Pottery and Ceramics Clay
Drying And Firing
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Most people working with pottery or ceramics don't really know the process the clay goes through when making a piece.
There are many stages that it goes through and knowing what they are could help you down the road in figuring out why you are having troubles.
Air DryingWet clay contains a large amount of water, with the least amount being about 25% water.
When clay starts to dry, water evaporates from it.
When it does, the particles of clay are drawn closer together which causes shrinkage.
A lot of the problems with clay are caused by uneven rates of drying, which creates stress in the clay.
Sometimes the stress shows up right away as cracks or warping, other times not until, during or even after firing.
It is very important to make sure your piece is dried as even as possible.
This is done by making sure that your piece is the same thickness throughout, drying slowly and even slowing down the drying of certain parts.
Clay that has very fine particle sizes will shrink more than the ones with larger particle sizes.
Porcelain clay has very fine particle sizes which makes it very plastic and also shrinks the most.
These bodies have the most strength when dried.
Clay that has a lot of grog in it, such as sculpture bodies shrink the least.
These bodies shrink less because they have lower water content to start with and also has channels through which moisture can escape toward the surface.
They are called open bodies.
After the water has evaporated from between the clay particles and all the remaining clay particles are in contact, shrinkage is complete.
This is called the leather hard stage.
The clay is still damp, but further drying will not cause any more shrinkage.
Initial Kiln DryingComplete drying will not take place until the piece is in the kiln.
This happens when the boiling point of water has been reached which is 212° F or 100° C.
This must happen slowly or the formation of steam within the body of the clay may cause it to explode.
That is the reason the early stages of firing is done slowly and with a peephole or lid open for steam to escape.
DehydrationThe next change that occurs is at about 662° F or 350° C, this is the point where the chemically combined water of the clay is driven off.
This is water that is part of the molecular structure of the clay, not the water that is between the particles of the clay.
This drying is completed by about 932° F or 500° C.
After this point you can no longer mix the clay with water to make new wet clay.
An irreversible chemical change has taken place, called dehydration.
No shrinkage can be seen during this stage.
Burn-offAnother thing that happens up to about 1652° F or 900° C is the burning off of organic and inorganic materials, such as carbon and sulphate's.
These are the fumes that are dangerous to breathe and the reason a kiln should be well ventilated even during bisque firing.
Quartz InversionAfter dehydration, the next change that happens is quartz inversion, which happens at 1064° F or 573° C.
At this temperature, quartz crystals rearrange themselves into a slightly different order.
A small and temporary increase in volume occurs at this point.
You will need some space around your pieces during firing, because they will expand.
Firing should done slow during this quartz inversion.
A large percentage of pottery or ceramics that is cracked during firing happens from fast firing through this stage.
The factory set program on electronic kilns usually slow down the firing at this stage for you.
If you don't have an electronic kiln, you will have to baby sit your kiln, at least that is what I call it!!
VitrificationThe next thing that happens is vitrification.
This is the hardening, tightening and finally the partial turning to glass of the clay.
Vitrification results from the melting of the all of the components of the clay.
The strength of fired clay is increased by the formation of new crystalline growth within the clay body, particularly the growth of mullite crystals.
Mullite is an aluminum silicate characterized by a long needle like crystals.
These crystals tie the structure together, giving it strength.
Your piece will shrink at the vitrification stage.
That is because of the diminished size of the particles as they approach fusion and to the closer arrangement of particles in their glassy state.
Firing shrinkage of a clay is usually about the same as the drying shrinkage.
Total shrinkage will usually be about 8 to 12%.
Clays vitrify at different temperatures depending on what they are made up of.
A red clay high in iron and other impurities might fire to hardness at about 1832° F or 1000° C and melt to liquid at 2282° F or 1250° C.
A kaolin body which is very free from impurities might not melt until over 3272° F or 1800° C!
By mixing proportions of different types of clays that melt at different temperatures, clay bodies are developed for different firing temperatures.
If you fire high enough, the clay will swell up and then turn into a liquid which would hold its shape and cool as a glass.
"Copyright BigCeramicStore.com, reprinted with permission."
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When you get all set up, just enjoy the hobby and have fun at it. Some people get pretty serious and start selling their creations at craft fairs and small stores, but others just like to create items for themselves, relatives, and friends. Whichever kind of handmade pottery you desire to endeavor, enjoy the hobby and have fun doing it.
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