What makes clay hard?

I'll try to answer your question in three ways: What makes clay "hard"(as in a solid), what makes fireclay heat resistant, and why it hardens.

What makes clay a solid is the strength of the intermolecular forces between the different molecules in it. Intermolecular forces, or IMF for short, don't act between just different molecules of the same compound, but with any compound around it. Generally if a substance is a solid at room temperature, then it has strong IMFs, and if it is liquid or gas, then it has weaker IMFs. There's a lot more that goes into and behind IMFs, and that's sort of the basic version. So if you're interested look it up on Google to learn more. So it is because of strong IMF's that SiO₂ and Al₂O₃ are able to "stick" to each other. But if I recall correctly, I've seen that firebricks aren't necessarily hard like a diamond, in fact rather brittle at times, but still an extremely heat resistant solid.

What makes fireclay so heat resistant is the types of bonds in the molecules that mostly make it up, SiO₂ (silica) and Al₂O₃(alumina). This is because alumina has much stronger bonds inside of each molecule than silica does. If you think of a diamond, you are actually thinking of just one big molecule, because all it's atoms are bonded together in a network. In many cases, alumina can bond together as big molecules like a diamond does as you can see in the picture below. Diamonds do it better though, as they are extremely hard unlike most alumina compounds.

So it is this what lends alumina it's high heat resistance. Silica is actually what makes up most sand and quartz. It is actually a very interesting compound, despite looking so simple. Similar to alumina, silica molecules can sometimes network together in a way you can find in quartz, and this makes them also very heat resistant. So both of these molecules can end up forming some big molecules compared to others, but the size of the particles these molecules make are still microscopic in size.

The post above me has a good explanation of the structure of clay.

And so for why fireclay (and other clays, for that matter), can go from a kind of slurry to a solid is because of chemical reactions that happen once the water is added. Fireclays are as you've seen not just alumina and silica, but there are some other metallic oxides and hydroxides in there as well. These too will have very strong ionic bonds, so they do not lessen the heat resistance of the bricks. So once you add water, it can hydrate many of the compounds in the fireclay slurry, an expand them slightly. Also, chemical reactions may happen too between some of the compounds in the mix and the water, similar to in plaster. And so as all the excess water evaporates, the IMFs present, new bonds made, and the hydrated compounds are what makes the brick a solid, heat resistant, brick.

Hope I was of help!

Helpful wikipedia pages:
https://en.wikipedia.org/wiki/Fire_clay
https://en.wikipedia.org/wiki/Clay_chemistry
https://en.wikipedia.org/wiki/Silicon_dioxide
https://en.wikipedia.org/wiki/Aluminium_oxide
https://en.wikipedia.org/wiki/Intermolecular_force

Note: If I've said anything wrong in my attempts to simplify someone please correct me.

" Intermolecular forces, or IMF for short"

Should any of your IM forces be caught or killed, the secretary will disavow any knowledge of your actions.

Quote: Originally posted by careysub

To my reading making alumina refractories is very doable. One thing that interests me is lining steel pipes with alumina slip for a cheap crucible. Differential expansion is an issue, but perhaps preventing bonding between the liner and the pipe, and using a thick lining might solve that problem.