Tony Hermsen on sun 1 nov 98
Toni, you should fire your planters to the highest temperature that your
terracotta will allow. Most planters are actually badly underfired. I would
think that you should go to at least cone 04, if your body will handle it go=
to
02. John Huggins in =22Pots For Plants and Gardens=22 ( a great book from =
the
Complete Potter Series) says =22What is important is that the clay is =
sufficiently
fired to develop its maximum strength and to reduce its moisture absorbtion =
to a
fairly low level. A good terracotta pot will absorb moisture, and this is =
one of
its advantages over non-porous materials, but it must not soak up water like=
a
sponge.=22 I agree with him entirely and have had good success over the =
years. He
also says, =22It is a mistake to think of unglazed terracotta garden pots as
biscuit ware.---------Biscuit firings by definition imply a second, usually
higher, firing and aim to produce a ware that is sufficiently strong to be
handled without breaking, while suitably porous to suck up and hold glaze =
slop.
Firing unglazed terracotta plant pots has quite a different purpose: to =
develop
maximum strength and low to minimum porousity.=22 Follow his recommendations=
and
your terracotta will last into posterity.
Tony Hermsen
Enderby, B.C.
Canada
http://www.jetstream.net/business/tonyh
nikom chimnok on tue 10 nov 98
Having followed the recent discussion of terra cotta planters, I am
thoroughly confused.
As I understand it, the market--at least in Europe and Northern
America--wants "frost proof" planters. That is, get it as soaking wet as it
can get, then freeze it, and it doesn't break. And this through numerous
cycles, like 5 winter's worth.
As I remember, it was Ron Roy who said that moisture absorbtion must
be less than 3% for clay to work out this way. More than that, and first the
glaze will craze (delayed moisture crazing) and eventually the pot will crack.
Technically, this seems difficult. Depending on the terra cotta it
may have to go to cone 1 or cone 5, and there aren't many terra cotta clays
that still look orange at that those temperatures. The typical old
formula--fire the bisque to cone 04 and the glaze to cone 06, certainly
doesn't work.
And then someone else comments that it is advantageous that a terra
cotta planter absorb water. Why? What purpose does that serve? To dry the
soil out? And if the planter absorbs that much water, how in the world could
you call it frost-proof?
Will someone please tell me what I am not understanding here?
The pottery I work at, by the way, is producing ware which is
over-fired--red clay once-fired to cone 6. It breaks like glass, and is
totally black inside. It may actually be iron silicate, from what I have
read someone on Clayart say. But it's real thick, and we get away with it.
The reason we fire this way is we don't have any lower temperature glazes
available.
Waiting to hear from the experts,
Nikom
koratpot@loxinfo.co.th
In Northeast Thailand, where it is now winter, and the weather is sunny and
beautiful, like a summer day in the States.
Carol Durnford on thu 12 nov 98
>----------------------------Original message----------------------------
>Having followed the recent discussion of terra cotta planters, I am
>thoroughly confused.
>
> As I understand it, the market--at least in Europe and Northern
>America--wants "frost proof" planters. That is, get it as soaking wet as it
>can get, then freeze it, and it doesn't break. And this through numerous
>cycles, like 5 winter's worth.
> As I remember, it was Ron Roy who said that moisture absorbtion must
>be less than 3% for clay to work out this way. More than that, and first the
>glaze will craze (delayed moisture crazing) and eventually the pot will crack.
> Technically, this seems difficult. Depending on the terra cotta it
>may have to go to cone 1 or cone 5, and there aren't many terra cotta clays
>that still look orange at that those temperatures. The typical old
>formula--fire the bisque to cone 04 and the glaze to cone 06, certainly
>doesn't work.
> And then someone else comments that it is advantageous that a terra
>cotta planter absorb water. Why? What purpose does that serve? To dry the
>soil out? And if the planter absorbs that much water, how in the world could
>you call it frost-proof?
> Will someone please tell me what I am not understanding here?
> The pottery I work at, by the way, is producing ware which is
>over-fired--red clay once-fired to cone 6. It breaks like glass, and is
>totally black inside. It may actually be iron silicate, from what I have
>read someone on Clayart say. But it's real thick, and we get away with it.
>The reason we fire this way is we don't have any lower temperature glazes
>available.
>Waiting to hear from the experts,
>Nikom
>koratpot@loxinfo.co.th
>In Northeast Thailand, where it is now winter, and the weather is sunny and
>beautiful, like a summer day in the States.
>
>
A planter should be porous for the sake of root growth. Plants need air
not just minerals and water. This requirement is different for different
species of plants. Ex.: Papyrus plants can grow in a water environment
while epiphyte orchids cannot. Even attempts to put a porous column
through the ceter of the pot to achieve this balance does not always work.
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