Stephani Stephenson on tue 7 jan 03
The following is from W. G. Lawrence "Ceramic Science for the Potter",
staring page 42
"Binns states that 3 properties are demanded of a clay for successful
pottery use. These are plasticity, porosity and suitable vitrification
Porosity of the clay controls the migration of water during drying.
Porosity can be improved by sand or grog additions. fine grained clays
have low porosity and and high plasticity. Coarse grained clays have
high porosity and low plasticity. there is always a compromise between
these two properties.
when a clay is fired, it becomes hard and strong due to glass formation.
Firing results in densifications and particle to particle adherence.
Absorption: a fired piece is weighed to the nearest .1 gram. It is then
placed in a suitable container and boiled for five hours in water. it is
removed from water and the excess wiped off, and the piece re-weighed.
the percent absorption is equal to (final weight- original weight X 100
)divided by the original weight
Lawrence shows graphs with examples of how absorption rates of various
clay bodies decrease as firing temperature goes up. It isn't a straight
decline. (picture a 45 degree angle decline.. it is not like that)
Rather the absorption drops off slowly then begins a steep precipitous
decline, even before the clay reaches full fired maturity. This is when
a multitude of changes are taking place in the silica/quartz structures,
feldspars and fluxes are melting and glassy phase is starting.
.A typical stoneware clay may actually begin a sharp decline in
absorption at around 1000 degrees centigrade, just over cone 06, even
though the optimum firing temperature for that body is somewhere
between 1200 ands 1300 degrees. In his graph the absorption rate drops
from 14% to 6% between 1000 and 1100 degrees, and is down to 4% at 1200
degrees, near 0% at near 1300 degrees centigrade. Use of fluxes in
lowfire bodies would tend to bring the curve down even more steeply for
(Interestingly, though Lawrence defines porosity as a property of drying
clay he does mention in a later chapter that at 1050 -1100 degrees
'porosity decreases rapidly' so could it be that he is using terms
interchangeably after all? All I can deduce from reading is that
absorption is the more precise and narrowly defined word, used
primarily when speaking of the absorption (boiled water ) test...
Anyone out there know differently????
also , I think that the reason boiled water is used in the absorption
test is that heated water has less surface tension and will migrate into
either smaller or more spaces more easily than cold water, which will
have a higher surface tension. that is why the boiled weight will differ
from the wet weight, as in the freeze thaw test.
So though Lawrence defines to porosity as a property of drying clay,
not fired clay, it makes sense that the two are related. Porosity in
the drying clay is important for even drying to take place, Lawrence
states that during drying" it is important the water not be removed
from the surface of the ware (via evaporation ) faster than it can be
replenished from the interior. The rate at which it can be replenished
depends on factors affecting the migration of water through the small
capillaries or interstices of the clay body. "(why fine grained clays
must be dried more slowly than coarse grained clays)
Porosity is also a factor in firing and allows one to fire thicker
pieces or fire faster in that pores in the clay allow for gasses to
escape during firing.
This porosity may also figure into the ability of fired clay to absorb
then 'shed water in outdoor installations.
It seems like you have a couple of options. One is to go for an entirely
or nearly entirely vitrified body such as porcelain or quarry tile
which will be impervious to water, i.e. waterproof.
Another option , for outdoor pottery,sculpture , tile and architectural
ceramics ,is to go with a somewhat porous body which will absorb then
drain off water or even allow for some expansion of freezing water in
the clay itself. this would explain the hardiness of various types of
brick in outdoor conditions I would think. I'd like to find out more
about that. This may be fine for unglazed ceramic but glazes add another
the difficulty in using a body which does allow water into its lil' ole
pores water occurs when you are glazing that body. You not only have to
think about how the clay will perform in wet and freezing conditions but
how the glaze will react.
Saturation through the back of the tile can be a problem in that the
clay will absorb water, expand and cause delayed crazing in the glaze.
Or water will seep into the tile from an already crazed glaze and cause
further expansion and additional crazing, and even flaking off of the
glaze, especially if freezing occurs and tile is wet. moisture seepage
can also cause efflorescence on the surface of the tile, if the water
seeping into the clay is mineral laden water.
However a LOT of tile is not porcelain or quarry tile, so you don't
have to knock yourself out , just consider what your tile will be used
a LOT of tile is not impervious to water, it is not fully vitrified.
for more info on types or grades of tile go to
A very important component in this equation is the setting of the tile,
especially since a lot of tile is not water proof, though it may be
water resistant. A good 100% coverage of thinset mud on the tile
substrate will protect the tile from moisture seepage from behind.
Prevention of moisture seepage from the back of the tile is crucial for
the longevity of your installation .
well I have windbagged eeeeeenough on this for now........