Most beer steins come from Germany where they were made at least as early as the 15th century. Outstanding pottery centers were Cologne, Sieberg and Frechen in Rhineland and Kruessen in Bavaria. Many small and important peasant potteries came and went, their wares, names and loctions long ago forgotten.
Today's collector is not likely to often come across a stein that is 300 years old, but he can expect to find steins made in imitation of those early products. Around 1850 stein production experienced a tremendous upswing and every imaginable kind were produced in the area of Westerwald called Kannenbackerland (or jug baker's land) which has a history of pottery making that stretches back hundreds of years.
by John McGregor from SCI Prosit Vol. 2, No. 18, June 1996
Stoneware goes back to about the 13th century when for the first time kilns could be brought to a temperature sufficient to cause vitrification. Vitrification is when the porous clay actually melts into a non-porous mass. This transformation from clay to stone occurs at about 1,200 degrees centigrade, or 2,200 degrees Fahrenheit. By introducing salt into the kiln at the height of the firing, just after vitrification has taken place, a hard glaze that is actually a type of glass, a sodium-alumina-silicate, is formed on the stoneware surface as the sodium in the salt and the silicates in the clay chemically combine. A proper glaze requires 2.5 kg, or about 5 lb. of salt per cubic meter of kiln space. To keep the salt from accumulating in the vessels, they are fired either upside down or in capsules. The salt glazed stoneware is now impervious to all liquids except hydrofluoric acid. Before the development of stoneware, ceramic vessels had to be given a glaze, usually a lead glaze, so that liquids could not seep through their porous walls. Because stoneware is vitrified and not porous, it does not actually require a glaze, but a glaze is added for aesthetic purposes, and of course it makes the job of cleaning much easier.
The stoneware clays in the west of Germany are white-yellow when they come out of the ground. The clay is finely ground and screened to remove any impurities. The clay is then mixed with quartz and fluxing agents in the following proportions, the exact percentages depending on the actual receipe used by any particular factory: clay, 30-70%, quartz, 30-60% and fluxing agents (lime, magnesia, alkalis) 5-25%. Once the clay has been prepared, it is mixed with water to one of two consistencies depending on the method of production. If plaster molds are to be used, the clay is mixed into a “slurry” that has the consistency of runny cake batter. This is then poured into the mold and the plaster absorbs the moisture out of the slurry, leaving a leathery shell. After an exact amount of time, the excess slurry is poured off and the remaining shell is removed from the mold. At this point, if the stein still needs a handle, it is added and the stein is set aside for a while to dry.
Westerwald Pottery or Stoneware is a distinctive type of salt glazed grey pottery from the de:Höhr-Grenzhausen and Hilgert area of Westerwaldkreis in Nordrhein-Westfalen. In the triangle between the Rhine and Lahn Rivers, high in the Westerwald (the Western Forest), clay is known as white gold. The clay is of such and excellent quality that the fame of the region was extolled far beyond its borders centuries ago; excavation finds have proven earthenware vessels were produced in the area as early as 800 B.C. This conglomeration of clay pits, potteries and potters was nicknamed "jug baker's land" or Kannenbäckerland in the eighteenth century. Besides the usual commonplace household utensils, fancy and artfully decorated jugs and steins were turned out by a number of famous master-potters starting in the 16th century.
The potters guild reached its first zenith in the 17th century,with the appearance of the typical Kannenbaeckerland grey-blue decorated and salt-glazed stoneware. In this process pots, jugs, and steins are fired in kilns for 42 hours until a temperature of 1200 degreees Celsius is reached, coarse salt ("Steinsalz") is shoveled into the furnace through the draft openings in the vaulted roof. An acid cloud is involved, and the released sodium forms the desired salt-glaze in combination with the fine quartz clay. By operating at reduced oxygen level during the firing operation the stoneware is given its typical grey color. At the end of the firing and glazing operation the oven must be cooled over a period days before the "baked" wares can be removed.
This info from The Westerwald Tradition by Dieter Hoffeller, translated by Jack Lowenstein in the Prosit for March 1984--page 1159.
An example of new Westerwald Pottery
In German-speaking regions a Stein may be known as a "Humpen" (if stoneware), "Steinkrug" (earthenware) or "Glaskrug" (glass). The "Maßkrug" (or "Maß" (ie.,"Mass") meaning measure )- refers to capacious 1-liter glass mugs familiar during Oktoberfest. In Munich the familiar term is "Keferloher" as Oktoberfest mugs were once made with Keferloh clay located in Grasbrunn near Munich. As "Ein Maß" means a measure, so "Maßkrug" is also a generic term meaning measure-crock; a line inside the mug denotes correct fluid volume. Today's 'measure', or standardized serving of beer is one liter (33.8 oz.), down from 19th.-century Maßen of up to 1.5 liters.