----- Original Message -----
From: "Khaimraj Seepersad"
To:
Sent: Thursday, May 27, 2004 4:34 PM
Subject: [IBC] Ceramic Pots 1
KHAIMRAJ,
( I'M GOING TO TYPE IN CAPITALS TO KEEP MY ANSWERS SEPERATE FROM YOUR BOOK
FACT QUOTES SO, PEOPLE DON"T SEND ME 15 NOTES ABOUT "YELLING" :))
I BELIEVE YOU ARE CONFUSING THE ORIGINAL POSTERS CONCERNS WITH WHAT YOU ARE
READING IN YOUR TEXT. BASICALLY THE "THERMAL SHOCK" YOU ARE READING ABOUT
IS "NOT" WHAT THE DISCUSSION WAS ABOUT, CONCERNING CLAY BODIES ABSORBING
MOISTURE THEN CRACKING IN WINTER DUE TO FREEZING CAPTURED MOISTURE IN THE
POROUS CLAY BODY.
If exceeded,past the %[seen above],the body will
have too much glass.It will then be more susceptible to
Thermal shock[the stress created within a ceramic object
by temperature change.Thermal shock is responsible
for the occurrence of cracks and is an OVERALL term,
pg.320 The Potter's Dictionary-Hamer 3rd edition.]
THE "THERMAL SHOCK" YOU DESCRIBE HERE ( PARAGRAPH ABOVE) IS FROM FIRING "TOO
FAST" ( OR COOLING TOO FAST) NOT FROM FREEZING THE CLAY BODY.
So freeze/thaw as experienced in winter or sun on a cold
pot or possible expansion of water in soil and so on
[which is what I meant and hoped would be understood
because there is a great deal of Japanese information dealing
with winter care.]Could after slow time damage a pot.
Especially since most pots already have fine cracks present
in the body after firing.
Raku bodies resist this damage because of the particles
within the body blocking the crack from physically passing.
BUT,RAKU BODIES ARE VERY POOR RISKS FOR BONSAI POTS IN COLD TEMP AREAS
BECAUSE OF THE CLAY TYPE USED TO AVOID THERMAL SHOCK....IT IS OPEN AND VERY
POROUS, THEREFORE MOISTURE IN THE BODY CAUSES POTS TO CRACK WHEN FROZEN,
BUTTHIS CLAY IS EXCELLENT IN 'THERMAL SHOCK" PROBLEM AVOIDANCE. THAT
IS WHY IT CAN BE TAKEN RED HOT FROM A KILN AND THROWN INTO STRAW R PAPERS OR
EVEN WATER. .
I don't know,and wasn't talking about the absorption of water
into the stoneware body and then freezing.
THIS IS WHAT THE ORIGINAL POSTER WAS CONCERNED ABOUT. SINCE YOU DON'T GET
HARD FREEZES, AND DON'T KNOW ABOUT THIS YOU SHOULDN'T SPEAK TO IT. THAT IS
EXACTLY WHAT YOU WERE TALKING ABOUT WHEN YOU MENTIONED THAT TOO MUCH
MISINFORMATION IS PASSED THROUGH IBC. YOU KNOW, WHEN YOU TOLD ME TO TRY TO
'GRASP THE IDEA" and TO DO A 'GENERAL STUDY UP"
On Porcelain.
In porcelain the body is homogenous;it does not contain
variously-sized particles of sand,grog etc.
[ The Potter's Dictionary -Hamer pg.312 ]
Stoneware isn't homogenous,but this article deals more with
glazes and but I think the above is enough to explain the difference
in response between the two clay types.
I have no way of know how porcelain pots handle cold.
AGAIN, THIS WAS THE ORIGINAL POSTERS CONCERN!
Glazed on the inside problems-
I think that breathe bit comes from high fired
but still say 8% porous earthenware ,with probably heavier
soils.
[Not everyone is too bothered by the salts deposited
on the outside of the pot.]
WELL YES THEY ARE, MOST WOULDN'T WANT THIS, BUT, THE POINT WAS THAT HIGHLY
POROUS BODIES ( WHICH WILL ALLOW THIS TO HAPPEN) WILL ALSO BE THE ONES TO
ABSORB WATER IN THE COLD AND FREEZE AND SHATTER. THIS WAS THE ORIGINAL
POSTERS CONCERN. MANY CHEAP POTS HAVE THIS EXACT PROBLEM
ALTHOUGH YOU HAVE BEEN IN CERAMICS FOR YEARS YOU ARE STILL MISUNDERSTANDING
THE RELATIONSHIP OF THE TERM "THERMAL SHOCK" IN REGARDS TO MANUFACTURING AND
DIRECT USE OF THE POTS. THERMAL SHOCK IS WHAT CAUSES A CLAY BODY TO SHATTER
IF HEATED TO FAST ( OR COOLED TO FAST). THIS IS WHY POTS ARE BISQUED VERY
SLOW AND HIGH FIRED WITH CAUTION BUT NOT AS SLOW AS BISQUE ( BECAUSE MOST
WATER HAS BEEN DRIVEN OUT IN BISQUEING) . IF YOU HEAT TOO FAST....THERMAL
SHOCK WILL DAMAGE THEM. ONCE A POT IS COMPLETED THE WINTER FREEZING EFFECT
IS "NOT" FROM THERMAL SHOCK, IT IS FROM POOR VITRIFICATION ALLOWING
MOISTURE INTO THE CLAY THUS CAUSING SHATTERING/CHIPPING/CHUNKING/ETC. DURING
WINTER MONTHS ( WHICH I DON'T BELIEVE YOU ARE HAVING LIKE WE DO). FURTHER,
THIS POORLY VITRIFIED CLAY ( OR CRAPPY CHEAP CLAY BODY) WHICH IS HIGHLY
POROUS WILL ALLOW WATER TO SEEP THROUGH THE CLAY AND ,AFTER DRYING ON THE
SURFACE, LEAVE MINERAL DEPOSITS.
Regards,
Dale
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