Glaze defects: Causes and Solutions

The glaze is a completing coating the surface that is melted and fused forever with the clay surface through the crystalline structure during firing. It increases the compressive strength of the ceramic body, in addition, it will provide an attractive appearance to the ceramic body.

 

They are typically applied to the ceramic body to make it impermeable, give sufficient mechanical strength, scratch resistance, chemical inertness, and be pleasing to the eye and touch. Overfiring, flux content, thermal expansion mismatch, underglaze, excessive powder production, and temperature ranges during drying and firing can all cause glaze flaws. We'll talk about glazing flaws, their causes, and how to fix them in this article.

1.                Peeling

The glaze is move away from the body.

Causes:

• Glaze is compressed too much by the body; the glaze actually peels off the ware on edge to relieve the stress.

• If compression is higher in a glaze layer, consequently breaking away of glaze from a ceramic ware takes place. This is caused by glaze composition that, its expansion coefficient is too low to match that of the body.

• Glaze peeling may occur if underglaze decoration or engobes forms a the barrier to the glaze-body reaction.
• If the glaze refuses to coat the body, the glaze may flake off (rare defect)

Remedy: To reduce the concentration of the body or raise that of glaze

2.               Pinholes

Pinholes are small holes in a glaze's surface that would go all the entire way through to the body.

Causes:

• Due to the presence of some organic matter during the firing cycle, gases may escape from the body.

• Pinholes are possible with overfired glazes.

Remedy:

• Adjustment of fusibility

• Lengthen the firing circle

• Apply the glaze less thickly

• Addition of a higher amount of flux to the glaze to make it more fluid

3.               Crawling

Crawling is the formation of small islands in the glaze.

Causes:

• Crawling is happened due to a higher magnitude of surface tension in the melting glaze.

• It develops when the glaze is too powdered and does not cling entirely to the surface of the clay. It can also occur when one glaze is applied over another, particularly if the first is allowed to dry too completely before the second application.

• Some fluxes like Zinc and Magnesium is likely to cause crawling when used in excess.

• Crawling takes place on the glaze because of the presence of grease, dust or any other dirts on the bisque at the time of firing.

• When underglaze is applied too heavily, it may lead to crawling of the covering layer of clear glaze.

Remedy:

• Reduce the addition of small amount of extra flux

• Calcining all or part of Zinc can help to fix this problem

4.                 Crazing

A network of fine cracks is present in the glaze.

Causes:

• It occurs because the thermal expansion coefficient of the glaze exceeds that of the body.

Remedy:

• Increase the silica in the body or glaze

• Decrease the feldspar in body or glaze

• Decrease any other material containing sodium or Potassium

• Increase the boron, lead oxide, alumina, i.e the content of clay

5.                Shivering

Opposite to crazing; if the the fired glaze is under compression, flaking of the body takes place mainly at the edges.

Causes:

• When a glaze shivers it cracks and pieces of glaze peel right off the piece, often the edges of a piece.

• This defect is an another mismatch in expansion coefficients and this is the opposite of crazing.

• In shivering, glaze expansion is too low when compared to a clay body.

Remedy:

• Adding a substance like a high expansion frit to the glazing to boost its expansion.

6.                Dunting

Dunting is a form of crack that develops as a result of forces that occur during the firing and cooling processes. Splitting of ceramic occurs as a result of silica inversion.

Causes:

• It is caused by a ware cooled too quickly after it has been fired (silica inversion temperatures) and amount of silica is higher in body and also the over firing of the body.

• These cracks are observed as long, clean, and cracks in body with sharp edges. If the ware is glazed, the edges are sharp.

• During cooling, stresses were released in cracks which were produced by thermal contraction mismatch.

Remedy:

• Careful consideration should be given while designing a ware

• Fire and gradually cool the body of the clay slowly in which the temperature ranges at inversion of silica takes place.

• Decrease the silica content of the body

• Decrease the firing temperature of the body

7.               Bubbles

Formation of bubbles within the body, during the process of firing.

Causes:

• At lower temperatures, carbonates, and hydrates (in body and glaze) are more likely to form gas bubbles because they are decomposed.

• If the body having a higher amount of flux, the formation of bubbles will take place.

• During the process of firing and after firing in a glaze, bubbles are present in a glaze. If the bubbles are larger and even not cures at the time of firing, they will be appear as a defect on the glaze surface

Remedy:

• Decrease the firing temperature

• Firing the process should be very slowly

• Reduce the flux content of the body

8.           Blistering

Blistering is a surface defect in fired ceramic glazes.

Causes:

• A blister is usually the result of either an excessive thick application of glaze or incomplete clay preparation, wedging, blunging, etc.

• These defects arise due to overfiring or to the use of soluble fluxes in the glaze. 

• Boric acid, Potassium carbonate, Sodium carbonate, Borax and Magnesium sulphate are the fluxes might cause this problem.

Remedy:

• Increase the soaking temperature (upto 30 minutes)

• More fluid glaze will heal blisters much better. For example: Nepheline syenite

• Firing temperature should be low

9.               Cracking

Formation of cracks in a bisque fired body.

Causes:

• The main cause of cracking is due to uneven drying or rapid drying.

• The other causes such as the plasticity of clay is poor, firing the body rapidly and over working of the clay mainly during the process of forming.

Remedy:

• Drying of the sample should be very slow

• Increase the plasticity of the body

• During forming, time of handling

Spangling/ orange peel

Peel like structures are present in the piece

Causes:

• Spangling or orange peel glaze surface is caused due to presence of gas bubbles in glaze. Under firing of glaze/ body, over firing of body/ glaze and high rapid firing are the main reason which leads to the formation of gas bubbles in glaze.

Remedy:

• Slow down the firing cycle

• Extend the soaking period

• The temperature range is biscuit firing is minimum 1000^oC

• With the help of pyrometric cones, monitor the firing temperatures

Devitrification

Formation of crystal from the glassy state

Causes:

• At the time of cooling, precipitation takes place. This precipitation is caused due to development of small crystals on the glaze surface (calcium or zinc silicates) and milky precipitate formation (calcium borate).

Remedy:

• Reduce the content of lime in glaze
• Addition of china clay to the glaze
• Instead of lead free glaze, use low solubility glaze

      Warping

The shape of greenware is different after drying and or firing process.

Causes:

Warping is caused due to uneven drying or uneven firing.

• Apart from this, it is formed because of fast firing, warped kiln shelves, thin and thick sections, improper geometry of the piece, and not sufficient supports at the time of drying as well as firing.

Remedy:

• The rate of drying and firing is even
• During forming process, maintain an even thickness
• Firing rate is 150^oC is adequate
• Taking care of kiln should be taken to arrange kiln for sufficient support for the piece.

 

We have seen some of the defects such as peeling, pinhole, crawling, crazing, shivering, dunting, bubbles, blisters, cracking and warping that arise in glaze with their causes and solutions in this article.