ABOUT THE GLASS
The English term stained glass is actually a misnomer. Early in its history in mainland Europe, painting on glass had transformed what began as a transparent, three-dimensional experience into a two-dimensional, if glowing, derivation from easel painting. When English travelers first observed French artisans painting their glass vitraux, they coined the English term stained glass. Actually, rather than staining or painting color onto glass, each glass color is created from a specific chemical recipe by the glass blower. See Blowing Sheets of Glass for a slideshow of how German mouthblown glass is created. See The Fabrication of Stained Glass for a slideshow overview of the basics of stained glass making.
CLICK any of the images on the left, for a captioned slideshow of the basic glass types used in ASG’s stained glass.
MOUTHBLOWN GLASS
Just as in Gothic times, mouthblown glass* begins as a molten, incandescent mass called a “gather” on the end of the glassblower’s blowpipe . Next the gather is blown and shaped into a cylinder. The cylinder is then cooled, removed from the blowpipe and its “top” and “bottom” are removed to create an open-ended cylinder. Finally the cylinder is split or cut from top to bottom so it can be reheated, opened out and flattened into a sheet of glass. The subtle variations in the thickness of mouthblown glass can produce watercolor-like gradations of color within a sheet. Because it is blown against air rather than a hard surface, antique glass has a more pristine, crystalline transparency than rolled glass (see below). An intentional texture is usually added to antique glass while it’s still on the blowpipe to simulate the straw onto which cylinders were opened in Gothic times. These “straw marks” or striations add sparkle to the sheets. Sometimes an air-entraining chemical is added to the molten glass causing tiny bubbles (seeds) to form. Sheets blown from such glass are said to be seedy.
* ASG’s glass palette draws from many sources, both domestic and European. A primary source for European mouthblown glass is Glashütte Lamberts in Waldsassen, Germany. Lamberts’ exquisite range of colors and textures makes the subtle watercolor-like gradations and figure-ground relationships in Smith’s designs possible. The photos you’ll see in Blowing Sheets of Glass appear courtesy of Glashütte Lamberts.
HISTORICAL ASIDE
Mouthblown glass is sometimes referred to as “Antique” glass. A little history will explain why: By the 16th century the technique for blowing sheets of glass was starting to be lost. Painting and staining on glass had become so widespread that less expensive rolled-glass (see below) had become the “canvas” of choice for glass painters. The fact that rolled glass lacked mouthblown glass’s pristine transparency and rich color gradations was acceptable to stained glass makers at that time since the subtleties of mouthblown glass were obscured during the painting process anyway. As it turns out, all was not lost. In the 1840’s the Gothic Revivalists discovered dusty Gothic texts that revealed the “lost” process for mouthblowing sheets of glass. They referred to glass blown using the old technique they had rediscovered as “Antique” glass. Because it is formed against air, the texture of mouthblown sheets has a lucidity and clarity not found in rolled glasses. With mouthblown glass once again available, the work produced by glass artists (such as Frank Lloyd Wright and Johannes Thorn Prikker) began to re-emphasize transparency with views out and the projection of sunlight into the experience of stained glass. Once again, stained glass could be experienced as a truly and fully three-dimensional window. For more visit “A History of Stained Glass”.
FLASHED MOUTHBLOWN GLASS
The earliest formulas for the ruby-red glass that smoulders in earliest Gothic windows produced an almost opaque black glass unless it was blown unusably thin. To get around this problem, flashed glass was invented. To make a sheet of flashed red, the glassblower started with two gathers of glass on the blowpipe. The first was a larger gather of clear glass. This was then encased in a thin gather (”flash”) of dark red glass. The resultant sheet of glass had a thin “flash” (layer) of red on a thicker clear base – the red was thin enough be visible as “red” while the thicker, clear, base allowed the sheets to be durable enough for stained glass.
Later, flashed glass began to be made with other transparent “flash” colors on a base of colorless or sometimes light-colored glasses. By starting with a darker than normal color, slight variations in thickness produce enhanced watercolor-like gradations that can be even more pronounced than in non-flashed sheets. It was soon discovered that the flashed side can be sand- or acid-etched to create graphic imagery, lettering or designs.
OPALS AND OPAKS
Another development was to combine a flash of white opalescent glass on a clear or colored (transparent) base. If the white flash is filmy enough to permit a certain degree of vision through the glass, the flashed sheet is referred to by the German glassbowers’ term Opal (pronounced “OH-paul”). If the white glass is so dense that no vision through the glass is possible (only diffused light is transmitted), the sheet is said to be Opak (“OH-pock”).
Opals and Opaks can extend the visibility and animation of a stained glass window to just about all lighting conditions – day or night, inside and out. This is because in transmitted light (back-lighting) the diffusing quality of the opalescent glass “grabs” light from all directions and sends light and sometimes color to the eye of the viewer. Conversely, in reflected (front-lighting), opalescent glass bounces light back to the viewer so that Opal or Opak glass remains visible when seen from the outside during daytime or when viewed from the inside at night.
Another interesting quality of so-called “white” opalescent glass is its “apparent” color in different lighting situations. In direct sun opalescent explodes as intense, radiant white. In bright indirect light, opalescent glass isn’t white at all, but shifts to a warm amber cast. In shadow or low-lighting, the same “white” glass appears to have a bluish tint.
Finally, Opals and Opaks expand the three-dimensional quality of the stained glass experience when used to play figure against a transparent glass ground.
OTHER TYPES OF MOUTHBLOWN GLASS:
SEEDY (also Rolled Seedy): A few seeds or bubbles can usually be found in any mouthblown sheet glass. Sometimes air-entraining chemicals are added to the glass recipe to intentionally increase the concentration of seeds.
REAMY: During the blowing process, the glass cylinder is distressed to create thickened waves and collapsed bubbles in the resultant sheets producing a frozen water-like appearance.
CRACKLE: During the blowing process, the cylinder is thermally shocked with water. This causes a controlled fracturing of the shocked side of the glass. This fractured patterning gives Crackle glass an alligator-skin sparkle.
STREAKY: Two or more colors (may be Opal or Opak) are flashed onto a clear or colored base. A great variety of waviness and streaking can be created in these types of glass.
SHADED: Flashed Shaded glass is produced by a flashed color being added, usually in bands so that the resultant sheet grades from the base color into the flash color (and sometimes back multiple times).
ROLLED GLASS
Rolled glass can made by pouring molten glass into a machine that rolls the glass into sheets of uniform thickness. It can also be rolled by hand – sort of like rolling out dough. Because of its uniform thickness and unavoidable texture, even so-called “smooth” rolled glass has a slight, wax-papery appearance when compared to the crystalline transparency of mouthblown glass. As the preferred canvas for glass painters beginning the middle Gothic period, rolled glass is sometimes referred to a cathedral glass.
There a numerous variations on this general description. Different textures can be pressed into one side of the sheets by adding textured molds to the rollers and/or by varying the speed and direction of the rollers. As with mouthblown glass, an air-entraining chemical is sometimes added to the glass formula causing tiny bubbles (seeds) to produce seedy rolled glass. If more than one color is poured behind the roller, the result is called “streaky” glass.
However, the very characteristics that make rolled glass generally less desirable* than mouthblown glass, can used in combination with mouthblown glass in a window, can add depth, interest and contrast.
* Desirability is, of course, in the eye of the beholder. The various makers of rolled glasses are justifiably very proud of the glass they make. Because of my preoccupation with three-dimensional transparency in glass, ASG only uses rolled glasses to contrast with and enhance the lucidity of mouthblown glass.
LEAD-CRYSTAL BEVELLED PRISMS
Austrian (i.e. Swarovski), lead-crystal, bevelled prisms are made from a glass formula containing up to 50% lead. This heavy, dense glass has an increased refractive index that is ideal for bending “white” sunlight into tiny spectra or rainbows that sprinkle unexpectedly across interior surfaces. During the year these rainbows will trace a path that tracks the sun’s seasonal round. These subtle accents are create unexpected surprises in many of ASG projects.
DICHROIC GLASS
Dichroic glass is made by vacuum-coating clear plate/float (or sometimes other) glass with heavy metal oxides. This coating creates color that shifts as the viewing angle changes. This becomes dramatically apparent when you walk past a window that includes dichroic glass – notice the apparent color change. In front lighting (i.e.: as seen from outside during the day) dichroic glass shifts again and becomes a reflective mirror. [SEE: Resurrection at St. Albert.]
PRESSED LENSES
These German lenses are hand-pressed and fire-polished. The may not be optically perfect, but are ideally suited for stained glass. One side is ground flat and polished, making it possible to laminate these lenses to another piece of glass in a window with optically-clear, UV adhesive. [SEE: Resolution Suite in the Admirals Club at DFW International Airport, Fire and Water at Houston’s Fire Station 37 and the U. S. Courthouse in Fargo]