Ceramic mug crack causes and prevention methods

Xin xiang custom ceramic mug manufacturer, we've had ceramic mug cracks appear in previous articles. Originally in “16 Flaws in the Making of Ceramic Mugs”, ceramic mug cracks are a type of defect in ceramic mugs. In “How to judge the quality of ceramic mugs”, the first criterion for judging the quality of ceramic mugs is cracks. Ceramic mugs produce cracks according to the overall division, can be divided into production link cracks and transportation damage. Transportation damage, is common, but also the easiest to understand. Because of the transportation process of bumping or violent transportation, resulting in ceramic coffee mugs produced broken or cracked. In the previous article we introduced how to avoid ceramic mugs transportation damage, if you are concerned about the transportation link, please refer to: “How to pack the ceramic mug so that it will not be damaged?”. So, what kinds of cracks are included in the production process? In fact, the appearance of cracks in the production of ceramic mugs throughout the production process. Including: raw material formulation, plastic molding, drying and trimming, glazing and firing. Next, Xin xiang ceramic factory for you to explain one by one.

1. Why does ceramic mug crack?

To understand ceramic mugs cracking, we must first understand the nature of ceramics. Ceramics are made of clay (raw materials) through molding, drying, and high temperature firing. This process is, in essence, a process in which the clay is stripped of physical and crystalline water, and the mineral structure undergoes a chemical reaction, eventually vitrifying and becoming hard. This process is full of “shrinkage” and “stress”. Cracking, is the tearing damage that occurs when the internal stress is greater than the structural strength of the material itself.

2. Several major forms of mug cracking

Ceramic mugs do not crack in just one mold; different forms of cracking point to different etiologies.

2.1. Penetrating Cracks (Cracking)

Manifestation: Cracks penetrate through the wall of the mug, completely splitting from the inner wall to the outer wall. Usually a dull cracking sound rather than a crisp porcelain sound is produced when tapping with the hand (this is also the case with ceramic mugs that have cracked due to impacts during transportation).

Characteristics: This is the most serious, can not be repaired defects, ceramic mug directly scrapped, can not be used normally.

2.2. Hairline Cracks (Shock Cracks)

Performance: Cracks as fine as hairline, sometimes difficult to detect with the naked eye immediately after kilning, but will bleed when tested with ink or pigment.

Characteristics: This type of cracking usually becomes visible late in the firing cooling period or after kiln discharge, and is a typical manifestation of uneven internal stress release.

2.3. Cracking at the Mouth Rim (Mouth Blowout)

Presentation: Cracks occurring at the rim of the mouth of the mug, sometimes in a radial pattern.

Characteristics: the mouth of the mug is the most varied shape and the thinnest part of the mug, which is also the part where the stress is most easily concentrated.

2.4 Cracking of the foot (bottom cracking)

Performance: Cracks occurring at the combination of the bottom and the body of the mug, or at the edge of the foot rim.

Characteristics: the bottom of the mug is the thickest, drying and firing internal and external contraction inconsistency, resulting in pulling crack.

2.5. Glaze cracks (cracks)

Performance: only the surface of the glaze layer appeared network cracks, while the body itself did not crack. In the previous article “Reasons for custom ceramic mugs cracking”, we talked about the specific reasons and solutions for glaze cracking.

Characteristics: This is usually caused by the “coefficient of expansion of the glaze being greater than that of the blank”. When cooling, the glaze shrinks faster than the body and is pulled apart. Although sometimes used as an artistic effect (as in the case of kiln flakes), in the case of mugs, it is a defect that affects hygiene and longevity.

3. The reasons for the cracking of the mug

ceramic coffee cup cracking, in addition to transportation damage, cracking caused in the production, we can categorize the reasons into four categories: raw materials, molding, drying and firing.

3.1. Raw material factors (clay “innate nature”)

Clay is the bone and blood of the ceramic mug, clay is the “foundation” of the ceramic mug, the quality of the clay is critical. If the foundation is not good, or the quality of the foundation is too poor, it is easy to cause the ceramic mug cracked in the later production. Clay in the previous article we have explained, clay is mainly composed of: kaolin, feldspar and quartz, plus a part of micro chat elements, such as: ball clay, CMC, etc., composition. These raw materials have to go through a number of processes, crushing, grinding, removing impurities, mixing, iron removal, dehydration before practicing mud. In the previous article “Ceramic mug raw materials - clay preparation process”, we have explained the specific clay preparation process. During the clay preparation process, if the recipe is not correct, the mug will crack. For example, too high a proportion of malleable raw materials such as kaolin or ball clay will cause the clay to be too “soft”, which in turn will result in a large shrinkage of the biscuit in the drying process, making it prone to deformation and cracking. If the proportion of quartz or clinker, which are barren raw materials, is too small, the clay will lack of support, and will also be easy to deform or crack in the drying process. If the mix of raw materials is not uniform, the contraction ratio of the clay is not consistent, and cracking will also occur during drying.

All of these are the clay in the production process is likely to lead to ceramic mug cracking reasons. In fact, after the completion of clay production, there is another link easy to lead to ceramic mug cracking, this link is often many ceramic mug production factory ignored. It is the clay aging, our industry commonly known as “boring material”. Clay aging, its main form of expression is to practice good clay, placed for a period of time. This link seems to be very simple and insignificant, but in fact it is very important. Lack of aging clay is like an army with insufficient training and poor internal communication, which will be easily dispersed once it goes into battle (into molding and firing). The purpose of aging is to create an elite force that will bring out the best in the clay. We will write an article about this later.

In the process of practicing clay, even if each link is accurate, precisely ignore the aging process of clay. Ceramic mugs made of such clay are also very prone to cracking during the drying and firing process.

Of course, there are other factors, such as air bubbles in the clay and uneven humidity, which can also cause the mugs to crack.

3.2. Molding and trimming factors (the production of the “acquired defects”)

Ceramic mug molding, we have talked about before, mainly is the pressure blank "Ceramic mug molding production process details" and grouting “Customized Ceramic Coffee Mugs - Grouting Process Explained”. Ceramic mugs come in a variety of shapes, depending on the shape of the mug required by the mug's purchaser. Ceramic mugs tend to be thinner at the mouth and thicker at the bottom. If not handled properly in the drying process, raw ceramic mugs are prone to cracking when dehydrated.

Accessories other than the body of the ceramic mug, such as the handle, are glued on. When bonding the handle, if the difference between the moisture content of the mug and the handle is too large, or the concentration of the joining mud is not suitable, or when bonding the handle is not standardized in terms of pressure and scraping operations, it will leave gaps, resulting in cracking later.

3.3. drying factors (moisture “gentle goodbye”)

artificial cause of ceramic mug cracking is relatively small, many cases of ceramic mug cracking is in the drying process. In the previous article “Drying and trimming process of ceramic mug clay blanks” we explained this in detail. Ceramic mug billet drying if the temperature is too high (for example, high temperature in summer), billet surface water will evaporate quickly and violently contracted, billet internal moisture has not come to release, will cause the hard shell of the ceramic mug billet by the internal burst, resulting in cracks. Even if the drying, billet did not crack, in the firing session, this did not complete a thorough drying of the billet in the firing, the internal physical water will also be rapidly vaporized, resulting in a strong steam pressure, directly from the inside of the mug to crack.

In fact, the wind will also cause ceramic mug cracking, if the billet for a long time by a fixed direction of the wind, then, the wind side of the fast drying, on the contrary, the leeward side of the slow drying, but also easy to cause uneven moisture content and lead to cracking. Similarly, when drying, if the mug is facing upwards, the mouth of the mug is thin and exposed to high winds, while the bottom of the mug is thick and exposed to low winds, which is also prone to uneven moisture content and lead to cracking.

3.4. Firing (the “ultimate test” of fire)

Firing is the most important process for ceramic mugs. In the previous article “In-depth analysis of the firing process of ceramic mugs”, firing is the test of the preliminary work of ceramic mugs, but also for the quality of ceramic mugs of good or bad determinants. If the preparatory work of the ceramic mug is not enough, then the firing of the ceramic mug must be unqualified; even if the preparatory work is well-prepared, the firing link for the various stages of control is not appropriate, but also can not generate a high-quality ceramic mug. Firing temperature, heat preservation and cooling, each link is not controlled properly may cause a large number of ceramic mug cracking. Heating up the early stage is to exclude the key period of residual physical water, if the temperature is too fast, the water is violently vaporized, it will cause shock or crack; insulation period if not enough time, the quartz occurs crystal transformation will not be uniform, resulting in shock crack. Cooling period is the main concern is the cooling amplitude, if the cooling amplitude is too large, the quartz in the cup in the reverse crystal transformation, the cup volume contraction, and the outer layer of the glaze has been fixed, can not be contracted, will produce cracks. Out of the kiln if the temperature is too high, cold air cooling rapidly, ceramic mugs are also prone to sharp contraction and shock crack.

4. Ways to prevent mugs from cracking

The third part explains the causes of mug cracking, and targeted refinement can reduce or avoid ceramic mugs from cracking. First of all, starting from raw material selection, practicing clay process under strict control, then trimming and drying links, pay attention to the details of the temperature and wind, and finally strictly control the firing time and temperature, you can produce qualified ceramic mugs.

5. Establish a quality traceability system

Ceramic mugs are products that have gone through a series of processes. For reducing ceramic mug cracking, China Ceramic Mug Factory has some unique quality control methods. First of all, each link to establish a monitoring and verification mechanism with specialized inspectors. One of the links found in the ceramic mug cracking, timely report and trace the cause, and then remedies. From raw materials, clay, glaze paste to drying, billet repair, and ultimately firing and other links, China Ceramic Factory to do every cup, every link, can be traced back to the person.

Summarize

mug cracking is a systematic project in ceramic production, which runs through every link from raw materials to finished products. China ceramic factory after nearly 30 years of ceramic mug production experience, has established a systematic knowledge of the whole production process and quality control awareness. Through stabilizing raw materials, standardizing operation, patient drying, scientific firing, and supplemented by rigorous data analysis and quality control traceability, we strive to produce more stable and higher quality ceramic mugs for our customers.

What is the single firing process in ceramic mugs production?

Xin xiang custom ceramic mug manufacturer, in our previous article, when introducing “glaze cracking” , we mentioned “single firing (primary firing).” So, what exactly is single firing? How does it relate to the “bisque firing” we discussed earlier? Single firing is a ceramic mug production technique—the most commonly used one. After glazing and applying decals, the ceramic mug's greenware undergoes only one firing to complete the mug. Bisque firing, as mentioned before, is a preliminary firing before the final firing. This process strengthens the greenware as a preparatory step. Ceramic mugs that have undergone bisque firing still require a second firing to become finished products.

1. What is Single-Firing?

As the name suggests, “single-firing” refers to the process where a ceramic mug's greenware, after being shaped and dried, undergoes only one high-temperature firing in the kiln to become the final product. Here, “main firing” refers to the “essential firing,” contrasting with preparatory firings like bisque firing. You can imagine it like baking a cake: the prepared batter (ceramic body + glaze) is placed directly into the kiln for a single firing, emerging as a complete, finished cake with its packaging (glaze).

2. Specific Process of Single-Firing

This process operates as a highly integrated production line. First, the ceramic factory shapes the prepared clay using molds for pressing or slip casting, resulting in dried greenware. These greenware pieces are extremely fragile, like chalk, and shatter easily upon contact. Workers then perform all decorative steps directly on these delicate greenware pieces: Glazing (dipping the mug body into glaze slurry) and decal application. Finally, carefully place this fragile, “dressed” (glazed and decorated) greenware into the kiln for a long, high-temperature firing. During this firing, the clay body becomes dense and hard (vitrified), while the glaze and decals melt into a glossy glassy layer, permanently bonding with the body. After cooling in the kiln, the result is a complete ceramic mug.

3. Advantages of Single-Firing

The primary advantage lies in its efficiency and cost-effectiveness. Since the entire process requires only one firing in the kiln , it significantly reduces time and energy costs while minimizing breakage risks associated with multiple handling and firings. It streamlines production steps, making it highly suitable for mass-produced, standardized daily ceramics like mugs. Consequently, the vast majority of ceramic mugs on the market utilize the single-firing process.

4. Advantages and disadvantages of single-firing?

Because everything is completed in a single firing, it demands extremely high control over all raw materials and processes, with risks concentrated in this critical single step. First, the body (clay) must be of exceptional quality, exhibiting minimal shrinkage and deformation during firing. Second, and most critically, the glaze and decal must be perfectly matched to the body. Their shrinkage rates and melting temperatures must fall within a highly coordinated range; otherwise, issues like the previously discussed glaze cracking, glaze pinholes, and body cracking. It's like a performance that must succeed on the first try—there are no second chances.

If china ceramic mug factory decides to adopt the single-firing process, the core work lies in preliminary formula debugging and sample testing. You must invest significant effort to find the glaze formula best suited to your clay body and select decals explicitly labeled as suitable for single firing. Each change in clay body or glaze requires rigorous retesting to ensure the three elements—body, glaze, and decal—are compatible in thermal expansion coefficients and melting temperatures. A stable and controllable kiln temperature curve is also critical to success. Once these foundations are solidly established, single-firing will become a powerful engine for efficient production in your factory.

Reasons for custom ceramic mugs cracking

Xin xiang custom ceramic mug manufacturer, when inspecting the quality of custom ceramic mugs, we sometimes encounter cups covered in spiderweb-like cracks or damaged logo patterns. These are examples of a common ceramic defect known as “crackling.” While mugs with this defect remain functional, they compromise aesthetic appeal, making them unsellable to buyers. Crackle is a common technical challenge in ceramic mug production. For detailed causes and solutions, let Xinxiang Ceramics explain:

1. What is Crackle

Crackle in ceramic cups refers to a common defect occurring during production. It describes a flaw where the glaze surface or decorative pattern exhibits cracking, peeling, or blistering. This term specifically denotes defects in the glaze layer itself, not merely damage to the applied decal paper. While the decal area on the mug is indeed prone to blistering, this term encompasses a broader range of issues. Essentially, it describes the separation or cracking that occurs between the mug's ceramic body and its glaze layer (including both transparent glaze and patterned decal glaze) due to poor bonding, either during firing or subsequent use. In custom ceramic coffee mugs, defects like body cracks, glaze separation from the body, or volcanic-like damage to decorative patterns all fall under ‘cracking.’ Imagine applying glue to a balloon: if the balloon shrinks while the glue remains stretched, the glue layer will crack and peel.

2. Manifestations of Cracking

Cracking appears in various forms, some visible upon kiln removal while others emerge during subsequent use. Freshly fired pieces may show hairline-fine, dense net-like cracks on patterns or glaze surfaces. Occasionally, small glaze fragments may chip off entirely, exposing the underlying body. In milder cases, crazing may be invisible to the naked eye. However, running a fingernail across the glaze surface produces a dull, scratchy sound (“creak”) instead of a smooth, crisp one—indicating internal cracking. In the worst scenarios, the entire glaze layer or decal peels off extensively, rendering the ceramic mug completely unusable.

3. Common Types of Cracking on Mug

Cracking occurs when the glaze or decorative paper layer fails to fully bond with the mug body. Three common scenarios exist: First, decorative paper cracking—the most frequent issue—manifests as fine spiderweb-like cracks along pattern edges, particularly in areas with dense pigments (like red or black), or as the entire design lifting or peeling off. The second is glaze cracking: even on plain, unpatterned mugs, the entire glaze surface may develop fine cracks or chips. The third is edge cracking: particularly prone to occur at the rim or base where glaze tends to accumulate or is thinner. This manifests as exposed ceramic body where the glaze layer is missing.

4. Causes of Cracking

There is only one fundamental cause of cracking: mismatched thermal expansion coefficients between the glaze layer and the ceramic body. You can think of it as the ceramic body and the surface glaze layer expanding and contracting at different rates during heating and cooling. The ceramic mug (greenware) and the glaze layer are two distinct materials. During firing in the kiln, both expand, yet they fuse tightly together. When firing concludes and cooling begins, if the mug contracts faster than the glaze layer—or contracts more significantly—it will “pull” the glaze layer along with it. If the glaze layer cannot keep pace with this contraction, it will be forcibly torn apart. Conversely, if the glaze layer contracts more severely, it will peel away from the body. This is the fundamental cause of glaze cracking. Of course, the glaze layer here refers not only to the base glaze layer of the ceramic mug but also includes the decal layer.

In previous articles, Xinxiang Ceramics Factory has discussed decals - "3 production methods and 2 decal methods for ceramic mug decals" . Standard decals typically consist of three layers: the base carrier paper, the middle pattern layer, and the outermost transparent film. This pattern layer is printed using a mixture of special ceramic pigments and glassy materials (the basic components of glaze). When fired in the kiln, the outer film burns away, while the pattern layer melts and fuses with the mug's white glaze. Upon cooling, it becomes permanently fixed to the mug. Thus, the pattern on the transfer paper is essentially a layer of colored glaze. The process of applying mug decals involves layering one or more layers of differently colored, differently composed “colored glazes” onto the pre-applied white glaze base. The decorative glaze on the decal paper is typically thicker and more pigmented, increasing the complexity of matching. Consequently, the base glaze, decal paper, and greenware must be compatible during firing; otherwise, they will “clash” in the kiln, causing decal cracking. The solution lies in meticulous formulation to ensure they become “good friends.”

5. Solutions for Crackling

Resolving crackling requires addressing the root cause—namely, the mismatch between the three materials. First, adjusting the formulations of the glaze and decal paper is paramount. Mug cup engineers must conduct experiments to develop glazes and decal glazes that match the thermal expansion coefficient of the ceramic body used by the factory. This ensures they contract “synchronously” during cooling. Second, optimizing the firing schedule is vital. Particularly during cooling, excessive rapid temperature drops must be avoided. Allow sufficient time for the body and glaze to contract slowly and steadily, preventing the generation of significant internal stresses from abrupt cooling. In the article “In-depth analysis of the firing process of ceramic mugs” China ceramic mug factories provides detailed insights into the cooling phase. (Ceramic Mug Cooling: High-temperature cooling to room temperature can be divided into three stages: rapid cooling (high temperature to 800°C), slow cooling (800°C to 200°C), and constant temperature (200°C to room temperature). The rapid cooling phase requires a fast temperature drop, but direct airflow must not blow onto the ceramic mugs. This rapid cooling facilitates glaze solidification and promotes the reverse transformation of quartz, converting α-quartz back to β-quartz. The slow cooling phase demands a gradual temperature reduction. After passing through the constant-temperature phase and dropping below 80°C, the mugs can be removed from the kiln.) Next, control the thickness of the glaze layer and the decal paper. Excessively thick glaze or overly layered decal glaze increases the risk of glaze cracking. Therefore, ensure uniformity in both the glazing and decal application processes. Finally, strengthen raw material and quality control. Ensuring the stability of each batch of clay bodies and glazes reduces variability at the source, enabling consistent production of high-quality products.

In summary, glaze cracking represents a classic technical challenge in ceramic production. Overcoming it requires systematically analyzing every stage—from the body and glaze to the decal application and firing process—to identify the optimal balance. Consistent mug quality relies on stable raw materials, mature techniques, and rigorous protocols. Above all, it hinges on the attitude of the ceramic producer.

Detailed explanation of the swing glaze for ceramic mugs

Xinxiang custom ceramic mug manufacturer, in the previous article, “Causes and solutions for running glaze in ceramic mugs,” when discussing glaze running in ceramic mugs, we mentioned in Section 4, “Which shapes of mugs are more prone to glaze running,” that special types of mugs usually use swing glazing for the interior glaze. Today, we will talk about swing glazing. Swing glazing is a glazing technique used in the manufacture of ceramic mugs. This method is mainly used on the inner walls of ceramic mugs.

1. What is swing glaze?

Glaze swing is also a method of applying glaze, similar to Dipping Glaze (Ceramic mug dip glaze process details) and Spraying Glaze (Detailed explanation of the glaze spraying process for ceramic mugs) and Painting Glaze (Detailed explanation of the painting glaze process for ceramic mugs), Pouring Glaze (Detailed explanation of the pouring glaze process for ceramic mugs), which are also types of glazing. Unlike other glazing techniques, glaze pouring is mainly used on the inner walls of ceramic mugs. During the process, workers pour a certain amount of glaze slurry into the interior of a dry, bisque-fired ceramic body. They then hold the body with their hands or using equipment and shake, tilt, and rotate it in various directions. This utilizes the glaze slurry's own fluidity and centrifugal force to evenly “drip” the glaze across the inner surface of the ceramic body. Imagine you are holding an empty mug and want to evenly coat the inside of the mug with sugar water. What would you do? You would probably pour in some sugar water, then gently shake and rotate the mug to allow the sugar water to flow inside the mug, covering every inch of the surface, and finally pour out the excess sugar water. Glazing is the application of this principle to ceramics! This is a method mainly used to apply glaze to the inside of vessels (such as mugs, bowls, vases, etc.). Finally, the excess glaze slurry is poured back into the glaze bucket for recycling. The key to the whole process is the “swinging” action, which allows the glaze slurry to flow and cover the inner wall like water waves.

2. Preparations before glazing

Before the actual glazing process, the ceramic mug factory needs to do some preparatory work. The purpose of these preparations is to improve the success rate of the glazing process and reduce the rate of defective products. These preparations are mainly divided into four parts:

2.1. Cleaning the ceramic mug blanks

In the previous article, we mentioned that after the clay is made into ceramic mug blanks, it is necessary to carry out “Drying and trimming process of ceramic mug clay blanks”  Ash may fall on the blanks during the drying process. If the dust on the mug is not cleaned, the adhesion of the glaze will be reduced, and the ceramic mug we hold in our hands will be prone to problems. In this process, an air gun is needed to clean the greenware. At the same time, the moisture content of the ceramic mug greenware needs to be tested. The most suitable moisture content for glazing is 3%-5%. If the moisture content of the greenware is too low, it will cause excessive water absorption and destroy the structure of the glaze; if the moisture content is too high, it is easy to cause problems such as pinholes and glaze flow.

2.2. Bisque firing of ceramic mug greenware

We have already explained in detail in previous articles whether ceramic mug greenware needs to be bisque fired. “What is the plain firing process for raw ceramic mugs?”  Ceramic mugs that are glazed should preferably be bisque fired. The bisque firing process can increase the strength of ceramic mugs and stabilize their water absorption rate, which are the two foundations for successful glazing. What problems will arise if glazing is applied to ordinary greenware instead of bisque-fired ceramic mugs? Glazing on raw clay is extremely difficult technically, with extremely demanding requirements for the clay body, glaze slurry, operating process, and drying conditions, which can easily lead to high breakage rates and glaze defects. Raw clay has a very high water absorption rate, and there are significant differences in the water absorption speed of different parts (such as the thickness of the mug wall). Additionally, when glaze slurry is applied to the green body, it can cause uneven water absorption, leading to deformation of the green body and uneven glaze surfaces.

2.3. Preparation of Glaze Materials

Quartz, feldspar, and kaolin are ground into powder, sieved, and mixed into a slurry to form the glaze slurry. These specific processes have been detailed in “Raw Materials, Production, Classification, and Quality of Glaze Materials”  . Before glazing, the glaze slurry must be stirred and tested, primarily to check its color and flowability.

2.4. Cleaning the workshop

The glazing workshop of a ceramic factory needs to be cleaned separately before formal glazing. This is similar to cleaning the greenware of ceramic mugs, in order to ensure the cleanliness of the glaze on the ceramic mugs. The main tasks are to clean up dust and control the temperature and humidity of the workshop. The workshop temperature is generally controlled at around 25°C, and the humidity is controlled at around 50%.

3. Glazing Process

After completing the preparatory work as described above, the actual glazing process begins:

3.1. Pouring the glaze: Workers hold the greenware (such as a mug) and pour an appropriate amount of glaze into it. The amount is critical. Too little will not cover the entire piece, while too much will be wasted and easily flow to places where glaze should not be applied (such as the outer rim or bottom of the mug).

3.2. Shaking/Rotating (Glazing): This is the core step! Workers swiftly and skillfully use wrist and arm movements to shake the greenware up and down, left and right, and front and back, while simultaneously tilting and rotating it. The goal is to make the glaze slurry flow and wash against the inner walls of the greenware like waves, ensuring every corner is covered to form an even glaze layer. This process is typically very quick, completed in a few seconds to a dozen seconds.

3.3. Draining Excess Glaze Slurry: Once the inner walls are uniformly coated, the clay body is immediately tilted (usually inverted) to drain all excess glaze slurry not absorbed by the clay body back into the glaze bucket. This step must be executed decisively and cleanly to prevent glaze slurry from accumulating at the rim or flowing onto the outer walls.

3.4. Cleaning the rim: After draining the glaze slurry, it is usually necessary to quickly wipe away any remaining glaze droplets from the rim of the body using a sponge or finger to prevent the rim from being stuck by glaze (kiln sticking) or affecting the appearance after firing.

3.5. Drying: After applying the interior glaze, the body is left to dry in preparation for the next steps (such as dipping or spraying the exterior with glaze).

4. When is dipping glaze suitable?

Due to the special nature of the dipping glaze process, it is only suitable for applying glaze to the interior of vessels. These vessels include ceramic mugs, ceramic bowls, ceramic jars, ceramic vases, etc. Applying glaze to the interior of these ceramic vessels is challenging, so the glaze dipping method is employed. Through glaze dipping, the interior of deep and narrow vessels, particularly the bottom and corners, can be effectively covered. These areas are difficult to treat using conventional spray glazing or dip glazing. Glazing is done entirely by hand, and skilled workers can more easily produce perfect ceramic products by controlling the concentration of the glaze slurry, the glazing time, the angle, and the force. This is because ceramic products themselves are works of art, including daily-use ceramics, ceramic mugs, etc. With the advancement of technology, machines have replaced manual labor, and automated assembly lines can produce more uniform ceramic products. However, this also reduces the artistic sense of ceramic products.

5. The difference between glazing, dipping, and spraying

Earlier, China Ceramic Mug Factory explained in detail several glazing techniques for ceramic mugs, including dipping, spraying, brushing, and pouring. The main glazing techniques for ceramic mugs are dipping and spraying, so what is the difference between glazing and these techniques? When we customize ceramic mugs, when should we choose dipping glaze, under what circumstances should we choose spraying glaze, and what kind of ceramic mugs should use splashing glaze?

In fact, through our previous knowledge, we can also roughly understand the characteristics of these three glazing techniques. In order to give everyone a more thorough understanding, Xinxiang will compare the three glazing techniques horizontally. Dip glazing is usually used to treat the exterior of ceramic mugs or the exterior and interior of conventionally shaped ceramic mugs, while spray glazing is usually used to treat the exterior of ceramic mugs and the interior of conventionally shaped mugs, especially those with complex shapes (Unusual mugs, embossed mugs, Gradient glaze mugs) on the outside of ceramic mugs. Glaze splashing is usually used to treat the inside of ceramic mugs, that is, to apply glaze to the inner wall of the ceramic mug. Conventional mug types can be treated with glaze splashing, but it is inefficient and wastes a lot of glaze. Only ceramic mugs with unconventional shapes can be treated with glaze splashing. Glazing is currently done manually by pouring glaze into the ceramic mug and rotating and shaking it to coat the inner wall with glaze. It consumes less glaze than spraying but more than dipping.

6. Summary

Glaze slurry coating is a key manual process in ceramic mug production that focuses on glazing the inner walls. It utilizes the fluidity of the glaze slurry and ensures that the deep cavity is evenly coated by pouring, shaking, and pouring out the excess glaze slurry. For mug production, glaze slurry coating is the most comprehensive method for treating the inner walls of mugs. Its advantages lie in its good coverage of complex internal structures and relative savings in glaze material, but its efficiency depends on the skill of the worker. For the outer wall of the mug, depending on the product design (whether a uniform color is required, whether a pattern is required, whether a special effect is required) and production requirements, either dipping (high efficiency, low cost) or spraying (more flexible, rich effects) is chosen to complete the process.

Causes and solutions for impurity in ceramic mugs

Xinxiang custom ceramic mug manufacturer, sometimes when we receive custom-made ceramic coffee mugs, we are bothered by small “bumps” on the surface of the mug. While these bumps do not affect the normal use of the ceramic coffee mug, they detract from its overall aesthetic appeal. In the industry, these “small bumps” are referred to as “impurity.” There are 5 causes for the formation of impurity, but preventing this phenomenon requires only 1 solution.

1.  What is impurity

You can think of impurity as “pimples” or “dirt” that should not be on the surface of a ceramic coffee mug. Simply put, it is small particles, small bumps, or embedded impurities that appear on the smooth glazed surface of the mug (or sometimes on the unglazed raw clay). These particles typically have a different color from the surrounding glaze (e.g., dark, brown, white) and feel rough and uneven, like a small grain of sand stuck to the surface. This is undoubtedly a flaw that affects both aesthetics and quality.

2. Forms of impurity

Impurity manifests in several distinct forms:

“Bubble” type: One or more small lumps protrude from the glaze surface, resembling small volcanoes. The tops of these lumps may crack or expose the impurities inside.

“Embedded” type: A dark (or light) small particle is clearly visible within the glaze, partially embedded and partially exposed, with a distinct edge.

“Explosion” type: The glaze around the impurity point may crack or develop small pits, like traces of a small explosion.

“Adhesion” type: Sometimes a visible foreign particle (such as a small piece of sand or dust) is loosely stuck to the surface of the mug and can be removed with a scratch, leaving a small pit. Strictly speaking, this is not typical slag.

3.  How are impurity marks formed

Impurity marks do not appear out of thin air. They are caused by impurities that can be “mixed in” or “generated” at every stage of mug production. Let's look at the production process step by step:

3.1. Raw Material Preparation and Clay Preparation

“Pollution at the source”: This is one of the most fundamental causes. The clay used to make ceramic bodies (mineral raw materials such as clay, feldspar, and quartz "Properties and functions of the 3 basic raw materials for ceramic mugs") may already contain hard particles such as iron filings, small stones, unground mineral particles, or even dust and sand particles mixed in during transportation and storage. If the raw materials are not sufficiently screened, de-ironized (using magnets to remove iron filings), and ground, these “impurities” will remain in the clay.

“Mixing contamination”: During the process of adding water and mixing the clay to make slurry (Ceramic mug raw materials - clay preparation process) , if the mixing tank or pipes are not clean, or if the environment is dusty, contaminants may also enter the mixture.

3.2. Shaping

Once the clay is ready, it is time to mold the ceramic mugs. The clay is fed into a machine and molded into the shape of ceramic mugs . or by injecting slurry into a mold to create a green body through slip casting. Molding mainly includes: the shape, size, thickness, and whether or not the ceramic mug has a handle and lid. There are two situations in which impurity is produced in this process.

“Dirty mold”: During slip casting, if the plaster mold has residual dry clay particles, dust, or damage causing debris, when the clay slurry is injected into the mold (Description of the 4 types of molds used in the making of ceramic mugs), these impurities will be wrapped into the surface of the mug blank. During press molding, if there are residual dry clay debris or dust on the surface of the metal mold, when the clay is placed in the mold, the plastic clay will be contaminated with impurities, and the raw ceramic mug will have debris on it.

“Environmental impurity”: If the forming workshop is dusty, or if the workbenches and tools are unclean, during the processes of demolding, handling, and trimming the wet, soft clay body, dust and small particles can easily fall onto the still-wet surface of the clay body and become “stuck.” This also results in impurity.

3.3. Drying and Trimming

After the ceramic mug blanks are molded, they must be dried and trimmed. The purpose is to refine the details of the ceramic mug greenware so that the final ceramic mug is more perfect. In fact, this is also an important step in the production of impurity.

“Dry greenware dusting”: The surface of the semi-dry greenware is like static electricity, which is particularly prone to adsorbing dust, lint, and gypsum powder (if the mold is being trimmed nearby) floating in the air. If the drying environment is poorly ventilated or unsanitary, the risk of slagging increases significantly.

“Scraping residue”: When workers use blades or sponges to trim the rim and bottom of the mug, if the scraped clay residue is not cleaned up in time and falls back onto the body, it may be pressed in or stick to it.

3.4. Glazing

In addition to impurities in the ceramic clay itself and dust in the environment, the raw materials of the glaze and the glazing process—spray glazing, dipping glazing, Brushing glaze process and Pouring glaze process , will also result in debris.

“Impure glaze”: Glaze (Instructions for making glaze in ceramic mugs) is like a layer of glass slurry. If the glaze is mixed with undissolved lumps, impurity particles (such as packaging bag fragments, dirt that falls in during mixing), or if the filter is broken and cannot block impurities, then the mug coated with glaze will have “slag” on it.

“Glaze bucket contamination”: The glaze buckets, mixing tools, or glaze sieve screens are unclean, with dried glaze lumps or impurities falling into them.

“Dirty glazing environment”: The glazing workshop is dusty, or there is dust and impurities on the workers' gloves and workbenches. When the mugs are dipped, sprayed, or brushed with glaze, impurities may fall onto the wet glaze surface and be trapped. Unstable air pressure during spraying may also blow dust from the environment onto the glaze surface.

3.5. Firing (high-temperature firing in a kiln)

“Unsanitary kiln”: This is a very important point! If the kiln (kiln walls, kiln cars, shelves, and pillars) has accumulated kiln ash, broken porcelain pieces, oxide scale, etc. from previous firings, the airflow disturbance (especially in gas kilns) during high-temperature firing can easily blow these dirty substances up, and they will then fall like snowflakes onto the glaze surface of the mugs being fired below. The glaze on the mugs melts and softens at high temperatures, and these impurities stick to and embed themselves in the glaze when they fall on it.

“Fuel-related problems”: If fuels such as gas or heavy oil are used, coal ash and oil droplets produced by incomplete combustion may also be carried by the airflow onto the products.

“Casing debris”: If a casing (a type of refractory container that protects the mug) is used for firing, the casing will age with prolonged use, and sand particles falling from the inner wall will also fall onto the mug and form debris.

4. The impact of impurity

Impurity is one of the most common defects in ceramic mugs, directly leading to: an unsightly appearance and rough texture. If the proportion of debris exceeds national standards, it will affect the pass rate of the batch of goods. In this case, China Ceramic Mug Factory inspects the ceramic mugs at every stage of the process and picks out the greenware with debris. Raw castings that can be repaired are repaired, and those that cannot be repaired are recycled and replenished. By spreading the inspection process across every stage of the process,  Ceramic Mug Group has reduced the scrap rate while ensuring the pass rate of ceramic mugs.

5. How to reduce impurity

The key is to maintain cleanliness throughout the entire process:

Strict raw material control: Select high-quality raw materials and strengthen iron removal and screening.

Environmental cleanup: Keep all workshops (especially molding, drying, glazing, and kiln areas) clean and tidy to reduce dust. Keep the floor moist or use dust collection equipment.

Regular cleaning of tools and molds: Thoroughly clean molds, workbenches, tools, glaze buckets, and glaze screens on a regular basis.

Kilns are a priority: Regularly clean the interior of kilns (kiln walls, shelves, supports, kiln cars), removing debris and accumulated dust. Inspect the condition of kiln shelves.

Operate according to standards: Workers must maintain personal hygiene (gloves, work clothes) and avoid introducing contamination during operations.

Understanding the causes of impurity buildup is key to resolving it. Controlling the hygiene and sources of impurities at every stage is the key to producing smooth and flawless mugs! Currently, most ceramic mug factories use assembly lines, and the hygiene of the workshop is guaranteed by the equipment. The decisive factor in the large difference in the pass rate of ceramic mugs between factories is actually people. It is the strength of the manager's control and supervision of the system, and the strength of the workers' implementation of the system and their sense of responsibility. Solving slag is not a difficult problem, but maintaining it is difficult.

Detailed explanation of the pouring glaze process for ceramic mugs

Xinxiang custom ceramic mug manufacturer, glazing is an important process in the production of ceramic mugs. This process is characterized by its suitability for glazing the outer surface of ceramic mugs, as it perfectly covers the connection between the handle and the body of the mug and creates a natural glaze flow and texture effect. Although glazing is not very efficient for mass production of ceramic coffee mugs, its unique texture effect makes it an irreplaceable glazing process for some ceramic mugs.

1. What is glazing

Glazing, as the name suggests, is a method of pouring glaze onto a ceramic body (an unfired mug) like pouring water. Imagine holding a pot of glaze and evenly pouring it onto the surface of a mug body placed on a turntable. The glaze flows naturally due to gravity, covering the parts of the blank that it comes into contact with. It's a bit like giving the mug a “shower,” except that it's glaze instead of water.

2. Glazing process

Glazing usually involves the following steps:

Secure the blank: First, the worker secures the dried mug blank (commonly referred to as a “green blank”) to be glazed on a rotating turntable or on a special stand. Make sure the mug is placed steadily.

Preparing the glaze: The glaze is pre-mixed and stirred to the appropriate consistency (similar to thick cream) in a dedicated glaze bucket. Workers use a container with a spout (such as a large glaze spoon, glaze pitcher, or small bucket) to scoop the appropriate amount of glaze.

Pouring: The worker holds the container of glaze and carefully pours it over the rim of the mug. At the same time, the mug is usually slowly rotated on the turntable. This is done so that the glaze flows evenly and continuously from the rim of the mug like a curtain of water, covering the outer wall of the mug.

Covering the mug: Under the force of gravity, the glaze flows down the outer wall of the mug. Workers need to control the speed of pouring and movement to ensure that the glaze completely and smoothly covers the entire outer surface of the mug, leaving no gaps or excessively thick areas. Special attention should be paid to the connection between the handle and the body of the mug, as these areas are prone to insufficient glaze coverage.

Treating the rim and inner wall: Glazing mainly treats the outer wall of the mug. During the pouring process, a small amount of glaze will flow into the mug. Workers need to precisely control the amount and timing of the pouring so that the glaze flowing into the mug just covers the inner wall and the inside of the rim, achieving the desired thickness of the inner glaze. Alternatively, the interior glaze may be applied separately by other means (such as dipping or spraying).

Draining and cleaning: After the glazing is complete, the workers stop pouring. The mug is left to stand for a short time or continues to rotate for a while to allow the excess glaze to drip back into the glaze bucket or collection tray. Finally, workers carefully check the glaze layer for evenness and completeness, and use a sponge or tool to carefully wipe away any glaze residue from the bottom of the mug where it touched the rack or turntable (if these areas are not wiped clean, they will stick to the kiln shelf after firing). The glazed mugs are then sent to dry and await firing.

3. When is glaze pouring suitable

Glaze pouring is particularly suitable for the following situations:

Vessels where the glaze is primarily applied to the exterior: A mug is a classic example! Glaze pouring can efficiently and evenly cover the entire exterior surface of the cup, including that tricky handle. Dip glazing the handle can easily trap air bubbles, and spray glazing may be uneven, but with pouring glazing, the glaze can flow naturally into every corner of the handle.

Medium or large greenware: For greenware of a certain size and weight, such as mugs, dip glazing by hand may be inconvenient or laborious, but pouring glazing is much easier and the greenware can be held steady for more stable operation.

Separate control of inner and outer glazes or only outer glaze required: If the production process requires different colors or types of glaze for the inner and outer walls of the mug (for example, the inner glaze needs to be more food-safe), pouring glaze makes it very easy to apply only the outer glaze (carefully controlling the glaze so that it does not flow into the inner wall) or to precisely control the amount of inner glaze that flows in. Even if the same glaze is used for both the inner and outer surfaces, glazing can simultaneously cover both surfaces.

Achieving specific glaze effects: Under the influence of gravity, glaze flows and can sometimes form natural, dynamic patterns or variations in thickness, creating unique effects that are difficult to achieve with other glazing methods.

Small-batch or artisanal production: Compared to automated spray glazing lines, glaze pouring equipment requires relatively lower investment and offers greater operational flexibility, making it ideal for small-to-medium-scale production or factories emphasizing artisanal craftsmanship.

Glazing involves applying glaze to ceramic bodies using a “showering” method. The core of the process involves pouring glaze + rotating the body, allowing the glaze to flow and cover the surface under the influence of gravity. For mug production, its biggest advantage is that it can perfectly and efficiently handle the outer wall of the mug and the complex handle structure, while also taking into account the glazing of the inner wall. It is particularly suitable for situations where the inner and outer glazes need to be separated or the outer wall needs to be treated with special care. China ceramic mug factory thinks although it requires skilled workers to control the uniformity and pay attention to glaze recovery, it is a very practical and widely used glazing technique.

Detailed explanation of the brushing glaze process for ceramic mugs

Xin xiang ceramic mug manufacturer, brushing glaze is an important process in glazing ceramic mugs. This process is characterized by high flexibility and a long history of use. In some cases, it is necessary to use the brushing glaze process when glazing mugs. However, this process requires high technical skills and has high production costs.

1. What is glazing?

Glazing, as the name suggests, is a method of “brushing” glaze onto the surface of a ceramic body with a brush (similar to a calligraphy brush or paintbrush). We have seen furniture being painted, but this is ceramic glaze being applied to ceramic mugs. It is one of the oldest, most direct, and most flexible methods of glazing. Unlike “dip glazing,” which involves immersing the entire body in glaze slurry, or “spray glazing,” which requires advanced equipment, brush glazing is akin to an artist painting a picture, applying the glaze layer by layer.

2. Applications of brush glazing

Brushing glaze is done by hand, so it is inefficient but highly flexible. Therefore, brushing glaze is suitable for ceramic mugs that are not produced in large quantities and for small orders. Because brushing glaze is highly flexible, this process is also suitable for ceramic mugs with complex glaze requirements, such as ceramic mugs with handles and bodies of different colors, or ceramic mugs with gradient colors on the body or handle. Brush glazing is also suitable for ceramic cups with multiple layers of glaze. For example, ceramic cups with wood grain, floral patterns, stone patterns, etc. on parts of the cup body, or ceramic cups with glitter glaze. Because brush glazing is usually done with a brush or paintbrush, it is also very suitable for repairing areas where glaze has been smeared or sprayed (such as the handle connection, cup rim, cup bottom, etc.). In addition, the brush glazing process is often used for complex mug shapes. For example, the brush glazing process is used for mugs that are too large, too small, or have complex shapes.

3. What is the brush glazing process?

3.1. Prepare the glaze slurry: Mix the glaze powder and water in a specific ratio and stir to a consistency similar to thick cream or yogurt. If the mixture is too thin, it may flow unevenly; if too thick, it may be difficult to apply and leave heavy brush marks. “Instructions for Making Glaze in Ceramic Mugs”

3.2. Prepare the body: Make sure that the mug body waiting to be glazed is completely dry (called “raw body”) and that the surface is clean and free of dust and oil stains. Typically, they undergo a “plain firing” process (What is the plain firing process for raw ceramic mugs? ) to make them sturdy and less prone to breakage, making it easier to absorb water (better glaze adhesion).

3.3. Glazing steps:

Glazing technicians hold a brush (commonly a wool brush or soft-bristle brush) and dip it into an appropriate amount of glaze slurry. Like painting, the glaze is evenly applied to the surface of the body. The direction of brushing should be consistent or cross-brushed to ensure full coverage. For mugs, pay special attention to the rim, the connection between the handle and the body, and the bottom rim, which are areas where glaze is easily missed or accumulated. Both the outer surface and inner cavity of the mug need to be carefully brushed. The glazing process usually requires 2-3 coats or even more to achieve the desired glaze thickness and even coverage. During multiple glazing applications, pay attention to the time intervals: After applying the first layer (base glaze), wait until the surface is slightly dry before applying the second layer. If the glaze surface is too wet when applying the second layer, it may damage the first layer, resulting in uneven glaze or peeling.

After glazing is complete, the ceramic mugs should be inspected and repaired: Carefully check whether the glaze surface of the ceramic mugs is uniform, and whether there are any missed spots, bubbles, or obvious brush marks. If there are any problems, repair them with a pen in a timely manner. Finally, place the glazed mugs on a drying rack to dry naturally, avoid touching them, and wait for them to be fired in the kiln. “In-depth analysis of the firing process of ceramic mugs” .

4. Pre-glazing and post-glazing work

4.1. Pre-glazing preparation:

Body preparation: Ensure that the green body is completely dry, cooled, and free of dust and grease (use a soft brush or compressed air to clean). Inspect the body for defects (cracks, pores), which must be repaired before glazing.

Glaze slurry preparation: Accurately measure the glaze materials and water according to the formula, mix thoroughly (preferably sieve) to the desired consistency, and let it settle to remove bubbles. Stir again before use.

Tool preparation: Prepare clean, appropriate brushes (different models may be needed for different areas), containers for holding the glaze slurry, stirring rods, clean water (for washing brushes), and racks for holding unglazed and glazed greenware (to prevent contact between pieces).

Operator preparation: The operator’s hands must be clean and dry. Understand the characteristics of the glaze slurry (such as drying speed) and the desired effect.

4.2. Post-glazing preparation:

Handle with care: Glazed pieces are extremely fragile (the glaze layer has not yet been fired), so they must be handled and placed with extreme care to avoid any bumps or fingerprints/scratches on the glaze surface.

Thorough drying: Place the glazed mugs in a well-ventilated, dust-free, safe (no people walking around and touching them) place to dry thoroughly and naturally. The drying time depends on the humidity of the environment and the thickness of the glaze layer. Make absolutely sure that they are completely dry before putting them in the kiln! Residual moisture will turn into steam when the kiln is heated, causing the glaze surface to crack and even the body to explode.

Final inspection: After drying, carefully inspect the glaze surface again for cracks, peeling, dust impurities, missed areas, or excessive buildup. Minor defects can be attempted to be repaired by gently sanding with extremely fine sandpaper (e.g., 800 grit or higher), but large-scale issues may require washing off and reapplying the glaze.

The glazing process is a test of the technician's experience. The quality of the glaze is usually determined by high-quality glaze materials, fine techniques, and strict management. With more than 20 years of professional ceramic mug production experience, China Ceramic Mug Factory has many experienced glazing technicians. We adhere to the concept of “details are fundamental, quality is life” to provide customized ceramic mugs for our customers.