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.
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
