This is an updated version of the product training course introduced by Supply House Times in 1979, authored by Don Arnold.
The term "vitreous" in connec-tion with china today concerns the manufacturing process in which molded fixtures are baked at extremely high temperatures, re-sulting in a material that is strong, nonporous and impervious to absorption. Though in general, the word "vitreous" means "glass-like," it is important to under-stand that, in plumbing china, the word is used in reference to the en-tire fixture, not just the surface glaze. The raw materials used to make china are basically a combination of special clays, mixed with a few other ingredients such as flint and feldspar, mixed in exact proportions with water (a mixture called "slip."). The slip is poured into a mold, conforming to the shape provided. The mold, be-ing a very porous material, absorbs water from the slip, resulting in a buildup of solid, semi-dry clay against its walls. After further curing, the clay so-lidification develops to the point where the product can be removed from the mold, and then cleaned up and com-pletely dried (but not yet heated). The next stop for the newly formed fixture is the glazing sta-tion, where a liquid similar to slip is sprayed in a thin, even layer over all exposed sur-faces. Color, if any, is combined with this glaze application. The coated fixture is then placed onto a car that slowly moves through a tunnel kiln (oven), transforming it into a solid, strong and nonporous struc-ture with a glassy permanent bond on the surface.
Cast iron fixtures are made by a process called sand casting, which means that shapes are made by pouring molten metal into cavities formed into tightly packed sand. Since sand casting produces a relatively rough finish, it is neces-sary to subject the molded sur-faces to a smoothing operation called dry blasting. This involves striking the surface with thousands of tiny metal balls (called shot) at high velocity, resulting in the creation of a smooth, uniform finish. The final step in the process is to apply the finish, commonly called porcelain or enamel. Usu-ally a combination of fine clay, quartz, feldspar and silica, it is very similar to the glaze applied to vit-reous china. After being applied to the exposed surfaces of the fixture in powder form, it is subjected to firing, which melts and fuses it uniformly into a glass-like coating. Prior to firing, pigments are mixed with these various finish ingredients to provide fixture color.
Enameled Steel / Stainless Steel
Another process used in manufacturing metal plumbing fixtures involves the forming of sheet metal in a cold state (as opposed to molten cast-ing). As a general definition, stamping is a term used to de-scribe the forming of products or parts from sheet metal. An operation which involves the forming of a shape with at least some degree of depth is commonly called drawing, which is per-formed on a draw press. Depending on the depth of the fixture to be formed, it is sometimes nec-essary to form the ultimate shape in progressive stages. In other words, the first press might draw the fixture to half its intended depth, and the next one will draw it the rest of the way. Beyond the basic forming of the fixture shape, there are usually other press operations involving the punching of holes and forming of rims. Certain products require the welding of sections together and often, the welding of mounting rails to the underside of the rims. Finally, the finishing operations take place. In the case of enameled steel, the composition and process is similar to that used with cast iron, though here the porcelain coating is sprayed on before firing (not a powder coating). With stainless steel, however, there is no surface coating applied. This is called a homoge-nous material makeup -- that is, the wall section is the same clear through. Except for the lowest quality models, stainless steel sinks are then subjected to a se-ries of grinding, polishing and buffing steps to produce an attractive and practical surface finish.
Reinforced Plastic & Fiberglass
There are two manufacturing processes commonly used in the production of "plastic" fixtures today. While they often look the same, the processes are quite different. As is often the case in our industry, the names aren't much help in sorting out the differences. Though both involve an underside reinforcement of fiberglass strands, one is called by that name, whereas the other is called acrylic. Here's how the two are made.
Fiberglass. As the original plastic process, this involves taking a highly polished mold form (male, in the case of most plumbing fixtures) and spraying it with a relatively thin polyester gel coat, which cures into the hard, smooth exterior sur-face of the product. Next, the molds are uniformly sprayed with a mixture of polyester resin and fi-berglass strands, developing a reinforcing structure on the back side of the initial gel coat layer. Reinforc-ing strips and braces (usually wood or metal) are placed on the under-side surfaces in strategic locations while the fiberglass mixture is still wet, adhering securely in place during the curing process. Curing is basically accom-plished under normal atmospheric conditions, though temperature and humidity are normally con-trolled at a constant rate. Finally, the product is removed from the mold, the edges are trimmed, and accessories, if any, are added. The process just described is one called "spray-up." An older approach, called "lay-up," produces a similar product. Here, rather than applying the fiberglass to the back of the gel coat layer by means of sprayed strands, it is placed on in cloth or mat form and made to conform to the configuration. This step is then followed up with additional spraying of resin.
Acrylic. Fixtures made using this process are called either acrylic or vac-uum-formed acrylic. Structurally, there is a similarity to the tradi-tional fiberglass lay-up approach, although the process begins in a very different way.
With a vacuum formed product, there is also a hard, smooth exterior layer, but rather than spraying a gel coat resin onto a male mold, the "skin" is formed by molding the basic shape out of a sheet of acrylic material pulled down into a female mold. To accomplish this, the sheet is heated until it becomes somewhat soft and pliable. When ready for forming, a vacuum in the mold begins "suck-ing" the sheet down into the desired form, often assisted by a male counter-part pushing from above. The re-sult is an acrylic shape that perfectly conforms to the configuration of the cavity. After the acrylic shell partially cools, it is removed to finish cooling in a holding area. From there, it is sprayed with resin and chopped strands of glass in much the same manner as the traditional fiber-glass designs. The installation of reinforcement boards and braces, trimming and adding of accesso-ries is also similar.
The key quality difference between these two approaches concerns its "skin thickness," with acrylic typically being significantly thicker (and harder) than fiberglass products.