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Troubleshooting Clay

 

Here are some of the most common problems and faults that makers may come across when making ceramics at all stages from greenware through to glazing:

Making/Greenware

 

Cracking: This is the most common problem that can occur when making. Clay can crack during the drying, bisque firing, or glaze firing stages and has many possible causes.

Why do ceramics crack when making them?

Ceramics can crack when making them due to several reasons, including:

  • Drying Too Quickly: If the clay dries out too quickly, it can shrink unevenly and cause cracking. It’s important to allow the clay to dry slowly and evenly.
  • Lack of Support: If the clay is not supported properly while it’s drying or being worked on, it can warp or crack.
  • Overworking: If the clay is overworked or stretched too thin, it can become weak and prone to cracking.
  • Thermal Shock: If the clay is exposed to extreme temperature changes too quickly, such as being moved from a hot kiln to a cold surface, it can crack due to thermal shock.
  • Inadequate Wedging: If the clay is not wedged properly to remove air pockets, it can cause uneven drying and cracking.
  • Excessive Moisture: If there is too much moisture in the clay, it can cause bloating, which can lead to cracking.
  • Structural Issues: If the design or form of the piece is not properly thought out, it can lead to inconsistencies or structural issues, which can cause cracking.

By taking care to address these potential causes of cracking, makers can reduce the likelihood of their ceramics cracking during the making process.

S-Cracks: S-Cracks are cracks that occur during the throwing process, usually when a potter pulls too hard on the clay while centring it on the wheel. The resulting crack will look like an S-shape and can weaken the structure of the piece.

Warping: Clay can warp if it is not properly supported or if it is not allowed to dry evenly. This can result in uneven surfaces or misshapen pieces.

Sticking: Clay can stick to the potter’s wheel or to other tools, making it difficult to work with. This can happen if the clay is too wet or if the wheel or tools are not properly lubricated or when using boards, formers and tools that aren’t porous.  Lining formers and working on porous surfaces can help.

Overworking: Overworking the clay can lead to a loss of plasticity, making it difficult to shape and form. It’s important to find the right balance between working the clay enough to achieve the desired form, but not so much that it becomes unworkable.

Dust and Debris: Dust and debris can become trapped in the clay, causing problems with the finished piece. This can happen if the workspace is not kept clean or if the clay is not properly wedged before use.

Inconsistent Texture: If the clay is not properly mixed or wedged, it can have an inconsistent texture, with areas that are too dry or too wet. This can lead to problems with shaping and forming the clay.

 

Bisque firing

 

Explosions from moisture and trapped air: When moisture or trapped air is present in the clay or glaze, it can cause the piece to explode during firing. This can happen if the piece is not properly dried before firing or if there are air pockets in the clay.

Slumping: Slumping happens when clay is not properly supported during firing and collapses or loses its shape.

Bloating: Bloating occurs when clay contains too much moisture and expands during firing, causing the piece to bulge or deform.

Poor Surface Quality: Clay can have an uneven or rough surface texture if it is not properly wedged, if there are impurities in the clay, or if it is not smoothed or polished before firing.

 

Glazing and glaze firing

 

Glaze Application Issues: When applying glaze to clay, common problems can include improper thickness, uneven application, improper fit between clay body and glaze, or bubbling/crawling of the glaze due to impurities.

Glaze pinholes: Glaze pinholes are small holes or bubbles that appear in the glaze after firing. This can be caused by impurities in the clay or glaze, incomplete coverage of the glaze, or excessive firing temperature.

Shivering: Shivering is a term used to describe the cracking or flaking off of glaze from the surface of the ceramic piece. This can occur if the glaze is not properly matched to the clay body or if the glaze is applied too thickly.

Crawling: Crawling is a term used to describe the separation of the glaze into small islands or clumps during firing. This can happen if the glaze is too thick or if the surface of the piece is not properly cleaned before glazing.

Glaze Bubbling- The glaze forms bubbles due to issues such as trapped air, insufficient adhesion or too thick application similar to pinholes.

Crazing: Crazing is a network of fine cracks that can appear on the surface of a piece after firing. This can happen if the clay body and glaze have different coefficients of expansion, causing the glaze to contract more than the clay during cooling.

Dunting: Dunting is a term used to describe the cracking of ceramic pieces due to thermal shock during the cooling process. It can occur if the cooling process is too rapid or if there is a temperature difference between the inside and outside of the piece. You can identify a cooling crack in glazed pieces as the edges of the glaze around the crack is sharp.  If the edges of the glaze are rounded and soft looking, then the crack occurred as the temperature was going up as the glazes has melted at the edges in the highest temperature.

Firing Too Hot or Too Cold: Firing clay at the wrong temperature can cause a variety of problems, such as bloating, cracking, warping, or discoloration. It’s important to follow recommended firing temperatures and schedules for the type of clay being used.

Firing Schedule Issues: If the firing schedule is not properly followed, it can result in problems with the finished piece. This can include under-firing, which can cause the piece to be brittle and weak, or over-firing, which can cause warping, cracking, or discoloration.

It’s important to note that some problems can occur at multiple stages of the process. For example, cracking can happen during the making/greenware stage as well as during the bisque firing stage.

 

A Note on Planning: Lack of planning can sometimes result in problems with the finished piece, such as uneven walls, misshapen forms, or problems with attachments. It can be important to have a clear plan for each piece before beginning to work with the clay although equally sometimes you just need to make it to figure out where the problems are likely to occur.  You can always make a fast sketch/maquette piece on a smaller scale to test ideas out.  These pieces are less precious as you’re building them expecting things to go wrong so you can learn before making the ‘finished’ piece.

 

In summary, there are several potential problems that can occur during the making, bisque firing, and glazing stages of ceramics. By understanding the potential causes of these problems and taking the appropriate steps to address them, ceramic students can create successful and structurally sound pieces. This troubleshooting guide provides a starting point for identifying and addressing these issues, and can help students develop their skills and knowledge in the art of ceramics.

 

What can be done to avoid these things happening?

There are several steps that can be taken to avoid or minimize problems when working with clay. Here are some general tips that can be applied to the different stages of the process:

Making/Greenware Stage:

  • Wedge the clay thoroughly to ensure even consistency and eliminate air pockets.
  • Use the appropriate amount of water for the clay type to avoid cracking or slumping.
  • Use supports or armatures to prevent slumping or warping while the clay is drying.
  • Slip and score the surface before attaching pieces to ensure a strong bond.
  • Cover the clay with plastic to slow down the drying process and prevent cracking or drying too quickly.
  • Keep the workspace clean and free of debris to avoid dust and debris in the clay.
  • Plan out the design and form of the piece before starting to work with the clay to avoid issues with consistency or lack of planning.

Bisque Firing Stage:

  • Make sure the bisque firing temperature and schedule is appropriate for the clay type and thickness of the piece.
  • Don’t rush taking them out of the kiln! Allow the piece to cool slowly to avoid thermal shock, which can cause cracking or warping.
  • Make sure the piece is dry before firing to avoid bloating or slumping.
  • Check the piece for any cracks or inconsistencies before glazing.

Glazing Stage:

  • Choose glazes that are appropriate for the clay type and firing temperature.
  • Apply the glaze evenly and at the appropriate thickness.
  • Make sure the glaze is completely dry before firing to avoid bubbling or crawling.
  • Use firing schedules appropriate for the glaze type and clay type.
  • Sand any rough areas or sharp edges before glazing to avoid chipping or peeling.

 

In addition to these tips, it’s important to practice good studio habits, such as keeping the workspace clean and organized, properly maintaining equipment, and following safety procedures. With practice and attention to detail, many of the common problems associated with working with clay can be easily avoided or minimized.

Written with the help of ChatGPT.