Bob Hayden on wed 21 mar 01
Hello,
I am new to this site but I am going to try to answer some of the =
questions posed in several of the places in regards to RAKU.=20
I will try to give an answer to the question of why RAKU potters dip =
their pots in water after removing them from the reduction kiln. First, =
this is how the original RAKU pots were done. The pot was taken out of =
the kiln, cooled in air a few minutes, and then plunged into water. In =
the original RAKU firing there was no reduction container, however they =
did fire the pots with wood or charcoal or probably coal. It (the RAKU =
pot) at that time was used for only the tea ceremony in Japan, so it was =
used immediately for making tea in and the tea was consumed immediately. =
Any glaze that was on the pot acquired a crackle that was highly prized =
by the Japanese. This also changed the color of the brown glaze to a =
so-called red glaze, improved the white color and also the black glaze =
in use. =20
Why were the pots used for the tea ceremony you might ask? Well the =
Shogun at that time liked the sound that the Wisk used in stirring the =
powdered tea made in the clay vessel that were fired by Chijaro, the =
first potter to do this type of firing. It was not called RAKU then. =
This potter was a roofing tile maker and he found that he could cool his =
tile quickly if they were removed from the kiln while still red hot and =
let the redness go out of them and then dousing them with water. This =
decreased his production cycle time for making tile because he didn't =
have to wait for them to cool naturally before he could send them on the =
way to be rooftops. Since he was also a potter of other items his bowl =
were sent to the reining tea master, which brought them to the Shogun. =
They were incorporated into the tea ceremony and eventually into making =
the pots during the ceremony and used immediately.
Paul Soldner and others here in the USA found out that if the glaze pots =
were reduced with organic matter that some very desirable effects could =
be obtained. The reason that we cool our pots quickly is to stop the =
reoxidation of the reduced glazes. If the metals in the hot pot are not =
protected from oxygen while they are still hot they will start to =
reoxidize back to their stable oxide state. (All metals except the =
noble metals of gold, silver, platinum, lead, and etc. will eventually =
oxidize in air to make an oxide of the metal. Just look how quickly =
Iron rusts). Copper will become copper oxide and will not have the =
copper metallic luster that it has just after it come out of the =
reduction container. In fact the copper will continual try to react =
with the oxygen in the air and eventually the copper luster will =
disappear from the pot. This is a very slow process and can take many =
years to happen. This will result in a bright copper turning to dark =
lusterless color. The only thing that will keep this from happening is =
if the glaze on the pot is a true glaze and not a copper stain glaze or =
if the piece is sprayed with a plastic or clear paint glaze. A true =
glaze contains A) glass like from silica, B) a flux to melt the glass, =
C) some clay to help the glaze adhere to the clay body, and D) some sort =
of colorant, such as a copper salt or any other metallic compound that =
can be used as a colorant. Those four things are essential to keep the =
reduced metallic luster present from going dull, as the silica coating =
will preserve the luster. The rapid cooling of the glazed piece is =
necessary if the reduction present is desired to be kept from =
reoxidizing. The glaze sets from the liquid state at a very high =
temperature. But the ingredients in the glaze are still reactive until =
they are at room temperature. Even then, some of them are reactive and =
subject to oxidation.
Let me give you an example of how the chemistry of glaze reduction =
works. Think of a blast furnace where the iron oxide pellets are being =
turned into liquid iron. The reaction that takes place is the iron =
oxide is heated in the presence of coke (an impure form of carbon) and =
without access to air, until there is a reaction of the coke with the =
air that is present to form carbon monoxide, the first state of carbon =
and oxygen reaction since there is not an excess of oxygen present. Now =
carbon monoxide is very, very reactive. It wants to go to carbon =
dioxide, which is very stable. Where does it get this oxygen from, from =
the iron oxide? What happens then is the iron is reduced to the metal =
and collects together until there is enough to pour out into the ingots. =
This is about the same thing that happens in the RAKU reduction =
container. The sawdust, leaves, paper, cardboard, etc, supplies the =
carbon that is used in the process. The hot pot is put in contact with =
the organic material, which ignites the carbonaceous stuff, and a flame =
erupts. Then when we put a lid on the container it puts out the flame. =
What results is that carbon monoxide is formed which burns to carbon =
dioxide by taking the oxygen from the metals in the glaze, leaving the =
pure metal behind as a luster or shiny metallic surface. There is =
nothing wrong or incorrect if the piece of reduced RAKU pottery is not =
cooled quickly in water but if you want the final luster results to last =
longer it is best quickly cooled to stop the reoxidation of the =
finish.The above is more fully explained at my web site if you are =
interested.
Bob Hayden, BS-In Chemistry, BFA in Ceramics.
=20
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