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What is annealing of glass? Name of the colour obtained when
i.) Ferric compounds
ii.) Cobalt compounds are added to the mixture of glass

Last updated date: 17th Jun 2024
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Hint: Annealing process of glass is similar to the annealing process of metal. The Annealing process of glass ensures the glass forming and shaping more efficiently. Temperature stress in glass causes spontaneous shattering or cracking.

Complete step by step solution:
Annealing refers to the heat treatment process of glass. Annealing process is basically used to minimise hardness and eliminate the internal stress. Annealing is a slow cooling process of the manufactured hot glass after holding some time. Annealing process minimises the internal stress, improves ductility and reduces the brittleness of manufactured hot glass.
- During the manufacturing of glass a large variation in the temperature develops stress on the surface of glass. Glass will expand on heating, while it will contract on cooling. So a controlled cooling process is necessary to prevent the damage like cracking, shattering of glass when glass is subjected to variable temperature.
- Mixing by spraying different coloured compounds such as ferric chloride on hot glass produces an iridescent (combination of pigment) effect on carnival glass (a type of pressed glass).

- Ferric chloride - ferric chloride is an orange or brown black coloured solid. So when we spray ferric chloride on melting glass, it forms a coloured coating on the surface of cooled glass.
- Cobalt compounds - cobalt compounds are blue coloured compounds, so when these compounds are sprayed on the surface of melting glass it forms a deep blue coloured coating on the surface of cooled glass.
These colouring compounds form a thin layer over the surface of glass which not only make them attractive but also prevents oxidation.

Note: Ferric chloride and cobalt compounds are coloured compounds which are mainly used as colouring agents in the chemical industry.
- The process of heating, cooling and forming induces stress in the glass and annealing process increases heat soak time period of glass.