The best technical definitions I can find are:
- Fireclay China: Mixture of clay and minerals, similar to vitreous china, which is molded, glazed and fired at intense temperatures.
- Vitreous China: Mixture of clay, feldspar, and quartz which is molded, glazed and "baked" at extremely high temperatures, resulting in a material which is strong, nonporous and impervious to absorption. The term refers to both the basic fixture material and the surface glaze used in the manufacture of such plumbing fixtures as lavatories, toilets, and bidets.
- Porcelain: A special type of clay either white or gray, to which kaolin (a white firing stiff clay) and white China stone (finely decayed granite, washed and prepared as small white blocks) is added. When fired at temperatures of 1,280 Celcius and over, the body vitrifies, i.e. it becomes completely impermeable.
Of course, there are some critical differences in construction techniques. Claw foot tubs, for instance, are built by coating an iron casting with porcelain frit and firing it at a high enough temperature to fuse the frit to the iron. Pure fireclay and vitreous china construction are not used here because claw foot tubs built with these materials could not withstand the weight and stresses of normal use. Conversely, kitchen sinks and pedestal bathroom sinks are generally no longer made with porcelain over cast iron. The strength and quality of modern ceramic production eliminates the need to reinforce these items with cast iron thus making them lighter and easier to manufacture.
A technical issue that clawfoot tub purchasers should be aware of is outgassing. Defined as the "venting of volatile gases from the heated interior of solid body sinks", outgassing occurs when the cast iron hull of a claw foot tub is placed into a kiln to fuse the porcelain frit to the tub. The increase in temperature causes gases to form. If the tub is not made very carefully, these gases will leave fine pinholes in the finished surface. These holes are not only unsightly but can also expose the cast iron base to water and air which will, over time, cause the claw foot tub interior to rust.
All clawfoot tubs that have the Vintage Tub and Bath name on them use the highest quality sanitary frits commercially available and are meticulously inspected by hand for defects to ensure that your tub is the finest possible quality.
National Gallery of Australia