In the process of debugging and using glazes, in addition to meeting specific decorative effects and performance indicators, they must also meet the most basic process requirements. We list and discuss the two most common problems in the process of using glazes.
1. The performance of glaze slurry is not good
Because the production of the ceramic factory is continuous, if there is a problem with the performance of the glaze slurry, various defects will appear in the process of glazing, which will directly affect the excellent rate of the manufacturer’s products. Important and the most basic performance. Let’s take the performance requirements of the bell jar glaze on the glaze slurry as an example. A good glaze slurry should have: good fluidity, no thixotropy, no precipitation, no bubbles in the glaze slurry, suitable moisture retention, and a certain strength when dry, etc. Process performance. Then let’s analyze the factors that affect the performance of the glaze slurry.
1) Water quality
The hardness and pH of water will affect the performance of glaze slurry. Generally, the influence of water quality is regional. Tap water in a certain area is generally relatively stable after treatment, but groundwater is generally unstable due to factors such as soluble salt content in rock layers and pollution. Stability, so the manufacturer’s ball mill glaze slurry is best to use tap water, which will be relatively stable.
2) Soluble salt content in raw materials
Generally, the precipitation of alkali metal and alkaline earth metal ions in water will affect the pH and potential balance in the glaze slurry. Therefore, in the selection of mineral raw materials, we try to use materials that have been processed by flotation, water washing, and water milling. It will be less, and the content of soluble salt in raw materials is also related to the overall formation of ore veins and the degree of weathering. Different mines have different soluble salt content. A simple method is to add water in a certain proportion and test the flow rate of glaze slurry after ball milling. , We try to use less or no raw materials with relatively poor flow rate.
3) Sodium carboxymethyl cellulose and sodium tripolyphosphate
The suspending agent used in our architectural ceramic glaze is sodium carboxymethylcellulose, generally referred to as CMC, the molecular chain length of CMC directly affects its viscosity in the glaze slurry, if the molecular chain is too long, the viscosity is good, but in the glaze slurry Bubbles are easy to appear in the medium and it is difficult to discharge. If the molecular chain is too short, the viscosity is limited and the bonding effect cannot be achieved, and the glaze slurry is easy to deteriorate after being placed for a period of time. Therefore, most of the cellulose used in our factories is medium and low viscosity cellulose. . The quality of sodium tripolyphosphate is directly related to the cost. At present, many products on the market are seriously adulterated, resulting in a sharp drop in degumming performance. Therefore, it is generally necessary to choose regular manufacturers to buy, otherwise the loss outweighs the gain!
4) Foreign impurities
Generally, some oil pollution and chemical flotation agents are inevitably brought in during the mining and processing of raw materials. Moreover, many artificial muds currently use some organic additives with relatively large molecular chains. Oil pollution directly causes concave glaze defects on the glaze surface. Flotation agents will affect the acid-base balance and affect the fluidity of the glaze slurry. Artificial mud additives generally have large molecular chains and are prone to bubbles.
5) Organic matter in raw materials
Mineral raw materials are inevitably brought into organic matter due to half-life, differentiation and other factors. Some of these organic matters are relatively difficult to dissolve in water, and sometimes there will be air bubbles, sieving and blocking.
2. The base glaze is not well matched:
The matching of body and glaze can be discussed from three aspects: matching of firing exhaust range, drying and firing shrinkage matching, and expansion coefficient matching. Let’s analyze them one by one:
1) Firing exhaust interval matching
During the heating process of the body and the glaze, a series of physical and chemical changes will occur with the increase of temperature, such as: adsorption of water, discharge of crystal water, oxidative decomposition of organic matter and decomposition of inorganic minerals, etc., specific reactions and decomposition The temperature has been experimented by senior scholars, and it is copied as follows for reference ① Room temperature -100 degrees Celsius, adsorbed water volatilizes;
② 200-118 degrees Celsius water evaporation between compartments ③ 350-650 degrees Celsius burn off organic matter, sulfate and sulfide decomposition ④ 450-650 degrees Celsius crystal recombination, crystal water removal ⑤ 573 degrees Celsius quartz conversion, volume change ⑥ 800-950 degrees Celsius calcite, dolomite decomposition, gas Exclude ⑦ 700 degrees Celsius to form new silicate and complex silicate phases.
The above corresponding decomposition temperature can only be used as a reference in actual production, because the grade of our raw materials is getting lower and lower, and, in order to reduce production costs, the kiln firing cycle is getting shorter and shorter. Therefore, for ceramic tiles, the corresponding decomposition reaction temperature will also be delayed in response to fast burning, and even concentrated exhaust in the high temperature zone will cause various defects. To cook dumplings, in order to make them cook quickly, we must work hard on the skin and stuffing, make the skin thinner, make less stuffing or get some stuffing that is easy to cook, etc. The same is true for ceramic tiles. Burning, body thinning, glaze firing range widening and so on. The relationship between body and glaze is the same as girls’ makeup. Those who have seen girls’ makeup should not be difficult to understand why there are bottom glazes and top glazes on the body. The fundamental purpose of makeup is not to hide ugliness and Beautify it! But if you accidentally sweat a little, your face will be stained, and you may be allergic. The same is true for ceramic tiles. They were originally burnt well, but pinholes appeared accidentally, so why do cosmetics pay attention to breathability and choose according to different skin types? Different cosmetics, in fact, our glazes are the same, for different bodies, we also have different glazes to adapt to them, ceramic tiles fired once, I mentioned in the previous article: It will be better to use more raw materials if the air is late and introduce bivalent alkaline earth metals with carbonate. If the green body is exhausted earlier, use more frits or introduce divalent alkaline earth metals with materials with less ignition loss. The principle of exhausting is: the exhausting temperature of the green body is generally lower than that of the glaze, so that the glazed surface is of course beautiful after the gas below is discharged, but it is difficult to achieve in actual production, and the softening point of the glaze must be properly moved back to facilitate Body exhaust.
2) Drying and firing shrinkage matching
Everyone wears clothes, and they must be relatively comfortable, or if there is a slight carelessness, the seams will be opened, and the glaze on the body is just like the clothes we wear, and it must fit well! Therefore, the drying shrinkage of the glaze should also match the green body, and it should not be too large or too small, otherwise cracks will appear during drying, and the finished brick will have defects. Of course, based on the experience and technical level of the current glaze workers It is said that this is not a difficult problem anymore, and the general debuggers are also very good at grasping the clay, so the above situation does not appear often, unless the above problems occur in some factories with extremely harsh production conditions.
3) Expansion coefficient matching
Generally, the expansion coefficient of the green body is slightly larger than that of the glaze, and the glaze is subjected to compressive stress after firing on the green body, so that the thermal stability of the glaze is better and it is not easy to crack. This is also the theory we must learn when we study silicates. A few days ago a friend asked me: why the expansion coefficient of the glaze is larger than that of the body, so the brick shape will be warped, but the expansion coefficient of the glaze is smaller than that of the body, so the brick shape is curved? It is reasonable to say that after being heated and expanded, the glaze is larger than the base and is curved, and the glaze is smaller than the base and is warped…
I’m not in a hurry to give an answer, let’s take a look at what the coefficient of thermal expansion is. First of all, it must be a value. What kind of value is it? It is the value of the volume of the substance that changes with temperature. Well, since it changes with “temperature”, it will change when the temperature rises and falls. The thermal expansion coefficient we usually call ceramics is actually the volume expansion coefficient. The coefficient of volume expansion is generally related to the coefficient of linear expansion, which is about 3 times the linear expansion. The measured expansion coefficient generally has a premise, that is, “in a certain temperature range”. For example, what kind of curve is the value of 20-400 degrees Celsius in general? If you insist on comparing the value of 400 degrees to 600 degrees Of course, no objective conclusion can be drawn from the comparison.
After understanding the concept of expansion coefficient, let’s go back to the original topic. After the tiles are heated in the kiln, they have both expansion and contraction stages. Let’s not consider the changes in the high temperature zone due to thermal expansion and contraction before. Why? Because, at high temperature, both the green body and the glaze are plastic. To put it bluntly, they are soft, and the influence of gravity is greater than their own tension. Ideally, the green body is straight and straight, and the expansion coefficient has little effect. After the ceramic tile passes through the high-temperature section, it undergoes rapid cooling and slow cooling, and the ceramic tile becomes hard from a plastic body. As the temperature decreases, the volume shrinks. Of course, the larger the expansion coefficient, the larger the shrinkage, and the smaller the expansion coefficient, the smaller the corresponding shrinkage. When the expansion coefficient of the body is greater than that of the glaze, the body shrinks more than the glaze during the cooling process, and the brick is curved; if the expansion coefficient of the body is smaller than that of the glaze, the body shrinks without the glaze during the cooling process. If there are too many bricks, the bricks will be upturned, so it is not difficult to explain the above questions!
Post time: Apr-25-2024