Bulletin - Vol 8 No. 5 September/October 2005

Pharology 101 - The Glass

by Denise Shultz, LoA President and Prism Editor

Noble and beautiful, glass is a material of both artists and engineers, a material without which a lighthouse could not even be imagined. Possessing unique physical and chemical properties it is indispensable in lighthouse technology.

Chemically, the glass is a mixture of three basic ingredients:

• Silicon dioxide (SiO₂) in a form of white sand
• Calcium carbonate (CaCO₃) or limestone
• Sodium carbonate (Na₂CO₃) or soda

Pure silicon dioxide when melted, makes an excellent glass which is transparent not only to visible but also infrared and ultraviolet light and resistant to heat and chemicals. At 1700ºC pure silica glass melting point is so high that its manufacture is very energy demanding and expensive as glass has to be heated well above its melting temperature to assure the quality. Adding soda can dramatically lower the melting point but makes the glass more soluble in water and prone to corrosion. The compromise is reached by adding lime, which makes the glass water resistant again. Soda can also be fully or partially substituted with potash or potassium carbonate (K₂CO₃) to enhance the optical properties.

The ratio at which these three basic materials are added varies according to what properties of glass are desired for the specific purpose.

Other chemicals could be added as well to enhance or suppress certain characteristics. Magnesium and aluminium for chemical resistance, selenium, cobalt, arsenic, cerium and antimony for refining the glass clarity, boron and aluminium for lowering the heat expansion coefficient and lead oxide for enhancing the optical properties. Other metallic oxides are used to add colour to the glass, for example cobalt for blue, chromium for green or yellow (depending on the compound), while manganese gives violet, nickel brown, iron olive green, uranium and lead yellow, selenium, tellurium and gold pink. The final colour of the product always depends on the concentration of the additive, the composition of the glass itself and the technology of the colouring process.

Time and exposure to sunlight can also change the colour of glass. The lighthouse lenses, when they were made, were required to be colourless and crystal clear. Yet, when we see them today, they have a yellow hue. It is because the trace elements like cerium or arsenic that were originally added to mask the colour of impurities that came with the raw materials react with energetic ultraviolet light and change their molecular structure.

When producing glass, 50 to 75% of broken recycled glass of the same type (called cullet) is added to the raw materials. In a furnace the cullet starts melting well before the other raw materials and by doing so acts as a solvent. That in turn further accelerates the melting and mixing process. The glass has to be heated at some point to a temperature close to 1500ºC at which it is as fluid as water. Such low viscosity is essential to allow the bubbles formed by trapped air and carbon dioxide liberated from the carbonates to escape. It also helps to homogenise the mixture.

When the molten glass is allowed to cool down, instead of arranging its molecules in an organised structure and forming crystals, it gets increasingly viscous until it solidifies, too quickly for the crystals to form. The result is an amorphous solid which is structurally like a liquid with its molecules locked in their position at random. The glass can not be allowed to cool too quickly though, because it would result in an uneven tension between the surface and the interior, freezing the internal stresses into the structure and making the glass too susceptible to even small surface shocks. The problem is overcome by slowly passing the glass through an annealing oven (lehr) with progressively lower temperatures.

Before it was replaced recently by plastics, the type of glass used most frequently in lighthouse engineering was optical glass.

For optical apparatus, certain properties like high transparency, clarity, low dispersion and high refraction index, are very desirable. Bubbles and striations could add to the glass item’s uniqueness if it is a work of art but in an optical lens, the glass has to be flawless and both bubbles and striae have to be avoided (even though many people would agree that a Fresnel lens IS a work of art). To do that, both manufacturing technology and the glass formula come into play.

Early 19th century manufacturers struggled with coming up with the product that would be good enough for use in large lighthouse lenses. Eventually, two types of glass evolved that were suitable for such a purpose.

Crown glass was developed in Venice and perfected in France. Even though today there are many different formulae for crown glass, basically it is a silica, soda and lime composite with high percentage of silica. For example the glass manufactured at Saint Gobain (France) for the construction of lighthouses was 72.1% silica, 12.2% soda and 15.7% lime with traces of alumina and iron oxide. Today, more sophisticated formulae are used and other oxides like boric, barium, aluminium and zinc are added, while soda is often replaced by potassium. Borosilicate glass, on the other hand, contains neither sodium or potassium or lime. It is mainly silica with lesser percentage of boric oxide and alumina (aluminium oxide). The crown glass has to be heated to a higher temperature then normal soda-lime-silica glass but is relatively hard, light and suitable for moulding and polishing. Its refractive index is between 1.49 and 1.61, average density 2.54 g/cm3.

Flint glass was first made in England and is named after the original source of silica, flint stone. Because it contains lead oxide, it is also known as lead glass or crystal glass. It is much heavier (the density of heavy flint could be up to 5.18 g/cm3, average 3.22 g/cm3), has a high refraction index (1.54 -1.78) and is more heat resistant, providing it has been properly annealed. It is more expensive to produce that crown glass.

While the French were decidedly on the side of crown glass, in England a discussion and experiments were taking place in 1820s to resolve the question of which one of these two types of glass would be more suitable for lighthouses. The result was ambiguous and both types were found to be suitable. Though most large lenses are said to be made from lighter crown glass, Chance Brothers produced many made of crystal glass. In Australia, examples of first order crystal glass Fresnel lens can be found at Cape Otway or Cape Schanck.

Resources:
Lighthouse Technology by Thomas Tag (well-known authority on lamps, lens apparatus and illumination for lighthouses, with articles on the subject appearing in the US Lighthouse Society’s Keeper’s Log, Lighthouse Digest and other publications.)
Encyclopaedia Britannica

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