20 June 2006

Berg’s Project: Glassmaking and Chemistry

To understand the scale of George Berg’s project and the ways that his formulas and processes did or did not develop during the fifteen years’ work represented by the experiment book, it is important to know something about the creation of glass.

In theory, any substance heated long enough and at a high enough temperature will melt, fuse, and, on cooling, form a glassy substance. Modern definitions of glass describe formulas that incorporate silica with metal oxides and note that a feature of its formation is that glass does not crystallize as it cools to a solid. In most eighteenth-century descriptions, glass is a substance made from a frit (a combination of sand and borax or other alkaline substances that has been ground, melted and reground) plus certain other ingredients—including what we call metal oxides, known in Berg’s time as calxes. The additions to the frit controlled such desirable characteristics as color, hardness, and clarity of the glass. Other variables that affected the characteristics included proportions of ingredients used, the size of the ground particles in the mixture, and both the temperature and length of time of heating. Glass gems needed to be harder than glass made for enameling. Certain imitation stones, including some chalcedonies and beryls, required a milky opaqueness, while false rubies had to be clear.[8] A degree of transparency was expected in colors used in enameling, as the color of the support material, whether gold, silver, copper, or white enamel, should have an effect on the finished piece. Each enamel formula was calibrated to adhere to the substrate used. Berg’s attention to these variables is apparent throughout the experiment book.

Berg also recorded his use of several different furnaces or kilns. The Falcon and Whitefriars glasshouses, two of the commercial glassmaking firms where he worked, were known for their production of the relatively hard cut-glasses. Berg also mentions his use of muffle or wind furnaces. These were typically found in the studios of goldsmiths, watchmakers, and other artisans who might regularly use enameling colors.[9]

The first experiments included in Berg’s first notebook are attempts to make glass gems, and he began to work with enamel formulas not long after. Throughout his fifteen years of experimentation Berg worked on both types of glass, occasionally in alternating weeks. The last experiments in the sixth notebook, however, show that Berg’s interest by then was almost exclusively enameling colors, especially those created for gold or silver jewelry or other trinkets.


At 22 June, 2006, Blogger Sarah Lowengard said...

Image Link

The Museum of London Picture Library has an image of the Falcon glassworks. It is dated 1840, and may not depict the interior as it was when Berg visited. http://www.museumoflondon.org.uk/piclib/pages/bigpicture.asp?id=102

At 14 July, 2006, Blogger Sarah Lowengard said...

The Swedish mineralogist Axel Cronstedt classified chalcedony under siliceous earths, calling it a type of white agate that “is a flint of a white colour, like milk diluted with water, more or less opaque” and harder than other agates. (Axel Fredrik Cronstedt, An Essay Towards a System of Mineralogy (London, 1770), p66)
Here is a modern definition of chalcedony, from the US Geological Survey website: http://minerals.usgs.gov/minerals/pubs/commodity/gemstones/sp14-95/chalcedony.html.
The Powerhouse Museum (Sydney, Australia) has in their online collections several examples: see, e.g. the “Head of a Roman” http://www.powerhousemuseum.com/collection/database/?irn=178585.

At 14 July, 2006, Blogger Sarah Lowengard said...

Image Link
The Schoenberg Center for Electronic Text and Image has images of wind and muffle furnaces. Try accessing this link http://dewey.library.upenn.edu/sceti/smith/imagedetail.cfm?PictureID=485&position=3&keywords=muffle&subcoll=, the main link to the E.F.Smith collection is http://dewey.library.upenn.edu/sceti/smith/index.cfm.


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