I haven’t seen toilets made, but I have seen porcelain tile made at the Crossville plant near Memphis. The raw materials are tested and stored in huge silos. The operators in the control room hit the button for the appropriate mix, the silos empty the materials into a huge steel barrel. Water is added along with lots of porcelain balls, like golf balls. This is the ball mill. It rotates like a giant rock tumbler. The balls mix the ingredients and grind everything finely. After five days or so, it is pumped out into sprayers.
The mix is sprayed into the air inside a huge upside down funnel with enormous fans (like Willie Wonka when they float up in the air). The air dries the spray in the air so it becomes fine dust of a specific size. The dust is collected and goes through screens to eliminate any over a certain size. The prepared clay dust goes into a silo until it is needed.
The mold for the tile is in two parts, both of steel, fitted to a huge press. The dust is hosed in, flattened off and the press goes chunk down on top, then kaCHUNK and it compresses the dust at some crazy amount I forget. What comes out is a piece of tile you can pick up and hold, but can break over your knee (I have a piece somewhere, it feels like talcum). Up to this point all waste can be totally recycled back into the mix because it has not been fired. It moves to glazing.
This tile was to be of mottled appearance. The underglaze of white was sprayed on as each tile moved down the belt, followed by repeated screens which dropped down and sprayed various earth tones. At the end was a stipple brush that randomly pushed the glaze into the crevices of the molded tile surface. This last step is a mark of quality. Without it you see the dot matrix of the screens, plus it avoids “cookie cutter” sameness. You can tell a cheap tile by looking closely.
The glazed tiles are stacked on carts and auto loaded into the kiln which never shuts down ever. The kiln is about a football field long and increases in temperature as it goes. They pulled a foot thick asbestos block out of the side and you could see the tiles cruising by on rollers. At this point it was vitrifying and it was red hot and flexible, sagging a bit between the rollers. Vitrification is right at the edge of usable material. Any more heat it would turn to cinder. The molecules of the clay interweave with each other and line up and it becomes hard and non-porous (not all tile is vitrified). The kiln cools as it approaches the end and the tiles are autostacked as they leave and the robots carry it to the next zone.
Then it is quality checked by pressure rollers that break it if it is weak. Then checked by human eye for defects. Then boxed up. A guy comes by, takes a few tiles off the top and goes into an all white room, lays them on the floor, gets a few samples of previous runs out of a cabinet. He compares the new ones and decides on the code that marks this particular shade run and the boxes are stamped with that shade number. Every run of tile will have a slightly different shade due to temperature and glazed variance. When you buy tile check that it has the same numbers box to box or there will be visible color lines.
Monocottura (sp?) means “single fired” marked the beginning of Italian dominance in tile manufacturing. The old method fired the clay body, then glazed it, then refired it. This resulted in brittle tile and limited the practical size of tile made. In the '80’s most floor tile was 8"x8". When the Italian tech caught hold, it became 12x12. As it improved, 16x16 and 18x18. Now you can buy 36x36.
The pressed clay dust (as opposed to poured liquid clay or “slip”) is to reduce moisture content as low as possible. Moisture causes shrinkage in firing and results in size variations. Some tile (Mexican Saltillo) is made from slip (then sun-dried and fired, sometimes, in an old car body fueled by rubber tires) and can vary in size 3/4" in a 12"x12" piece. Porcelain has less shrinkage than clay body tiles, so a dust pressed porcelain is quite exact and can be jointed at a “toothpick joint” without “catstepping” (inability to maintain straight lines at a given joint size).
More than you ever wanted to know.