Ceramic mug orange peel glaze phenomenon research and solution

Xin xiang custom ceramic mug manufacturer, many ceramic mug buyers get ordered mugs, its body is not smooth, (exceptions for special needs such as frosted mug, matte mug, etc.). Ceramic mugs like this, in fact, belong to the unqualified products. China ceramics factory stands in the producer's point of view, for ceramic mug surface appears similar to the unevenness of the orange peel phenomenon (commonly known as “orange peel glaze”), to explain the reasons for its formation. The main reasons include: glaze formula, firing process and billet quality.

1. what does orange peel glaze look like?

Imagine the skin of a fresh orange: covered with tiny bumps, uneven to the touch. Ceramic mug if the appearance of orange peel glaze, looks like the orange peel “printed” on the cup. Specific performance: surface roughness: the original should be smooth surface of the mug appeared wavy undulation; dull luster: like a layer of mist, reflective effect is poor; feel bad: finger scratching can obviously feel the grain. This defect not only affects the beauty of the ceramic mug, but also reduces the grade of the cup, and even lead to serious glaze flaking.

2. Why does orange peel glaze?

Ceramic mugs appear orange peel glaze phenomenon, very similar to our failure to do the cake. When making a cake, if the quality of the cream, flour problems, or baking temperature is not controlled, it is easy to make a failed cake. The orange peel glaze of ceramic mugs is also due to the glaze and firing process.

2.1. glaze viscosity

In the previous article about ceramic mug glaze (Instructions for making glaze in ceramic mugs), we introduced. Before a glaze is put into the kiln and fired, it is a paste-like liquid. When a mug is coated with glaze and fired in a kiln, it becomes a smooth “glass layer” on the surface of the mug. The role of kaolin in the glaze, play the role of the binder. The glaze paste adjusts the viscosity of the glaze by adjusting the content of kaolin and water. If the proportion of kaolin exceeds 15%, then its viscosity is too strong, and the phenomenon of orange peel glaze will easily appear. If the glaze is not finely ground enough, the diameter of the particles inside is too large. Then, such a ceramic mug will be covered with fine bumps, feel uneven, and have a poor luster.

2.2. Glaze too thick

Ceramic mug glazing process, we have talked about it before. It is mainly divided into: dip glaze, spray glaze and brush glaze. In the glazing process, whether the glaze is uniform, can lead to ceramic mug flow glaze. Similarly, whether the glaze thickness meets the requirements also determines whether the mug will appear orange peel glaze phenomenon. If the glaze is applied too thinly, especially with darker colors, you will notice that the mug will have a vaguely ceramic undertone. If the glaze is applied too thickly, exceeding the safe thickness of 0.25 mm, the ceramic mug will be prone to orange peel glaze.

2.3. Firing

The firing process is what transforms a mug from “clay” to “ceramic”. In the previous article, we introduced the firing process of various equipment, temperature and humidity control (In-depth analysis of the firing process of ceramic mugs) . So, how does the firing process lead to the orange peel glaze on ceramic mugs? Mainly because: in the heating stage, the heating speed is too fast or missing insulation stage, resulting in the glaze is not uniformly melted on the beginning of solidification; another reason is that the kiln does not reach the high temperature of the glaze melting (for example: the kiln is no longer warmed at 1250 ℃, resulting in the glaze is not completely melted); the last reason is that the kiln is too fast to cool down, high-temperature heat preservation time is not enough, resulting in the glaze suddenly shrinkage caused by the orange peel glaze phenomenon.

2.4 Failure of Raw Billet

In addition to the above 3 reasons, leading to the phenomenon of orange peel glaze, there is another kind of ceramic mug raw billet reason. Glaze, glaze, firing, these three processes are acting on the ceramic mug billet. Ceramic mug raw material if the iron content is higher than 1%, iron and glaze will produce exclusion reaction, the phenomenon of orange peel glaze. If there is dust on the surface of the ceramic mug raw billet, resulting in reduced glaze adhesion during the glazing process, will also produce the phenomenon of orange peel glaze.

3. How to solve the orange peel glaze

According to the practical experience of China ceramic factory, the following measures can reduce the incidence of orange peel glaze from 20% to 3%:

3.1 Improve the glaze formula

Add “lubricant” to the glaze - borax and lithium faience, used to stabilize the viscosity of the glaze. In the powdering stage of the glaze material, the precision and grinding time of the ball mill should be ensured, and in the sieving stage, the fineness of the glaze material should be ensured. In addition, the use of stable quality raw materials, pay special attention to the content of iron and sulfur.

3.2 Control glaze application

At present, China ceramic factory adopts automatic glazing assembly line to make the glazing process and quality inspection more rigorous. Through the automated assembly line, the glazing process is guaranteed to be uniform, and the glaze thickness error is within 0.01mm. Through this, the glazing process of ceramic mugs is guaranteed to be problem-free.

3.3. Kiln automation

China's previous kilns were mostly based on traditional kilns, and the firing process was mainly dependent on the experience of the technicians. Nowadays, kilns use automation from fuel to temperature control. The heating up time, firing temperature, holding time and cooling down process are precisely controlled by computer.

3.4. Clean Raw Billets

The ceramic factory itself is with the concentration of clay and ceramics, so the production plant is relatively dusty. Ceramic mugs need to be dehumidified and dried after the production of raw blanks (Drying and trimming process of ceramic mug clay blanks). Raw billet in the drying stage, it is inevitable to fall on the dust, in the later part of the glaze link will be prone to problems. Ceramic factory for this problem using measures are mainly, control the environment of the drying plant; glaze before the use of air guns to clean the billet.

4. Frequently Asked Questions

Q1：Home to buy a mug with orange peel glaze, can I still use it?

A: Mild orange peel glaze does not affect the use, but heavy may lead to glaze off, it is recommended to replace.

Q2：How to judge the orange peel glaze with naked eyes?

A：Rotate the mug to the light, the smooth glaze will reflect evenly, and the orange peel glaze will have bright and dark ripples.

Q3：Why do some handmade mugs intentionally do orange peel effect?

A: The deliberately created texture is different from the defects, which are usually evenly distributed and treated with anti-peeling.

By understanding the glaze characteristics and controlling the production process, orange peel glaze can be completely avoided. This is not only a technical problem, but also a test of craftsmanship - China Ceramics Factory strives for excellence in every link in order to guarantee that every mug is smooth as a mirror and warm to the heart.

Defects in customized ceramic mugs - pinholes

Xin xiang custom ceramic mug manufacturer, pinhole is a ceramic glaze on the emergence of tiny holes (diameter usually 0.1-1mm), pinhole is the essence of the firing process gas can not be discharged from the glaze layer in a timely manner due to the specific causes can be traced back to the 5 major links.

1. What is a ceramic mug pinhole

Pinhole is a ceramic glaze on the emergence of tiny holes (diameter usually 0.1-1mm), shaped like a needle prick marks, when the seriousness of the dense honeycomb. According to the stage of occurrence can be divided into two categories: open air holes: through the glaze layer directly to the billet, the cross-section was funnel-shaped, easy to hide dirt; closed air holes: the glaze layer is not penetrated, the surface only left pits, affecting the appearance of gloss. Pinhole, as in the baking of cakes, the internal air bubbles are not discharged, the surface of the formation of collapsed pits.

2. pinhole is the essence of the firing process gas can not be discharged from the glaze layer in a timely manner due to the specific causes can be traced back to the 5 major links:

2.1. Raw material impurities generated by the gas

Pyrite (FeS₂) in the blank or humus in the clay, decompose at 300-800 ℃ to release SO₂, CO₂ gas. The use of cheap kaolin with sulfur content > 0.1%, the pinhole rate after firing surges to more than 20%.

2.2. Defects in glaze preparation

When the grinding fineness of the glaze is not enough, the glaze particles>30μm, the gas channel is formed in the gap between the particles, resulting in the glaze containing gas. Glaze paste contains air bubbles: excessive mixing introduces air, no defoamer (such as n-butanol) is added, and air holes are left after the glaze layer is dried. The glaze should be sieved through a 400 mesh sieve (pore size 38μm) during the screening stage to reduce pinholes by 60%. Glaze paste in the mixing process to add defoamer, not stirred too much, and storage should also refer to the previously described standards for storage.

2.3 Firing too quickly

Ceramic mug billet firing in the kiln, water evaporation period (room temperature ~ 300 ℃), if the rate of heating > 150 ℃ / h, then the ceramic mug billet inside the steam will break through the glaze layer to form pinholes. In the decomposition and oxidation period (300-900 ℃), the rate of heating > 200 ℃ / h, sulfur or carbide will decompose gas, the formation of pinholes.

2.4 Lack of heat preservation

If a 30-minute holding section is not set up for the raw ceramic mug during the firing heating phase, specifically before 900°C, gases will remain in the glaze and form pinholes.

2.5. Excessive air pressure in the kiln

Inside the kiln, when the exhaust fan power is insufficient, resulting in air pressure in the kiln > 10Pa, it will prevent the escape of gas, resulting in the ceramic mug gas can not be discharged normally, forming pinholes.

3. The formation of pinholes is mainly due to the above five reasons. So, how can ceramic mug factory improve in order to reduce the chances of pinholes in ceramic mugs?

3.1. raw material pretreatment

Since the sulfur in the clay raw materials can lead to the emergence of pinholes, then in the raw material preparation stage, we need to target the reduction of sulfur content in the clay, to control it within 0.1%. Specific methods include, washing and flotation of kaolin (removal of sulfides), or adding 1% barium carbonate (BaCO₃) high-temperature sulfur fixation; billet sealed aging for more than 15 days to promote the decomposition of organic matter, reducing the amount of fired gas production.

3.2. Glaze optimization

The grinding degree of the glaze is not enough particle gap is too large, it will be easy to make the air mixed into it, resulting in pinholes. Then, we have to glaze raw materials in the screening stage over 400 mesh sieve to reduce large particles. In the raw material grinding stage, the ball mill is used to grind to the 10,000-hole sieve residue <0.05%. In addition, add 0.5% magnesium oxide (MgO) to expand the melting range of the glaze and extend the gas discharge time. In the glaze slurry storage stage, 5% zirconium silicate (ZrSiO₄) can be added to improve the glaze high-temperature viscosity and close the external gas.

3.3. Firing temperature control

There are several temperature nodes mentioned above in the process of raising the raw billet from room temperature to 900°C. One is room temperature-300℃, to control the speed of heating, in the season that is not winter, to control the heating time should not be less than 2 hours. In 300-900 ℃, to control the time should not be less than 3 hours. In addition, every 100 ℃, to set aside 5 minutes of insulation period. Used to allow the ceramic mug billet in the body of the organic matter or sulfide, can be fully decomposed, and there is enough time for gas discharge. 900-1250 ℃, need 4 hours. Holding is also necessary during the vitrification stage of the glaze.

3.4. Control of kiln air pressure

The kiln needs to have an oxidizing flame and a reducing flame during firing. When ceramic mugs are fired, their content varies somewhat depending on the stage. The overall principle is to be a temperature section of a temperature section of the firing, in the firing of air pressure should be controlled at negative pressure -5 ~ 0Pa, the most appropriate. Can make the fired ceramic mug more stable, less pinhole defects.

In summary, the formation of pinholes in ceramic mugs, the root cause is that the gas is not discharged from the raw billet. The formation of pinholes is twofold, one is the raw materials, including clay and glaze raw materials, and the second is improper control of the firing process. Pinholes in ceramic mugs can not be avoided, China Ceramic Mug Factory, through strict control of the quality of raw materials and improve the precision of firing control, to minimize the chances of pinholes, to provide customers with high quality and stability of ceramic mugs.

In-depth analysis of the firing process of ceramic mugs

Xinxiang custom ceramic mug manufacturer, firing is the core part of the production of ceramic mugs, which directly determines the strength, appearance and safety of the mug. Simply put, firing is the transformation of loose clay, glaze and decal paper into a dense ceramic body through high temperature. This process involves complex physical and chemical changes. A little carelessness, ceramic mugs will appear: cracking, deformation, glaze defects, burst flowers and other problems. For ceramic mug factory, the firing stage of the error may cause the kiln scrapped, the whole kiln ceramic mug billet scrapped, the loss of up to tens of thousands of dollars. Therefore, it is important to master the scientific principles and operational details of the firing process.

1. Raw billet and the kiln's scientific matching

In the ceramic mug into the firing stage, the need for adequate preparation. These preparations mainly include: ceramic mug raw blanks, kilns, heat insulation boards and other hardware facilities; engineers, duty technicians, inspectors, etc., but also need to be based on the season, temperature, humidity and other natural factors, to develop a specific firing program such as firing temperature, firing time, lifting time and spreading heat time.

1.1. “Compatibility” of billet and glaze

- Matching coefficients of thermal expansion: the degree of expansion of the billet and glaze when heated must be close. If the coefficient of expansion of the glaze is greater than that of the billet (e.g., if there is too much quartz in the glaze), the glaze will be “cracked” during cooling, resulting in cracks; otherwise, the glaze will flake off.

- Firing temperature matching: high-temperature glaze (1280 ℃) must be matched with high-temperature body (such as high kaolin content of hard clay), low-temperature glaze (800 ℃) can be used in ordinary clay, otherwise the body is not sintered and the glaze has been melted, the cup crumbles when pinched.

1.2. Selection of kiln

- Tunnel kiln: suitable for single-species mass production, but poor temperature control flexibility. The principle is similar to the “assembly line oven”, the use of waste heat to preheat the new cups into the kiln, energy consumption is low.

- Shuttle kiln: suitable for small batch and many varieties, by adjusting the flame distribution of the gas lance (oxidizing flame or reducing flame), special glaze colors can be fired (such as celadon reduction firing).

- Electric kiln: precise temperature control (±5℃), suitable for low-temperature firing of flower paper containing metallic pigments, but the electric wire is easy to wear out under high temperature, and the long-term cost is higher.

These three modes of kilns are chosen according to the specifics of the ceramic mugs to be fired this time. For example, is this time to make samples, small orders, or mass production. Many customers of our factory have also asked why the sample production time is so short, but the real order production time is so long? This is because the choice of kiln is not the same. Tunnel kiln, once into the production of large orders, its production can not be interrupted. One order must be completed before another can be scheduled. So, it seems like the lead time for large shipment production is longer. However, for the ceramic mugs with return orders, we can book the kiln in advance, in which case the production cycle is shortened.

2. Changes of ceramic mugs during the firing process

2.1. Dehydration period (room temperature~300℃) - Ceramic mugs are mainly evaporated cleanly in the preheating period. Raw billets and glazes need to be fired at high temperatures, and the moisture inside the mug is an unsettling factor. It is important to drain the moisture in the preliminary stage to facilitate the stabilization of the shape of the ceramic mug and the melting of the glaze. In the pre-watering stage, the kiln is required to heat up more slowly, exactly how long it takes, but also with the thickness of this ceramic mug, the season and the room temperature to decide. The dehydration stage has a principle of “rather slow, not fast”, rapid heating, which can lead to mug cracking. In the dehydration stage, the treatment of water vapor is also a challenge. Discharge of water vapor can not be allowed to condense again inside the kiln, otherwise, it is easy to cause ceramic mug glaze with “tear”.

2.2. Oxidation period (300 ~ 900 ℃) - ceramic mug in this stage is mainly chemical changes. With low-temperature dehydration, ceramic mug billet began to formally enter the stage of change. This period, the organic matter in the ceramic mug raw billet for carbonization. For example, plant particles such as roots and grass clippings in the clay are burned to produce carbon dioxide. At this time the mug emits black smoke. Sulfides in the raw material are also oxidized. For example, FeS₂ (pyrite) becomes Fe₂O₃ and SO₂ gas, and if not sufficiently oxidized, the glaze will yellow after firing. Oxygen content should be ensured during this period, as the oxygen required for the oxidizing reaction of the raw billet increases. If the oxygen content is insufficient, it will easily lead to the firing of ceramic mugs will appear black spots, smoke and yellowing.

2.3. Bolitization (900°C to the highest temperature) - At this high temperature stage, the clay is officially transformed into ceramic. With the help of high temperature, the quartz in the raw material will undergo some changes, mainly β-quartz becomes α-quartz, and its volume expands by 0.8%; the feldspar in the raw material also melts at 1100℃, and it will turn into a glassy phase to fill the pores of the ceramic body. The glaze also changes at this temperature to a state similar to magma, which evenly covers and fills the surface of the mug as if it were coated with a film. In the high temperature period, the temperature control is very critical, too high or too low temperature can not make the glaze to meet the standard of qualified products. Real-time monitoring of temperature should be carried out during this period. After many ceramic mugs are fired, the glaze feels like snot and is not uniform enough. That's when the temperature is too high during the firing stage.

2.4. Cooling period -- Although the mug has been fired, the cooling period is the last stage, but also the most important one. Otherwise, you may face the “goal kick, the ball flew” embarrassing situation. High-temperature cooling to room temperature can be divided into three stages: fast cooling period (high temperature - 800 ℃), slow cooling period (800-200 ℃), and constant temperature period (200 ℃ - room temperature). The fast cooling period requires rapid cooling, but does not allow the wind to blow directly on the ceramic mug. The rapid cooling is for the solidification of the glaze, and also for the reverse transformation of the quartz, where the α-quartz changes back to β-quartz. Slow cooling period requires a slow cooling speed, and eventually through the constant temperature period temperature drops to 80 ℃ or less, then you can leave the kiln.

3. Precautions for firing ceramic mugs

3.1. ceramic cup billet before putting into the kiln, to mouth down on the heat insulation pad, the cup directly spaced more than 3cm. in the placement of ceramic cups, to check whether the billet has deformation, cracks and other quality problems, quality problems of the cup is not put into the kiln. The reason for placing the mugs with their mouths facing downwards is to avoid dust falling into the mugs in the kiln, and at the same time, it is not easy to deform the mugs during the firing process.

3.2 During the rainy season or when there is a lot of humidity in the air, the kiln should be preheated to remove the moisture, otherwise the mugs will show “tear marks”. In the event of a sudden power failure or gas failure during the firing process, all valves should be closed immediately and the kiln door should not be opened until the gas is restored.

3.3. Cracks in mugs are caused by uneven drying of the billet, residual moisture inside, or too rapid cooling during the cooling period. During the rainy season, the drying time of the billet should be extended, and the cooling time should be extended at the same time.

3.4. When the mouth of the mug is deformed or the body of the mug is deformed, it is caused by the softening of the billet during the high temperature stage due to too rapid heating, which leads to the deformation of the mug body due to gravity. Or because of the uneven thickness of the mouth and body of the mug during the production stage of the billet.

3.5. Pinholes in ceramic mugs. This is caused by the organic matter in the glaze not being burned out. The glaze needs to be filtered through a 200-mesh sieve during the glaze making process. Another reason is that after the glaze is finished and before firing, the glaze is floating. Ceramic mugs need to be dusted with an air gun before entering the kiln to avoid pinholes.

Kiln firing of ceramic mugs is an art, and this technique firing technique was available in ancient China. Historically, earthen kilns were used, along with handmade ceramic products, and this art has been passed down to the present day. Industrialization progressed with various kilns facilitating the mass production of standardized ceramic mugs. In recent years, with the rapid advancement of artificial intelligence and automation technology, the kilns have become more precise and convenient to control. Xinxiang Ceramics Factory has also introduced a number of kilns with the latest technology, automatic billet trimming production lines, automatic glazing production lines and so on. Therefore, we are able to provide customers with the best ceramic coffee mugs with stable quality.