Contents. Introduction and Acknowledgments xiii. 1 Clay and Claybodies 1. 2 Handbuilding 14

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2 Contents Introduction and Acknowledgments xiii 1 Clay and Claybodies 1 The 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 4 Primary and Secondary Clays 4 Claybodies 5 Claybody Components 5 Accessory 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 9 Water of Plasticity 9 Mixing and Recycling Clay 10 The 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 14 Wedging 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 23 Making 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

3 vi contents Slab Construction 29 Rolled 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 40 Choice 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 42 Centering 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 51 What 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 54 Critical Points in Throwing 55 Throwing off the Hump 57 Throwing 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 69 Teapot Lids 70 Teapot Spouts 70 Teapot Handles 71 Thrown-and-Altered Forms 72 Throwing 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 74 Paddling 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 78 Finishing 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 89 Water to Plaster Tables 90 The Use of Cottles 91 Using Strips of Sheet Metal or Linoleum for Mold Forms 91

4 contents vii 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 92 Open-Pour Slip-Casting Molds 93 Multipiece Slip-Casting Molds 93 Multipiece Flick/Smear Molds from Plastic Clay Prototypes 93 Multipiece Molds from Rigid (Not Plastic Clay) Prototypes 94 Mixing and Pouring Casting Slip 96 Pouring Your Molds 96 5 Surface Decoration on Greenware 98 Decorative Effects during Forming 98 Impressed Decoration 99 Subtractive Methods 100 Additive Methods 101 Burnishing and Polishing 102 Slips and Slip Decoration 102 How 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 106 Polychrome 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 109 Making 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 113 Basalt 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 Glazes and Glazing 120 Introduction to Glazing 120 Glaze Color 121 Glaze Transparency and Surface 121 Approaching Glaze Design 121 Glaze-Firing Ranges 122 Referring 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 124 Using a Triple-Beam Gram Scale 125 Glazing Methods 127 Using Resist Compounds 127 Using Resists for Glaze Decoration 128 Contamination of Glazes 129

5 viii contents 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 132 Brushing 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 137 Underglazes 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 139 What Are Glass and Glaze? 140 Oxides, Oxidation, and Reduction 140 Reoxidation 141 Components of a Glaze 141 The 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 148 Coloring Oxides 149 Common Traditional Glazes 150 Salt and Soda Glazing 151 The Chemistry and Physics of Glaze Firing 152 Reactions 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 157 Making 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? Kilns and Firing 160 Types of Firings 161 Types of Kilns 161 Electric Kilns 162 Fuel Kilns 162 Wood Kilns 163 General Kiln and Firing Practices 163 Firing Logs 163 Ventilation 163 Don t Burn Yourself! 163 Opening Hot Kilns 164 Care of Refractory Surfaces 164 Preparing and Loading Kilns 164 Electric 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 168 Selecting 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 173 Firing 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

6 contents ix 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 179 Controlling 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 182 Raku Firing 182 Salt and Soda Firing 184 Single-Firing 186 Wood Kilns and Wood Firing 187 The 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 194 How 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 197 Electric Heating Elements 198 Reduction-Firing in an Electric Kiln 198 Temperature Control and Shutoff Devices on Electric Kilns 199 Electric Kiln Venting Systems 201 Kiln-Wall Thickness/Construction and Temperature Rating 201 Size and Design of Top-Loader Kilns 201 Element Support Systems 201 Installation Requirements for Top-Loader Electric Kilns 201 Heavy-Duty Industrial Electric Kilns 202 Purchasing a Used Electric Kiln 202 Maintenance and Repair of Electric Kilns 203 Problems with Corrosion 203 Dawson Kiln Sitter Problems 203 Electrical Problems and Repairs 204 Electrical Terminals and Wires 204 Switch Replacement 205 Power Supply Problems 205 Element Replacement 205 Refractory Repairs on Electric Kilns 207 Fuel Kiln Selection, Design, Construction, and Repair 207 Choosing the Right Kiln Design 208 Kiln Proportions 208 Proportions for Downdraft Kilns 208 Kiln Size 209 Commercially Made Gas Kilns 209 Gas Kiln Installation 209 Venting Fuel Kilns 210 Venting Updraft Kilns 210 Venting Downdraft Kilns 211 Burner Systems 211 Gas Burner Systems 211 Gas Burner Ignition and Safety Systems 211 Programmable Controllers on Gas Kilns 213 Gas-Line Pressure: Variations and Measurement 213 Gas Burners and Entrained Air 214 Atmospheric/Natural Draft Burners 215 Simple Tube Burners 215 Flame-Retention Problems 216 Gas-Air Mixing and Turbulence: Flame- Retention Burner Tips 216 Venturi Burners 217 Pilot Burners 218 Power Burners 218 Oil-Burner Systems 219 Drip-Feed Oil Burners 219 Atomizer Oil Burners 219 Safety Systems with Oil Burners 220

7 x contents Refractory Materials Used in Kiln Construction 220 Hardbrick 220 Cutting Hardbrick and Kiln Shelves 221 Insulating 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 237 Building 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 239 Refractory Repairs 239 The Damper 240 Repairing Burner Components Mixed Media in Ceramics 242 Possible Mixed-Media Materials 244 Flat Stuff 244 Long Stuff 244 Miscellaneous Stuff 244 Odd Found Objects 245 Fastening and Forming 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 248 About 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 250 Avoid Wet Floors 250 Repetitive Motion Disorders; Carpal Tunnel Syndrome 250 Taking Care of Your Back 251 Skin Care 251 Lighting 252 Equipment Safety 252 Leave Machinery in Proper Shutdown Condition 252 Always Observe Proper Machinery Safety 252 Studio Ventilation 253 Ventilation 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 Studio Design, Setup, and Operation 256 Studio Design and Setup 256 Concerns in an Existing Structure 257 Studio Size 257 Plan for the Future 257 Studio Lighting 257 Wiring 258 Plumbing 258 Specific-Use Areas 258 Clay Storage/Processing Area 258 Throwing Area 258 Handbuilding Area 259 Damp-Box and/or Dry-Box 259 Ware Storage 259

8 contents xi 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 278 Researching and Approaching Shops and Galleries 279 Home/Studio Sales 280 Studio Showrooms and Attached Galleries 281 Holiday Sales in Shopping Malls 281 Advertising 281 Local Advertising and Studio Newsletters 281 Color Cards 281 Color Sheets and Brochures 282 Personal Websites 282 Studio Tools and Equipment 282 Tooling 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 305 An Inexpensive and Efficient Clay-Mixing Option 305 Stiffening Slurry 306

9 xii contents Clay Mixers 306 Dough Mixers 306 The Soldner Mixer 306 The Pugmill 307 Pottery Wheels 308 Kick Wheels 308 Variable-Speed Electric Wheels 308 Other Studio Equipment 308 Clay Extruders 308 Slab Rollers 309 Scales for Weighing Clay and Glaze Materials 309 Plumbing Traps 310 Spray Booths 311 Studio Furniture 312 Clay-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

10 Chapter 1 Clay and Claybodies The Nature of Clay We 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 considers 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 substance that can be worked in many ways, but it can be safely handled only in industrial circumstances, and the related environmental concerns 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 versatility 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

11 2 clay: A Studio Handbook 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 permanence 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 embarrassment 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 positive 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? 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 discover the possibilities, we must take chances and experience lots of failure and mistakes. Just as every question 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 experimenting 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 guard and will throw you for a loop every time. But as long as you maintain a spirit of discovery and curiosity, the clay will reward you frequently and generously. Clay and Claybodies: Some Basic Questions Answered What 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 minerals 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 particles. The end result of the decomposition of granite and feldspar produces 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 fraction of particles less than 2 microns in size a micron is of a millimeter or 8 100,000 of an inch. That means that we could pack up to half a billion clay particles into one cubic millimeter.

12 clay and claybodies 3 Why Do Different Clays Behave Differently? Different clays behave differently depending on the range and distribution 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 particles in water suspension. Floc cu lation involves the addition of a minute amount ( 1 4 to 1 2 of 1%) of soluble metallic salts such as epsom salts (magnesium sulfate), which render the water slightly acidic, giving 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 especially important in slip-casting bodies, 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 particles stick together more effectively, improving the plastic working qualities. 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 process than the development of organic activity in the clay, and usually happens over a period of a few weeks. It is a very beneficial process and can be greatly speeded up by soaking the particles thoroughly 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, glassformers, and refractories or stabilizers. The fluxes or melting agents lower the maturing temperature and assist in formation of glass, the essential binder in all ceramics. The primary fluxes appearing 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 glassformer 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 temperatures can lead to the formation of cristobalite, a crystalline form of silica, which drastically increases thermal expansion and lowers thermal shock resistance.

13 4 clay: A Studio Handbook The refractories or stabilizers provide the physical matrix of clay, the particles that the fluxes and glassformers bind together. Increasing refractory content raises the maturing temperature and reduces formation 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) crystals in high-fire bodies, which creates 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 discussing varieties of clay and claybodies, we must understand the phenomena known as sintering and vitrification. 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 porcelain 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 glassformers begin to interact. Once the clay is sintered, it can no longer be slaked down and reused. As soon as even minimal sintering has occurred at dull red heat, the clay can be considered 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 glassyphase, which strengthens the sintered 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 sintering 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 shrinkage in vitrified wares, and inversely, the lack of firing shrinkage in nonvitrified wares. In all fired wares, the sintered connection between refractory particles gives basic physical structure, which prevents thermoplastic deformation (warping) at firing temperatures, whereas in vitrified wares the glassy-phase gives density, impermeability, and strength. In earthenware clays and claybodies, there is usually too much flux present for simple sintering to provide structure above low-fire temperatures. If the firing temperature of a true earthenware clay exceeds about 2000 degrees F, the fluxes will usually overpower the sintered connections, and the body begins to deform and bloat as the constituent materials flow and volatilize. Similarly, when stoneware or porcelain is overfired, the glassy phase begins to dissolve the physical structure, causing slumping and warping. Classification of Clays Primary and Secondary Clays All clays are classified as primary (also referred to as residual) or secondary (also referred to as sedimentary 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 determined by the degree of subsurface metamorphic activity, by acids seeping down from the surface, and by heating and cooling. The ideal formula of pure kaolin is Al 2 O 3 2SiO 2 2H 2 O, but this would give an extremely high-temperature kaolin. In reality, most kaolins are contaminated with unbroken-down feldspar and free silica. 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

14 clay and claybodies 5 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 serious 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 considerable 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 common 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 bodies. 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, introducing more impurities, finer particle size, and higher flux content (primarily 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 bodies 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. Claybodies Pure natural clays almost always have some shortcomings. Claybodies are mixtures of clay and other materials designed to accomplish 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 claybody, 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 additions 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

15 6 clay: A Studio Handbook the primary flux is feldspar, which provides sodium, potassium, calcium, 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. Refractories As mentioned earlier, different clays have varying degrees of refractoriness. In order to control the maturing 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 materials like sand and grog that open up the claybody, giving improved forming 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 shrinkage than grog and possibly contributing to free silica and resulting cristobalite formation in high-fired wares, decreasing thermal shock resistance. Plasticizers Many claybodies benefit from the addition of accessory plasticizers. Porcelain bodies often need these additives, especially pure whitefiring porcelains, which usually con- tain only kaolins as the primary clay component. Small additions of bentonite, macaloid, or Veegum T will increase plasticity and workability of the clay on the wheel and in handbuilding. See Appendix II for additional information on these materials. Colorants Our concern here is with colorants found or used in common claybodies, rather than specialized colored clays. The most common colorant in clay is iron. As mentioned, iron becomes a powerful flux in highfired 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 appreciably with less fluxing than pure iron oxide. Very small additions of powerful colorants like cobalt will significantly modify color. Five percent granular manganese dioxide will give speckles in oxidation-fired wares at any temperature. Five percent granular rutile or granular ilmenite will increase iron speckles in reduction high-fired wares. Common Types of Claybodies Earthenware 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 clay plus sand or grog to give structure 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 whiteware bodies rather than earthenware. The most popular whiteware body, composed of ball clay and talc, is actually very similar to one used by the Egyptians 5000 years ago. Talc is a sheet-lattice magnesium silicate with properties similar 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 earthenware or whiteware clay the raku and bonfire processes often use highly refractory stoneware bodies that are simply underfired at lowfire 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 temperatures (cone 4 and above), with a fully developed glassy-phase and

16 clay and claybodies 7 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 pyroplastic. This gives the great density and strength and low absorption needed for functional wares, but the pyroplastic flexibility of such bodies makes them completely inappropriate 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 encourage 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 phosphate). 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 glassyphase. This creates translucence in the fired claybody, but it also lowers the maturing temperature to around cone 6. With what basically amounts to an overdeveloped glassyphase, bone china bodies are very prone to warpage unless fired on flat shelves with no hot spots in the firing. Actually, any reasonably wellfluxed cone 10 porcelain thrown very thin will give some translucence without the disadvantages of bone china. Stoneware claybodies use natural stoneware clay and/or fireclay as a base, with additions of ball clay, kaolin, 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 stoneware 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 shrinkage. 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 lowheat 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 thermal 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 sintered structure, reducing thermal shock resistance and encouraging pyroplastic deformation. The natural flux component of most ball clays, stoneware clays, and fireclays is usually adequate to form a sufficient glassy-phase to make a very strong sintered matrix, while also absorbing free silica, which reduces cristobalite 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

17 8 clay: A Studio Handbook wet and left untouched until hardened. Such a mix would not be appropriate for shelves and furniture, 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 mix of fireclay and a mullite grog like kyanite or cordierite will form a very complete felted mass of mullite crystals with an adequate glassyphase. Cone 10 furniture should be fired to cone 11 or 12 with a long soaking to maximize the glassyphase 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 promoted 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 recommended. 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 amazing strength and can be handled 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 paperclay, consult Rosette Gault s book on the subject. Variations in Claybodies for Different Applications and Firing Processes Any claybody that gives certain favorable qualities may be modified for different applications or methods, such as wheel work, handbuilding, 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 correct 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 bodies that have been discarded as unworkable, when a suitable flocculant (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. 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 during wet-working, but will also increase water absorption during throwing. Remember that plasticity is significantly 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 preparation 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 claybody with most of the same qualities as a throwing body, but plasticity is not quite as much of an issue. Structure is often far more important, 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 drastically reduces drying and firing shrinkage and their associated flaws and faults. For the most demanding forming methods, such as large softslab 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