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Introduction and Acknowledgments …… xiii
1 Clay and Claybodies ……………………… 1The Nature of Clay ………………………………… 1
Clay and Claybodies: Some Basic Questions Answered …………………………… 2
What Is Clay? ………………………………………… 2
What Makes Clay Behave the Way It Does? What Makes It Plastic? ……………………………… 2
Why Do Different Clays Behave Differently? …… 3
How Does Particle Size Affect Drying and Firing? …………………………………………… 3
What Are Flocculation and Deflocculation? ……… 3
Why is Aged Clay More Workable? ………………… 3
What Are the Basic Structural Components of Clay? ……………………………………………… 3
What Happens When Clay Is Fired? What Are Sintering and Vitrification? ………………………… 4
Classification of Clays ……………………………… 4Primary and Secondary Clays ……………………… 4
Claybodies ………………………………………… 5
Claybody Components …………………………… 5Accessory Fluxes ……………………………………… 5
Refractories …………………………………………… 6
Tempering Materials or Fillers ……………………… 6
Plasticizers ……………………………………………… 6
Colorants ……………………………………………… 6
Common Types of Claybodies …………………… 6
Variations in Claybodies for Different Applications and Firing Processes …………… 8
Analysis of Clay Properties ……………………… 9Water of Plasticity …………………………………… 9
Mixing and Recycling Clay ……………………… 10The Low-Tech Approach ……………………………… 11
Clay Mixers and Pug Mills …………………………… 11
The Hopper Mixer or “Dough Mixer” ……………… 12
The Soldner Mixer …………………………………… 12
The Pugmill …………………………………………… 13
2 Handbuilding …………………………… 14Wedging the Clay ………………………………… 15
Cylinder Wedging …………………………………… 15
Cone Wedging ………………………………………… 16
The Cut-and-Slap Method …………………………… 18
Wedging Large Amounts of Clay …………………… 18
Handbuilding: General Guidelines and Suggestions ………………………………… 18
Making Pinch Forms ……………………………… 20
Coil Construction ………………………………… 23Making Round-Bottom Coil Pots with or
without a Puki ……………………………………… 24
Coiling the Walls ……………………………………… 24
Closing the Mouth of a Coil Form ………………… 26
Paddle-and-Anvil and Rib-and-Hand Forming Methods …………………………………… 28
Coil-Built Sculpture …………………………………… 29
Slab Construction ………………………………… 29Rolled Slabs and “Memory” ………………………… 29
Combining Slab and Thrown Components ……… 29
Rolling Out Slabs ……………………………………… 30
Rolling Slabs by Hand ………………………………… 30
Making Very Thin Slabs ……………………………… 30
Soft-Slab Construction ……………………………… 31
Soft-Slab Cylinders …………………………………… 31
Soft-Slab Covered Boxes ……………………………… 31
Slumped Slab Lids for Soft-Slab and Stiff-Slab Vessels ……………………………………… 32
Slump-Molds …………………………………………… 32
Soft-Slab Masks ………………………………………… 33
Soft-Slab Sculpture …………………………………… 34
Stiff-Slab Construction ……………………………… 35
Stiff-Slab Boxes ………………………………………… 35
Stiff-Slab Sculpture …………………………………… 37
An Unconventional Approach to Slabs …………… 38
Making Tiles ………………………………………… 38
3 Throwing ………………………………… 40Choice of Wheels and Seats ……………………… 41
Throwing Right-Handed vs. Left-Handed ……… 42
Wedging and Preparing Balls of Clay ………… 42
Clay Consistency …………………………………… 42
Correct Position for Centering ………………… 42Centering ……………………………………………… 43
Wheel Wedging …………………………………… 45
Penetrating the Lump …………………………… 46
Measuring the Thickness of the Bottom ……… 46
The Claw—Widening the Bottom ……………… 46
Recentering ………………………………………… 49
Compacting and Leveling the Bottom ………… 49
Lifting the Walls …………………………………… 49
Lubrication While Throwing……………………… 50
Compressing the Rim ……………………………… 50
Trimming Excess Clay or Irregularity from the Rim …………………………………………… 50
Skill Development with Cylinders ……………… 51What To Do with the Basic Cylinder ……………… 51
Remove All Water ………………………………… 51
Trim Excess Clay from the Base ………………… 52
Removing the Pot from the Wheel …………… 53
Throwing on Bats ………………………………… 54
Throwing on Canvas ……………………………… 54Critical Points in Throwing ………………………… 55
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Throwing off the Hump ………………………… 57Throwing Bowls ……………………………………… 59
Throwing Plates ……………………………………… 60
Throwing Pitchers, Vases, Jars, Bottles, and Jugs …………………………………………… 62
Vessel Proportions …………………………………… 62
“necking In” a Vessel ………………………………… 63
Throwing Pitchers and Vases ………………………… 65
Forming the Spout on a Pitcher …………………… 65
Throwing Bottles and Jugs …………………………… 66
Making Lidded Vessels ……………………………… 67
“Grinding-In” Your Lids ……………………………… 69
Making Teapots …………………………………… 69Teapot Lids ……………………………………………… 70
Teapot Spouts ………………………………………… 70
Teapot Handles ………………………………………… 71
Thrown-and-Altered Forms ……………………… 72Throwing Oval, Square, or
Polygonal Forms …………………………………… 72
Throwing Components to Be Assembled ………… 73
Cutting Darts ………………………………………… 73
Lids for Thrown-and-Altered Vessels ……………… 74
Feet on Altered Forms ………………………………… 74
Throwing and Using Closed Forms …………… 74Paddling and Rib-Shaping Thrown Forms ………… 75
Throwing Large Pots—Coil Throwing and Multipiece Vessels ……………………………… 76
Production Throwing ……………………………… 77
Drying Your Pots …………………………………… 77
Finishing the Bottoms of Your Pots …………… 78Finishing without Trimming—the
Rolled edge …………………………………………… 78
Trimming Your Pots …………………………………… 79
Trimming Platters and Wide, Low Bowls ………… 80
Trimming Bottle and Vase Forms …………………… 80
The Giffin Grip ………………………………………… 82
To Sign or not to Sign ………………………………… 82
Making and Applying Handles ………………… 82
4 Plaster Working, Mold Making, and Slip Casting ……………………………… 88
Plaster in Drainpipes: A Plumbing Nightmare— How to Clean Up ………………………………… 89
Measuring, Mixing, and Pouring Plaster ……… 89Water to Plaster Tables ……………………………… 90
The Use of Cottles ………………………………… 91
Using Strips of Sheet Metal or Linoleum for Mold Forms ……………………………………… 91
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The Concept of Draft ……………………………… 91
Mold-Release Agents (Parting Agents) ………… 91
Drying of Molds …………………………………… 91
Making and Using Plaster Press Molds ………… 92
Making Slip-Casting Molds ……………………… 92Open-Pour Slip-Casting Molds ……………………… 93
Multipiece Slip-Casting Molds …………………… 93Multipiece Flick/Smear Molds from Plastic
Clay Prototypes ……………………………………… 93
Multipiece Molds from Rigid (not Plastic Clay) Prototypes …………………………………… 94
Mixing and Pouring Casting Slip ……………… 96Pouring Your Molds …………………………………… 96
5 Surface Decoration on Greenware …… 98Decorative Effects during Forming ……………… 98
Impressed Decoration ………………………… 99
Subtractive Methods …………………………… 100
Additive Methods ……………………………… 101
Burnishing and Polishing ……………………… 102
Slips and Slip Decoration ……………………… 102How to Select a Slip Formula ……………………… 102
Flocculation and Deflocculation ………………… 103
Mixing a Slip without a Deflocculant …………… 104
Mixing a Slip with a Flocculant ………………… 104
Adding a Flocculant to a Liquid Slip …………… 104
Mixing a Slip with a Deflocculant ……………… 104
Desired Consistency of a Slip Mixture ………… 104
Adding Colorants to White Slip ………………… 105
Slip-Decorating Techniques …………………… 106Polychrome Slip Painting ………………………… 106
Sgraffito ……………………………………………… 106
Slip Trailing ………………………………………… 106
Feather Combing ………………………………………107
Slip Marbling ………………………………………… 107
Mishima (Slip Inlaying) …………………………… 108
Slip Layering ………………………………………… 108
Slip-Resist Techniques ……………………………… 108
Slip Texturing ……………………………………… 108
Slip Stamping ……………………………………… 108
Pate-sur-Pate (Paste-on-Paste) …………………… 108
The Wonders of Terra Sigillata ……………… 109Making Terra Sigillata: Batch Mixing,
Deflocculants, and Specific Gravity …………… 109
Initial Settling ……………………………………… 110
Decanting the Suspension ………………………… 110
Concentrating the Suspension …………………… 111
Yield from Different Clays ………………………… 111
Application and Desired Specific Gravity ……… 112
Polishing Terra Sigillata …………………………… 112
Firing Temperatures ………………………………… 112
Coloring Terra Sigillatas …………………………… 112
Colored Clay Techniques ……………………… 113Basalt Body ………………………………………… 114
Clay Marquetry ……………………………………… 114
Clay Murrini ………………………………………… 114
Lamination of Colored Clays …………………… 116
Layered Colored Clays …………………………… 116
Marbleized and Grained Colored Clays ………… 117
Rocklike effects in Colored Clay ………………… 117
neriage ……………………………………………… 117
nerikomi …………………………………………… 118
Pate-sur-Pate ………………………………………… 118
Slip effects with Colored Clays …………………… 118
Sprigged Colored Clay …………………………… 118
Swirlware …………………………………………… 118
6 Glazes and Glazing …………………… 120Introduction to Glazing ………………………… 120
Glaze Color ………………………………………… 121
Glaze Transparency and Surface ………………… 121
Approaching Glaze Design …………………… 121
Glaze-Firing Ranges …………………………… 122Referring to Glazes by the Firing Cone ………… 122
Very Low-Fire ……………………………………… 122
Low-Fire ……………………………………………… 122
Low-Mid-Range……………………………………… 122
Mid-Range …………………………………………… 123
High-Fire …………………………………………… 123
Multirange Firing …………………………………… 123
Glaze Variations, by Design and by Accident …………………………………… 123
The Choice of Whether to Buy or Mix Glazes ……………………………………… 123
Organizing Glaze Recipes: Card Files and Software …………………………………… 124
Converting Glaze Recipes to Standardized Form …………………………… 124
Mixing Glazes …………………………………… 124Using a Triple-Beam Gram Scale ………………… 125
Glazing Methods ………………………………… 127Using Resist Compounds ………………………… 127
Using Resists for Glaze Decoration ……………… 128
Contamination of Glazes ………………………… 129
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Glaze Consistency and Thickness of Application …………………………………… 129
Glaze effects Resulting from Thickness of Application …………………………………… 130
Using Multiple Glazes ……………………………… 131
Using Oxide Washes and Patinas …………… 131
Glaze Application ……………………………… 132Brushing and Sponging Glazes …………………… 132
Dipping Glazes ……………………………………… 133
Pouring Glazes ……………………………………… 134
Spraying Glazes ……………………………………… 134
Avoiding Problems during Glazing, and Dealing with Them When They Occur ……………… 135
Multiple Firings, and Reglazing Glaze-Fired Wares ………………………………… 136
Checklist of Guidelines for Glazing ………… 136
Commercial Glaze Products …………………… 137Underglazes ………………………………………… 137
Stains ………………………………………………… 137
Low-Fire Commercial Glazes ……………………… 138
Mid-Range and High-Fire Commercial Glazes ……………………………… 138
Lusters ………………………………………………… 138
China Paints (Overglaze enamels) ……………… 139
Glazes: The Technical Side …………………… 139What Are Glass and Glaze? ……………………… 140
Oxides, Oxidation, and Reduction ……………… 140
Reoxidation ………………………………………… 141
Components of a Glaze ………………………… 141The Glass Formers: Acidic Oxides—Silica ……… 141
Refractories: Stabilizers—The neutral Oxides— Alumina …………………………………………… 142
Fluxes: Basic Oxides—Coefficient of expansion—eutectics …………………………… 142
Glaze Modifiers ……………………………………… 146
Miscellaneous Components ……………………… 146
Primary Chemical Variations in Glazes for Different Firing Ranges ……………………… 146
Adjusting the Qualities of a Glaze …………… 147
Glaze Color ……………………………………… 148Coloring Oxides …………………………………… 149
Common Traditional Glazes …………………… 150
Salt and Soda Glazing ………………………… 151
The Chemistry and Physics of Glaze Firing … 152Reactions and Properties during Heating ……… 152
Reactions and Properties in the Fluid State …… 152
Reactions and Properties as the Glaze Starts to Cool ……………………………………………… 152
Glaze Faults ……………………………………… 154
Testing Glaze Materials and Glazes ………… 157Making Test Tiles …………………………………… 158
Testing Glaze Hardness …………………………… 158
Testing Durability of Fired Wares ………………… 158
Ceramic Calculation Software, Unity Formulas, and Limit Formulas …………………………… 158
How Do We Use Ceramic Calculation and the Unity Formula? …………………………………… 159
7 Kilns and Firing ………………………… 160Types of Firings ………………………………… 161
Types of Kilns …………………………………… 161electric Kilns ………………………………………… 162
Fuel Kilns …………………………………………… 162
Wood Kilns ………………………………………… 163
General Kiln and Firing Practices …………… 163Firing Logs …………………………………………… 163
Ventilation …………………………………………… 163
Don’t Burn Yourself! ……………………………… 163
Opening Hot Kilns ………………………………… 164
Care of Refractory Surfaces ……………………… 164
Preparing and Loading Kilns ………………… 164electric Kiln Preparations ………………………… 164
Gas Kiln Preparations ……………………………… 164
Kiln Shelves and Furniture ………………………… 164
Cleaning Shelves and Applying Shelf Wash ………………………………………… 165
Temperature Measurement: Pyrometers and Pyrometric Cones ………………………………… 166
Making Proper Cone Packs ……………………… 168
Loading Kilns …………………………………… 168Selecting and Placing Kiln Furniture …………… 168
Loading a Bisque-Firing …………………………… 169
Loading a Glaze-Firing …………………………… 170
Determining Appropriate Firing and Cooling Ramps ………………………………… 171
Bisque-Firing Ramps ……………………………… 171
Glaze-Firing Ramps ………………………………… 172
Cooling Ramps ……………………………………… 173
Firing Theory and Practice …………………… 173Firing Clay: Chemical and
Physical Changes ………………………………… 173
The Sources and effects of Heat ………………… 174
Heat Units—Calories and BTUs ………………… 175
The Combustion of Fuels ………………………… 175
Convection Currents and Back Pressure in Fuel Kilns ………………………………………… 176
Oxidizing, neutral, and Reducing Atmospheres in Fuel-Burning Kilns …………………………… 176
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Different Fuels and Surface exposure …………… 177
The Firebox—The Heart of a Fuel Kiln ………… 178
Flames and Flame Path in the Combustion Zone ………………………………… 178
Primary and Secondary Air ……………………… 179
Firing Fuel Kilns ………………………………… 179Controlling Temperature in Fuel Kilns ………… 179
Controlling and Correcting Temperature and Atmosphere in an Updraft Kiln ………………… 180
Controlling and Correcting Temperature and Atmosphere in a Downdraft Kiln ……………… 181
Watching the Flame Shape ……………………… 182
Specialized Firing Processes …………………… 182Raku Firing ………………………………………… 182
Salt and Soda Firing ………………………………… 184
Single-Firing ………………………………………… 186
Wood Kilns and Wood Firing ………………… 187The Coal Bed ………………………………………… 187
Air Ports ……………………………………………… 188
Types of Wood Fireboxes and Grate Systems …………………………………… 188
Watching the Ports ………………………………… 190
Small Wood Kilns …………………………………… 190
Promoting Flashing and Residual Ash Deposition …………………………………… 191
Choice of Wood Types and Sizes ………………… 191
Regulating Oxidation and Reduction with Wood ………………………………………… 192
Sagger Firing …………………………………… 192
Sawdust Smoking ……………………………… 193
Bonfire Firing …………………………………… 194How To Do a Bonfire Firing ……………………… 195
Selecting the Clay and Preparing the Wares ………………………………………… 195
Selecting and Preparing the Fuel and Manure ……………………………………… 195
Preparing the Pit for Blackware Firing …………… 195
Firing the Wares Directly in the Bonfire ………… 196
Pit Firing ……………………………………………… 196
Bonfiring with a Grate, Cage, or Drum ………… 196
Stacking and Covering the Wares………………… 196
Kindling the Bonfire ……………………………… 197
The Oxidizing Bonfire ……………………………… 197
The Blackware Bonfire …………………………… 197
Cleaning the Wares ………………………………… 197
Postfiring Polishing ………………………………… 197
Electric Kiln Selection, Design, and Repair … 197electric Heating elements ………………………… 198
Reduction-Firing in an electric Kiln ……………… 198
Temperature Control and Shutoff Devices on electric Kilns ……………………………………… 199
electric Kiln Venting Systems …………………… 201Kiln-Wall Thickness/Construction and
Temperature Rating ……………………………… 201Size and Design of Top-Loader Kilns …………… 201element Support Systems ………………………… 201Installation Requirements for Top-Loader
electric Kilns ……………………………………… 201Heavy-Duty Industrial electric Kilns …………… 202Purchasing a Used electric Kiln …………………… 202
Maintenance and Repair of Electric Kilns …… 203Problems with Corrosion ………………………… 203Dawson Kiln Sitter Problems ……………………… 203electrical Problems and Repairs ………………… 204electrical Terminals and Wires …………………… 204Switch Replacement ……………………………… 205Power Supply Problems …………………………… 205element Replacement ……………………………… 205Refractory Repairs on electric Kilns ……………… 207
Fuel Kiln Selection, Design, Construction, and Repair ……………………………………… 207
Choosing the Right Kiln Design ………………… 208Kiln Proportions …………………………………… 208Proportions for Downdraft Kilns ………………… 208Kiln Size ……………………………………………… 209Commercially Made Gas Kilns …………………… 209Gas Kiln Installation ……………………………… 209Venting Fuel Kilns ………………………………… 210Venting Updraft Kilns ……………………………… 210Venting Downdraft Kilns ………………………… 211
Burner Systems ………………………………… 211Gas Burner Systems ………………………………… 211Gas Burner Ignition and Safety Systems ………… 211Programmable Controllers on Gas Kilns ……… 213Gas-Line Pressure: Variations
and Measurement ………………………………… 213Gas Burners and entrained Air …………………… 214Atmospheric/natural Draft Burners ……………… 215Simple Tube Burners ……………………………… 215Flame-Retention Problems ………………………… 216Gas-Air Mixing and Turbulence: Flame-
Retention Burner Tips …………………………… 216Venturi Burners……………………………………… 217Pilot Burners ………………………………………… 218Power Burners ……………………………………… 218
Oil-Burner Systems ……………………………… 219Drip-Feed Oil Burners ……………………………… 219Atomizer Oil Burners ……………………………… 219Safety Systems with Oil Burners ………………… 220
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Refractory Materials Used in Kiln Construction ……………………………… 220
Hardbrick …………………………………………… 220
Cutting Hardbrick and Kiln Shelves ………… 221Insulating Firebrick ………………………………… 222
Ceramic Fiber Products …………………………… 222
Castable Refractories ……………………………… 223
Mortars and Kiln Cements ………………………… 224
Refractory Kiln Coatings…………………………… 224
Where to Get Refractory Materials ……………… 224
Kiln-Roof Spanning Systems ………………… 225
Fiber Kiln Construction ………………………… 226
IFB Gas Kiln Construction from the Ground Up ……………………………………… 227
The Kiln Foundation ……………………………… 227
The Kiln Floor ……………………………………… 227
Brick Wall Construction …………………………… 227
Designing and Constructing Burner Ports and Flue Opening ……………………………………… 228
Burner Placement: Fireboxes and Bag Walls ……………………………………… 228
Steel Support Framework ………………………… 229
Building the Sprung Arch ………………………… 230
The Arch Form ……………………………………… 231
Laying the Arch …………………………………… 232
Insulating and Reinforcing the Arch …………… 232
Building the Chimney on a Downdraft Kiln …………………………………… 233
Design and Placement of the Damper ………… 234
Door Construction ………………………………… 234
Making Peepholes ………………………………… 237
Gas Plumbing …………………………………… 237Building Your Own natural-Draft Burners ……… 238
Making Your Own Flame-Retention Tips ……… 239
Building Power Burners …………………………… 239
Mounting Burners on the Kiln …………………… 239
Repairing Gas Kilns ……………………………… 239Refractory Repairs ………………………………… 239
The Damper ………………………………………… 240
Repairing Burner Components …………………… 240
8 Mixed Media in Ceramics …………… 242Possible Mixed-Media Materials ……………… 244
Flat “Stuff” …………………………………………… 244
Long “Stuff” ………………………………………… 244
Miscellaneous “Stuff” ……………………………… 244
Odd Found Objects ………………………………… 245
Fastening and Forming …………………………… 245
9 Studio Safety and Sensible Studio Practice ………………………… 246
Studio Safety Checklist ………………………… 246
Toxic and Hazardous Materials in Clays and Glazes ……………………………………… 247
Disposing Toxic Materials ………………………… 248
Dust/Dirt Management ………………………… 248About Dust Masks ………………………………… 248
Floor and Surface Cleaning ……………………… 249
Dust in Handling Clay and Glaze Materials …………………………………… 250
Dust Problems While Grinding and Cleaning Wares and Kiln Furniture ……………………… 250
Stationary Dust Filters in the Studio …………… 250
Other Studio Health Issues …………………… 250Avoid Wet Floors …………………………………… 250
Repetitive Motion Disorders; Carpal Tunnel Syndrome ………………………………… 250
Taking Care of Your Back ………………………… 251
Skin Care …………………………………………… 251
Lighting ……………………………………………… 252
Equipment Safety ……………………………… 252Leave Machinery in Proper
Shutdown Condition …………………………… 252
Always Observe Proper Machinery Safety ……… 252
Studio Ventilation ……………………………… 253Ventilation needs during Clay and
Glaze Mixing ……………………………………… 253
Ventilating Hot Wax Fumes ……………………… 254
Ventilating Glaze Overspray ……………………… 254
Ventilation for Kilns ……………………………… 254
Safety with Kilns and Firing …………………… 254
10 Studio Design, Setup, and Operation ………………………… 256
Studio Design and Setup ……………………… 256Concerns in an existing Structure ……………… 257
Studio Size …………………………………………… 257
Plan for the Future ………………………………… 257
Studio Lighting ……………………………………… 257
Wiring ………………………………………………… 258
Plumbing …………………………………………… 258
Specific-Use Areas ……………………………… 258Clay Storage/Processing Area ……………………… 258
Throwing Area ……………………………………… 258
Handbuilding Area ………………………………… 259
Damp-Box and/or Dry-Box ……………………… 259
Ware Storage ………………………………………… 259
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Glazing/Decorating Area ………………………… 260
Kiln Area/Room …………………………………… 260
Space for Packing and Shipping ………………… 260
Proper Packing and Shipping ………………… 260
Design, Setup, and Operation of Specialized Studios …………………………… 262
The Amateur or Hobby Studio …………………… 262
The Cooperative or Group Studio ……………… 263
The Professional Studio for an Individual Artist/Artisan ……………………………………… 264
The Professional Studio with employees or Students Present ………………………………… 265
The Professional Studio with an Attached Gallery ………………………………… 266
The Academic Studio ……………………………… 266
Studio Equilibrium ……………………………… 267
Resources for Students, Studio Artists, and Educators ………………………………… 268
Exhibition, Presentation, Marketing, and Sales ……………………………………… 269
What Am I Getting Into? ………………………… 269
Resumes and Artist’s Statements ………………… 269
Presenting Your Work in Photographs and exhibitions …………………………………… 270
Photographing Your Work ………………………… 270
Presenting Your Work in exhibitions …………… 272
Marketing and Exhibiting Your Work— Good Work Sells ……………………………… 273
Know Your Market ………………………………… 273
Pricing Your Work ………………………………… 273
exhibition Opportunities ………………………… 274
Applying to Competitive exhibitions …………… 274
Other exhibition Opportunities ………………… 275
Marketing Choices: Retail, Wholesale, and Consignment ………………………………… 275
Art/Craft Shows …………………………………… 276
High-end Art/Craft Shows ………………………… 276
Trade Shows and Wholesale Reps ………………… 278
Sales on the Internet ……………………………… 278
Small-Studio Marketing Options ……………… 278Researching and Approaching Shops
and Galleries ……………………………………… 279
Home/Studio Sales ………………………………… 280
Studio Showrooms and Attached Galleries …… 281
Holiday Sales in Shopping Malls ………………… 281
Advertising ……………………………………… 281Local Advertising and Studio newsletters ……… 281
Color Cards ………………………………………… 281
Color Sheets and Brochures ……………………… 282
Personal Websites …………………………………… 282
Studio Tools and Equipment ………………… 282Tooling Up: The Tools to Make the Tools ……… 283
equipment Maintenance and Repair …………… 286
Clay Studio Tools: Buy, Make, Find, Improvise ………………………………… 286
Banding Wheels and Turntables ………………… 287
Bats for Throwing ………………………………… 287
Canvas as an Alternative to Bats ………………… 290
Brushes ……………………………………………… 290
Combing/Texturing/Scoring Tools ……………… 291
Cutoff String ………………………………………… 291
Cutoff Wires ………………………………………… 292
Drills for Clay ……………………………………… 292
Drill Mixer …………………………………………… 292
Feather-Combing Tool …………………………… 292
Fluting Tool ………………………………………… 293
Glaze-Mixing Whisk ……………………………… 294
Hole Punches ……………………………………… 294
Jug Finger (Potter’s Finger) ………………………… 294
Knives for Clayworking …………………………… 295
Modeling Tools ……………………………………… 295
needle Tools ………………………………………… 295
Paddles and Anvils ………………………………… 295
Patterned Paddles …………………………………… 296
Template Ribs ……………………………………… 296
Ribs …………………………………………………… 296
Rollers and Rolling Pins …………………………… 297
Saw for Clay ………………………………………… 297
Scraping and Abrading Tools ……………………… 297
Sieves for Glaze/Slip ………………………………… 298
Shrinkage Ruler ……………………………………… 298
Slip-Trailing Vessels ………………………………… 299
Sponges ……………………………………………… 300
Sponge Stamps ……………………………………… 300
Sponge Stick ………………………………………… 300
Stamps and Roulettes (Coggles) ………………… 300
Throwing Gauges …………………………………… 301
Throwing Stick ……………………………………… 302
Trimming Tools …………………………………… 303
Veneer/Slab Slicer …………………………………… 303
Wire Frame for Cutting/Blending Clay ………… 304
Studio Fixtures and Equipment ……………… 305
Clay Preparation, Processing, and Recycling 305An Inexpensive and efficient
Clay-Mixing Option ……………………………… 305
Stiffening Slurry …………………………………… 306
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Clay Mixers ……………………………………… 306Dough Mixers ……………………………………… 306
The Soldner Mixer ………………………………… 306
The Pugmill ………………………………………… 307
Pottery Wheels ………………………………… 308Kick Wheels ………………………………………… 308
Variable-Speed electric Wheels …………………… 308
Other Studio Equipment ……………………… 308Clay extruders ……………………………………… 308
Slab Rollers…………………………………………… 309
Scales for Weighing Clay and Glaze Materials …………………………………… 309
Plumbing Traps ……………………………………… 310
Spray Booths ………………………………………… 311
Studio Furniture ………………………………… 312Clay-Working Surfaces …………………………… 312
Wedging Tables ……………………………………… 312
Storage Containers ………………………………… 313
Benches, Chairs, and Stools ……………………… 314
Ware Carts …………………………………………… 314
Damp-Boxes and Drying Cabinets ……………… 315
Appendix I Glossary of Terms ………… 316
Appendix II Glossary of Ceramic Raw Materials ……………… 340
Appendix III Repairing, Fastening, and Mounting …………………… 348
Appendix IV Useful Charts and Information ………………… 353
Temperature equivalents for Orton Pyrometric Cones ………………………………… 353
Temperature Conversion ………………………… 355
Weights and Measurements ……………………… 355
Index ………………………………………… 357
The Nature of ClayWe who work and play in clay have chosen well. Clay is among the most abundant and inexpensive materials on earth. The natural processes that weather and decay igneous rocks have been generous in providing us with extensive clay deposits in a variety of forms. Clay is abundantly available almost everywhere on earth, awaiting our need, often requiring little processing.
Clay is a remarkable material for so many reasons. When one consid-ers other art media, it becomes clear. Aluminum, bronze, and iron can be welded, hot-forged, or melted and cast in molds. Glass may be cut and assembled when cold, or it may be slumped, stretched, or blown
when very hot. Wood may be sawn, carved, or assembled with glue or fasteners. Plastic is a fascinating sub-stance that can be worked in many ways, but it can be safely handled only in industrial circumstances, and the related environmental con-cerns are many. All of these materials and processes require elaborate and expensive tools and equipment. But clay is different. There is no other art or craft material that has the versatil-ity and possibility of clay. We can cast it, throw it, extrude it, model it, roll it, pinch it, press it, slump it, stamp it, pull it, and push it. We can use it to create any form or shape, tiny or monumental, organic or rectilinear, thin and fragile, or thick and heavy. It is the most malleable and forgiving
Chapter 1
Clay and Claybodies
Clay: a Studio handbook2
guard and will throw you for a loop every time. But as long as you main-tain a spirit of discovery and curios-ity, the clay will reward you fre-quently and generously.
Clay and Claybodies: Some Basic Questions AnsweredWhat Is Clay?Clay is the end result of the natural decomposition of certain igneous rocks. The major parent rocks are feldspathic—primarily granite and feldspar. In decomposition these rocks yield aluminum silicate min-erals with a sheet lattice molecular structure versus a framework lattice. With a sheet lattice structure, the molecular bond is strong in only two dimensions, and the material fractures easily into thin, flat parti-cles. The end result of the decom-position of granite and feldspar pro-duces microscopic flat clay crystals called platelets .
What Makes Clay Behave the Way It Does? What Makes It Plastic? It is the nature of the microscopic clay platelets that when they are wet, they have a tendency to stick together and to slide smoothly against one another. The most plastic clays are those with the smallest particle size. Good plasticity in clay usually requires a large frac-tion of particles less than 2 microns in size—a micron is 1⁄1000 of a milli-meter or 8⁄100,000 of an inch. That means that we could pack up to half a billion clay particles into one cubic millimeter.
It is assumed that those using this handbook will already have some fam iliarity with the properties of clay. We know how frustrating our clay can be for the beginner, as soon as he or she moves beyond the initial infatuation with the seductive malleability of the material. We know that the individual artist’s evolution in this medium is one of symbiosis and cooperation with the clay. But we also know that the clay appreciates a vigorous, commanding approach. We do not know what we can do until we find out what we cannot do, and in order to fully dis-cover the possibilities, we must take chances and experience lots of fail-ure and mistakes. Just as every ques-tion can lead to truth, every failure can lead to knowledge, as long as we examine our results and retain our proactive commitment to the clay. When in doubt, make something. Never allow frustration or failure to drive you from this medium. When I am frustrated by the medium, as I am occasionally even after almost 30 years of professional involvement in clay, I make something completely different from whatever induced my frustration. If I am working on the wheel, I make pinch pots, or I undertake a monumental coil pot. It restores peace in the mind and in the studio.
If the preceding paragraphs have a moral, it is do not ever stop experi-menting and exploring. Do not be satisfied with a single direction in your work. Do not become smug with any aspect of the medium, no matter how well you think you know it. The clay will catch you off
of art materials. It asks little of us, but with commitment and respect on our part, it rewards us generously.
When subjected to a simple firing process, clay is transformed to hard, impermeable stone, and what was once so malleable and impermanent might now remain stable and unchanged for millennia. As if the mere workability and fired perma-nence of clay were not enough, we can also apply an unending variety of mineral coatings that fuse into glassy glaze surfaces of unlimited color and texture. When all viewed together, it does seem an embarrass-ment of riches.
In painting and drawing, artists talk about “the terror of the blank page.” When one is faced with a blank canvas or page, the first mark divides the frame into areas of posi-tive and negative space. Major and often irreversible compositional decisions take place in those first few gestures. This is not the case with clay, and we are released from any such irreversible finality. When you place a lump of clay in anyone’s hand, the response is automatic. The hand closes and squeezes the clay, and a unique sculptural form is produced, subtly different from any other before. Few of us stop at that point, for the clay encourages us to apply different forces, responding to every push and pull. Until the clay begins to stiffen, there are no rules, and no externally imposed finality. We can undo what we have done, and we can immediately return any form or shape to a simple lump and begin anew. What other art media allows this extraordinary leeway?
3Clay and ClaybodieS
Why Do Different Clays Behave Differently? Different clays behave differently depending on the range and distri-bution of particle size and the presence of nonclay contaminants, primarily organic materials and nonplastic minerals.
How Does Particle Size Affect Drying and Firing?The size and shape of clay particles help determine plasticity, but they also have profound effects in dry ing and firing the clay. The evaporation of the water layer that exists be tween each particle in the plastic state is what causes drying shrinkage. The finer the particle size, the more water layers are present, and there fore the greater the water content, and the greater the drying shrinkage. But at the same time, the finer the particle size, the more contact points between particles in the dry state, which gives greater dry strength in greenware and more bonding surfaces in the early stages of the firing. The ideal condition, therefore, is to have a mixture of sizes of clay particles. This creates as much contact surface as possible between particles, giving good plasticity, dry strength, and bisque strength, and yet it minimizes the water content and resulting shrinkage.
What Are Flocculation and Deflocculation? These terms refer to the addition of particular materials that change the electrical charge on mineral parti-cles in water suspension. Floc cu la-tion involves the addition of a min-
ute amount (1⁄4 to 1⁄2 of 1%) of soluble metallic salts such as epsom salts (magnesium sulfate), which render the water slightly acidic, giv-ing opposite electrical charges to the particles, causing them to attract, and making a stickier, more plastic mass. Defloc cu lation involves the addition of a minute amount (1⁄4 to 1⁄2 of 1%) of soluble alkaline material (soda ash, sodium silicate, calgon), which gives same electrical charges to the particles, causing them to repel. This would be a great disadvantage in a claybody, but a distinct advantage in some clay slips, where it greatly benefits suspension and flowing properties. It is espe-cially important in slip-casting bod-ies, where deflocculation makes a smoothly flowing liquid with far lower water content, which means lower shrinkage in drying.
Why is Aged Clay More Workable?As clay ages, organic activity increases. The byproducts of this organic activity are acidic, which flocculates the clay, making the par-ticles stick together more effectively, improving the plastic working qual-ities. Some potters add vinegar to clay to speed up this process, but the acetic acid simply evaporates quickly. When plasticity is a real concern, as with pure white porcelain bodies, it is an excellent idea to add epsom salts (1⁄2 of 1% of dry materials weight) to flocculate the clay slightly, counteracting any natural alkalinity in the kaolins. Another issue with aged clay is the thorough wetting of the particles. This is a quicker pro-
cess than the development of organic activity in the clay, and usu-ally happens over a period of a few weeks. It is a very beneficial process and can be greatly speeded up by soaking the particles thor-oughly to begin with, by mixing the clay a little wet, or by mixing it as a slurry and stiffening it to plastic consistency.
What Are the Basic Structural Components of Clay?All clay contains a combination of fluxes or melting agents, glass-
formers, and refractories or sta-bilizers. The fluxes or melting agents lower the maturing tem-perature and assist in formation of glass, the essential binder in all ceramics. The primary fluxes appear-ing in natural clays are feldspars and iron. The higher concentrations of iron in common or local clays help them fire hard and durable even at low-fire temperatures.
Glass-formers react with fluxes to form glass. The primary glass- former is silica, but the pure material melts at a very high temperature. The fluxes act on the silica, bringing the melting temperature down to a usable range. The proportion of flux and glass-former must be properly balanced—the addition of too much flux gives a weak glass. Too much silica leaves excess free silica in fired wares, which at high-fire tempera-tures can lead to the formation of cristobalite, a crystalline form of silica, which drastically increases thermal expansion and lowers ther-mal shock resistance.
Clay: a Studio handbook4
when stoneware or porcelain is overfired, the glassy phase begins to dissolve the physical structure, caus-ing slumping and warping.
Classification of Clays
Primary and Secondary ClaysAll clays are classified as primary
(also referred to as residual) or sec-
ondary (also referred to as sedi-mentary or deposited).
Primary clays are those that remain at the physical site where the parent rock decomposed, and include the purest kaolins or china clays. They usually fire pure white in color, but due to coarser particle size they tend to be less plastic. There is some variation in plasticity between different primary kaolins, because fineness of particles is deter-mined by the degree of subsurface metamorphic activity, by acids seep-ing down from the surface, and by heating and cooling. The ideal for-mula of pure kaolin is Al2O32SiO22H2O, but this would give an extremely high-tem-perature kaolin. In reality, most kaolins are contaminated with un-broken-down feldspar and free sil-ica. This decreases plasticity even more, but it also provides flux to lower the maturing temperature.
Secondary clays are those that have been transported away from the parent rock by wind, water, or glacial activity. This includes all the rest of the naturally occurring clays—ball clays, earthenware clays, stoneware clays, fire clays, bentonite, and slip clays. So-called secondary or deposited kaolins (Georgia and
even minimal sintering has occurred at dull red heat, the clay can be con-sidered fired, but if firing were ceased at that point the result would be a very weak mass.
As temperature increases towards the bisque-firing range, the fluxes and glass-formers begin to interact, forming the beginnings of a glassy-
phase, which strengthens the sin-tered connections between the refractory particles, but without filling the air spaces in the body.
As temperature continues to climb towards the high-fire range, the fluxes and glass-formers form a more complete glassy-phase, which gradually fills in the spaces between sintered particles. Vitrification is sin-tering in the presence of a fully developed glass-phase, where the air spaces between particles are almost completely filled in. The filling of air spaces accounts for firing shrink-age in vitrified wares, and inversely, the lack of firing shrinkage in non-vitrified wares. In all fired wares, the sintered connection between refrac-tory particles gives basic physical structure, which prevents thermo-plastic deformation (warping) at firing temperatures, whereas in vit-rified wares the glassy-phase gives density, impermeability, and strength. In earthenware clays and claybodies, there is usually too much flux pres-ent for simple sintering to provide structure above low-fire tempera-tures. If the firing temperature of a true earthenware clay exceeds about 2000 degrees F, the fluxes will usu-ally overpower the sintered connec-tions, and the body begins to deform and bloat as the constituent materi-als flow and volatilize. Similarly,
The refractories or stabilizers pro-vide the physical matrix of clay, the particles that the fluxes and glass-formers bind together. Increasing refractory content raises the matur-ing temperature and reduces forma-tion of glass. The primary refractory in both clays and glazes is alumina, but pure alumina is rarely added. To increase refractoriness of a claybody, we normally add a high-alumina clay like fireclay or kaolin. Adequate refractory content, combined with an appropriate proportion of silica and flux, encourages the formation of mullite (aluminum silicate) crys-tals in high-fire bodies, which cre-ates an interlocking “felted” matrix, giving a very strong body resistant to thermoplastic deformation, and a strong clay-glaze interface.
What Happens When Clay Is Fired? What Are Sintering and Vitrification?The chemical and physical changes that occur in clay during firing are discussed in detail in the chapter on kilns and firing, but before discuss-ing varieties of clay and claybodies, we must understand the phenomena known as sintering and vitrifica-
tion. Of all the physical/chemical changes that accompany the firing of clay, these two are the most important. Fired earthenware or bisque-fired stone ware and porce-lain is sintered but not vitrified. When clay is fired to red heat, it becomes sintered, as increasing heat causes the particles to stick together even before the fluxes and glass-formers begin to interact. Once the clay is sintered, it can no longer be slaked down and reused. As soon as
5Clay and ClaybodieS
Florida kaolins, like Tile 6) have been transported from the parent rock, but only a short distance. They still remain very pure, but tend to be much more plastic than primary kaolins (like Grolleg).
Kaolins or china clays include the generally pure white primary kaolins and the slightly less white but more plastic secondary kaolins, both mentioned previously. These clays are a major component of most high-fire porcelain claybodies and are frequently used in stoneware bodies to lighten the fired color.
Ball clays have been transported by wind or water and deposited in swampy areas, where organic acids have broken down the particles to ultrafine size and have introduced organic contaminants. They are extremely plastic, but if used alone the extreme shrinkage causes seri-ous cracking. In combination with other clays or nonplastics they often account for 15 to 25% of a claybody and occasionally as much as 50%. Ball clays are similar to kaolins after firing, but most contain consider-able iron contamination, and although they fire white at low temperatures, in high fire they tend to yellow in oxidation and gray in reduction.
Earthenware clay is the com-mon surface clay found throughout the world. It contains high amounts of flux contaminants, primarily iron, which gives the fired wares both their strength and the characteristic red terra cotta color. Well below 2000 degrees F, iron can begin forming a glassy-phase in contact with silica, which gives great strength to most earthenware bod-
ies. True earthenware cannot fully vitrify, as the high percentage of powerful fluxes will usually cause deformation and bloating before vitrification can occur.
Stoneware clays are simply kaolins that have been transported farther from parent rock, introduc-ing more impurities, finer particle size, and higher flux content (pri-marily calcium, feldspar, and iron), which lowers maturing temperature enough to bring on full vitrification at standard high-fire temperatures. Fired color varies from gray to buff.
Fireclays are similar to stoneware clays, but contain less flux, especially calcium and feldspar. When fired by themselves, they are not fully vitrified even at standard high-fire temperatures. Some fireclays have very fine particles and are there - fore very plastic, whereas others are coarse and granular, giving greater thermal shock resistance but poor plasticity. The former are preferable in throwing bodies, and the latter are used in sculpture and raku bod-ies and in kiln furniture.
Bentonite has the finest particle size of any natural clay and is formed from decomposition of the airborne ash from volcanic eruptions. It is contains more silica and less alumina than kaolins, with varying traces of iron. It is very useful as a plasticizer in claybodies or as a suspension agent in slips or glazes, but must be used in quantities no more than 3% of the dry batch weight. Greater amounts will almost certainly cause cracking in drying.
Slip clays are naturally occurring clays that contain enough iron that at high-fire temperatures they will
melt to form a glaze with no other additives. Some common slip clays are Albany slip, Alberta slip, Bar nard, and Blackbird. The classic brown/ black liner glaze found in Early American jugs, crocks, and churns was a slip-clay glaze. Albany slip has traditionally been the most popular, but it is no longer available. Alberta slip is the current substitute.
ClaybodiesPure natural clays almost always have some shortcomings. Clay-bodies are mixtures of clay and other materials designed to accom-plish specific goals like plasticity in throwing, stability in large-scale work, thermal shock resistance, dry and fired strength, or vitrification and density. When designing a clay-body, always begin with clays whose natural qualities are closest to the desired goals. This usually involves a combination of clays selected for the qualities listed above, with addi-tions of nonclay materials such as fluxes, glass-formers, refractories, and tempering materials (grog, sand, etc.). The additions of nonclay materials usually do not total more than 50% of the claybody. Keep in mind that although small particles give plasticity, they also give high shrinkage. The best claybodies usually contain a broad spectrum of particle sizes.
Claybody Components
Accessory Fluxes The fluxes contained in naturally mined clays are often inadequate for our needs, so we frequently add accessory fluxes. In high-fire bodies
Clay: a Studio handbook6
clay plus sand or grog to give struc-ture and often with fireclay or stoneware clay to increase firing temperature and reduce the chances of deformation and bloating. Natural earthenware clays tend to be very plastic due to a broad distribution of particle size, and therefore rarely require the addition of ball clay. However, modern low-fire bodies are often white, excluding the use of natural earthenware clays, and they are generally referred to as white-ware bodies rather than earthen-ware. The most popular whiteware body, composed of 50-50 ball clay and talc, is actually very similar to one used by the Egyptians 5000 years ago. Talc is a sheet-lattice mag-nesium silicate with properties simi-lar to clay, but it is highly thermal shock-resistant, even without any sand or grog.
It is important to point out that a low-temperature firing process does not necessarily mean an earthen-ware or whiteware clay—the raku and bonfire processes often use highly refractory stoneware bodies that are simply underfired at low-fire temperatures and are therefore very porous and open, giving high thermal shock resistance. Low firing is especially appropriate for large sculptural work, as there is little or no shrinkage in low firing, and common problems with cracking and warpage are minimized.
Vitreous claybodies, including porcelain and stoneware, are those that become truly vitrified at mid range and high-fire tempera-tures (cone 4 and above), with a fully developed glassy-phase and
tain only kaolins as the primary clay component. Small additions of ben-tonite, macaloid, or Veegum T will increase plasticity and workability of the clay on the wheel and in hand-building. See Appendix II for addi-tional information on these materi-als.
Colorants Our concern here is with colorants found or used in common claybod-ies, rather than specialized colored clays. The most common colorant in clay is iron. As mentioned, iron becomes a powerful flux in high-fired or reduction-fired wares, and in such situations any considerable iron content must be considered in total flux content. As a general rule, when a darker color is desired it is better to add darker clay, such as Redart or a slip clay. Most slip clays contain both iron and manganese and will darken the claybody appre-ciably with less fluxing than pure iron oxide. Very small additions of powerful colorants like cobalt will significantly modify color. Five per-cent granular manganese dioxide will give speckles in oxidation-fired wares at any temperature. Five per-cent granular rutile or granular ilmenite will increase iron speckles in reduction high-fired wares.
Common Types of ClaybodiesEarthenware claybodies remain porous at low-fire, and yet at higher temperatures will likely deform and bloat before vitrification. Traditional earthenware bodies are usually red or buff, a blend of iron-rich surface
the primary flux is feldspar, which provides sodium, potassium, cal-cium, and/or lithium. In low-fire bodies, feldspars often still play an important fluxing role, usually boosted by a calcium-borate frit such as Ferro 3134, which has a composition very similar to Gerstley borate, but is insoluble. See the chap ter on glazes and glazing for a more thorough discussion of fluxes.
RefractoriesAs mentioned earlier, different clays have varying degrees of refractori-ness. In order to control the matur-ing temperature of a claybody we regulate the balance of fluxes and clays and the types of clays. Kaolins and fireclays are the most refractory clays.
Tempering Materials or Fillers These are the gritty granular mate-rials like sand and grog that open up the claybody, giving improved form-ing strength, less shrinkage, more even drying, and greater thermal shock resistance. In high-fired wares, many people prefer to use grog, as silica sand will fuse partially into the glassy phase, giving greater shrink-age than grog and possibly contrib-uting to free silica and resulting cristobalite formation in high-fired wares, decreasing thermal shock resistance.
PlasticizersMany claybodies benefit from the addition of accessory plasticizers. Porcelain bodies often need these additives, especially pure white- firing porcelains, which usually con-
7Clay and ClaybodieS
little tendency to deform or bloat. In a well-designed stoneware or porcelain body for functional wares, pushed towards the upper end of its firing range (usually cone 11), the glassy-phase is so well developed that the sintered network barely retains its structural stability, and otherwise the body is quite pyro-plastic. This gives the great density and strength and low absorption needed for functional wares, but the pyroplastic flexibility of such bodies makes them completely inappropri-ate for refractory pieces (bricks, kiln furniture) or large sculptural forms. Keep in mind that most cone 10 claybodies are usually designed for cones 8 to 11, and when fired to a high cone 10 or to cone 11, they are at their upper limit of structural integrity. If overfired even a small amount, they can warp, slump, or bloat badly.
Porcelain claybodies include gritless high-fire bodies that fire close to pure white. Pure primary kaolins rarely perform well in any forming method. Additives are needed to increase plasticity, lower the firing temperature, and encour-age glassy-phase and vitrification. Up to 25% of ball clay will increase plasticity, but will also give a slight yellow cast in oxidation or gray in reduction. The whitest porcelains usually use up to 50% kaolin as the primary clay component, often with the addition of bentonite, macaloid, or Veegum T to increase plasticity. Up to 25% feldspar lowers maturing temperature to reasonable high-fire levels, and up to 25% flint provides a more complete glassy-phase and
denser vitrification. Pure kaolins are sometimes slightly alkaline, and therefore porcelain bodies should be flocculated with epsom salts.
Under certain circumstances, fired porcelain can be translucent. True bone china (traditional translucent porcelain) is so-titled due to the addition of bone ash (calcium phos-phate). Phosphorus is technically a glass-former, but combined in correct proportions with silica and calcium it acts as a powerful flux, contributing to a very active glassy-phase. This creates translucence in the fired claybody, but it also low-ers the maturing temperature to around cone 6. With what basically amounts to an overdeveloped glassy-phase, bone china bodies are very prone to warpage unless fired on flat shelves with no hot spots in the fir-ing. Actually, any reasonably well-fluxed cone 10 porcelain thrown very thin will give some translu-cence without the disadvantages of bone china.
Stoneware claybodies use natural stoneware clay and/or fireclay as a base, with additions of ball clay, kao-lin, flint, fluxes, and/or grog or sand. Whiteness is rarely an issue, so the materials are selected for desirable performance in forming and firing, regardless of color. Natural stone-ware clays and plastic fireclays with the addition of ball clay produce an extremely plastic throwing body. Ad dition of sand or grog gives tooth or structure in the plastic state and reduces slumping during throwing or handbuilding, allows thinner, taller wares with greater horizontal extension, and reduces drying shrink-
age. Depending on the refractoriness of the clays, feldspar and free silica are often added to control maturing temperature and glassy-phase.
Refractory claybodies are those used for making firebrick and kiln furniture. They differ widely depending on application. For low-heat use, almost any claybody will work well. For all other refractory applications, earthenware clay is inappropriate, and free silica, silica sand, and all fluxes (especially iron) should be minimized. In repeated or prolonged high firing, free silica converts to cristobalite (crystalline silica), severely increasing the ther-mal expansion on the hot face, resulting in spalling (peeling away of surface layers). Excess flux will encourage an active glassy-phase, which is an advantage for functional wares, but for refractory pieces it fills the pores and dissolves the sin-tered structure, reducing thermal shock resistance and encouraging pyroplastic deformation. The natural flux component of most ball clays, stoneware clays, and fireclays is usu-ally adequate to form a sufficient glassy-phase to make a very strong sintered matrix, while also absorb-ing free silica, which reduces cristo-balite formation. Also, at least a partial glassy-phase is necessary to encourage formation of mullite crystals, which gives critically important structure highly resistant to high-temperature pyroplastic deformation. For hot-face firebrick an appropriate mix would be 80% fine grog and 20% plastic fireclay or low-iron ball clay, which should be mixed and molded while quite
Clay: a Studio handbook8
Stiffer plastic clay stands up better and absorbs water slower, but it is more difficult to work and can lead to muscle/joint problems. Instead, 5 to 25% additions of tempering ma -terials or fillers, such as grog or sand, will improve physical structure dur-ing wet-working, but will also increase water absorption during throwing.
Remember that plasticity is sig-nificantly increased with aging—whenever possible prepare your clay at least a month ahead of time and stockpile it. Many serious studio potters always stay at least six months ahead of themselves on clay prepa-ration in order to ensure sufficiently aged clay. If you have ever had really well-aged clay, you know what a joy it is to work with. It is resilient and responsive and absorbs less water.
Handbuilding requires a clay-body with most of the same quali-ties as a throwing body, but plasticity is not quite as much of an issue. Structure is often far more impor-tant, and large additions of grog are common. Water absorption is not a problem, because water is rarely added. Not only does the added filler increase structural integrity during forming, but it also drasti-cally reduces drying and firing shrinkage and their associated flaws and faults. For the most demanding forming methods, such as large soft-slab or stiff-slab construction, the addition of chopped nylon fiber (one loose handful per 100 lbs. of dry material) will drastically increase both wet and dry strength. The fiber, as it comes from the bag, should be broken up by hand and thoroughly mixed into the plastic clay. Do not
amazing strength and can be han-dled in such a way that would immediately tear normal clay slabs. The clay shows extraordinary dry strength, and very large forms may be built extremely thin and light. For more information on paper-clay, consult Rosette Gault’s book on the subject.
Variations in Claybodies for Different Applications and Firing ProcessesAny claybody that gives certain favorable qualities may be modified for different applications or meth-ods, such as wheel work, handbuild-ing, or slip casting.
Wheel work, of course, requires a primary emphasis on plasticity, and the three critical considerations are clay-particle size, percentage of clay in the body, and ionic charge of the particles. Adding up to 25% (of original dry batch weight) ball clay and up to 3% bentonite will im prove plasticity, as will plasticizers like macaloid or Veegum T. Most plastic clays (ball clay, stoneware clay, plastic fireclay) tend to be slightly acidic, which gives the cor-rect ionic charge for a claybody. Kaolins are often slightly alkaline, and this will have a deflocculating effect, which if not counteracted will give a very “short” claybody. Many are the “short” porcelain bod-ies that have been discarded as unworkable, when a suitable floc-culant (epsom salts, 1⁄2 of 1% of dry batch weight) would have solved the problem.
Another issue in wheel work is structure, which is the result of water content and tempering ma terial.
wet and left untouched until hard-ened. Such a mix would not be appropriate for shelves and furni-ture, which must be hard and dense with no deformation under load. For this a slightly more developed glassy-phase is needed, requiring a higher clay content. A 50-50 mix of fireclay and a mullite grog like kya-nite or cordierite will form a very complete “felted mass” of mullite crystals with an adequate glassy-phase. Cone 10 furniture should be fired to cone 11 or 12 with a long soaking to maximize the glassy-phase and encourage the formation and interlacing of mullite crystals.
Paper clay is an exciting new arrival on the studio ceramics scene. This unique claybody, actively pro-moted by Seattle clay artist Rosette Gault, consists of any claybody with a hefty portion of paper pulp mixed in. For making small quantities, a kitchen blender, food processor, or jiffy mixer may be used to make pulp from toilet paper or newspaper. The drained pulp is blended into a thick slurry of the desired claybody, usually 1⁄3 (by volume) drained pulp to 2⁄3 slurry. For small test batches the blender or food processor works well, but for larger amounts a large impeller-mixer on a 1⁄2″ drill is rec-ommended. The resulting slurry is then stiffened on plaster bats to the desired working consistency.
The reinforcing effect of the paper pulp gives paper-clay extraordinary working properties. Paper-clay components can be joined when bone dry, using paper-clay slurry, and pieces of drastically different moisture content can even be attached. Paper-clay slabs have
9Clay and ClaybodieS
attempt to use the chopped fiberglass
available for use in concrete mixes.
This material is carcinogenic and works itself into the skin and can result in severe inflammation.
Slip Casting Slips for casting are discussed in greater detail elsewhere in this text. Any claybody may be adapted for slip casting, but any grit should be omitted, because it would settle out in the mold. All casting slips must be deflocculated, which improves flowing character-istics and reduces water content, thereby reducing water absorption by the mold and reducing shrinkage and resulting problems in drying.
Raku and bonfire claybodies
require low thermal expansion and high thermal shock resistance in order to survive these demanding firing processes. Since prehistory we have made such claybodies by open-ing them up with fillers so that they remain porous at the maturing tem-perature of the firing. Filler materi-als include sand, grog, crushed sea shells, or volcanic ash. A raku body is usually just a porous, underfired stoneware body with as much as 30% tempering materials. Actually, most claybodies will stand up to the raku process as long as they are not quenched in water. A bonfire body is often even coarser, containing 50% or more filler materials.
Salt-Soda Firing Most mid-range and high-fire claybodies work well in salt and soda firing processes, but in general it is best to avoid high-iron bodies, unless a very dark finish is desired. Salt and soda tend to supercharge the action of iron as a flux, and even a modest iron con-tent can produce a very dark surface.
Kaolins often promote beautiful flashing in salt and soda, and this effect is often accomplished with a kaolin slip rather than by using a porcelain claybody.
Because the sodium from vaporiz-ing salt or soda interacts with silica in the clay, the level of silica content significantly affects surface quality. It is not a good idea to load up a clay-body with excessive silica, risking cristobalite formation. However, if you are using grit in your claybody anyway, you might consider using sand instead of grog. Sand in the clay-body tends to promote orange-peel effect, as the individual sand granules interact with the sodium vapors.
Wood Firing As in salt and soda firing, high iron content can result in a very dark finish in wood firing. However, if you wish to maximize subtle fly ash effects, a fair iron con-tent in the clay will interact with this fly ash, giving warm flashing and variegated surface effects. Japanese Bizen wares are famous for these effects, and Bizen clay charac-teristically contains 5 to 7% iron. This iron content is usually achieved by the addition of a high-iron stone-ware clay, such as lizella or carbondale. See the materials glossary in Appendix II for more information on these clays.
If warm red-brown flashing effects are desired, a kaolin body or certain kaolin slips tend to respond very well. However, accessory plasticizers will seal the surface completely, lim-iting or preventing such flashing.
Porcelain bodies may flash to beautiful warm orange-brown tones in wood-firing. Some kaolins are more prone to warm colors. Avery
kaolin was legendary, but is no longer being mined. It is reported that Helmer kaolin is a good substitute. Again, an appropriate kaolin slip will often produce similar flashing.
Analysis of Clay PropertiesEven with only a very small sample mixed by hand there are a number of simple tests that can tell you a great deal about any claybody.
Water of Plasticity This is the amount of water required to bring a particular dry claybody to its most plastic state. If the clay is composed of fairly coarse particles, like a pure primary kaolin or a non-plastic fireclay, then it will take less water to wet all the particles and achieve its most plastic state. On the other hand, if the clay has extremely fine particles, like most ball clays, it will take a great deal more water to wet the surfaces of all the particles. Thus, a clay with a higher water of plasticity percentage will necessarily be more plastic, but it will also shrink more in drying. To measure the water of plasticity place exactly 100 grams of dry powdered clay on a flat plate (a glazed tile works great) and weigh it again, recording the combined weight of clay and plate. Begin dispensing drops of water onto the clay, mixing it in thor-oughly with a spatula or knife, until you reach the desired plasticity. Make sure all the dry clay is worked up into the mass, make sure all clay is removed from the mixing tools and added to the mass, weigh it again, and subtract the weight of the plate. The number of grams over
Clay: a Studio handbook10
no glassy-phase is present, may have extremely low shrinkage. Even in a claybody with a very well-devel-oped glassy-phase, firing shrinkage and the chances of warpage may be significantly reduced by the addi-tion of grog. In dealing with struc-ture during initial forming, grog and sand behave similarly, but in high-firing sand contributes to the glassy-phase and to free silica content, whereas grog remains relatively inert.
To measure shrinkage roll out a strip of plastic clay, cut one edge to a straight line, and mark the edge to a specific measure, usually at least four inches. When the slab is bone-dry take a second measurement. Measure again after bisque-firing, and take a final measurement after the glaze-fire. From these you can figure the percentage shrinkage in each step, as well as the cumulative shrinkage overall.
Mixing and Recycling ClayThe specifics of clay mixing equipment are discussed in Chapter 10 “Studio Design, Setup, and Operation.”
Any studio artist or academic stu-dio must face the decision whether to mix one’s own clay from dry materials or to purchase commer-cially available moist-bagged clay. The objective and direction of the individual artist or studio is the de ciding factor. If you rely heavily on experimentation and change, en tailing frequent alteration of your claybodies, then it would only make sense to have the necessary
body, the greater the drying shrink-age. As you can see, the “water of plasticity” figure is an excellent indi-cator of both plasticity and drying shrinkage. One can normally expect drying shrinkage of 4 to 10%. As mentioned previously, the clay mass shrinks as the water layer disappears from between the clay particles. The more particles, the more shrinkage. Having a broad distribution of par-ticle sizes reduces shrinkage to some extent, and the presence of sand and/or grog particles, being much larger and more irregular than clay particles, tends to keep the body open, reducing drying shrinkage. Grog is generally high-fired, essen-tially preshrunk, and therefore reduces firing shrinkage as well.
Usually shrinkage between bone-dry and bisque-fired is minimal, rarely exceeding 1⁄2%. Shrinkage in low-fire glaze-firing is usually ex tremely low, except when calcium-borate body fluxes have been added to give a denser fired product. Also, terracotta bodies fired in a reduction low-fire atmosphere can experience shrinkage almost as great as in a high-fired body.
Shrinkage is again considerable during high-firing, as the fluxes and glass-formers fuse into the glassy-phase, filling the spaces between the refractory particles. The degree of shrinkage depends on the flux content and the size and quantity of refractory particles. Of all high-fire bodies the high-kaolin porcelains generally have the great-est high-firing shrinkage, as much as 8%, whereas refractory bodies for firebrick or kiln repair, where almost
100 is the percentage water of plas-ticity. It should be noted that although water of plasticity is a use-ful measure for comparing claybod-ies, it is not an absolute measure of the amount of water needed for ideal plasticity, because initial wet-ting does not result in complete saturation of the clay particles. Thus, a clay that seems to be mixed to its proper consistency might be consid-erably stiffer several days or weeks later when the water has completely penetrated the clay particles.
Actual Plasticity For a useful test of the actual working plasticity of the clay, form a thin coil of the clay and bend it in a loop. If the coil cracks in this test, the clay is consid-ered to have low plasticity, but remember that both aging and thor-ough wedging will improve the plasticity of any claybody.
Porosity, referred to in the indus-try as absorption, is a very important consideration in many claybodies and is easily measured. Carefully weigh a fired but unglazed sample of the clay or claybody (fired to its intended maturing temperature). Place the sample in a pan of water, bring it to a boil, allow to cool over-night, blot off the surface water, and weigh it again. The percentage increase in weight represents the porosity or absorption of the clay or claybody. Earthenware usually has 5 to 14% absorption, stoneware 2 to 6%, and porcelain 1 to 3%.
Shrinkage occurs in all claybod-ies as they are dried and fired. Considerable shrinkage occurs as water evaporates before the bone-dry stage. The more plastic the clay
11Clay and ClaybodieS
facilities and equipment to mix your own clay. If you are operating an academic studio where students are trained to be professional ceramic artists or educators, then you must teach them how to mix clay from raw materials. They need that skill, and the process will also teach them much about raw materials.
On the other hand, if you are a studio potter with a well-established direction in your work, a commer-cially produced claybody may suit you very well and will release you from the time spent mixing your own clay, the expense of elaborate mixing equipment, and the neces-sary space to accommodate it. When purchased in large quantities, the price of commercially prepared claybodies can be very low. But large amounts of recycle are an inevitable byproduct of almost any ceramic studio, and if you do not recycle this material in a responsible manner, you will be wasting a great deal of money over time.
There is a broad range of strategies for mixing clay, from the most low-tech labor-intensive, to the more high-tech involving sophisticated clay-mixing equipment. All have their merits, and surprisingly, some of the low-tech approaches may be the most suitable for many studio potters. A primary concern is the degree to which the aging of the clay is important to the individual artist or studio. If well-aged clay is very important, then the clay should be mixed or purchased ahead of time, which requires more storage space. Another option is to mix the clay as a slurry and stiffen it to plastic con-
sistency, which wets the particles far more thoroughly and accelerates the onset of organic activity, giving a very plastic claybody much more quickly.
Whichever mixing method you choose, it makes sense to adapt your recipe for greatest convenience. Keep in mind that most materials come in 50-lb. or 100-lb. bags. Whenever possible, adjust all the separate amounts in your recipe proportionally, so that as many materials as possible are in 50-lb. or 100-lb. quantities. With small batches, this is obviously impossible, but with large batches this will dras-tically simplify the process of weigh-ing out raw materials.
Remember that the kind of preci-sion essential in glaze mixing is unnecessary in making clay. In fact, in several past circumstances I have measured out claybody materials by a convenient volume (cup, scoop, etc.) rather than by weight. The results did not differ significantly from those obtained when measur-ing by weight. For weighing clay-making materials, a platform scale or shipping scale is ideal.
The Low-Tech ApproachTraditionally, without the benefit of sophisticated machinery, clay was often mixed as a slurry, and this is still a very viable option. In the sim-plest approach, water and dry mate-rials are placed in a large wood, brick, or stone trough, and the pot-ter simply works the mixture with hands, feet, or a garden hoe to blend the slurry. Appalachian potters used a large wooden drum with a slow-
turning mixer shaft attached to an overhead sweep pulled by a horse or mule. Modern potters can accom-plish the same thing more efficiently using an inexpensive drill-mounted mixer or motor-driven blunger forblending even large amounts of slurry in plastic barrels. See Clay Mixing in Chapter 10, “Studio Design, Setup, and Operation,” for more information on this process.
Clay Mixers and Pug MillsIn many studio situations it is essen-tial to be able to mix custom clay-bodies that are ready to use. And in all studios, you are faced with the inevitable dilemma of recycling scrap clay, and with the right machine, you can easily deal with scrap clay and occasionally mix custom clay-bodies when they are needed.
All clay mixers are potentially dangerous machines if used improp-erly. Whether in an academic, com-mercial, or home studio, all clay-mixing machines should be fitted with lockable switches to prevent the possibility of them being turned on by unauthorized persons.
In general terms, a plastic clay-body is about 25% water, which translates to 33% of the dry materi-als weight. For all processes that produce a plastic claybody, it is use-ful to use that number as a starting point. If you choose to mix 300 lbs. of plastic clay, it will require approx-imately 75 lbs. of water. It is gener-ally wise to keep some dry-mix claybody on hand, in case you add all your materials or all your water and end up with a claybody that is too soft.
Clay: a Studio handbook12
a quantity of your claybody in a cardboard drum (as described in Chapter 10), and with the mixer running, begin dispensing dry-mix into the hopper from one end to the other. Keep it up until the clay approaches the appropriate plastic stage, and then begin checking it as previously described.
With any clay mixer in a coopera-tive or academic studio, there is often some disagreement as to how much the hopper should be cleaned each time. Unless the machine is frequently used for mixing porce-lain or whiteware, it is really a waste of time to clean it thoroughly. In most situations, it is expected that each user will scrape clean the hop-per walls and mixing blades, so that they are free of any large chunks of clay that when dry could not be easily absorbed into the next batch. A thorough cleaning is needed only in preparation for mixing porcelain or whiteware bodies. Otherwise, whatever residue is left from the previous batch will just blend into the next claybody with no ill effects.
The Soldner MixerNamed after its designer, clay guru Paul Soldner, this is the most popu-lar heavy-duty mixer for the serious ceramic artist or academic studio. It consists of a rotating reinforced concrete tub three feet in diameter and two feet deep, with stationary interior bars that mix and blend the clay quickly and effectively. This unit easily mixes batches of 250 pounds dry-weight, up to three batches per hour. The Soldner mixer works best when the water is added first. Close the lid, start the ma chine,
evenly from one end of the mixer to the other, preferably with the mixer running. If you add too much water at once, slurry will be created, which will lubricate the clay excessively, and effective mixing will cease until the slurry is absorbed into the clay, which can take a long time. It is far better to be patient in adding water to prevent this from happening. Most hopper-type mixers tend to leave patches of poorly mixed clay against the outside walls, especially if the blades are worn. It is wise to have a straight hoe or some large heavy-duty spatulas on hand. During the latter stages of mixing, stop the mixer several times, preferably with a double shutoff switch setup, and use the straight hoe or spatulas to work down deep along the wall, bringing any unmixed material up into the mix.
When the clay begins to resemble plastic consistency, periodically shut off the mixer, take out a small ball of clay, and toss it back and forth between your hands. If it sticks, it is still too wet. When you can toss the ball freely without it sticking, the clay is still a bit on the soft side, but remember that the particles will continue to absorb water for a period of time, and the clay will stiffen up a bit. With this in mind, always consider the clay ready when it is still a bit softer than you like it. Ultimately, only trial and error will show you exactly how to mix your clay. Keep in mind that clay that is too stiff is a great deal more trouble to deal with than clay that is too soft.
When recycling slaked scrap clay with a hopper-type mixer, fill the hopper halfway with scrap. Dry-mix
There are three common types of clay processors on the market: the horizontal-shaft hopper mixer (including the classic dough mixer), the vertical-axis rotating-drum Soldner mixer, and the pug mill.
The Hopper Mixer or “Dough Mixer”This type is simply an adaptation of the traditional commercial dough mixer and consists of an open or lidded hopper with a horizontal mixing shaft mounted with agitator blades. These mixers are effective, but they can be extremely danger-ous. Any mixers of this type should be equipped with a key-lock shutoff switch. The current commercially available mixers based on this design have a perforated lid with a con-nected shutoff switch that shuts down the machine whenever the lid is lifted. The perforated lid allows the addition of water or dry materials while the mixer is running.
Before handling dry materials and mixing clay, put on a proper dust mask and turn on appropriate exhaust fans.
In a hopper-type mixer, it is nor-mal to add your dry materials first, unless you are reprocessing scrap. Pour in the finest clays first, then the coarser clays, nonplastic powders, and finally any sand or grog. Pour each dry material evenly from one end of the mixer to the other, and turn on the motor to dry-mix the materials. If the mixer has a reverse switch, reverse the direction of the blades several times to ensure com-plete mixing. When dry-mixing is complete, begin adding water with a hose or bucket. Dispense the water
13Clay and ClaybodieS
and add the dry materials through the grillwork in the lid. Add the fin-est clays first, then the coarser clays, the powdered nonplastics, and finally any sand or grog. As explained previously, stop mixing when the clay is slightly softer than the desired working condition.
As is the case with the hopper mixer, when recycling slaked scrap, fill the mixer about halfway, turn it on and let it run for a while, and then begin adding dry-mixed claybody until you achieve the desired consistency.
Whether mixing from dry ma terials or from recycled slurry, be careful not to add too much dry material at one time. If you do, the mixer will begin to bog down, and the thermal overload switch on the motor may shut it down altogether. If this happens, dribble a little water over the clay, wait a few minutes and then the machine will restart. As it runs, continue to dribble water very slowly over the clay. Be cautious of adding too much water all at once, because this will form a slurry that lubricates the barrel, allowing it to
turn freely, while all the clay remains stuck to the mixing bars in a sta-tionary lump. In such a case, you can just wait for the excess slurry to be absorbed or dribble a little dry clay down the inside wall of the mixer drum to absorb the slurry. When the mixer gets bogged down with over-stiff clay, it may take some patience to get the clay to absorb sufficient water to return to normal operation.
The PugmillThe simplest common analogy for a pugmill is an oversized meat grinder. The clay is fed into a hopper, forced through a horizontal barrel by a series of rotating blades, and finally extruded from a restricted opening at the end of the barrel (which can also be set up as a power extruder). Although not really appropriate for mixing clay from dry materials, the pugmill is very suitable for final blending of plastic claybodies, for processing clay mixed as slurry and stiffened to plastic consistency, and for processing slaked, stiffened recycle. It can also be used for blend-ing several claybodies together, which
gives you many more options with only a few varieties of moist-bagged clay on hand.
The pugmill is especially handy for adjusting moisture content in clay. It is normal for there to be some variation in moisture content in bagged clay, especially if it has been stored for an extended period of time. When pugging bagged clay, you can alternately charge soft and stiff clay, or dip chunks of stiff clay in water or slurry before charging, and the pugmill will effectively blend the clays and produce the desired consistency.
The best pugmills are the vacuum de-airing models, which extract all air from the clay. At the same time, they tend to draw off some mois-ture, so it is usually necessary to feed very moist clay in order to get the desired consistency.
As with the dough mixer, any pug-mill should be installed with a lock-able wall switch, and should always be locked when not in use. Most new heavy-duty pugmills come already fitted with a key switch.
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