Tom Sawyer on sun 25 may 03
Thanks to everyone who responded to my initial question concerning
someone's statement made that paperclay was to be avoided for functional
work. I would like to raise a question again concerning the use of paper
clay for functional pieces.
It has been mentioned over the past few days during the course of this
discussion that paper clay was more porous; someone else has pointed out
that glazing could offer protection when using earthenware or stoneware
paperclay. Someone else suggested paper clay was more absorptive than
regular clay. To me absorption and porosity are different concepts; it
may be that paper clay is both more absorptive and porous; at this point
I am a little uncertain of these terms.
Let me explain. A cylinder composed of hollow glass or metal rods is
porous, if those tubes are open at both ends but that it might be wrong
to say the glass or metal is absorptive. If these rods were composed of
capillary sized tubes they would fill with water when immersed; would we
then say that the glass or metal rods absorbed water?
A question I have is, is stoneware paper clay if fired to vitrefication
analogous to glass or metal capillary tubes? If, for example, if we make
a cup out of stoneware paper clay and fire to vitrefication temperatures
carefully glazing the inside of the cup, I believe we might argue that
the cup is not porous. Now when I think of water or liquids being
absorbed into clay, I think of the liquid intermixing with the clay
molecules but does this really happen if the clay is vitrefied? When
considering capillary action, it is one thing to have a capillary tube
measured in inches and another in microns particularly where these
micron sized tubes are randomly oriented, as they are with paperclay. If
I were to place the foot of an unglazed paperclay cup/glass in water
what would be the height that the moisture would reach? Would it
permeate all the way to the top? If it were to stay largely confined to
the footed area and the upper portions were glazed what then are the
implications of using paperclay for functional wares? For example, if
one uses paper clay for functional items such as a cup and there is only
a small unglazed foot what are the practical implications?
Tom Sawyer
tsawyer@cfl.rr.com
Cindi Anderson on sun 25 may 03
I asked this or a similar question before but nobody really answered it. I
mentioned that someone at Laguna said what you are saying, that absorption
and porosity were different. He said there were some clay bodies that were
not absorptive (<1%), but leaked, and others that had higher absorption
(3-4%), but did not leak. I would be grateful if someone could explain
this.
Cindi
Fremont, CA
John Hesselberth on sun 25 may 03
Hi Cindi,
I suppose porosity and absorption could be different, but, from a
potter's viewpoint, I'm not sure it makes practical sense to try to
differentiate between them. Absorption is the amount of water a fired
clay will "take up". To measure it you usually start with a test bar
fresh out of the kiln that you know is dry. You weigh it. Then you
treat it so it will absorb as much water as it can. This is usually
done by boiling it under water for a couple hours and letting it cool
while still under water. Then you quickly dry off the surface water and
weigh it again. The difference is the amount of water it absorbed.
Whether or not a fired clay leaks will depend on whether or not the
clay is vitrified which will also be pretty directly related to how
much water it absorbs. The higher the absorption the more likely it is
to leak. I suppose there could be a structure that was fully vitrified
but had tiny continuous channels all the way through it so it would
leak but have fairly low absorption--maybe that is what this person was
visualizing.
To give you some numbers, well vitrified stoneware clays have
absorptions of 2-3% or less and they don't leak. Porcelain is usually
1% or less and it doesn't leak. Earthenware has an absorption of 10-15%
and it normally leaks. I haven't personally seen low absorption fired
clay bodies that leak, but maybe Laguna has some--you'd have to ask
them.
Regards,
John
On Sunday, May 25, 2003, at 01:55 PM, Cindi Anderson wrote:
> I asked this or a similar question before but nobody really answered
> it. I
> mentioned that someone at Laguna said what you are saying, that
> absorption
> and porosity were different. He said there were some clay bodies that
> were
> not absorptive (<1%), but leaked, and others that had higher absorption
> (3-4%), but did not leak. I would be grateful if someone could explain
> this.
>
> Cindi
> Fremont, CA
>
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