially the ash from gramineous plants contains more silicic acid. When silicic acid is needed, rice husk ash or plant ash is specially used. \x0d\\x0d\Calcium oxide is a flux in the glaze, which can lower the melting temperature of the glaze and make it vitrified at a lower temperature. The calcium oxide required in the glaze is now mainly made of limestone (CaCO3). In the past, plants or rice husks were mixed with lime and calcined into ash, commonly known as "glaze ash". This ash contains about 30 to 50% calcium oxide. According to the analysis results of ancient ceramics made by some scientific workers in our country, lime glaze has been made as early as the Shang Dynasty in the 16th to 11th centuries BC. The calcium oxide content is mostly between 16% and 20%. After the Southern Song Dynasty, lime-alkali glaze was used instead. \x0d\\x0d\The content of sodium and potassium in rocks and soil is small, so they can also be obtained by using ash. When plant ash is dissolved in water, it becomes gray water. If it is boiled, a white powder will be obtained, which contains a large amount of potassium oxide. For example, when an aqueous solution of seaweed ash is boiled, sodium oxide can be obtained. Nowadays, the main source of potassium and sodium required in the raw materials of ceramic glaze is the use of feldspar. \x0d\\x0d\Some magnesium oxide is obtained from siderite (magnesium carbonate) and dolomite (calcium carbonate, magnesium), and some use talc (magnesium silicate). Because our country's limestone often contains a large amount of magnesium carbonate, when lime is used in our country's ceramics, magnesium will naturally be added. \x0d\\x0d\Lead and zinc used to be naturally produced lead carbonate (cerusite) and zinc silicate (hemimorphite), but now pure lead oxide or zinc oxide is often used to match the glaze. \x0d\\x0d\As for silica and alumina, because they are the main components of soil and rocks, they naturally contain a lot when using these raw materials. Unless silicon oxide is particularly needed, pure silicon-containing quartz is used. The famous Ru kiln in the Song Dynasty of my country, according to some records, used agate. In fact, agate is also a kind of pure silica, which has a slightly different structure from quartz. Although some people can't help but doubt whether this precious product has been deliberately used, but if you think about it carefully, not all the agate produced can be used for decoration, and the remaining useless parts can still be used as glaze. For example, now Jingdezhen Some use agate to prepare color glazes. \x0d\\x0d\The plant ash or grain ash mentioned above contains a large amount of silica, and also contains a small amount of alumina, calcium oxide, sodium, potassium, etc., and can be made by mixing it with lime and adding an appropriate amount of china clay. glaze. Some ancient Chinese ceramics were glazed by the natural fall of kiln dust deposited on the body; some were deliberately hung with more ash to make them naturally glaze; in addition, some were hung with mortar to form glazes. Especially when I found out later. Using only ash is not only easy for the glaze to flow, but also causes wrinkles or spots, or is dull due to calcium crystallization. The result is very boring. Gradually, it was noticed that when materials containing a large amount of silicic acid, such as quartz or feldspar, were mixed, there would be no such disadvantages, so the feldspar glaze was invented. For example, the Xing Kiln and Yue Kiln of the Tang Dynasty, the Ding Kiln and Yingqing Kiln of the Song Dynasty, the Ru Kiln and the Longquan Kiln of the Ming Dynasty, and the Jingdezhen Kiln porcelain of the Ming Dynasty were all gradually developed from this. It has given great influence to mankind in the East and even the entire world. There is no denying the benefits