Janet H Walker on sun 28 feb 99
...A concern of mine, in making slab pieces with bare bottoms, is the
porosity of the bottoms. They can retain water when washed...
This is a difficult issue to sort out and there seems to be a lot of
confusion about the difference between absorption, porosity, and
maturity or vitrification in fired clay bodies.
Fact: fired but unglazed clay pots can be perfectly watertight
Fact: fired fully glazed (both sides) clay pieces can allow water
to seep through.
Now, howzat?
Put some water in a bowl or vase and let it sit on paper. Check the
paper for dampness. If it gets wet right away, you probably have a
crack! What you are looking for is whether the damp stays dry or is
damp after 12 or 24 hours. If it gets damp, then what you have is
called seeping, which can happen with any clay body that is still
porous or absorbant after firing. Mostly this happens with
earthenware but it is possible with high fire clay bodies. It
depends on the formulation of the body.
If you get seeping, then the fired clay is absorbing water. I
worked in a community studio at one point where the clay they were
using was still porous after firing at cone 8. The pots seeped.
The pieces I had would take about a day to get damp on the
underneath side. So I ruined tables with them because I would
forget. I liked some of them but need to throw them out or use them
for dry weeds.
Anyhow, to be clear, the porosity of a fired body has nothing to do
with what temperature it is bisqued. It is affected by the final
firing temperature but it is really a function of the recipe used
for the clay body. A mature body can be porous. This takes time to
get one's head around. I have heard potters with 15 years
experience but no technical study get this wrong.
"Mature" does not mean "non-porous". It just means that if you fire
it any hotter, the clay will start to collapse and go glassy. The
absorption happens because teeny air spaces remain between the
particles in the fired body and water can get into those. If the
clay is denser (i.e. no way to get into those air pockets) then the
fired pieces are not absorbing.
In order to understand what is going on with your clay, you need to
measure the absoption. Take a test piece, any size or shape, not
too small and fire it with no glaze on it in your normal glaze
firing. Weigh it immediately while still warm before it has a
chance to absorb any moisture from the air. Use your most
accurate/sensitive scale.
Now you put it in water and bring it to a boil for two hours. Then
take it out, cool it by running cold water over it, dry off the
surface water and weigh it again. It will almost certainly be
heavier (only some porcelains have no absoption). The amount of
difference indicates how much water has been taken into the body.
Take the wet weight minus the dry weight and multiply by 100. Then
divide by the dry weight and now you have a percentage absorption
number. The manufacturers typically list a shrinkage and absorption
for each of their clays. check with your distributor who should
have a catalog that shows the percents. If it is above say 2% you
could experience seeping with the body. A rule of thumb is to look
for a stoneware with 1 to 2% absorption. I have (since that nasty
experience long ago) always made sure to buy clays with low
absoption at the cone I want to fire them to.
Hope this sheds some light. It is a complicated subject. Spend
some quality time with Hamer's dictionary which has lots of little
drawings of clay particles at different stages of drying and firing.
Regards,
Jan Walker
Ron Roy on wed 3 mar 99
Hi Jan - just to clarify a few points.
>Put some water in a bowl or vase and let it sit on paper. Check the
>paper for dampness. If it gets wet right away, you probably have a
>crack! What you are looking for is whether the damp stays dry or is
>damp after 12 or 24 hours. If it gets damp, then what you have is
>called seeping, which can happen with any clay body that is still
>porous or absorbent after firing. Mostly this happens with
>earthenware but it is possible with high fire clay bodies. It
>depends on the formulation of the body.
RR - and/or whether you are firing it high enough - could be you are not
using cones and/or firing too fast.
>Anyhow, to be clear, the porosity of a fired body has nothing to do
>with what temperature it is bisqued. It is affected by the final
>firing temperature but it is really a function of the recipe used
>for the clay body. A mature body can be porous.
If the clay is fired to maturity - I say 2% absorbency or less for water
holding - it will not leak unless there are some holes in it.
>
>"Mature" does not mean "non-porous". It just means that if you fire
>it any hotter, the clay will start to collapse and go glassy. The
>absorption happens because teeny air spaces remain between the
>particles in the fired body and water can get into those. If the
>clay is denser (i.e. no way to get into those air pockets) then the
>fired pieces are not absorbing.
RR - I'm not sure I am right in trying to make this clearer - but. A clay
body is mature when it no longer will let water through - stonewares can
have some absorption but generally porcelains are vitrified - it's the
reason porcelains warp more then stonewares.
RR - In other words - it seems to me that mature does mean non - porous.
Over mature would mean the clay has been fired past it's optimum and will
then soften too much or start to break down. I'm avoiding the word glassy -
but it is applicable to stoneware but not to porcelain. So what if
porcelain is underfired - say with 2% absorbency you say - well perhaps you
should call it porcelainious stoneware for instance.
>In order to understand what is going on with your clay, you need to
>measure the absorption. Take a test piece, any size or shape, not
>too small and fire it with no glaze on it in your normal glaze
>firing. Weigh it immediately while still warm before it has a
>chance to absorb any moisture from the air. Use your most
>accurate/sensitive scale.
>
>Now you put it in water and bring it to a boil for two hours. Then
>take it out, cool it by running cold water over it, dry off the
>surface water and weigh it again. It will almost certainly be
>heavier (only some porcelains have no absorption). The amount of
>difference indicates how much water has been taken into the body.
>
>Take the wet weight minus the dry weight and multiply by 100. Then
>divide by the dry weight and now you have a percentage absorption
>number. The manufacturers typically list a shrinkage and absorption
>for each of their clays. check with your distributor who should
>have a catalog that shows the percents. If it is above say 2% you
>could experience seeping with the body. A rule of thumb is to look
>for a stoneware with 1 to 2% absorption. I have (since that nasty
>experience long ago) always made sure to buy clays with low
>absorption at the cone I want to fire them to.
RR - What Jan has said in these last three paragraphs is correct as I see
it. If a clay maker says their clay has a proper maturity for functional
ware over say 3 or 4 cones - don't bet the farm on it - I have never seen a
clay body that will work properly and consistently over a range of cones.
It's difficult enough keeping clays working at one cone - never mind 3 or 4
- even when testing constantly.
RR
Ron Roy
93 Pegasus Trail
Scarborough, Ontario
Canada M1G 3N8
Tel: 416-439-2621
Fax: 416-438-7849
Web page: http://digitalfire.com/education/people/ronroy.htm
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