Knowledge Zone

Underglaze can be applied in two ways. You can apply the underglaze colour directly on to greenware, fire and then add more colour if desired, or just apply a transparent glaze if you are happy with the colour depth. Alternatively, you can apply underglaze to bisque and apply the transparent glaze before firing. When using underglaze on greenware we advise to do a firing before adding the transparent glaze, as the greenware can absorb the transparent glaze and crack during firing. When applying the transparent glaze on top of the un-fired underglaze layer it would be beneficial to sponge a thin layer on first to avoid smudging your design. After that has dried you can apply the transparent glaze with a brush. Unlike other glazes underglaze colours can be mixed together to produce new colours.

Stains are blended metal and ceramic oxides ground into fine powder which are more stable than colouring oxides. The colours can be replicated more consistently than raw colouring oxides. At lower temperatures they tend to be brighter but most will withstand high firing temperatures.

Stains cannot produce the interesting effects such as speckles that raw oxides can but instead a much more reliable and homogenous colour. Stains do not suspend well in water alone so a medium is recommended for brushwork.

Cracking is usually the result of stress in the clay. Clay shrinks as it dries and then during firing it expands and contracts. This can sometimes result in too much stress for the clay to handle and it then cracks. The way to avoid this is to try and reduce the stress as much as possible:

• Be aware of the position you leave the piece in when drying, for example allowing the pot to rest upside down on its rim, can later result in the rim cracking as it dries.
• Choose a suitable clay – some clays handle stress better than others
• Your design may encourage cracks, for example sharp corners can create more stress
• Drying your piece too quickly
• Firing the piece too many times can make the clay crack
• Thickness of the clay – if you have areas on your piece that are thicker than others, as the clay dries the stress is uneven and parts of your piece will shrink faster than others.
• Uneven layers of glaze can create stress, for example having a thin layer on the outside but a thick layer on the outside
• Glaze pooling – if the glaze pools at the bottom it creates tension and may split across the bottom

This is when a piece of glaze cracks off with some clay attached. This happens because the clay has been over-sponged taking the fine clay particles away and leaving the groggier clay which is not elastic enough to absorb the stress.

This looks like very fine cracks in the glaze and is caused by incompatibility between the clay and the glaze. Stress builds up because the glaze and clay have different shrinkage rates and once the piece cools the cracks show up

This type of cracking results from stress caused during firing or cooling. Firing needs to be conducted slowly between 220 ºC – 573ºC. At these temperatures the silica molecules are rearranged, in the glaze this does not really have an impact as all the particles dissolve, but in the clay only some particles melt in the feldspar glass (Further reading available in the ThinkTank – Silica Inversion). Dunting can also occur during cooling and there are 3 main reasons:

1. If there is a high silica content in the clay body, the body can contract suddenly during silica inversion (573 ºC) but because different parts of piece reach silica inversion at different times the body will contract at different rates and creates stress resulting in cracking.
2. As the piece cools through the lower end of the silica inversion (226 ºC) contraction happens again resulting in the same stress
3. This can occur months or years after firing as a result of thermal shock (Further reading available in the ThinkTank – Thermal Shock). This is when the clay and glaze expand at different rates (ie body contracts faster than glaze) when exposed to different temperatures.

Occurring at the bottom in an ‘S’ shape. Usually occurs on pieces that have been thrown and is caused by the bottom being too wet whilst throwing. To avoid ‘S’ cracks try to keep the bottom of the piece as dry as possible whilst throwing.

Tiny holes in the glaze which go all the way through the glaze to the clay body. These are caused by gas escaping from the clay body whilst being fired. To avoid this during the bisque firing, fire the clay hotter than you are going to fire the glaze at. A longer firing cycle with a 15 minute soak may also stop this occurring.

When glaze ‘crawls’ and exposes bare bisque. This can occur because there is grease or other dirt on the bisque before the glaze is applied. It can also be due to a fault with the glaze or caused by shrinkage. To resolve crawling we recommend applying the glaze thinly or thin the glaze.

Blisters in the glaze occur when it has been applied too thick, the glaze has been allowed to dry thoroughly before firing or the clay body is too dense trapping air in the piece.

Glazes can settle out, meaning the heavier components of the glaze drop to the bottom of the container. The glaze needs to be properly re-mixed to ensure no key ingredients are missing in application. The causes of this are too much water in the glaze which causes the suspension medium to become ineffective. It can also be due to bacteria growing which consumes the medium and leads to loss of suspension. Storing the glaze properly is vital to avoiding bacteria contamination amongst other things.

Do not return used glaze back to the original container. Do not introduce objects into the original container (such as brushes). Glaze should not be stored in hot or direct sunlight areas as this can encourage bacteria to form. It also should not be allowed to freeze as this can affect the suspension medium.

Settled out glazes that are not completely hard can be re-suspended using Epsom Salts. First the Epsom Salts should be dissolved into warm water (keep adding the salts until no more will dissolve). Slowly add the salt solution to the glaze, constantly stirring until it stays suspended.

Quartz or Silica inversion is the sudden change to the crystal structure of Quartz from α-quartz at room temperature to form β-quartz at 573ºC. This process is called inversion and has linear expansion of 0.45%. The inversion process can lead to cracking if ware cools too quickly through the inversion temperature. Cracks occur from varying thermal expansion within the clay body. As some of the quartz particles dissolve in the feldspar glass it creates different thermal expansion and contraction and heat may not be equally distributed throughout the whole piece. The cracking is more likely to occur when cooling rather than when the piece is heating up because the particles surrounding the quartz as they enter inversion are less capable of absorbing the volume change. To avoid this, we recommend cooling rates of 50 °C/hour.

The stress caused by the volume changes by sudden changes in temperature is called thermal shock. For example putting a frozen dish into a hot oven can trigger stress from the quick change in temperature. To avoid thermal shock it is important to make sure the thermal expansion of the glaze is compatible with the clay body. Ceramics can be easily tested for their resistance to thermal shock. An example would be the boiling water to ice water test.