Xin xiang 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” Xinxiang Ceramic Mug Factory 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.