The Basics

of Porcelain Manufacturing

10½" Bowl with fruit decor

The history of the initial production of porcelain in Europe by Tschirnhaus and Böttger is well documented and need not be repeated here. The important aspect of their work, the fusion of a mixture of kaolin and silica with a flux of borax (or feldspar), is that the product formed has many useful properties. In thin sections, the vitreous product is translucent. When properly glazed and fired, the surface becomes very hard, often harder than most steels. Over the years, modifications have been made to make specialty use porcelains, generally by changing the flux used to fuse the principal components of the body or the composition of the glaze.

There are three reasons why Reinhold Schlegelmilch was so successful in selling into the American marketplace. First, high quality kaolin is essential in making porcelain, for it comprises about 50% of the initial starting materials. Deposits in America capable of supplying the needs of a large commercial enterprise were not known. Effectively, there was no competition from American factories. Second, the manufacture of objects out of porcelain is exceptionally labor intensive. It is very likely that child labor was involved in the manufacture of most of Reinhold's products. Third, the temperature needed to vitrify the shaped articles is very high, between 2500 and 2700 degrees Fahrenheit. A great deal of fuel is needed to reach and maintain these high temperatures. In addition, the ovens and the capsules that contain the shaped articles must be fabricated to withstand the heating. The location of Reinhold's second factory in Tillowitz was in part due to very low labor, raw materials, and transportation costs that allowed the firm to make excellent quality porcelain at a competitive price.

One aspect of decorated porcelains that is often misunderstood is whether the color is applied under or over the glaze. Up to the early 1900's, cobalt blue was about the only color that could be successfully painted on blanks at the biscuit stage of manufacture (after the initial firing). The color tends to run during the glaze firing, and to compensate for this, gold is often applied over the fuzzy borders of the decorating pattern of the finished piece. Whether a blue color is under or over the glaze may be seen by inspection at10X magnification. Colored enamels used for decoration were made from minerals that could withstand the temperatures (1300-1400 degrees F) needed to bond them to the glazed surface. Under magnification, one can always see that they rise above the smooth glazed surface. Gold decoration is always applied over glaze after the enamels are applied and fired.

One important characteristic of porcelain is that the finished piece is much smaller than the clay body from which it was made. Most, but not all of the contraction occurs during the glaze firing. Consequently, making a casting mold from an existing object will eventually yield a product that is from 10 and 16 percent smaller than the original. Fortunately, this phenomenon works to the advantage of the collector for it is a strong deterrent to the counterfeiting of rare objects.

Another consequence of the contraction during the conversion from clay to porcelain is that an object may be fractured in the first firing step. Fractures seem to be common in Reinhold's products made after 1900. When a dried object with a fracture is coated with glaze, the glaze seeps into the fracture. In many instances, the two edges of the fracture bond together during the glaze firing step. Although one can still see evidence of the fracture, usually in the form of a line when viewed at an angle, the porcelain is structurally sound. Close inspection under magnification reveals that the fracture has been replaced by a channel that is partially filled with glaze. This glazed porcelain will exhibit a persistent resonance when lightly struck unless the full length of the fracture has not been completely healed.