Introduction to Clay

1. Introduction to Clay Not Your Ordinary “Mud”

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.

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 2232-2381 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

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

17. Writing Activity • In your sketchbook, choose one of the discussed topics, about which you’d like to write a paragraph. • Use your own words and be descriptive. Try not to re-write the definitions found here. • 3-5 Sentences.

18. Example: “Clay Formation Process” When hot magma from under the earth’s crust cools, it becomes a solid called “igneous rock.” Over millions of years, through a process known as “mechanical weathering,” this igneous rock is broken down from boulders to rocks, rocks to pebbles, and finally pebbles to small particles called platelets. These platelets are mixed with organic matter and clay is formed.

19. Part 2: Clay Properties

20. 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.

21. Clay Terms to Know • Plasticity • Shrinkage • Porosity • Aging • Wedging • Vitrification

22. 3 Major Properties • Plasticity • Shrinkage • Porosity These 3 properties must be considered before choosing a clay body.

23. PROPERTY #1: PLASTICITY

24. Plasticity is. . . • 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. Create the Coil Plastic Not Plastic

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

26. 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.

27. 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.

28. 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.

29. PROPERTY #2: SHRINKAGE

30. 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.

31. 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

32. 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. 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.

33. 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.

34. 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.

35. PROPERTY #3: POROSITY

36. 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

37. 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.

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.

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. Porcelain White Hot Heat 2400 degrees F. Stoneware 2200 degrees F Earthenware or Bisqueware Red Hot Heat 1800 degrees F. Firing Temperatures

45. 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