1. Not Your Ordinary “Mud”
Introduction to Clay
Not Your Ordinary “Mud”
Image Resource: studiopotter.org

2. Part I: What is Clay?

3. At the end of the section, you will be able to. . .
… define clay.
… describe the natural process that form clay.
… identify primary and secondary clays.
… Identify porcelain, stoneware, and earthenware as being primary or secondary clays.

4. Clay Terms to Know Clay Igneous Rock Weathering Primary Clays
Secondary Clays
Porcelain Clay
Stoneware Clay
Earthenware Clay

5. What is Clay?

6. DEFINITION
Clay is . . .
. . .a fine mixture of decomposed igneous rock minerals and organic matter
Igneous Rock

7. How is clay formed?
Melted rock is cooled and solidified into igneous rock (mother rock) which makes up earth’s crust.
Mechanical weathering over millions of years breaks down the rock from boulders into stones, to pebbles to fine, small particles known as platelets.
Weathering

8. Mechanical Weathering actions include…
Freezing and thawing
Grinding of glaciers
Pounding of rain
Flow of rushing streams
Probing of tree roots
Acidic reactions with decomposed organic matter (chemical weathering)

9. Erosion also. . .
. . .aids in moving particles away from the site of origin resulting in 2 types of clay deposits.
Image Resource:

10. 2 Types of Clay Deposits
Primary Clays
Secondary Clays

11. Primary Clays
Clays which remain at the site (mother rock) where they were formed.
Fewer impurities
Coarser grained texture (less weathered)
Highly refractory (resistant to heat/high fire)
Low plasticity (harder to work with)
Whiter fired color (Porcelain)

12. Primary Clay Example: Porcelain
Bright white clay – very pure
Translucent (light passes through)
High Fire – matures at 2419 degrees F.
A very hard clay
Vitrifies = non-porous
Smooth – very fine, not as plastic
Feels very dry (harder to throw)
Still white when bisque

13. Secondary Clays
Clays which have been transported by wind, glacier, water, etc. away from site of origin
Finely ground (more weathered)
Contaminated with organic matter
More plastic
Stoneware & Earthenware

14. Secondary Clay Example: Stoneware
Gray to tan or brown when moist
Non-translucent
Matures at degrees F.
Vitrifies = non-porous
Pinkish color when bisque
Very durable = dinnerware safe
Used by Burke Pottery

15. Secondary Clay Example: Earthenware
Reddish brown clay that matures at a low temperature (approximately 1830 degrees, which is red hot heat.)
Also known as terra cotta (baked earth)
Flower pot clay
Remains porous
Does not vitrify
Image resource (picasso)

16. Identify the Clay Body Type (How do you know?)
Earthenware
Stoneware
Porcelain
Image Resource:
REMEMBER: E.S.P.

17. Student Activity: Comic Strip
Materials: Clay Formation Comic Strip Worksheet
Pencils & colored pencils
Directions: Create a comic strip that visually describes the natural processes that transform igneous rock into primary and secondary clay deposits. Label each process and deposit within each cell of the comic strip.

18. Part 2: Clay Properties

19. At the end of this section you will be able to. . .
… identify the characteristics of the three major properties of clay: Plasticity, Shrinkage, and Porosity … Perform test to determine a clay body’s plasticity, shrinkage, and porosity.

20. Clay Terms to Know Plasticity Shrinkage Porosity Aging Wedging
Vitrification

21. 3 Major Properties Plasticity Shrinkage Porosity
These 3 properties must be considered before choosing a clay body.

22. PROPERTY #1: PLASTICITY

23. Plasticity is. . .
Create the Coil
The ability to hold together while being shaped (workability)
Does it stretch and bend without breaking?
Test It! Wrap a coil around your finger. If it cracks, the clay is not plastic.
Plastic
Image Resource: Omaha North High School Art Department
Not Plastic

24. How do you improve clay’s plasticity ?
AGING, WEDGING, & ADDING MOISTURE

25. What is aging?
Storing well mixed clay for 3 plus weeks which causes a chemical breakdown of the organic matter contained in the clay.
Older is better!
Historical Fact: The Chinese aged clay for whole generations.

26. How does wedging help? Aligns clay particles Equalizes moisture
Removes air bubbles
Makes clay texture uniform
Tip: You should always wedge first before using your clay.

27. How do I add moisture? All clay contains water.
Clay dehydrates when exposed to air.
Add water to clay to make it more plastic.
Too little or too much water causes clay to become less plastic.
Tip: Mist clay with water bottle or dampen it with a wet sponge. Always wrap your projects with plastic before storing.

28. PROPERTY #2: SHRINKAGE

29. Shrinkage is. . .
Contraction of clay (or glaze) during both drying and firing processes.
Clay shrinks 12-15%
Shrinkage depends on the clay body’s composition.
Tip: It is important to understand the shrinkage of your clay body since it has a tendency of cracking and warping during this process.

30. How does shrinkage occur?
Water between clay particles moves to the surface and evaporates
Clay particles move closer together until touching.
Shrinkage stops and remaining water between pores evaporates.
Tip: Drying should occur more slowly during stages A and B as seen in the diagram
Resource: studiopotter.org

31. What happens if shrinkage occurs unevenly?
When a drying tile rests on a smooth surface, the upper face will dry first.
The tile will curl up because the upper face is smaller than the lower face (uneven shrinkage).
Later when the tile is completely dry the curl remains as the body is too rigid to completely straighten out.
Resource: studiopotter.org
Tip: In addition to warping, uneven shrinkage may also cause cracking. These defects may be prevented with slow overall drying or delay the drying of certain parts by covering.

32. Activity: Determining Shrinkage Rate
Make a cylinder 5” tall and 5” in diameter with ½” thick walls for each clay body.
Place the cylinder on a piece of paper, tracing around the base.
Measure and record the exact diameter, height, and thickness of the wall.
Re-measure the form after firing. Subtract the difference and divide by the original wet measurements to get the percentage of shrinkage.
Optional: Make a 2” x 11”x 1/2” slab. Score a 10” line down the middle. Measure and divide the difference by 10 to get the shrinkage rate.
Resource: Experience Clay, Page 187

33. Result Example
If the shrinkage rate is 10%, then the final glaze-fired piece of any object made from that clay body will only be 90% of the original wet clay size.

34. PROPERTY #3: POROSITY

35. Porosity is… The capacity of a clay body to absorb moisture
Based on the amount of space in the structure of the fired clay body
Image Resource: Omaha North High School Art Department

36. High Porosity allows more water into or through clay body
High Porosity allows more water into or through clay body. Keep it inside during the winter and use a trivet to protect your coffee table!
Low Porosity, or vitrified clay, allows little or no water into or through the clay body. Vitrification: When a clay is fired to its maturing temperature it loses its porosity and transforms into a hard, nonabsorbent, glasslike state.
Image Resource: Omaha North High School Art Department
Vitrification: The stage during firing when a clay or glaze loses its porosity and transforms into a hard, nonabsorbent, glasslike state.

37. Science Connection: Hydroculture and Clay Pebbles
Hydroculture is the method of growing plants in a nutrient solution instead of soil. Plants, like the salad you buy in the grocery store, are grown on light expanded clay aggregate. These “clay pebbles” not only provide support for the root system, but because of their high porosity, they also function as a wick that brings water and nutrients to the roots without them being directly immersed in the solution.
Resource:

38. Part 3: Studio Processes Used To Transform Clay

39. At the end of this section you will be able to . . .
… identify and list the various processes for transforming clay into ceramic forms. … identify and categorize the physical characteristics of each stage of clay.

40. Clay Terms to Know Kiln Cone Slurry Wet Leatherhard Greenware
Bisqueware
Glazeware

41. Mixing & Recycling Clay
Clay may be recycled up through greenware stage.
Image Resources: Omaha North High School Art Department

42. Firing: The Electric Kiln
The Kiln is a chamber to heat clay
It creates chemical changes to the clay and glazes which create greater strength and permanency

43. The Firing Cycle 8-12 hours to heat + 8-12 hours to cool
16-24 hours for firing cycle

44. Firing Temperatures Stoneware 2200 degrees F Porcelain White Hot Heat
Earthenware or
Bisqueware
Red Hot Heat
1800 degrees F.
Cone: An object made of ceramic materials that melts at a certain temperature used to show the temperature in a kiln during firing. Cone 022 is a very low heat, while cone 12 is a white hot heat. There is no cone 0

45. After the clay has been fired it CANNOT be recycled!
Important Tip!
After the clay has been fired it CANNOT be recycled!

46. Stages of Clay
1. Slurry (potter’s glue): Liquid clay 2. Wet/Soft: Plastic clay 3. Leatherhard: not easily distorted, maintains form and can be smoothed, carved, and added to 4. Greenware (bone-dry): Water is evaporated; form is brittle 5. Bisqueware: once-fired, maintains porosity 6. Glazeware: fired more than once, vitreous

47. Student Activity
Materials: Stages of Clay worksheet Various examples of clay at different stages Directions: Fill out the Stages of Clay worksheet by Studying the physical characteristics of each stage of clay.

48. The End