How Do You Prevent Clay From Cracking?

Clay cracking is a common frustration for potters, sculptors, and ceramic artists. As clay dries and is fired, it undergoes physical and chemical changes that can lead to stresses, resulting in cracks and breakage. Cracks compromise the structural integrity of clay pieces and can ruin hours of hard work.

There are several different types of cracks that can occur at various stages of the ceramic process. Drying cracks happen as wet clay dries and shrinks. Thermal cracks happen from too rapid temperature changes during firing. Mechanical and structural cracks are caused by flaws in the clay body or pieces being too thin or stressed.

While some cracking may be unavoidable, there are several strategies ceramic artists can use to minimize cracks. Careful clay preparation, controlled drying and firing, and intentional design choices can help prevent cracking issues. This guide will overview the main causes of cracking and provide tips to help you avoid heartache and produce flawless ceramic art.

Table of Contents

Types of Cracking

There are three main types of cracks that can occur in clay:

Drying Cracks

Drying cracks happen as clay loses moisture during the drying process. As clay dries, it shrinks. If the clay dries too quickly or unevenly, cracks can form on the surface or inside the clay body. Drying cracks tend to be small and shallow.

Thermal Cracks

Thermal cracks occur from too rapid temperature changes during firing. If clay heats up or cools down too quickly, it can expand or contract unevenly, causing cracks. Thermal cracks can vary in size and depth depending on factors like clay thickness.

Structural Cracks

Structural cracks stem from weak spots in the physical structure of the clay piece. Weak structural integrity can be caused by issues like air bubbles in the clay, uneven thickness in the walls, or excess weight being unsupported. Structural cracks tend to be large and can often lead to complete breaks.

Causes of Cracking

There are three main types of cracks that can form in clay: drying cracks, thermal cracks, and structural cracks. Each type of crack has different causes.

Drying cracks occur as clay loses moisture. As clay dries, the material shrinks. If certain areas dry faster than others, cracks can form. Factors that contribute to uneven drying and drying cracks include:

Using a clay body with high shrinkage
Letting clay dry too quickly
Inconsistent thickness in the clay piece
Drafts or direct heat/sunlight during drying

Thermal cracks happen during the firing process as clay undergoes thermal expansion and contraction. The rapid temperature changes place stress on the clay that can cause cracks. Factors leading to thermal cracking include:

Firing too quickly at high temperatures
Inadequate burnout of organics in clay body
Firing thick and thin sections together
Resting ware directly on kiln shelves

Structural cracks occur due to weak spots in the physical structure of the clay piece. Heavy or improperly supported sections can crack under their own weight. Insufficient clay thickness or uneven distribution can also lead to cracks. Other structural causes include:

Excessive weight in a certain area
Joining two separately thrown pieces that are misaligned
Improper clay wedging or preparation
Insufficient internal support for thin or large pieces

Preventing Drying Cracks

Drying cracks occur when the clay dries too quickly and the surface shrinks faster than the interior. To prevent drying cracks, it’s important to store and dry clay properly.

Clay should be stored in a cool, humid environment. Ideally, the clay storage area should maintain a humidity level between 50-70%. You can place a humidifier in the storage area or simply keep the clay in a sealed plastic bag with a damp sponge or towel. This helps the clay retain moisture.

When drying clay, control the rate of moisture loss. Allow thorough but slow drying. Don’t force clay to dry too quickly near heat sources like direct sunlight or heating vents. Drastic changes in temperature or humidity can also shock the clay. Maintain consistent workshop conditions.

As clay dries, moisture migrates from the interior to the surface. If the surface dries too fast, it can form a hardened shell before the interior has finished drying. Mist the clay regularly during the drying process to maintain surface moisture. Drape plastic over the clay or place it in a partially sealed bag to slow the drying rate.

These simple practices of proper clay storage, controlled drying, and surface misting help prevent drying cracks by maintaining consistent moisture levels throughout the clay body during storage and drying.

Preventing Thermal Cracks

Thermal cracks occur when the clay body is subjected to rapid temperature changes during the firing process. As clay heats up, it expands, and as it cools down, it contracts. If the temperature rises or drops too quickly, stresses build up in the clay body leading to cracks. Thermal cracks can be prevented by:

Controlling Kiln Temperature Carefully

It’s important to follow the manufacturer’s recommended heating and cooling schedules for the specific clay body and glazes being fired. In general, aim to heat up and cool down the kiln at a gradual rate, usually no more than 200°F per hour. Rapid temperature changes put too much stress on the ware.

Preheating Clay Pieces

Consider preheating clay pieces to 150-200°F before putting them in a hot kiln. This reduces the temperature differential and thermal shock. Place pieces to be preheated in a warm oven or fire up the kiln to a low temperature first.

Glazing Pieces

Applying an appropriate glaze helps prevent thermal cracks as it forms a protective layer over the clay body. The glaze and clay should have similar coefficients of expansion to avoid building up internal stresses.

Preventing Structural Cracks

Structural cracks occur when the clay body is too weak to support itself and breaks under its own weight. There are a few ways to prevent structural cracks in clay:

Reinforce pieces with armature. Adding metal or wooden supports inside clay pieces helps reinforce thin areas prone to cracking. For example, inserting wire, rods, mesh, or wood pieces into sculptures, handles, spouts, or other thin protrusions gives interior strength and stability.

Avoid thin sections. Structural cracking happens most often when clay is too thin. Try to maintain thickness of at least 1/4″ throughout pieces. Build up thin areas by adding coils or slabs of clay to reinforce weak points.

Let pieces dry slowly and thoroughly before bisque firing. Rushing the drying process causes uneven shrinkage that leads to interior stresses and cracking. Allow clay to dry naturally without direct heat, and bisque fire only when completely dry.

Bisque fire properly. Follow the clay manufacturer’s recommended bisque schedule. Firing too fast can cause cracking and weaken clay. Slow bisque firing also prevents future cracks during glaze firing.

Clay Selection

When choosing clay for a project, consider the clay body’s shrinkage rate and porosity. A clay that shrinks minimally and has moderate porosity is less likely to crack. Look for stoneware or porcelain clays that exhibit:

Low-moderate shrinkage of 10-15% from wet to fired state.
Moderate porosity with medium sized particles.
Minimal impurities like iron that can cause excessive expansion.
A smooth, consistent texture without large grog or particles.

Test a small sample to ensure the clay has suitable drying and firing behavior for your project. Avoid clays prone to excessive shrinkage, cracking, or warping.

Clay Preparation

Properly preparing the clay is an important step in preventing cracks. Here are some key techniques:

Knead thoroughly – Spend at least 5-10 minutes vigorously kneading and wedging the clay before beginning to shape it. This helps blend ingredients evenly, remove air pockets, and align the clay particles for strength.

Wedge clay – Cut the clay into chunks and slam it against the table over and over in a slicing motion. This helps blend water throughout the clay and aligns the clay particles.

Mix in grog or fiber – Adding grog (ground fired clay) or fiber (paper, plant matter) gives the clay more stability as it dries and fires. The grog/fiber help reinforce the clay body against cracking.

Taking the time to properly prepare the clay makes it less prone to cracking issues down the line. Work any dry pockets thoroughly and knead until the clay feels smooth, supple, and unified.

Drying & Firing

Proper drying and firing techniques are essential for preventing cracks in clay pieces. Here are some tips:

Control humidity – Dry clay in a space with consistent humidity between 40-60%. Drastic humidity fluctuations cause uneven drying that leads to cracks. Use a humidifier or dehumidifier to maintain an ideal humidity level.

Dry slowly – Avoid quick drying which causes the exterior to harden while the interior is still wet. Dry clay over several days or weeks, keeping it wrapped in plastic when not drying. Control airflow to slow moisture evaporation.

Fire at low ramp speeds – Fast temperature spikes during firing lead to cracking. Use slow firing cycles that increase no more than 200°F per hour. This allows moisture to evaporate safely and the clay to adjust to shrinking at an even pace.

Repairing Cracks

Clay cracks happen but they can often be repaired. There are several methods you can use to fix cracks in dried or fired clay.

One technique is scoring. Use a sharp tool to cut V-shaped grooves along the length of the crack on both sides. The grooves help fill material adhere to the clay. Fill the crack with water or slip and smooth over once dry.

Slip is a useful material for repairing cracks. Mix together clay and water to make a liquid clay slip. Apply the thick mixture over cracks and smooth out. The slip will dry and harden to patch cracks.

Small cracks can be mended with clay staples. Roll out a thin clay coil and cut into 1 inch pieces. Wet both sides of the coil and place over the crack, pressing gently to adhere. Clay staples are also great for cracks in leather hard clay.

Larger cracks may need more support. Cut blue tape or fiberglass window screen to fit over the crack, then apply slip to fully cover and seal. Remove once fully dry. The tape/screen provides structure as the slip hardens.

With patience and the right technique, most clay cracks can be repaired for functional and decorative purposes.

How Do You Prevent Clay From Cracking?