chemical water?

[mr. piper]

what is the difference between water present in ceramics that is leached out in the kiln firing process at one temperature, and the "chemical water" that is removed at a different, higher temperature. scuse me i thought all water was a chemical and had a boiling point constant for a constant atmospheric pressure?

[Calvesy]

Clay is a silicate, a mineral that contains hydrogen and oxygen. Temperatures much higher than 100 Celsius cause chemical changes as it turns to ceramic, releasing hydrogen and oxygen which bond forming steam. When you breathe out there is water and CO2 even if your food was dry and not fizzy.

[brachiopod]

Clay minerals also consist of hydrated silicates (usually alumo-silicates) where water is chemically bonded within the crystal lattice. It takes a considerably higher temperature than 100 �C for thermal decomposition of the mineral to occur.

An anology is if you ever heated (blue) copper sulphate back in your school chemistry lessons. You could drive off the water of crystallisation to form the white, anhydrous copper sulphate, but it involved heating it over a very hot bunsen flame for several minutes.

Although it's not exactly the same process, (as the reaction in the copper sulphate case is easily reversed by adding water) - but you get the idea, in that it's not just a case of the clay 'being wet', but that the water is chemically bonded within the clay minerals.