This is a very large industry world wide, second only to ceramic tile in clay and energy use. The knowledge of how to make sanitary ware can be found on every industrial continent. Although alot of it is made in China (e.g. Chaozhou is a small city in Southern China that has hundreds of sanitary ware factories) many other countries have very highly developed processes and innovative products (we are impressed by many in the middle east).
Oversize particles in a typical manufactured porcelain body
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.
Crawling of the glaze on a typical sanitary ware porcelain item
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. The suggested change sources much more of the Al2O3 from kaolin and substitutes other fluxes for part of the lost K2O (since the glaze also had a high thermal expansion).
Over deflocculated slip causes instability in toilet tank
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.
How well zircon glaze can cover
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.