The nitty gritty on ceramics
Aside from the continuing West Circle Steam Renovation Project, construction around campus this summer has been rather slow. For Campus Archaeology, this means we have spent a lot of time in the lab. While shovel testing around campus is exciting, spending time in the lab, cataloging, organizing, and researching artifacts can be just as fulfilling. We have spent a few weeks now getting to know the artifacts in the lab, in doing so we have had some confusion in identifying ceramics, or the best way to label general ceramics. In an effort to clarify our questions I thought I’d do some research on the hierarchy of ceramics.
The general term “ceramic” can be defined as any inorganic, nonmetallic solid that has been created by the action of heating, then cooling (wikipedia). This definition would include glass as a ceramic, though in terms of archaeological typology it is easier to just call glass, glass…so as not to be confused. While ceramics include industrial material, like brick, piping, electrical conduits, this post focuses on houseware ceramics.
If ceramic is the top tier, the all encompassing term, then “whiteware” is immediately below. Whiteware, can is defined as a ceramic that consists of the three mineral types- clay, silica, and feldspar. The clay acts as the plastic component during the construction process and the solidifying agent in the firing process. The silica (generally in the flint form) gives the whiteware strength, and the feldspar acts as an agent to lower the melting temperature (http://www.britannica.com/EBchecked/topic/642810/whiteware). Fine whiteware, like porcelain, uses high-quality, pure kaolin clay, that once fired turns white, hence “whiteware.” Kaolin with impurities, like iron oxides, results in gray or tan colors.
Whiteware can be further broken down into several categories: earthenware, stoneware, and porcelain. These categories are loosely based on the degree of vitrification of each product; earthenware is nonvitreous and porcelain is vitreous, while stoneware is somewhere in the middle. Degree of vitrification correlates conversely with porosity.
Earthenware also has several subcategories. Some that CAP often comes across are: creamware, pearlware, terracotta, yellowware, and ironstone ware. Creamware is named for its cream color, resulting from the lead glaze. Terracotta (which includes building bricks, and other industrial material) is most easily identified by its bright orange color and its tendency to be unglazed. Ironstone ware gets its namesake from strength. One common type of ironstone we find at CAP is transferware; this is a technique that creates designs using one color against a white background. Yellowware is less commonly found around campus, and is identified by the yellow color resulting from the yellow clays used. Pearlware, is essentially just creamware with cobalt added to the glaze to give it a bluish tint. Unlike creamware, the term pearlware wasn’t used by the manufacturers, pearlware is mainly a designation given by archaeologists and collectors…so it is highly variable.
Stoneware is generally defined by how it differs from earthenware and porcelain. Stoneware is more vitreous than earthenware, but less than porcelain. Stoneware is more opaque than porcelain. It is a very heavy duty ceramic that is named because of the stone-like appearance after firing. There are a variety of stoneware types, but CAP usually finds American Stoneware, which was salt glazed and referred to as “crock” ware.
The final category of ceramics that CAP encounters is porcelain. Porcelain is completely vitrified, not porous, white, translucent, and resonant (unlike other whiteware). Sometimes porcelain is so broadly defined that some stoneware is considered porcelain, but for CAP we have our own way of drawing the line…if the ceramic does NOT stick to your tongue, then it’s porcelain, if it does….then it’s not porcelain.
Anyway, so that is my quick and dirty family tree of ceramics. I’m sure that many of these terms overlap, or may be put into other categories, but this is CAP’s typology system.
Author: Kate Frederick
http://www.thepotteries.org/features/pearlware.htm
http://en.wikipedia.org/wiki/White_Ware
http://en.wikipedia.org/wiki/Stoneware
http://www.britannica.com/EBchecked/topic/642810/whiteware