Example of the oversize particles from a 100 gram wet sieve analysis test of a powdered sample of a porcelain body made from North American refined materials. Although these materials are sold as 200 mesh, that designation does not mean that there are no particles coarser than 200 mesh. Here there are significant numbers of particles on the 100 and even 70 mesh screens. These contain some darker particles that could produce fired specks (if they are iron and not lignite); that goodness in this case they do not. Oversize particle is a fact of life in bodies made from refined materials and used by potters and hobbyists. Industrial manufacturers (e.g. tile, tableware, sanitaryware) commonly process the materials further, slurrying them and screening or ball milling; this is done to guarantee defect-free glazed surfaces.
The original recipe had a very low clay content, sourcing almost all of its Al2O3 from feldspar instead. Although the glaze slurry was maintained at 1.78 specific gravity (an incredibly high value) and thus would have had very low shrinkage, it did not stick and harden well enough to the ware. Why? Lack of clay content in the glaze. The fix was to source much more of the Al2O3 from kaolin instead of feldspar. The reduction in feldspar shorted the glaze on KNaO and SiO2 so these were sourced from a frit and pure silica instead (the calculations to do this were done in Insight-live.com). The change also provided opportunity to substitute some of the KNaO with lower expansion CaO. This reduced the thermal expansion and reduced crazing issues.
Sanitary ware factories optimize their slips to have the lowest possible specific gravity for production volume reasons. Potters would be happy with 1.7 SG whereas numbers approaching 1.9 SG are common in factories. They often teeter on the edge of issues like this (sections softening causing localize warping) and inexperienced technicians can be unaware of the critical balances needed to prevent loss in production.
This sanitary ware tank lid was made in China. Notice how thick the white glaze is being applied to cover the iron containing body below. This is a testament to how opaque a zircon opacified glaze can be. Of course, high percentages create a stiffer glaze melt and conditions can more often combine to produce crawling like this.