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From what I have recently learned about ceramics, clay is essentially sheets of silicon and oxygen, bonded by covalent bonds. Between them, water keeps clay from forming a full lattice of $\ce{SiO4^{2-}}$. What I am wondering is, does clay absorb the water because of the water's polarity, i.e. are water molecules bonding with hydrogen bonds to the negatively charged oxygen atoms of the ceramic sheets? Or is water there because pressure from the environment forces it in between the sheets (which I know would then be a physics question)?

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I am no expert regarding clay (which is a surprisingly varied field) and will therefore give a more general answer. I suspect that there is also quite complex interaction with ions on the surfaces of the layers and such things.

Materials can be generally categorized on a rather continuous scale, the extremes of which are called hydrophilic and hydrophobic (or lipophobic/lipophilic which are synonyms).

If something is hydrophilic, it "likes" to be wet, i.e. it is energetically favourable for the surface to be wet because this minimized surface energy of the system. The opposite applies to hydrophobic substances.

As you have observed yourself, clay is quite hydrophilic.

One can indeed estimate whether a material is hydrophilic based on its chemistry: water likes to form hydrogen bonds and water complexes (e.g. around Na+). Therefore things are more hydrophilic the more of said things they have. A good example is that sugars and carbohydrates are very soluble in water despite being large, whereas even small alkanes are barely soluble.

The explanation involving air pressure makes no sense, because to move matter there has to be a pressure difference. But if you imagine clay as a porous substance, the pores might as well be filled with air and there is an equilibrium. In fact (as easily observed when burning earthware) clay gets more voluminous with higher water content.

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  • $\begingroup$ I was thinking in hydrophilic terms, yes. Your points are exactly why I was thinking that this could be the reason. By pressure, however, I never meant AIR pressure; clay forms (as I understand it) in water, and the thought was that the pressure from water (oceans, groundwater, etc.) pressed itself inside the sheets and simply stayed there when they were exhumed for use. $\endgroup$ – Henry Stone Apr 24 '16 at 15:04
  • $\begingroup$ There still is no pressure difference; I either don't understand you or you have an erroneous understanding about pressure. $\endgroup$ – caconyrn Apr 24 '16 at 15:43
  • $\begingroup$ I assume there is no pressure issue, then. The thought was that the pressure of deep waters above could press water through the seabed or nearby layers, similar to the pressure of groundwater. I guess that is not the case. $\endgroup$ – Henry Stone Apr 25 '16 at 12:57
  • $\begingroup$ Somehow you still sound confused, but I guess you have to add more information if that is the case. I did never figure out what exactly confused you :( $\endgroup$ – caconyrn Apr 27 '16 at 0:06
  • $\begingroup$ I perfectly understand your explanation, it's very well explained. But it is the basic premise. I just need to know if that premise applies to clay or not. $\endgroup$ – Henry Stone Apr 27 '16 at 7:47

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