Xinxiang 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 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, Xinxiang Ceramics 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, Xinxiang Ceramics 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.