Ceramicware Defects Handbook/Pinhole

Defects Description


Pinholes are usually a small holes that appears on the surface of ceramic wares and mainly caused by the materials that burnt out during firing leaving a minuscule crater on the clay bodies.

Causes & Solutions
There are many possible sources of defects such as:
 * Airborne particles stuck onto the surface before firing
 * Solution: Ensure the firing kiln are cleaned at least once a week and generally ensure proper cleaning is done in the production floor where the unfired wares travel


 * Contaminated sands used during slip mixing.
 * Solution: Need to sieve the slips before casting


 * Foreign contaminants such as fibers from cleaning clothes, particles of sponge or foreign debris from water that are burnt off when firing leaving a hole on its place.
 * Solution: Try replacing the consumable materials (such as sponge) to materials which are less likely to leave foreign contaminants (such as silicone scrubber)


 * Poor pugging due to faulty de-airing machine (vacuum parts). The vacuum should extract any of the air bubbles inside the clay cakes.
 * Solution: Check the vacuum functions of de-airing machine are still functioning according to the manual


 * Rapid firing may cause trapped remnants air bubbles inside clay to burst through the surface
 * Solution: Try the slower firing allows the air to percolate to the surface without rupturing clays causing pinhole


 * Direct uses of reclaimed slip cast. Slips drained from moulds may pick up dust and dirt from pouring holes, casting throughs and etc.
 * Solution: Only mix the reclaimed slips to main stocks if the slips are ensured in good condition; otherwise return to blunger


 * Insufficiently blunged slips (especially if dried scrap clays are added)
 * Solution: Increase the blunging time and blunger speed and manual checking on slips to see if the slips are properly blunged.


 * Thick glaze coating where the bubbles may form as bubbles may take times to travel from bottom of layers to surface
 * Solution: Thinning the layers to reduce the travels of bubbles


 * Short heat soaking time causing the glaze is still bubbling at end of glost firing
 * Solution: Extend the soaking time at max temperature by half-hour or more to allow the pinholing to recover and glaze to stabilize


 * Contamination in glaze tubs due to dried glaze falls into the glaze slips during next dipping sessions. The dried glaze has the air pocket which will cause air pockets
 * Solution: Cover the glaze tubs when not in use, the sides of tubs should be wiped with clothes or hands