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.

Detailed explanation of the glaze spraying process for ceramic mugs

Xinxiang ceramic mug manufacturer, spray glazing is a surface treatment technique used in the production of ceramic mugs. A spray gun is used to atomize the glaze into tiny particles, which are then evenly sprayed onto the surface of the ceramic blank. Glaze is a glass-like coating that, when fired, gives ceramics smooth, waterproof, and wear-resistant properties, while also serving a decorative purpose. Spray glazing is suitable for complex shapes (such as unconventional mugs) or ceramic mugs that require fine gradation effects. Compared with traditional dip glazing, it allows for more flexible control of glaze thickness and coverage.

1. Spray glazing operation process

The spray glazing process is more advanced than dip glazing. Currently, ceramic mug factories commonly use two types of spray glazing processes: manual spray glazing and automatic spray glazing. Manual spraying requires a spray gun and air pump. The technician holds the spray gun, keeping it 20-30 cm away from the mug, with an air pressure of 0.3-0.5 MPA, and moves the spray gun at a constant speed, first covering the edges and recesses, then spraying the entire surface to ensure an even glaze layer. Complex areas (such as the inside of the mug handle) need to be sprayed repeatedly. Automatic assembly line operation: Place the ceramic mug blank upside down on the rack and send it to the conveyor belt. When the rack moves the ceramic mug under the spray gun, it rotates at a constant speed for 2-5 seconds (the dwell time is determined according to the different glaze materials and requirements) to spray the glaze. After spraying the glaze, check the ceramic mug blanks. If there are any areas missing glaze, use a brush or sponge to fill in the glaze. Finally, clean up the excess glaze at the bottom of the blanks to prevent them from sticking to the kiln plate during firing.

2. The difference between spray glazing and dip glazing

Operating method: Glaze spraying relies on equipment for application, while glaze dipping involves directly immersing the green body into glaze slurry to absorb it. In automated production lines, the cost of glaze spraying and glaze dipping lines differs.

Applicable scenarios: Spraying glaze is suitable for complex shapes or partial glazing (such as mugs with transitional colors); dipping glaze is suitable for simple shapes and ceramic mugs with low glaze color requirements. Dipping glaze is highly efficient but not as good as spraying glaze in terms of detail.

Glaze layer effect: Spray glazing can control the thickness of the glaze layer to achieve gradations or textures; dipping glazing produces an even glaze layer but tends to accumulate glaze on the edges.

Cost and difficulty: Spray glazing requires equipment investment and high operating skills; dipping glazing is low in cost and suitable for the production of conventional ceramic mugs.

3. Precautions fspray glazing

or The spray glazing process uses a spray gun to apply glaze to ceramic mugs. The glaze slurry has strict requirements. If it is too thick, it will clog the spray gun, and if it is too thin, it will not provide sufficient coverage. In addition, the glaze slurry needs to be stirred regularly to prevent sedimentation. The spray gun needs to be thoroughly cleaned after each use to prevent residual glaze from clogging the spray gun and to prevent residual glaze from being sprayed onto the greenware after drying, causing debris to fall on the ceramic mug. During the glaze spraying process, avoid spraying at a fixed point for a long time (which can easily cause glaze flow). The key is to coordinate the movement speed of the rack with the air pressure of the air gun. Additionally, workers in the spraying workshop must wear masks and goggles to avoid inhaling glaze dust.

4. Preparation before spraying glaze

Body treatment: After drying, the ceramic mug body must be completely cooled, and surface dust must be removed with compressed air.

Glaze Preparation: Adjust the glaze slurry. For specific instructions, please refer to “Instructions for Making Glaze in Ceramic Mugs”

Test spraying: Test spray on scrap clay or bisque-fired pieces to observe glaze adhesion. Adjust the rotation speed and time of the stand, as well as the air pressure of the spray gun.

5. Post-glazing treatment

After spraying the glaze, the raw ceramic mug needs to be dried. During the drying stage, the raw mug should be kept out of direct sunlight or strong winds, and it is best to dry it naturally in the shade for 6-12 hours (depending on the humidity). After applying the glaze, the glazed surface needs to be inspected, mainly to observe whether the glaze is uniform under strong light, with a focus on checking areas prone to leakage, such as the handle connection and the rim of the mug. In addition to inspection, the glaze on the bottom of the mug should be removed so that the part of the ceramic mug that comes into contact with the kiln plate remains free of glaze. Finally, the ceramic mug blanks are placed on the kiln plate with spacing between them and wait to be fired in the kiln.

6. Six major defects that are likely to occur

6.1. Uneven glaze surface

Some substandard colored glazed ceramic mugs have uneven glaze thickness on the surface, which can be distinguished by the naked eye. The orange peel glaze phenomenon of ceramic mugs that we discussed earlier is a relatively obvious example. For the specific causes of this issue, please refer to “Research and Solutions for the Orange Peel Glaze Phenomenon in Ceramic Mugs”  .

6.2. Glaze cracking

After the ceramic mug is glazed and dried, cracks appear. These cracks are mainly divided into horizontal, vertical, and mesh patterns. The main reasons for this are: a. The glaze layer is too thick (>0.5mm) or the glaze has been sprayed multiple times. Because the glaze layer is too thick, cracks are likely to appear on the glaze surface during the drying stage of the ceramic mug. b. The ceramic mug dries too quickly during the glazing and drying stage. The main reasons for this are that the temperature in the drying workshop is too high or there is a strong wind blowing directly on it.

6.3.  Grainy glaze surface

There are obvious bumps on the glaze surface of the ceramic mug. The main reason for this phenomenon is the glaze. The glaze is too coarse, and the particles in the glaze slurry are sprayed onto the ceramic mug. Another reason is that the spray gun is not cleaned regularly, causing the previous glaze to clump together and be sprayed onto the ceramic mug.

6.4. Glaze flow

This phenomenon is similar to the first type of uneven glaze surface. Glaze flow mainly occurs at the bottom and rim of the mug. If we disregard the glaze itself and only consider the glazing process, then local glazing will lead to glaze flow.

6.5. Pinholes/bubbles

Pinhole and bubble defects in ceramic mugs during the glazing process are mainly caused by bubbles and impurities in the glaze, or dust on the greenware.

6.6. Glaze peeling

If the glaze peels off a spray-glazed ceramic mug, there are two possible causes. There are two reasons for this: one is that the green body is contaminated with oil or dust; the other is that the moisture content of the green body is too low. This prevents the glaze from adhering properly to the green body, causing the glaze layer to peel off.

Spray glazing is a process that is often used when customizing ceramic mugs. This process has obvious advantages when dealing with mugs of special shapes and mugs with special requirements for the glaze surface. The essence of good glaze spraying lies in the combination of “standardization” and “refinement.” From raw material selection and glaze preparation to the glazing process, every step is controlled by the people at Xinxiang Ceramics Factory. China Ceramics Mug Factory believes that only by converting process parameters into executable SOPs and continuously optimizing every detail can the best balance between efficiency and quality be found.

Ceramic mug dip glaze process details

Xin xiang custom ceramic mug manufactuer, in the previous section we talked about the phenomenon of orange peel glaze on ceramic mugs. One reason for this is the glazing process. Our common ceramic mugs are glazed, professionally this kind of mug is called color glaze mug. Many people said, we see the color glaze mug very little ah, are like: sublimation mug, printing mug, travel cup, stoneware mug, etc.. In fact, these ceramic mugs have a layer of glaze on the body, strictly speaking, they all belong to the color glaze mug. Only, on the basis of color glaze mugs, they have their own independent characteristics, so they are named after their characteristics. Well, let's talk about the glazing process of ceramic mugs, the glaze developed into a glaze paste, attached to the ceramic mug billet process, called glazing. Common glazing methods include dip glaze, spray glaze, brush glaze, pouring glaze and so on. Today we focus on a talk: dip glaze process.

1. Glaze before the preparation

Before the formal dip glaze, ceramic mug factory is to do some preparatory work. The purpose of these preparations is to ensure the success of the glazing process, reduce the rate of defective products. These preparations are mainly divided into 4 parts:

1.1. Clean ceramic mug raw blanks

In the previous article we will have, after the clay made of ceramic mug billet, to carry out the “Ceramic mug clay billet drying and billet repair process”  During the drying process, the blanks may be dusted. If the dust on the mug is not cleaned up, then the adhesion of the glaze is reduced and the ceramic mug we get in our hands is prone to problems. In this process, the raw blanks need to be cleaned with an air gun. At the same time, but also to detect the water content of the raw ceramic mug, the most suitable glaze water content is: 3% -5%. Raw billet water content is too low, it will be excessive water absorption, damage the structure of the glaze; water content is too high is prone to pinholes, flowing glaze and other problems.

1.2. Raw ceramic mugs

Whether the raw ceramic mugs should be plain fired, we have explained in the previous article. “What is plain firing of raw ceramic mugs?”.  Some ceramic mugs with complex shapes or special requirements for the thickness of the mug, need to be fired before glazing. Vegetable firing process, can enhance the quality of the ceramic mug billet, in the later glaze, hand-painted, paste underglaze flower paper and other processes, is essential, this link is not all ceramic cups, only for high-end ceramic cups.

1.3. Prepare glaze

Quartzite, feldspar, kaolin, after powdering, sieving, mixing slurry, etc., made of glaze paste, these specific operations in the “glaze raw materials, production, classification and quality” explained in detail. Before glazing, the glaze paste should be stirred and tested, mainly checking the color and fluidity of the glaze paste.

1.4. Cleaning the workshop

The glazing workshop in a ceramic factory is cleaned separately before formal glazing. It is the same as ceramic mug raw cleaning, in order to guarantee the cleanliness of the glaze on the ceramic mug. The main thing is to clean up the dust, control workshop temperature and humidity. Workshop temperature is generally controlled at about 25 ℃, humidity control at about 50%.

2. Ceramic mug dip glaze

2.1. Basic principle

Utilizing the water-absorbing properties of ceramic mug blanks, the surface is uniformly adhered to by immersing the glaze paste, similar to the principle of painting oil paintings. The glaze paste penetrates into the pores of the ceramic mug under capillary action, forming a 0.2-0.3mm thick glaze layer.

2.2 Scope of application

Dip glaze on the shape of the ceramic mug is required, regular cylindrical, conical ceramic mugs, using dip glaze. Dip glaze because the operation is simple, with automatic dip glaze equipment, for large orders, often used dip glaze process.

2.3 Operation steps

Mechanical arm grips the upper edge of the mouth of the mug at 1/3, the raw billet is vertically immersed in the glaze paste, keep the glaze surface 5mm away from the mouth of the mug, hold it for 2 seconds, and then lift it away from the glaze surface at an even speed. After lifting, the mechanical arm rotates 3-5 turns at 60 rpm to shake off the excess glaze. During this process, keep the mouth of the mug tilted downward by 15 degrees to discharge the accumulated glaze using gravity.

3. Drying and finishing

3.1 Drying

After glazing, place the raw ceramic mugs horizontally on the grid drying rack, and require the ambient temperature to be about 25℃ and humidity to be about 50%, drying for 2 hours. Then, raise the ambient temperature to 40℃, humidity 30%, and activate the circulating fan to accelerate the drying for 5 hours.

3.2. Finishing

Use sponge, scraper and other tools to clean up the mouth and bottom of the mug, excess glaze.

4. Quality Inspection

Grade A standard: glaze without pinholes, bubbles (diameter <0.2mm); thickness fluctuation <±0.05mm; mug roundness error <0.3mm.

5. Common Problems

5.1: Glaze accumulation at the mouth of the mug (“glaze circle”)

Causes: insufficient rotating speed of dumping glaze / too deep immersion glaze

Countermeasures: Increase the speed to 80 rpm; install limiters to control the depth of immersion.

5.2: Lack of glaze on the handle

Causes: Air bubbles attached / Mechanical arm clamping obscured

Countermeasures: ultrasonic vibration to remove bubbles before dipping glaze.

5.3: Glaze layer peeling off

Phenomenon: after drying the glaze appears spider web cracks

Root cause treatment: adjust the glaze paste formula (add thickener)

5.4: Uneven thickness

Means of detection: every 30 pieces with a thickness gauge sampling test

Process optimization: the specific gravity of glaze paste is stabilized at 1.45; add automatic replenishment pump to maintain a constant liquid level.

5.5: Glaze pollution

Pollution source tracking: 60% from workshop dust; 30% from impurities in the glaze tank.

6. Analysis of advantages and disadvantages of glaze dipping method

6.1 Core advantages

Outstanding efficiency: single machine handles 800-1200 pieces per hour

Low cost: the utilization rate of glaze material＞95%.

Stable quality: pass rate up to 92%-95%.

6.2 Inherent limitations

Weak adaptability: not suitable for relief/shaped mugs

Edge defects: the mouth of the mug needs to be processed twice.

The immersion glaze method is still the mainstream glazing process for ceramic mugs after centuries, and its essence lies in the combination of “standardization” and “refinement”. As the old craftsman said: “dip glaze three points by equipment, seven points in the management.” Even the most sophisticated equipment, but also need people to manage. The production of ceramic mugs is a process of detailing. From raw material selection, glaze preparation to glaze process, each link has Xinxiang ceramic factory people on the details of the control.  China ceramic mug factory believes that the only way to find the best balance between efficiency and quality is to transform the process parameters into enforceable SOPs and continuously optimize every detail.

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.