In the glasing y-wroght: Medieval Glass as Material Medium and Message

The 2016 Harvard Medieval Material Culture Lectures and Workshops were held Monday, April 4- through Thursday, April 7 in Cambridge, Massachusetts. Featured lecturers were Elizabeth Pastan of Emory University and Patrick Degyrse of University of Leuven.

On Thursday morning at the Fogg Museum, Elizabeth Pastan, Professor of Art History at Emory University along with Katherine Eremin, Research Scientist of Harvard Art Museums and Charlotte Gray, Art Historian of Harvard University presented The Craft of Medieval Glass: Decorative Glass in the Harvard Art Museums. Their focus was medieval stained glass.

In their presentation it was explained how technology based equipment used for XRF analysis could be brought on site to research the chemical makeup of individual pieces of a stained glass window in order to determine whether which pieces were original, and which were part of a restoration. It furthermore allowed the glass to be assigned a date and region of origin. Chemical processes were also explained for creating clear glass (white glass), transparent colored glass, and opaque colored glass. During the presentation smaller stained glass samples were illuminated over a light box for close inspection. Methods for cutting and assembling stained glass panels were also explained and a section of lead came, the metal structure which supports a glass panel, was on hand for inspection.

Next, methods for firing images and text onto a glass surface were explained. Using silver stain (silver nitrate) and vitreous paint, the front and reverse sides of a piece of glass could be treated in order for an artisan to reach a desired effect. This process could also have been repeated multiple times if necessary.

The relationship between stained glass designer and stained glass artisan was then described. In some cases the relationship could be contentious, however one example was shown of an artisan’s faithful interpretation of the artist’s original design. In terms of installation, if a window were to be located high above ground, a full-scale cartoon would be placed on the floor of the actual site where the glass pieces could be laid out to ensure accuracy and efficiency during an installation.

The presentation moved into the Naumburg Room, a two-story Jacobean hall, deigned with special light wells for displaying and viewing recently restored stained glass windows. The highlight of the two-hour presentation was the ehibition of a restored late-12th Century roundel from the Canterbury Cathedral, depicting the life of Thomas Beckett. The decision making process of the restoration as well how to support, display, and illuminate the work were discussed at length.

For stained glass enthusiasts, two valuable resources are:

www.corpusvitrearum.org

www.therosewindow.com

Naumburg Room, Fogg Museum, Boston

Scenes from the life of Thomas Beckett.

Later that afternoon Patrick Degryse, Professor of Archaeometry at University of Leuven presented The Technique and Trade of Glass in Antiquity and the Middle Ages at the Harvard Semitic Museum. The museum’s basement level is not only used for storage but for hands-on research and ongoing projects where currently approximately 400 glass objects are in the process of being catalogued. By performing isotopic analysis, Degryse attempts to trace the glass back to its source.

Unlike the earlier stained glass presentation, these glass objects were formed from molten glass taken directly from a furnace. The workshop’s participants were invited to observe and handle about a dozen of these glass objects. The glass was explained in terms of its fundamental properties of which silica (sand), makes up approximately 75% of the material. Other ingredients would have included plant ash and sodium carbonate. Sand samples taken from various regions of the Mediterranean were then examined by the workshop participants who then theorized which samples would be most suitable for glassmaking.

Next, working with modeling clay, the participants had an opportunity to simulate techniques for working with hot glass. The three techniques discussed were core-trailed glass, mold formed glass, and blown glass. According to Degryse glassmaking goes back to at least 3000 BC, but glassblowing did not develop until approximately 100 BC.

One very important point in the history of the development of glassmaking is the enormous consumption of fuel. Molten glass is kept at approximately 1600 F, a temperature which would need to be maintained over a period of days, even weeks, and would require the cutting of thousands of trees, essentially deforesting the area where the glass shop was built. Therefore glass shops would move their location nearer to fuel sources. Glass furnaces necessarily took an enormous toll on local forests resulting in medieval laws prohibiting the use of glass furnaces.

If you have not seen it, glassblowing is a fascinating process to watch. Visit the website for the Corning Museum of Glass in Corning, NY and click on their video links.

www.cmog.org

Roman mold formed glass

core trailed glass

By Michael Weldon