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Crazing in Pottery Glaze: Why It Happens And How To Avoid It

 

What is crazing?

Crazing is the effect on pottery which causes it to have a web of tiny cracks over its surface. These are not cracks in the actual structure of the pottery but actually an effect on the glazed part of the pottery.

Although crazing is generally a surface affliction, it can weaken the integrity of your piece in time, as it is opening up the glazed piece of your pottery and thus weakening its overall structure.

Crazed pots are also not suitable for foodstuffs as they may leak if the clay body is exposed, and potentially harbor bacteria in the cracks. For more information on this, check out our article on how to make food safe pottery.

 

What are the key causes of crazing?

Crazing generally occurs with age but there are other factors which cause immediate crazing which include:

  • Temperature and humidity changes which causes the glaze to crack
  • It can be caused by moisture getting into the glaze and forcing cracks in the glaze
  • It can be caused by being bumped or knocked repeatedly, causing small cracks in the glaze

Crazing can also occur when the glaze shrinks more than the body of the ware. This happens generally as the wares cool after firing. It is upon the cooling of the kiln and the contraction of the wares that cracks form. Heating and then cooling too rapidly can cause the glaze to shrink too quickly and cracks appear more readily.

 

How can you avoid crazing and what can you do to fix it?

There are a few main ways of avoiding crazing on your pottery piece. One of these methods is either changing the glazing or changing the clay. This may sound simplistic but the expansion and contraction rates are generally the underlying cause and so experimenting with different types can lead you to a solution.

 

Changing the glaze

The aim of changing the glaze makeup is to reduce the expansion of the glaze (and therefore to stop it contracting as much on cooling). In simple terms, this means adding materials with low levels of expansion and decreasing materials with high levels of expansion. This can be a tricky process and may take some experimentation to get the right balance. Here are some tips for changing the makeup of the glaze to avoid crazing:

  1. Increase the silica
  2. Decrease the feldspar
  3. Decrease any materials containing potash/soda
  4. Increase the boric oxide
  5. Increase the alumina

In addition to the above, thinning down the glaze may also help reduce or eliminate crazing. The thicker the glaze, the more liable it is to craze.

 

Changing the clay

One of the key things you can do to the body of the clay is to add silica, which helps to dry out the body of the clay and helps it to expand and contract with the glaze.

Too much silica can cause more cracks however, and too little can make the clay difficult to glaze, so there is some experimentation required as to what works for you and your chosen finish.

This advice is fine if you have the option of testing (and wasting) clay regularly, but many potters do not have this luxury or finance to experiment. Changing the formulation of the glaze may be an easier and less expensive option.

 

Do not over-fire or under-fire the ware

The recommended firing schedule for a small piece to avoid crazing of the ware is 150°C per hour up to 600°C.  From 600°C to 1020°C, the program should last for a schedule of 2-3 hours e.g about 200°C per hour with a 20 minute soak at the end of the firing. Larger pieces will need to be adjusted accordingly.

Crazing can also be reduced by increasing the firing time and slowing down the cooling. One example of how to achieve this is to extend the last section of the firing section to a few hours so that the glaze expands slowly. This will also give the clay body the best chance to tighten and achieve a good glaze fit.

You should also avoid opening the kiln too soon to ensure the wares are sufficiently cooled and do not expand by cooling too quickly.

Again, experimentation may be required to ensure that you have a program that suits. To achieve a more accurate and even temperature and cooling effect, why not consider an electric fired kiln? Soul Ceramics supply a range of electric fired kilns by which you can easily set the length and heat of a program and therefore have much more control over the firing cycle. This will be of particular value when trying to avoid crazing.

 

Ultimately, crazing can and does happen over time, even to professionally produced pieces. To avoid instant crazing, experimentation with different recipes of glaze and clay will help you to understand what works for you and also adjusting the temperature and length of your firing cycle will help to avoid this occurrence if you control the parameters carefully.

 

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