Two crafts built on fire
Pottery and glassmaking both turn common earth materials into hard goods by heating them. Potters worked clay; glassmakers worked sand and an alkali. Both needed kilns or furnaces, a supply of fuel and skill in judging heat. The goods differed in use: pottery supplied kitchens, tables and building sites, while glass, apart from simple vessels, was more of a luxury and was prized above all in church windows.
Pottery
Medieval potters made cooking pots, jugs, bowls, storage jars, lamps, roof and floor tiles and, in some regions, drinking vessels. The quality range was wide, from plain, locally made earthenware to finely finished tableware that was traded over a distance. Pottery is among the most commonly found objects on archaeological sites, because broken fragments last in the ground when wood and cloth do not, which makes it an important source for dating layers and tracing trade.
Potters dug clay from local deposits, cleaned and kneaded it, and sometimes added sand or other material to improve its behavior in the fire. Early medieval potters in many areas shaped vessels by hand or on a slow wheel. Faster wheels spread more widely in the later Middle Ages in many regions, allowing more regular shapes and quicker production. Vessels were dried and then fired in a kiln. Firing temperature, atmosphere and clay type affected hardness, color and how well the vessel held liquid.
Many pots were left unglazed. Others were covered with lead-based glaze, which gave a glassy, water-resistant surface and a yellow, green or brown tone depending on the additives. Some regions developed stoneware, a harder ceramic fired at higher temperatures that was more watertight, with centers of production in parts of what is now Germany and the Low Countries. Decorated tiles, made by pressing patterned stamps into clay, covered the floors of some churches and wealthy houses.
Potters worked in villages and towns, sometimes seasonally, and kilns were often placed on the edge of settlements because of smoke and fire risk. Many potters seem to have worked on a small scale, though larger production centers existed.
Glass
Glass was made from silica, usually in the form of sand, mixed with an alkali that lowered the melting point. In much of northern Europe, the alkali came from the ash of burned wood or bracken. This produced what is called forest glass, which was often green or yellowish because of impurities such as iron. In the Mediterranean, ash from certain coastal plants was commonly used, producing a clearer glass. Venice and its Murano glasshouses later became famous for fine glass, which fits into the broader story of the Venetian republic and its trade.
Glassmaking required very high temperatures and a great deal of fuel, so glasshouses in the north were often located in wooded areas, and workshops sometimes moved when local timber ran short. Glassblowers gathered molten glass on the end of a hollow iron pipe and shaped it by blowing, swinging and tooling. They made bottles, drinking vessels, lamps and flasks, and window panes. Cheaper everyday containers were often pottery, wood or metal rather than glass.
Window glass was commonly made in one of two ways. Crown glass involved spinning a blown bubble open into a flat disc. Cylinder or muff glass involved blowing a long cylinder, cutting it open and flattening it. Either way, the panes were small, and large uniform sheets were not possible, which is why windows were assembled from many pieces.
Stained glass
Stained glass windows combined colored glass with a supporting framework, usually lead. Color was added to the glass while it was molten, by mixing in metal oxides. Different oxides were associated with different colors, though results depended on the recipe and the furnace. Cobalt was one source of blue, for example, and copper compounds could produce reds or greens depending on how they were used. The process of making each color was expensive, and individual colors were sometimes made by specialist suppliers.
A window’s production followed several steps. The artist first prepared a design, often as a drawing or full-size pattern on a board. Colored glass was then cut into shapes to fit the design. Details such as faces, hair, drapery folds and lettering were painted on with a dark paint made from ground glass and metal oxides, then fixed by firing in a kiln so that it fused to the surface. Later in the period, silver compounds were used to produce a yellow stain on the glass surface. Pieces were then joined with grooved strips of lead, called cames, soldered at the joints, and the panels were set in iron frames within the window opening.
This craft developed alongside the large windows of the Gothic style. As structural advances allowed thinner walls and bigger openings, as in Gothic architecture, the amount of glass in great churches increased dramatically. Windows told biblical stories, lives of saints and moral lessons in pictures, and they are often cited as one way images served those who could not read, part of the wider religious iconography of churches.
Workshops and organization
Stained glass makers often worked near the building site, or traveled with a project. Master glaziers directed teams who prepared patterns, cut, painted and assembled. Church patrons, including bishops, chapters, guilds and wealthy individuals, paid for windows, and donors are sometimes shown in the glass itself. Glaziers also did plain glazing, and in time formed guilds of their own in various towns, often grouped with painters or other trades depending on the place.
Survival is uneven. Wars, weather, iconoclasm, neglect and later restoration have destroyed or altered many windows, and cathedrals such as medieval cathedrals in France and England preserve only part of what they once held. Because restorers have sometimes replaced or rearranged panels, historians study windows carefully to separate original work from later repairs.
What is less certain
Some medieval technical writing about glass and pigments survives, but its recipes are often incomplete and its terms hard to interpret. Scientific analysis of surviving glass has revealed composition differences between regions, but a general picture should not be pressed too far on any single window or workshop.