Properties of Clay Johnson High School Beginning Ceramics
Properties of Clay + Johnson High School Beginning Ceramics by Ms Marni Oberpriller
QUESTION: How can understanding the properties of clay + improve the quality of my work in Ceramics?
+ What is clay? Properties of clay n A form of glass n Is made of decomposed and compressed igneous rock. This type of rock makes up most of the Earth’s surface n Primary Clays are pure materials that are at the original site of the igneous rock: Kaolin, Porcelain n Secondary Clays have been moved from the original site by water erosion and deposited somewhere else and mixed with other materials n Clay particles are flat. Made up of silica (which melts) and alumina (which allows it to melt at a low temp). Non-Ceramic types of clay: Play Doh, Model Magic (air dry) , Plasticine (oil-base), Polymer: Sculpy/Fimo (form of plastic)
+ Everything you will do with clay will involve these three variables (changes): n. Moisture n. Plasticity n Amount of water in the clay. n n States of moisture range from slurry to bone-dry Ability to stretch, hold the form it is stretched into. n Clay has plasticity because the particles are flat and stick together when wet. n. Heat n Clay becomes “permanent” at a relatively low temperature, 1800* F
+ What is Ceramics? Ceramics is the art of making objects out of clay. -- Objects or sculptures are built out of clay and then “fired” in a kiln, a special ceramics oven. Then the object can be glazed and fired again to add color or gloss to the piece
+ History of Ceramics Theory 1 People first began to mold clay as a diversion (something to do). Theory 2 People used clay for practical purposes. (cooking and transporting food, storage) Of all the arts, Ceramics has perhaps the longest history, dating back to when people first learned to control fire
+ Minnesota Ceramics WOODLAND PERIOD: 500 BC to 1000 AD Illinois, Wisconsin, Minnesota, Canada History First use of Pottery: 500 BC • Humans began to cultivate plants, form permanent villages • Pottery tempered with crushed shell, improved technology, and allowed for new ceramic forms • Surface designs came with more free time and desire to embellish utilitarian objects • Used rope, paddles, and scrapers
+ Early Techniques Studio Clay Early Potters n Processed – impurities removed n Had to locate clay deposits n Prepared – blended for certain temperatures n Raw materials needed to be dug by hand from the ground n There were sometimes impurities that had to be removed.
+ Working with Clay: Clay Properties (what clay can do) n. Plasticity n. Shrinkage n. Texture n. Moisture Handling the clay Safety Tips
+ Plasticity Is the ability to stretch and hold a form it is stretched into. Clay is plastic because the particles are flat and stick together when wet. n Numerous factors can make clay more plastic: n Spray clay with water n Store the clay in a bag with water n Mix (wedge) new plastic clay with the old n Properties of clay that is Plastic n Difficult to remove “clay boogers” n Clay body: darkest grey n Cold to touch
+ Shrinkage An irreversible reduction in the size and volume of a ceramic piece or glaze during the drying and firing process. n All clays shrink as they dry n Clay can crack when it shrinks n Clay needs to dry evenly. n Clay of different thicknesses dry at different rates and can cause cracking. n Clay needs to dry slowly n Grog n is made up of small particles of clay that have already been fired. n Grog decreases the amount that a clay body will shrink.
+ Texture adds visual interest with light and shadow to the surface of the clay n Texture of a clay body can range from coarse to smooth n Some Ceramists add sand or grog to a clay body to change the texture of their clay n Grog decreases the amount that a clay body will shrink
+ Moisture Is the amount of water in a clay body. n Moisture can be added back into clay by: n Spray clay with water, dampen the clay with a wet sponge or a damp paper towel n Store the clay in plastic to retain moisture n Add too much water and the clay loses plasticity and cannot hold form. n Properties of clay: n Clay begins to dehydrate or lose moisture when exposed to air. The clay body becomes more fragile and brittle. n Once the clay is fired, it becomes permanent, harder, and less likely to break
+ Wedging the Clay Process of preparing the clay for use. Similar to kneading dough. n Wedging accomplishes three things: n Removes any air pockets that are present in the clay body. n Final mixing process that evens out any inconsistencies in clay and moisture. n Align the clay particles making “throwing” easier n Wedge at the work table (not on wheel heads) and scrape clay from the surface when finished.
+ Clay Safety Preventing dust! n Why we clean up our work spaces n Little bits of clay that end up on the surface of your table or wheel are harmless. However, when they are brushed onto the floor, they crumble and break down into smaller and smaller particles which then scatter in to the air we breathe. n Capture clay dust with water n Use a wet sponge to clean tables, tools, wheels and sinks. n Sweeping with a broom or brush makes more dust.
+ Stages of Clay: How the clay changes as it dries and is fired. n. Wet clay: Slurry and Slip n. Plastic n. Leather-hard n. Bone-dry n. Green-ware n. Bisque-ware n. Glaze-ware
+ Wet Clay: We do not want to build with clay at this stage. The clay is too soft and has no strength or structure. n Properties of clay at the Wet stage: n Clay is soft and flexible because it is 75 % water. n Clay cannot hold a form, n Clay body: darkest grey n Cold to touch n Clay can be reclaimed at this stage.
+ Plastic Clay: How the clay changes as it dries and is fired. n Plastic n Clay is pliable and forgiving. n A workable consistency for coils, slabs, and throwing. n Properties of clay at the Plastic stage: n Clay body is 50% water n Difficult to remove “clay boogers” n Clay body: darkest grey n Cold to touch n Clay can be reclaimed at this stage.
+ Leather-hard: A part of the drying process when clay can be carefully handled with out danger of being malformed but is still workable. We want our work to remain at this stage until we finish our assignments. n Clay is drier, stronger, and more stable like shoe leather. n Properties of clay at the Plastic stage: n Clay body is 30% to 40% water n Best stage to remove “clay boogers, ” surface carving and burnishing. n Handles and spouts can be attached to wheel-thrown pieces. n Clay body: grey n Cold to touch n Clay can be reclaimed at this stage.
+ Bone-Dry: A part of the drying process when clay can be carefully handled with out danger of being misformed but is still plastic enough to be altered n Avoid working with clay at this stage. n Properties of clay at the Plastic stage: n All physical water has evaporated, clay is dry. n Best stage to remove “clay boogers” and surface carving and trimming. n Can hold the form, VERY FRAGILE and BRITTLE n Clay body: light grey and chalky looking n Warmer to touch – can be room temperature n Last stage to reclaim the clay
+ Green-ware: Clay that has not been fired. These post are dried upside down to allow the base to dry to avoid cracking.
+ Firing: The process of exposing clay to high heat. During the firing, the clay goes through a change where it fuses together. Into a solid piece. Unfired clay will dissolve in water, but clay becomes resistant to water after firing.
+ Bisque-ware Our JHS stoneware clay body turns pink after it has been bisque-fired.
+ Glaze-ware Our clay body turns grey-brown after it has been glaze -fired (in the large gas kiln). • After a piece has been glazed. It will be fired a second time. • Clay that is exposed to high heat becomes VITREOUS and repels water. • Vitreous clay is dense and glass-like • Ware can not be reclaimed.
+ Warping Is stress caused by uneven thickness or drying of the clay. Memory All clay has what is called memory. This is the tendency for clay to return to a previous shape or form.
+ Ceramic Techniques The different ways that clay can be formed. n. Pinch n. Coil n. Slab n. Wheel or “Throwing” n. Slip-cast and Press-mold n. Recycling Clay
- Slides: 35