WEEK 6 - Clay Flashcards
Clay
1
Q
What is Clay?
A
Clay can mean:
- A mineral from the phyllosilicate group (like micas – flat, sheet-like minerals).
- A grain size – very tiny particles (less than 0.002 mm).
- A mass of minerals (mostly clay minerals) that act plastic (moldable) when wet.
2
Q
What is a Ceramic?
A
- In art, clay is used to make ceramics
- A ceramic is a hard, brittle, nonmetallic material made from clay + Earth materials.
- It’s hardened by high heat (firing).
- Made of tiny silicate crystals in a glassy cement.
- Ceramics have a crystalline structure (unlike glass).
3
Q
Structure of Clay Minerals
A
Built from 3 basic parts:
- Sheets of silica tetrahedra (silicon + oxygen)
- Sheets of alumina octahedra (aluminum + oxygen)
- Positive ions (like hydrogen, potassium, iron, magnesium) hold the sheets together
4
Q
Two Common Clay Minerals
A
Kaolinite and Illite
5
Q
Kaolinite vs. Illite (Clay Minerals)
A
- Both are made of silica + alumina sheets.
Kaolinite:
→ Layers = Alumina, Silica, Hydrogen ions
Illite:
→ Layers = Alumina, Silica, Potassium ions, Silica, Alumina
- Silica + alumina sheets are covalently bonded (strong bonds).
- Layers are held together by ionic bonds (from hydrogen or metal ions like potassium).
6
Q
Why Clay is Easy to Shape (Good Workability)
A
- Clay grains are shaped like thin, wide plates (like hexagons).
- Their flat, broad shape helps them slide past each other easily when wet.
- This gives clay its plastic, moldable quality.
7
Q
Properties of Clay: Ability to Be Shaped
A
- Clay crystals are flat and plate-like (even when broken).
- They have perfect cleavage in one direction.
- Under light pressure, the plates slide past each other.
- Water helps them slide, making clay easy to mold.
8
Q
Properties of Clay: Cohesion (Sticks Together)
A
- When slightly wet, clay is very sticky.
- Clay grains have a large surface area (flat + broad).
- Thin water films cling to the surfaces.
- Surface tension pulls grains together — like wet playing cards sticking.
9
Q
Where Clays Come From
A
- Clay minerals form when other minerals weather (break down).
- Especially from feldspar (a common mineral in rocks).
Example: Feldspar → clay (you can see dark clay flecks in weathered crystals).
10
Q
Types of Clay
A
Primary and Secondary
11
Q
Types of Clay: Primary Clay
A
- Also called china clay or kaolin
- Found near the site of weathering (where it originally formed)
- Not moved much — little or no transport
- Made mostly of the mineral kaolinite
12
Q
Types of Clay: Secondary Clay
A
- Also called sedimentary clay
- Moved away from the site of weathering
- Transported by wind, water, or ice
- Contains kaolinite + other clays (like quartz, iron oxides, organic matter)
13
Q
Which Clays are used for Ceramics?
A
- Ceramic clays are usually blends from different sources.
- Blending gives better results, making a “clay body”.
- Potters can change the texture and color by blending different clays.
14
Q
Types of Clay Bodies
A
- Earthenware
- Stoneware
- Porcelain
15
Q
What is Grog? (Firesand / Chamotte)
A
- Grog = crushed quartz or pre-fired clay
- Made from things like crushed bricks
- Added to some clay bodies
Helps in 2 ways:
→ Holds heat (good for cookware)
→ Adds texture (“tooth”) and a rustic look
- Comes in different sizes for different uses
16
Q
Why Add Grog to Clay?
A
- Grog is stiff (not soft like clay) → adds strength to soft clay
- Doesn’t hold water → helps reduce shrinkage
- More grog = less shrinkage
- Less grog = more shrinkage
- Helps gases escape during drying, preventing cracks
17
Q
What is Earthenware?
A
- Made from secondary sources
- Good for thick items (like mugs, plates, flower pots)
- Cheapest to make (secondary clays are common)
- Color: white to terra cotta (if iron oxide is present)
- Texture: fine to rough (depends on grog amount)
18
Q
What is Stoneware?
A
- Mostly made from primary sources
- Used for stronger vessels than earthenware
- Also used for non-porous items (like floor tiles, pipes)
- Often has a coarse texture (contains grog)
- Color: usually brown (from iron oxide)
- Can show more glaze effects (like fine speckles)
19
Q
What is Porcelain?
A
- Made from primary clays
- Used for very thin but strong items (like dinnerware)
- Texture: smooth (contains no grog)
- First made in Asia (10th century) → why it’s called “China”
- Sometimes mixed with bone for a translucent look → “bone china”
20
Q
Shaping Methods: How to Get Clay into Ceramic
A
- Handbuilding
- Throwing
- Slip-casting
21
Q
Shaping of Clay Methods Explained
A
Handbuilding:
- Freeform method (no potter’s wheel)
Throwing:
- Shaped on a spinning potter’s wheel
Slipcasting:
- Liquid clay (“slip”) is poured into a mold to form the shape
22
Q
Air Drying (Before Firing)
A
- Clay dries to “greenware” stage before firing
- Wet clay has at least 25% water
- As water evaporates, clay shrinks (particles move closer)
- Uneven drying causes cracks or warping
- To avoid this, keep the clay thickness even
23
Q
Firing (Kiln Drying)
A
- Final drying happens in the kiln (not fully dry before this)
- At 100°C, water boils off from between clay particles
- Must be done slowly — fast steam buildup can cause bursting
- Organic matter + sulfur compounds burn off between 300–800°C
24
Q
The Big Transformation (Firing Clay)
A
- After water is gone, clay chemically changes between 350–800°C
- Hydrogen + oxygen once part of the clay are released as water vapor
- Clay minerals (like kaolinite) turn into mullite and quartz
Example reaction:
Kaolinite → Mullite + Quartz + Water
25
The Effect of Changing Crystal Structure (Sintering)
- Crystal Changes = Stronger Clay (Sintering)
- Around 900°C, mullite crystals start to form
- Mullite = needle-like aluminum silicate crystals
- These crystals lace through the clay, making it stronger and more cohesive
26
Vitrification (Final Stage of Firing)
- Mullite crystals grow and silica melts
- Molten silica fills gaps between the crystals
- When cooled, silica becomes glass, holding the structure together
- Result: a hard, durable ceramic made of fused minerals
27
Why Do Clays Fire at Different Temperatures?
- Different clays vitrify (melt & fuse) at different temps
- The firing temp must match the clay’s vitrification point
28
Primary Clay Temperatures (Porcelain)
- Made of nearly pure kaolinite
- Vitrify at high temps (up to 1,400°C)
- High melting point
29
Secondary Clay Temperatures (Earthenware)
- Contain iron oxides → act as a flux
- Vitrify at lower temps (as low as 1,000°C)
- Flux = lowers melting point
30
Typical Firing Temperatures
- Earthenware: 1,000–1,200°C
- Stoneware: 1,100–1,300°C
- Porcelain: 1,200–1,400°C
31
What is Bisqueware?
- Clay that’s been fired once but not glazed yet (unglazed ceramic)
- Also called "biscuit ware"
- Sometimes left as-is (e.g. flowerpots → stays porous)
- Usually glazed after for sealing + decoration
32
What is Glazing?
- Glazing = adding a glass-like coating to a ceramic after firing
Purpose:
- Beautifies the piece
- Makes it waterproof
33
What’s in a Glaze?
- Glass-forming minerals (e.g. silica, feldspar)
- Stiffeners (like clay)
- Fluxes (lower melting point, like calcite and dolomite)
34
How Does Glazing Work?
- When fired, the glaze melts (up to 2000°C)
- It cools to form a glass coating on the ceramic
35
How Does Glaze Colour Change?
- Impurities in natural minerals
- Additives
36
Common Glaze Colour Additives
- Copper oxide: greens and blues
- Cobalt oxide: blues and violet
- Iron oxide: yellow, orange, and red
- Manganese dioxide: deep purple
