Xin xiang Ceramic Mug Manufacturer, we received custom-made ceramic mugs, and some of them had areas that should have been covered with glaze but were missing glaze. Ceramic mugs like these are considered substandard products. This type of glaze defect is called glaze crawling, which is a common glaze defect in custom-made ceramic coffee mugs. This defect often appears at the connection between the handle and the body of the mug, as well as at the bottom and rim of the mug. What causes glaze crawling in ceramic mugs, and how can ceramic factories avoid this defect during production? Today, Xinxiang Ceramic Mug Factory will explain it to you slowly:

1. What is glaze crawling?

Imagine that you brush a layer of glaze on a ceramic mug blank (just like painting it with paint). After firing, the ideal state is that the entire surface of the mug is covered with a smooth, even, glassy glaze layer. But glaze crawling ruins it! It refers to the glaze layer failing to properly cover the body surface during or after firing, instead “retracting” from certain localized areas like water droplets on a lotus leaf. As a result, the areas that should have been covered by the glaze layer expose the underlying unglazed, rough clay body, while the edges of the glaze layer form distinct, irregular, coastline-like boundaries. In simple terms, the glaze has “run away,” leaving behind patches of unglazed “bald spots.”

2. What does glaze crawling look like?

It is easy to spot glaze crawling on a fired mug. The most obvious sign is one or more areas on the mug that are not covered by glaze, exposing the original color and texture of the clay or porcelain (usually matte and rough). These areas can be seen with the naked eye, and when you touch them, you will find that they are clearly different from other areas. The edges of glaze crawling are also very distinct. The edges of these “bald spots” do not fade gradually but form sharp, irregular boundaries, as if the glaze had suddenly broken off at that point. Both visual inspection and tactile examination can easily distinguish these areas.

3. Location of glaze crawling

In theory, glaze crawling can occur in any area of a ceramic mug. However, some areas are more prone to glaze crawling, such as the rim of the mug, the area where the handle joins the body, the bottom of the mug, and areas where there are small dents or bumps on the body. Why are these areas prone to glaze crawling? Essentially, these areas either tend to accumulate dust or undergo significant shape changes, leading to reduced glaze adhesion.

4. What causes glaze crawling?

We have previously discussed the basics of glaze (Instructions for making glaze in ceramic mugs) and the firing process (In-depth analysis of the firing process of ceramic mugs). Glaze melts into a liquid at high temperatures and should flow evenly to cover the surface of the clay body. Its “crawling” or retreat is primarily due to two reasons:

First, the glaze and the clay body are incompatible (rejection/poor wetting): This is the most common cause. As you may have observed, water spreads out when poured onto clean glass (good wetting), but forms droplets when dropped onto wax paper (poor wetting). Similarly, before glazing a ceramic mug, if the surface of the body is contaminated with dust, oil stains (such as grease from your hands), wax marks, or fine powder left over from previous grinding, these substances act as a “barrier film” that prevents the molten glaze from approaching the body, causing it to shrink and gather in these dirty areas, forming glaze crawling.

Second, the body is too dry/absorbs water too quickly: If the body is extremely dry before glazing (especially in hot or dry workshop conditions) or has low density and high water absorption, the glaze slurry is instantly “absorbed” upon application. The glaze particles do not have time to spread evenly or adhere firmly before drying on the surface. This layer of dried glaze has a loose structure, and when it is actually melted into a liquid during firing, it cracks and shrinks like dry mud, unable to form a glaze layer with good fluidity to cover the entire ceramic mug. This situation can also cause glaze crawling.

Third, the glaze and the green body are “incompatible” (the thermal expansion coefficients do not match): After the glaze is fired and cooled, if the degree of crawling of the glaze is much greater than that of the body, the glaze layer will be “tightened.” When the tensile force is too great, the glaze layer may be “pulled” off the surface of the body, forming a crack in the glaze layer, which is called glaze crawling. Cracks and crawling occur especially at the edges or corners of ceramic mugs. This requires ceramic manufacturers to control the crawling ratio of each batch of ceramic raw materials and glaze materials to minimize such occurrences.

Fourth, glaze formulation issues: if the glaze concentration is too high or the viscosity is too great during formulation, it can also cause glaze crawling. If the glaze slurry is too viscous or thick, whether applied via spray glazing (Ceramic mug dip glaze process details) or dip glazing (Detailed explanation of the glaze spraying process for ceramic mugs), the glaze will have poor flowability when applied to the body, making it difficult to spread evenly. Thicker areas may be acceptable, but thinner areas or curved sections will have uneven glaze distribution. During firing, these weaker areas are more prone to crawling, resulting in glaze crawling. Of course, if the glaze layer is too thick, glaze running may occur. Xinxiang Ceramics Factory will provide further details on glaze running (Running Glaze) in future articles.

Fifth, as mentioned in previous articles, if the glaze material is not ground finely enough during the grinding stage, it may cause kiln dusting (Kiln Dusting - Causes and solutions for impurity in ceramic mugs) and pinholes (Defects in customized ceramic mugs - pinholes). So, does simply increasing the grinding degree of the glaze material and grinding it too finely solve all these issues? Actually, there is an old Chinese saying: “Too much of a good thing is as bad as a lack of it.” If the glaze is ground too finely: The glaze particles are ground too finely, making the glaze more prone to clumping and generating excessive crawling stress during drying, leading to micro-cracks in the glaze layer before firing. During melting, the glaze shrinks from the cracks. Excessively fine grinding of the glaze is also one of the causes of glaze crawling.

5. Methods to reduce glaze crawling

Based on the above understanding, we now know the causes of glaze crawling and can implement targeted improvements to address the issue. The key to resolving glaze crawling lies in ensuring better integration between the glaze and the green body.

First: Maintain the cleanliness of the green body to remove any “barriers” between the glaze and the green body, facilitating better integration. Among these, dust removal is the key! Keep the workshop clean to reduce the amount of dust in the air that floats back onto the blank. Use dry, clean compressed air or a soft brush (such as a wool brush) to carefully blow/brush away all dust and powder from the surface of the blank. Operators must wear clean gloves to avoid direct contact with the areas to be glazed (especially the rim and handle base).

Second: Maintain the humidity of the greenware. In our article on the (Drying and trimming process of ceramic mug clay blanks), we discussed the ideal moisture content of greenware, which is 18%-22%. During the hot summer or dry autumn, moisture content may drop too low, so it is necessary to “hydrate” the greenware. Moisturizing before glazing: Before glazing (e.g., a few hours or a day in advance), use a clean spray bottle to evenly apply a thin layer of water mist (wet finishing) to the dry blank, or briefly place the blank in a room with moderate humidity (steaming). The purpose is to allow the surface of the greenware to absorb a small amount of moisture, reducing its excessive absorbency. This ensures that when the glaze slurry is applied, the moisture is not immediately absorbed, giving the glaze particles time to distribute evenly and adhere to the surface of the greenware. Ensure that the moisture is applied evenly and avoid pooling.

Third: Optimize the performance of the glaze slurry. For the glaze slurry, two key properties must be prioritized. The first is the concentration and viscosity of the glaze slurry: Adjust the concentration and viscosity of the glaze slurry precisely based on the glazing method (spraying, dipping, pouring, or pouring), seasonal temperature, and humidity. Ensure the glaze slurry has good flowability and can evenly cover the body. If it is too thick, add water to dilute it (in small amounts multiple times); if it is too thin, add glaze powder or a thickening agent (such as CMC). The second is controlling the fineness of the glaze material: Regularly test the fineness of the glaze slurry (using the sieve residue method or sedimentation method) to ensure it meets process requirements. Avoid over-grinding, which can result in excessive fineness, and ensure there are no residues in the glaze slurry, as these can cause debris to settle.

Fourth: optimize the firing process. To prevent glaze crawling, appropriately extend the heating time before glaze melting (oxidation period): During the heating stage before the glaze begins to melt (glaze melting temperature), especially during the phase where moisture and organic compounds are expelled, slowing down the heating rate helps fully release trace gases, moisture, and organic compounds adsorbed on the interior and surface of the body, reducing the likelihood of defects on the glaze surface (including glaze crawling) caused by their sudden release during glaze melting. However, this requires experimental adjustment based on the characteristics of the kiln and the product.

6. Summary

Glaze crawling occurs when the glaze fails to adhere properly to the body during firing, causing it to shrink into a clump and expose “bald spots.” The primary causes include the presence of dust, oil, or excessive dryness in the body (which rapidly absorbs moisture from the glaze slurry), or sometimes due to the glaze slurry being too thick or ground too finely. The solution is to thoroughly blow clean the body before glazing, wear gloves when operating, add an appropriate amount of water to the dry body, and adjust the concentration of the glaze slurry to make it flow well. By focusing on the two core points of “cleaning” and “moistening,” you can greatly reduce the phenomenon of glaze crawling on mugs!