Clay
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Transcript of Clay
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CLAY!
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What is it?
Fine particles of dirt which float in a stream or river then sink to the bottom- breaks off from rocks
River valleys tend to have a lot of clay.
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Ancient CulturesAncient man discovered that clay could be shaped and modeled into forms both
decorative and functional. Some of the earliest pottery has been found in the Middle East and dates to
between 8,000-7,000 BCE. Japanese ceramics have flourished since over 4,000 years agoNative American pottery shards have been found dating back over 3,000 years.
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Types of ClayPRIMARY:
(Kaolins)– Has been in the
same place for millions of years.
– Not good for building.
– Always WHITE in color.
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Kaolin (primary) clay
Not used as much in ceramics, except for porcelain
But, is used in paper making, toothpaste, cosmetics, and paint!
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Types of ClaySECONDARY:
– “Art” clay- can be worked and shaped
– Variety of colors: reds, yellows, whites, blacks, greys
– Carried into riverbeds- goes to a ‘secondary’ place from the original rock and contains more impurities/smaller particles
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Secondary Clay
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US Clay “Map”
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Preparing Clay for UseWedging- process of
removing air from the clay. – Necessary to remove
air from the clay in order to keep it from exploding in the kiln
– Wedging is done by kneaded or ‘throwing’ the clay down in the appropriate wedging area.
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Preparing Clay for Use
Dry Time– Unless clay is kept wet with a
squirt bottle it will dry out. IT MUST BE COVERED when you are still working on it.
– Cracks will start to appear and the clay will be difficult to work with.
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Preparing Clay for Use
Slip and Score– When joining 2 pieces of clay
together it is necessary to do the “3 S’s”: SLIP and SCORE, SMOOTH each piece.
– Slip is like glue for clay– Scoring helps the pieces fit
together like puzzle pieces– Smoothing makes the pieces
STRONGER and look like seamless
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Preparing Clay for UseSlip and Score
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Preparing Clay for Use
Hollowing Out– In order to prevent
breakage and explosions in the kiln, clay must be HOLLOWED OUT if it is over ½ inch thick.
– Hollowing out also allows the clay to dry faster.
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Preparing Clay for Use Hollowing Out
– Newspaper can be used to hold the shape of the project once it is hollowed out.
– YOU MUST POKE HOLES where you’ve hollowed out, otherwise you ’ve created a giant air bubble and it will explode!
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Stages of Clay
Leather-hard The condition of a clay body when much of the moisture has evaporated and shrinkage has just ended, but the clay is not totally dry. Carving, burnishing, or joining slabs are often done at this stage. Bone DryClay that has the moisture completely evaporated. It can no longer be scored and slipped withouta compromising the strength of the piece.
BisqueClay that has been fired to a state hard enough for glazing
Greenware Unfired pottery or sculpture.
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Firing
Clay must be left in the open air to dry (GREENWARE) before it can be FIRED in the kiln. (BISQUEWARE)
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Greenware
Clay is ‘leather hard’
Most of water has evaporated
Most fragile state
Has not been fired yet
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Bisqueware Clay that has
undergone one firing Clay is now
CHEMICALLY ALTERED. Cannot be worked anymore/be turned back into slip or clay.
Sturdier than greenware
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Glazeware
Bisqueware that goes through a second firing, with glaze applied
“Final” firing
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Kilns
Ovens used to fire clay
1. Pit kilns- (prehistoric)- simplest. Shallow pit dug in the ground. Made pots very fragile and porous.
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Kilns2. Beehive Kilns-
greater heat, pots are stacked in a chamber with fire burning from underneath.
3. Natural Gas Kilns- more modern. Similar to Beehive kiln except natural gas is used instead of firewood.
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Kilns4. Electric Kilns- 20th century. Heated by an electric current that heats coils throughout.
- Pyrometric Cones- pyramids made of ceramic materials that tell the actual temperature of the kiln during firing. Used to check to accuracy of the firing
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Cone chart
Named after the pyrometric cones, to tell how hot the kiln was before we could accurately measure the temperature
Generally refers to certain temperatures. Clays all have a firing ‘cone’ as well as glazes. (Make sure they match up!)
The clay you are using is cone 04 to cone 06. This means it fires at about 1,855-1,971 degrees F
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Clay Disasters