Filtration and iron removal directly affect glaze cleanliness, body uniformity and the final acceptance rate of ceramic sanitary ware.
Screening removes coarse particles, gypsum fragments and foreign material from pipelines, while magnetic separation captures free iron and iron-bearing minerals.
If these contaminants enter body slip or glaze, firing can reveal them as black specks, pinholes, bubbles, discoloration or local weak points that increase cracking and rejection risks.
Filtration passes slip or glaze through a specified screen to limit oversized particles and non-magnetic debris. Iron removal uses permanent magnets or high-intensity electromagnetic separators to capture ferrous particles and weakly magnetic minerals.
The two processes are complementary. A screen cannot reliably remove very fine magnetic contamination, while a magnetic separator cannot capture every piece of gypsum, plastic, fibre or other non-magnetic debris. Effective control therefore requires staged protection after milling, before storage and again before casting or glaze application.
Contamination disrupts body and glaze uniformity. A study published in the Journal of the European Ceramic Society examined pollutants introduced into sanitary ware bodies and glazes. It linked contaminants including gypsum, iron, metals and organic materials with spots, pinholes and bubbles produced by impurity degassing. This confirms that defects discovered after firing may have entered the process much earlier. (Research paper)
Iron is especially visible on white sanitary ware. Fine iron may come from raw materials, transport, grinding-media wear, pumps, pipelines or maintenance tools. After firing, it can appear as dark specks, reduce glaze whiteness or form a weak local region. Bunting’s sanitary ware guidance recommends magnetic separation at several stages, including raw-material preparation, after body-slip screening and before glaze spraying. (Technical reference)
Pinholes are not only a kiln problem. Digitalfire explains that gas-producing particles in a ceramic body can release gases through molten glaze, leaving holes that fail to heal. Better firing may reduce the symptom, but it cannot fully compensate for unstable screening or upstream contamination.
Verify screen specification, residue level, damage and cleaning frequency.
Record the contamination collected by each magnetic separator.
Match separator strength and flow path to slip viscosity and production rate.
Prevent pumps, pipelines and maintenance tools from reintroducing metal.
Trace black specks, pinholes and cracks to the material batch and process location.
Sunlets combines sanitary ware production with ceramic-equipment development, allowing impurity control to cover raw-material preparation, slip and glaze transfer, casting and firing as one connected process.
Filtration and iron removal are not auxiliary cleaning steps. They are preventive quality controls that stop small contaminants before they become expensive fired defects. Stable screening, staged magnetic separation and defect traceability provide a stronger foundation for glaze quality, structural consistency and production yield.
Why do black specks remain when the screen is working correctly?
The source may be fine magnetic contamination or metal introduced downstream by worn pumps and pipelines.
Is the strongest magnetic separator always the best choice?
No. Performance also depends on slip viscosity, flow rate, contact path, separator location and cleaning frequency.