Why will clay, when heated to high temperatures, harden and become waterproof?

Question

I have a question

Why will clay, when heated to high temperatures, harden and become waterproof?

Answer 1

Among other things in the clay body, the silica content is heated to a state where it is more viscous, though the structural integrity of the body is maintained. As the viscosity of the individual pieces of silica increases, the particles are disordered and come together in a more liquid form. At the point of cooling, vitrification occurs and the individual particles have joined together in a more ordered state. Due to the new order of particles in the vitrified clay body, water cannot pass, provided the integrity of the vitrified clay body is maintained.

At approximately 2385-2420 deg.F the SiO2, among other things, begins to turn from solid particles and crystals into molten SiO2. In the heating process the organic material in the clay is burning off, the water is evaporating, and gaps are being created. However, at the point of many compounds turning more viscous/ or closer to liquid, gaps are also being filled. During the cooling process, the molten compounds have combined, filled gaps, and to some extent maybe even crystallized. There has definitely been some rearranging of structure, fusing many individual particles into one body. There are both physical and chemical changes that occur in the firing.

*It is important to know about viscosity and vitrification to fully understand this simplified explanation.

Answer 2

From http://matse1.matse.illinois.edu/ceramics/prin.html we get this piece of information from a summary:

The processing of crystalline ceramics follows the basic steps that have been used for ages to make clay products. The materials are selected, prepared, formed into a desired shape, and sintered at high temperatures.

What is sintering?

From http://www.substech.com/dokuwiki/doku.php?id=sintering_of_ceramics we get this definition of ceramic sintering;

Sintering (Firing) of ceramic materials is the method involving consolidation of ceramic powder particles by heating the “green” compact part to a high temperature below the melting point, when the material of the separate particles difuse to the neghbouring powder particles.

From the same page this suggests an answer to your question about waterproofing:

During the diffusion process the pores, taking place in the “green compact”, diminish or even close up, resulting in densification of the part, improvement of its mechanical properties.

Answer 3

Clay minerals form as the products of low-temperature chemical weathering reactions near the earth's surface. Clay minerals contain hydrogen and oxygen in the form of hydroxyl (for example the composition of kaolinite: Al2Si2O5[OH]4). If kaolinite is heated above 550-600C, there is a series of chemical reactions that consumes the clay as a reactant and produces water and other minerals as products. The products of such a relatively low-temperature treatment can be stronger and more moisture resistant than the starting material because of the recrystallization that accompanies the clay dehydration, although not as strong and impervious as produced by firing at higher temperature.

At a temperatures of 1000C and higher, chemical reactions producing anhydrous alumina and silica oxide drive off more water. At high temperature, the diffusion transport along grain boundaries (and even through crystals) required for sintering can become significant.

As an aside, it was fortunate for the development of human culture that relatively water-resistant and strong bricks and pots can be manufactured from relatively low temperature firing of clay. The hydrous fluids produced by these reactions at a more easily attainable temperature facilitates solution and re-deposition mass-transport through the microstructure which makes the piece stronger and impermeable.