Crazing is a network of fine cracks that forms in the glaze of a ceramic piece. It looks like a web of hairline fractures spread across the surface, sometimes barely visible until the piece gets wet or dirty and the cracks pick up staining. The root cause is a mismatch between how much the glaze and the clay body shrink as they cool after firing. When the glaze contracts more than the clay underneath, it gets stretched like a too-tight skin, and that tension eventually cracks it.
Why Crazing Happens
Every material expands when heated and contracts when cooled, but different materials do so at different rates. In ceramics, the glaze and the clay body each have their own rate of thermal expansion. When a piece comes out of the kiln and begins cooling, the glaze solidifies and bonds to the clay body. If the glaze shrinks faster or more than the body, it ends up under tension. Once that tensile stress exceeds what the glaze can handle, it fractures into the characteristic web pattern.
The degree of mismatch determines the severity. A slight difference might produce only a few faint lines. A large mismatch creates a dense, obvious crack network. Potters control this by adjusting glaze chemistry, particularly the ratio of silica and flux materials, to bring the glaze’s expansion rate closer to the clay body’s. Firing temperature also plays a role: higher temperatures can change how the glaze and body interact, shifting the stress balance in either direction.
Delayed Crazing: Why It Shows Up Later
One of the more frustrating aspects of crazing is that it doesn’t always appear right away. A piece can look perfect out of the kiln, then develop cracks days, weeks, or even months later. This delayed crazing has a specific cause: moisture expansion of the clay body.
After firing, porous ceramic bodies slowly absorb water from the atmosphere. This isn’t visible to the eye, but it causes the clay body to expand slightly over time. Research into this phenomenon dates back over a century, and the mechanism is well understood. Water molecules enter the open pores of the ceramic surface through a process of rehydration, and the body gradually swells. The glaze, being glass, doesn’t absorb moisture and stays the same size. As the body underneath grows, it puts increasing tension on the glaze until cracks form.
This is particularly common in low-fired ceramics like earthenware and some terracotta, where the body remains more porous. One study measured moisture expansion of 0.1% in a porous clay body, which was enough to produce new cracks in glazes that had survived the initial cooling without damage. The crack network in these cases is often unstable, meaning new, thinner cracks continue to appear over time as the body keeps absorbing moisture. Stoneware and porcelain, fired at higher temperatures, are much denser and far less susceptible to this kind of long-term expansion.
Is Crazing Just Cosmetic?
For most pottery and tableware, crazing is primarily a surface defect. The cracks are confined to the thin glaze layer and don’t penetrate into the clay body itself. A crazed mug or bowl won’t shatter because of the crazing alone. However, crazing does compromise the glaze’s function as a sealed, waterproof barrier. Liquids can seep into the cracks and stain the piece, and over time the edges of the cracks can catch on sponges or feel rough.
In structural or industrial ceramics, surface cracking is taken much more seriously. Radial cracks can link with other flaws in the material and grow into critical fractures, potentially causing catastrophic failure under load. But for a handmade bowl on your kitchen shelf, the concern is practical and hygienic rather than structural.
Food Safety Concerns
This is where crazing matters most for everyday users. Those tiny cracks create spaces where bacteria can lodge, and unlike a smooth glaze surface, the insides of the cracks are difficult to clean thoroughly. A hot, soapy wash will kill most bacteria on the surface, but organic material trapped deep in a crack can serve as a growth medium.
The risk level depends on how you use the piece. The highest concern is with food storage containers, especially for wet foods and liquids. If you store soup or sauce in a crazed ceramic pot for hours, a small bacterial colony hiding in a crack can multiply in the surrounding food. For this reason, any ceramic used for storing food should be completely free of crazing on the inside surface, including the rim.
Plates, cups, and bowls used for serving and immediately washed carry lower risk. Consistent washing in hot, soapy water keeps bacterial growth in check. But in commercial food service settings, crazed pieces are generally considered unacceptable because the cleaning cycle is less controlled and the stakes of contamination are higher. If you have a favorite crazed mug you drink coffee from every morning and wash right after, the practical risk is low. If you’re selling pottery or using it to store leftovers, crazing on food-contact surfaces is a real problem.
Intentional Crackle Glazes
Not all crazing is a mistake. Potters have deliberately produced crackle patterns for over a thousand years, treating the web of cracks as a decorative feature rather than a flaw. The most celebrated examples come from China’s Song Dynasty (960 to 1279 AD), particularly Guan and Ge wares, where the crackle pattern was the only decorative element on the piece.
Guan ware typically features large, bold cracks, while Ge ware is known for a distinctive double crackle network. The major cracks were deliberately stained black with ink or tea, and the finer secondary cracks appeared gold-brown, creating a striking two-tone effect. Potters achieved these patterns by intentionally formulating glazes with a higher expansion rate than the body, then controlling the crack density through the relative thickness of glaze to body, firing temperature, and kiln atmosphere.
Modern potters still use this technique, often called “crackle glaze” to distinguish it from accidental crazing. The visual effect is the same, but the intent and control are different. Some potters apply crackle glazes only to decorative or non-food-contact surfaces, getting the aesthetic without the hygiene concerns. Others use them on the exterior of functional pieces while keeping the interior glaze craze-free.
How to Prevent Unwanted Crazing
Prevention comes down to matching the thermal expansion of your glaze to your clay body. In practice, this means adjusting glaze recipes. Increasing silica in a glaze generally lowers its expansion rate, reducing the tendency to craze. Reducing high-expansion flux materials like sodium and potassium has a similar effect. Many potters rely on published glaze calculation software to predict expansion coefficients before mixing and firing.
Firing to the correct temperature for your clay body also matters. Underfiring leaves the body more porous and more vulnerable to moisture expansion down the road. A well-vitrified stoneware body that has been fired to maturity absorbs very little atmospheric moisture and is far less likely to develop delayed crazing.
Testing is essential. The simplest method is to fire test tiles and then stress them by boiling and quickly cooling them (thermal shock testing). If the glaze crazes under this stress, it’s under too much tension and needs reformulation. Some potters also examine pieces under a strong light at an angle, which reveals fine crazing that might otherwise go unnoticed for weeks.
For existing pieces that have already crazed, there’s no fix. You can’t re-seal a crazed glaze without refiring with a different glaze. If the piece is functional and the crazing is on the food-contact side, the practical choice is to retire it to decorative use or limit it to dry foods and immediate serving rather than storage.