Ceramic glazes are glass forms made by melting and cooling down the mixture of oxides. Sometimes you may face with little holes on the surface of your glazes with the various shape and size of pins. That’s why they are called pinholes. This would distort the appearance of your form, may reach to the body and reflect its color, or in case of high amounts, they may hold bacteria turning your functional ware to be not food safe anymore. Many parameters could cause these unwanted pinholes
Possible Sources Of Pinholes;
- Clay body causing pinholes
- Glazed firing process
- Glazing process
- Particle size distribution of the glaze
Solutions
- Smoothing the surface: After final trimming, smoothing the surface of your greenware at the leather hard stage with a rubber finishing tool is good for closing the pores which may be triggering the pinhole formations.
- Longer bisque firing : If you increase the bisque firing period, you let gas exhausting to be completed. So safer glaze firing.
- Less stacking bisque ware into the kiln: Generally in case of bisque firing, we tend to place more objects into the kiln than placing glazed wares. Sometimes we place one pot into the other so tightly pack the kiln. In electric kilns, there is less air circulation, hence with more loaded kilns will have less oxygen during the firing process. That cause trapping Carbon molecules as they cannot find enough Oxygen atoms to bind. Those molecules trapped inside the body, may find only opportunity to escape during the glazed firing and causing pinholes
- Let your kiln breath : During bisque firing, keeping the chimney open until the mid-temperatures, lets the gases out. According to the kiln model this could also be a peephole.
- Clean your green ware carefully with a moist sponge after drying. The dust particles left on your green ware, will stick on your bisque ware after firing. Then they will cause pinholes during glaze firing.
- Clay raw materials: If you have pinholes first time with the same clay that you always use, you can ask your supplier if they changed a raw material.
- Glaze Carbon and water content : Same as bisque ware, glazes also needs to gas out, so must be given enough time during firing process
- Refractors in your glazes : For your firing temperature, If one or more refractor components of your glaze is in excess amounts in your glaze formula, they may cause pinholes too (ex: Alumina)
- A good particle size distribution of the all components helps gas out, so grinding procedure and sieving where necessary is important
- Glazes with less viscosity produce less pinholes as it melts earlier so lets the gases out completely
- Thickness of the glaze is important. A thin layer of glaze would not block the gases as thick layered glazes. So thinner glaze application is better for preventing the pinholes
- Lovering the final top temperature a little bit; If the pinholes are all over the surface with very high quantity, it may be a sign that the temperature is too high to boil the glaze. In that case, lover temperatures may be tried to solve the problem.
- No free falling : Another solution by the firing process is not letting cool down from the top temperature immediately but waiting at lover temperature (around 50C less) for a half or an hour then shuting off the kiln. This would help the glaze surface to be in liquid phase for a longer time but without boiling, hence letting pinholes disappear in a sufficient time.