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WonderScience fact

About 90% of the glass made today is soda-lime glass: roughly 73% silica from sand, 14% soda and 9% lime, melted together and cooled before the atoms can line up into crystals. The result is an amorphous solid, hard like a crystal but with its atoms jumbled like a liquid's.

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Pure silica only melts at about 1,700 °C and is hard to work, so glassmakers add soda (sodium carbonate), which lowers the melting temperature. Soda alone would make glass that slowly dissolves in water, so lime, from limestone, is added to make it durable. Modern furnaces run at up to about 1,575 °C. Glassmaking goes back more than 4,000 years in Mesopotamia and Egypt, where the first glass objects were beads. Blowing molten glass through a pipe, invented in the 1st century BC, made cups and bottles cheap. Most window glass today is float glass, made by pouring molten glass onto a bath of molten tin, a method developed in England by Alastair Pilkington's team in the 1950s. The old story that church windows are thicker at the bottom because glass slowly flows is a myth: at room temperature glass does not flow measurably.

Türkçe oku
Bugün üretilen camın yaklaşık yüzde 90'ı soda-kireç camıdır. Karışımın kabaca yüzde 73'ü kumdan gelen silika, yüzde 14'ü soda, yüzde 9'u kireçtir; hepsi birlikte eritilir ve atomlar kristal düzenine geçemeden soğutulur. Ortaya amorf bir katı çıkar: Kristal kadar serttir, ama atomları sıvıdaki gibi karmakarışıktır.

Saf silika ancak 1.700 °C dolayında erir ve işlenmesi zordur. Bu yüzden camcılar erime sıcaklığını düşüren sodayı (sodyum karbonat) ekler. Yalnız soda katılsa cam suda yavaş yavaş çözünürdü; kireç taşından elde edilen kireç camı dayanıklı kılar. Modern fırınlar 1.575 °C'ye kadar çıkar. Cam yapımının Mezopotamya ve Mısır'da 4.000 yılı aşkın bir geçmişi vardır; ilk cam eşyalar boncuklardı. MÖ 1. yüzyılda bulunan cam üfleme, erimiş camı bir boruyla şişirip biçimlendirmeyi mümkün kıldı ve bardakla şişeyi ucuzlattı. Bugünkü pencere camlarının çoğu, erimiş camın erimiş kalay banyosunun üzerine dökülmesiyle yapılan float camdır; yöntemi 1950'lerde İngiltere'de Alastair Pilkington'ın ekibi geliştirdi. Eski kilise pencerelerinin altta daha kalın olmasını camın yavaş yavaş akmasına bağlayan hikâye ise yanlıştır: Oda sıcaklığında cam ölçülebilir biçimde akmaz.

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