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

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This is a good introduction: en.wikipedia.org/wiki/Sintering Maybe You prefer the french version of this page. –  Georg Oct 11 '11 at 14:11

4 Answers 4

To make the clay "waterproof," the first step is to get the chemically bonded water out of it. That happens as temps rise over 350°C, up to about 800°C. The organic compounds (carbon) have already burned away by that temperature, and what happens next is that other compounds begin to melt and fill the voids left by the attached water molecules (at the micro level.) The silica starts to melt shortly after, and bonds differently to the alumina, with long, flat layers of each connected by rows of oxygen molecules that line up with each other. That is called vitrification, and by then it has hardened, "plugged the holes" (so to speak) and lined up as a barrier to smaller molecules. The other thing that makes clays "waterproof" can happen when excess silica forms glass sheets that coat the clay and prevent water from getting to the clay at all. That is the point of glazes. (Glaze means glass.) The silica can begin to "flow" below its normal melting temperature by adding a flux. Fluxes also change the characteristics of clay to produce different hardening and melting temperatures. Iron oxide is one. It is in most clay and causes the melting (and maturing) temps to drop. That's a simplified overview, but you can easily find more online.

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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.

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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.

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This doesn't tell us why these things happen, but it at least gives us a solid basis for the conversation. –  timquinn Dec 29 '12 at 18:18

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.

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This answer is ridiculous--- you have said nothing besides big words, and "it happens because it happens". Translating: the silica "viscous"ifies (which doesn't mean anything. You should have said "melts"), dissolves the impurities, refreezes in a glass. –  Ron Maimon Oct 11 '11 at 16:52
    
Ok, "melts" could be used, but doesn't glass become more viscous as it melts? Sorry, I'm not just trying to use "big words." I was trying to use key words to describe specific points of the process. Could you please provide an answer that would contribute to the better understanding of this process? –  A. Mendoza Oct 11 '11 at 17:26
    
@Mendoza-- I overreacted. But I think "melts" is wrong too--- the melting temperature for pure glass I think is higher than a clay oven. Aren't you just decreasing the water content quickly, so that it is similar to evaporation? But perhaps you are doing this after first dissolving the object in the water. I was only going by the fact that the answer left me as mystified as I was when I read the question, even after reading the wikipedia pages on vitrification. Maybe you are right, and this is a silica melting-refrezing transition, but please clarify. –  Ron Maimon Oct 11 '11 at 17:31
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No problem Ron. At approximately 2385-2420 deg.F the SiO2 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. –  A. Mendoza Oct 11 '11 at 18:22
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@Georg: You are ridiculous--- I have seen clay baking, and the oven was nowhere near as hot as an oven which melts silicon-dioxide glass (pure glass, to distinguish from glass mixed with mud and dirt, i.e clay)--- in particular it doesn't glow white. For all I know, the only purpose of the oven was to hasten water removal, but perhaps there is real chemistry. –  Ron Maimon Nov 12 '11 at 8:15

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