Pottery
107 flashcards by MentisLibrary
Ceramics from clay to kiln: clay bodies and shrinkage, wedging, handbuilding, wheel throwing and trimming, drying and bisque, glaze chemistry and faults, cones and firing, and studio safety.
- What makes clay plastic?
- Its particles are extremely fine platelets of hydrous aluminium silicate. A film of water lets them slide over one another and then hold the new shape.
- What are the three main families of clay body?
- Earthenware, stoneware and porcelain, in rising order of firing temperature and vitrification.
- What are the characteristics of earthenware?
- A low-fire body, commonly matured between cone 06 and cone 02, that stays porous after firing and is often red or buff from iron.
- What are the characteristics of stoneware?
- A body maturing around cone 5 to cone 10 that vitrifies to a dense, strong, nearly non-absorbent ceramic in grey, buff or brown.
- What makes porcelain different from stoneware?
- It is rich in kaolin, fires white, becomes translucent when thin, and is far less plastic and more prone to warping and slumping.
- What is vitrification?
- The point in firing where enough of the body melts into glass to fill the pores and lock the particles together.
- What is grog?
- Fired clay ground to a grit and added back into a clay body.
- What is the difference between ball clay, kaolin and fireclay?
- Ball clay is very fine and highly plastic, kaolin is pure and white but not very plastic, and fireclay is coarse and refractory.
- What is slip?
- Clay suspended in water, thin enough to pour or brush.
- What does a deflocculant do in casting slip?
- A small addition such as sodium silicate or soda ash makes the slip pour freely with far less water in it.
- Why must plaster never get into your clay?
- Plaster fragments absorb water and expand after firing, blowing small craters out of the surface of finished ware.
- How do you reclaim scrap clay?
- Let the scraps dry out completely, slake them in water until they slump, then stiffen the slurry on a plaster bat or canvas and wedge it up.
- How much does clay shrink from wet to fired?
- Commonly 10 to 14 percent overall, depending on the body, split between drying shrinkage and firing shrinkage.
- What is wedging, and what does it do?
- Kneading clay in a repeating pattern to even out its moisture, break up lumps and drive out trapped air.
- What is the difference between ram's head and spiral wedging?
- Ram's head rocks the clay forward in a symmetric roll; spiral wedging turns it as you push, producing a shell-shaped spiral.
- Do air bubbles make pots explode in the kiln?
- Not usually. Explosions are almost always caused by water turning to steam in ware that is not bone dry, or by very uneven wall thickness.
- Name the stages clay passes through from wet to finished.
- Slip, plastic, leather-hard, bone dry greenware, bisque, and finally glaze fired.
- What does leather-hard mean?
- Firm enough to hold its shape and take handling, still damp enough to carve, join and trim.
- How is a pinch pot made?
- Press a thumb into a ball of clay to open it, then squeeze the wall evenly between thumb and fingers while turning it, working from the base upward.
- How do you build a coil pot so it does not come apart?
- Score and slip each coil to the one below, then smear the joins together on both the inside and the outside before adding the next coil.
- What is slab building?
- Construction from flat sheets of clay, rolled or cut, joined at their edges.
- What does score and slip mean, and why does it work?
- Roughen both mating surfaces with a needle or serrated rib, wet them with slip, and press them together with a slight twist.
- Why must joined pieces be at the same moisture level?
- Because wetter clay shrinks more as it dries, so a damp addition on a stiffer body pulls itself apart at the seam.
- What is the difference between a hump mould and a slump mould?
- A hump mould is a form you drape clay over; a slump mould is a hollow you lay clay into.
- Why do you compress a rolled slab?
- Rolling stretches and aligns the particles in one direction, and compressing with a rib evens the stress out.
- Why do handbuilt forms often crack at inside corners?
- Sharp corners concentrate the stress of drying shrinkage, and thin corners dry faster than the walls around them.
- What is centering on the wheel?
- Working the clay until it runs true, with no wobble, so the mass is symmetrical around the axis of the wheel.
- What is coning up and down on the wheel?
- Squeezing the clay into a tall cone and then pressing it back down, repeated a few times.
- How much clay should be left in the floor when you open a pot?
- Enough to trim a foot later, commonly about a quarter of an inch, checked with a needle tool.
- Why do you compress the floor of a thrown pot?
- Because the floor is left with its particles pulled outward, and compressing with a rib or fingers realigns them.