The Intriguing History and Science of Wavy Antique Glass Windows
The windows of old houses and cathedrals are often known for their distinct, wavy appearance. This unique quality is not an intentional design choice but rather a result of the methods used in glass manufacturing before the advent of modern techniques. Let's explore the fascinating history and science behind these wavy antique glass windows.
Understanding Glass Flow and Manufacturing Techniques
Until the modern era of glass manufacturing with the introduction of float glass, the quality and characteristics of glass were much different. One of the early methods involved pouring molten glass onto a rotating table to spread it out. This process resulted in ripples and a thicker center due to the uneven distribution of the glass during cooling.
Another traditional method involved blowing a large bubble of molten glass, cutting the bubble open, and then flattening the result. This technique, known as crown glass, produced wavy and uneven surfaces due to the manual manipulation involved in the process. The irregularity in thickness and surface quality was further complicated by the presence of small bubbles and inclusions that were inherent in the manufacturing process.
The Transition to Modern Manufacturing: Float Glass
The advent of float glass dramatically changed the glass industry. Molten glass is now rolled into sheets and allowed to settle on a pool of molten tin. This process, known as the Pilkington process, ensures that the glass becomes perfectly flat as the tin supports the glass evenly during cooling. The result is a much smoother and higher-quality glass surface, free from the wavy patterns and imperfections common in antique glass windows.
Science Behind Glass and the Manufacturing Process
While it might seem counterintuitive, glass actually flows very slowly over time. This is due to the way in which glass is structured on a molecular level. Unlike a typical liquid, which is made up of molecules that are close together and can flow smoothly, glass has a structure that is more crystalline in nature. In this state, the molecules are not entirely fixed in place, but they can still move over incredibly long periods. This causes the glass to slowly deform over time, but the changes are imperceptible on human timescales, such as the few millennia we experience.
However, when dealing with large pieces of glass, such as those used in telescopes, the flow can create a noticeable bulge. This phenomenon is known as "bournage," and it is a result of the glass being supported by the edge only. To counteract this, many modern large telescopes use mirrors instead of lenses, as the support structure of the mirror helps to minimize the effect of the glass flow. Mirrors can be supported from their surfaces, making them more stable and less prone to deformation.
Historical Traditions in Glass Manufacturing
The evolution of glass manufacturing techniques over the centuries has led to the production of different types of glass. Early clear glass was called crown glass, while an inferior but more commonly used glass was known as cylinder glass. Crown glass was made by spinning a glass bubble, while cylinder glass was produced by blowing cylinders that were then flattened into sheets. Both types of glass often had imperfections such as bubbles and surface irregularities, which contributed to the unique appearance of antique glass windows.
The preference for small panes in old windows can be attributed to the limitations of the manufacturing processes in the past. The techniques available did not permit the production of large sheets of glass, hence the use of smaller panes to create the windows.
For those interested in more detailed information about the history and production methods of old glass, you may want to visit the Old House Authority website, which offers a wealth of information and insights.