Glass-Making-art

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Glass-Making-art - 5/15/17

"The Method to the Madness: Creating Glass in the Medieval Period" by Leofwynne le glasyer.

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NOTICE -

This article was added to this set of files, called Stefan's Florilegium, with the permission of the author.

These files are available on the Internet at: http://www.florilegium.org

While the author will likely give permission for this work to be reprinted in SCA type publications, please check with the author first or check for any permissions granted at the end of this file.

Thank you,

Mark S. Harris...AKA:..Stefan li Rous

stefan at florilegium.org

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The Method to the Madness:

Creating Glass in the Medieval Period

by THL Leofwynne le glasyer

Glass has been around for millennia. The earliest glass has been dated to Egyptian times, around 3000 BC and before. The Egyptians were making glass artifacts, including perfume bottles, vessels, and containers, and the craft spread across the Middle East. Traders brought glass objects, primarily made of glass pressed or blown in molds, rather than blown on the end of a blowpipe, and the knowledge for production, from the major glass center of Alexandria to Greece and Rome. The Romans refined glassmaking significantly, developing the methods for making clear glass, as well as methods for intentionally coloring glass, and utilizing flat glass in early windows. By the Middle Ages, glass was being blown and made into flat panels for piecing, painting, and etching. The traditional methods for coloring glass were used for centuries, and the process for making sheets of glass did not change significantly until the modern era.

Glass in its purest form is silica -- silicon dioxide. If silica is heated to above 3000 degrees F, it vitrifies, or becomes perfect glass. Vitrification of silica requires extremely high temperatures, and this was not easily done with ancient and medieval kilns. With additions of a flux and a stabilizer, glass could be made at a much lower temperature, and still retain strength. The Romans found that adding soda and lime to the silica mixture resulted in a glass that was more durable, and could be made at a lower temperature (around 2100°F). This knowledge was, however, lost to the Anglo-Saxons when the Romans left. It was not rediscovered until around the fifth century, when glassmaking was revived in Western Europe, particularly in Gaul. Early medieval glass was made with a 50% silica, 50% lime and potash mixture. This resulted in a glass that vitrified easily, but was brittle, and was easily eroded or discolored over time.

Fig. 1

Fig. 2

Making sheets of glass

There are two methods of making sheet glass from the molten mixture. The first method, the "muff" method, makes a rectangularly-shaped piece of sheet glass. A glassblower gathers molten glass on the end of a pontil rod (fig. 1). The glass is then shaped with the pontil rod and blown into a cylindrical shape (fig. 2).

Once the correct shape is achieved, the glassworker cuts off both ends of the cylinder (fig. 3). The resulting piece is sliced down the center, and opened up to create a rectangle (fig. 4). A wooden spatula was used to flatten the glass piece.

Fig. 3 Fig. 4

The second method is called the "crown," or "spun," or "sheet" method. Molten glass is again gathered on the pontil rod, and a cylindrical shape is blown. Once the piece attains a desired size, another iron rod is attached at the end of the cylinder (fig. 5), and the pontil rod removed. The glass is then continually rotated on the new rod until it flattens out and reaches the desired circular shape and size (fig. 6). It is cut off the rod and cooled. Glass made in this way retains a raised mark in the center from the shaping rod attachment. It is sometimes called "bullseye" glass because of its round shape and center mark. Both of these methods are still in use today.

Fig. 5

Fig. 6

Coloring Glass

From the earliest manufacture of glass to modern-day production, metal oxides are the primary method for coloring glass. In the early manufacture of glass, coloring was often a hit-or-miss affair, as early glassmaking materials were not always pure, and metals were present in the component parts, unbeknownst to the glazier. This resulted in situations where, as glass ingredients were heated through the molten stage, different colors could be obtained by heating or cooling the mixture, or by letting it stand at a particular temperature. In this case, the glazier had no particular foreknowledge of metallic oxides present in the glass component mixture, it was more a case of observing the glass in the pots as it cooked. Certain materials were known, however, to produce certain colors, and these substances could be added to the molten glass to produce specific colors.

Some metallic salts and oxides used for coloring glass

Copper

Chromium

Manganese

Iron/

Uranium

Cobalt

Gold/Copper/

Selenium

Blue

Yellow Greens

Purple derivations

Brown Yellows

Green

Yellows

Blue

Red

Use of these metallic salts and oxides is still the way glass is colored even today.

There are two basic types of colored glass used during the medieval period: pot glass, and flashed glass. Pot glass, or pot-metal glass, has had the color dispersed throughout, so that the entire piece of glass is solid-colored. Flashed glass has a thin layer of another colored glass permanently bonded to it; the blown glass bubble or cylinder was dipped into a pot of molten colored glass to coat it. Etching or abrasion was used to expose the underlying color.

The Construction of a Window

Before a window could be made, several preparatory items had to be completed. A design had to be developed and sketched, and lead had to be molded and shaped for placing between the sections of glass, and soldering sticks had to be made.

Medieval glaziers made their sketches on a whitewashed board. The design was drawn on with lead, and this was used as a pattern for cutting and shaping the glass to make the window. It was reusable, as another coat of whitewash was all that was needed to make the surface fresh for another window.

Fig. 7

Fig. 8

Lead was molded into cames, or strips, by casting into sand, or by pouring into carved molds. The resulting pieces were H-shaped channels into which glass could be fitted to hold the window together (fig. 7).

Solder was made by castinga mixture of one part lead and fice parts tin into molds to make sticks.

After the materials were made, and the glass was created, construction could begin. Sheet glass, once cooled and annealed, was cut using a red-hot dividing iron, which was drawn along the surface of the glass, creating thermal shock conditions and causing the glass to crack. The pieces of glass, broken out by the use of the hot iron, were then shaped using a grozing iron (fig. 8), to smooth irregular edges, and help fit the glass into the pattern.

Once the cut pieces of glass were smoothed, and fit into the pattern on the board, molded lead cames were placed around each piece of glass. Nails were driven into the board around the lead-covered pieces of glass, to prevent them from moving while soldering.

The glazier then used another hot iron, and melted the solder at the junctions of all the lead came. The panel was carefully turned onto its other side, and the soldering process was repeated. The window was then ready to be installed.

Reference List

Books

Brisac, Catherine. A Thousand Years of Stained Glass. Edison, NJ: Chartwell Books, 2001.

Brown, Sarah and David O'Connor. Medieval Craftsmen: Glass Painters. Toronto: University of Toronto Press, 1991.

Halliday, Sonia and Laura Lushington. Stained Glass. Sydney: Colporteur Press, 1982.

Osborne, June. History Handbooks: Stained Glass in England. Great Britain: Sutton Publishing, 1997.

Scobey, Joan M. Stained Glass Traditions and Techniques. New York: The Dial Press, 1979.

Theophilus. On Divers Arts. Translated by John G. Hawthorne and Cyril Stanley. New York: Dover Publications, 1963.

Web Resources

History of Stained Glass

<http://www.stainedglass.org/main_pages/association_pages/historySG.html>;

A brief history of stained glass from the Stained Glass Association.

Medieval stained glass windows from Esslingen am Neckar (Germany)

<http://home.bawue.de/~wmwerner/essling/english/glas01.html>;

German glass from the 13th and 14th centuries.

Michelli's History of Stained Glass, Medieval

<http://www.ariadne.org/studio/michelli/sgmedieval.html>;

This site has some good commentary and lots of pictures of period glass.

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Copyright 2016 by Christine Brandel. <leofwynne at verizon.net>. Permission is granted for republication in SCA-related publications, provided the author is credited. Addresses change, but a reasonable attempt should be made to ensure that the author is notified of the publication and if possible receives a copy.

If this article is reprinted in a publication, please place a notice in the publication that you found this article in the Florilegium. I would also appreciate an email to myself, so that I can track which articles are being reprinted. Thanks. -Stefan.