[Clayart] Water in clay ( was: Microwave heating)

[Paul Gerhold]

You are not trying to force the water out of the pores. The partial pressure of the water in the pores will equilibrate with the partial pressure of the water in the surrounding air. Just Physics 101.  Same principle that drys clay sitting in a room, just on smaller scale.

Paul

Sent from my iPad

> On Nov 9, 2016, at 12:45 PM, Girrell, Bruce <bigirrell at microlinetc.com> wrote:
> 
> Paul Gerhold wrote: As far as I know fired clay has pores and not capillaries.
> 
> Clay does indeed have pore space, and a lot of it. These pores are highly connected in the raw clay. As clay is fired, it sinters and the pores and the connecting channels become smaller, but they don't all go away. The manner in which fluids move between pores is governed by capillary forces and surface tension of the fluid(s)* involved. The pore-pore interstices are small and short so they're not at all like blood vessel capillaries, but they are still governed by capillary forces and that's what we're talking about here.
> 
> Paul Gerhold wrote: I also question the assumption that moisture once absorbed into clay will stay there indefinitely.
> 
> Once a piece of clay has been fired, the electrical forces from the surface of the clay are no longer but you're still left with the capillary forces and those can be difficult to overcome. Consider a droplet of water filling a pore in some object. While some pores may be isolated, especially after firing, they will not be water-filled simply because of the firing. The pores we are interested in are the ones that can still be reached by water (and subsequently be heated by the microwave). Thus, by definition, there is some form of fluid communication among the pores. These passages represent what remains of the grain-to-grain contacts and spaces after the firing. 
> 
> To simplify a bit, imagine a bunch of ping pong balls in a bucket. If you were able to shrink yourself and go in among the ping pong balls, you would see little tetrahedral pores between them and tiny triangular spaces between the pores. Those little spaces connecting the pores are the capillaries. In the real world, nothing is anywhere as neat as that, but the pores still exist and the little channels connecting them still exist.
> 
> Now consider a drop of water filling one of the pores, with everything around it mostly dry. In order to get the water out of the pore, you have to deform it sufficiently to force it through the small opening. Why can't you do it just a few molecules at a time? Water likes itself and you must add energy to free H2O molecules from the surface of the water droplet as a gas. That actually takes quite a bit of energy, such as reheating the cup. The water droplets on their own will not evaporate from inside the pores. There is not enough energy to drive the water 1) off of the droplet as a gas or 2) through the small opening (which requires deforming the droplet). _Eventually_ the fired clay can dry out from off-gassing, but from the standpoint of human experience, the water stays in there indefinitely, as there is no reason (force) to go elsewhere.
> 
> Bruce Girrell
> 
> * If you're really up for some mind-bending math, try calculating forces with a three-phase system (water-oil-gas) present in the pores. This is the usual case for underground hydrocarbon reservoirs.
> 
> 
> 
> -----Original Message-----
> From: Clayart [mailto:clayart-bounces at lists.clayartworld.com] On Behalf Of Paul Gerhold
> Sent: Wednesday, November 09, 2016 8:04 AM
> To: Clayart international pottery discussion forum <clayart at lists.clayartworld.com>
> Subject: Re: [Clayart] Microwave heating
> 
> Your statement was that capillary action in fired clay will keep moisture in the clay indefinitely.  As far as I know fired clay has pores and not capillaries.  And yes, I am following the conversation which is why I question a clay theory based on the assumption that capillary action has any relevance to clay. 
> 
> And I also question the assumption that moisture once absorbed into clay will stay there indefinitely. My opinion is that the moisture in the pores will gradually equilibrate with the moisture content in the surrounding air.
> 
> Paul
> 
>