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

Part I: What is Clay?

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.

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

What is Clay?

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

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

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)

Erosion also. . . . . .aids in moving particles away from the site of origin resulting in 2 types of clay deposits. Image Resource: http://www.axner.com/

2 Types of Clay Deposits Primary Clays Secondary Clays

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)

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

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

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

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) http://www.masterworksfineart.com/inventory/picasso/prev_picasso1924.jpg

Identify the Clay Body Type (How do you know?) Earthenware Stoneware Porcelain Image Resource: http://www.axner.com/ REMEMBER: E.S.P.

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.

Part 2: Clay Properties

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.

Clay Terms to Know Plasticity Shrinkage Porosity Aging Wedging Vitrification

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

PROPERTY #1: PLASTICITY

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

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

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.

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.

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.

PROPERTY #2: SHRINKAGE

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.

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

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.

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

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.

PROPERTY #3: POROSITY

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

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.

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: http://www.microperfusion.com/hydrofun/starter.htm

Part 3: Studio Processes Used To Transform Clay

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.

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

Mixing & Recycling Clay Clay may be recycled up through greenware stage. Image Resources: Omaha North High School Art Department http://www.nigercurrents.ca/media/mixing.jpg http://gallery.photo.net/photo/4413754-lg.jpg

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

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

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

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

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

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.

The End