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Adjusting Glaze Expansion by Calculation to Solve Shivering
Alberta Slip, 20 Years of Substitution for Albany Slip
An Overview of Ceramic Stains
Are You in Control of Your Production Process?
Are Your Glazes Food Safe or are They Leachable?
Attack on Glass: Corrosion Attack Mechanisms
Ball Milling Glazes, Bodies, Engobes
Binders for Ceramic Bodies
Bringing Out the Big Guns in Craze Control: MgO (G1215U)
Ceramic Glazes Today
Ceramic Material Nomenclature
Ceramic Tile Clay Body Formulation
Changing Our View of Glazes
Chemistry vs. Matrix Blending to Create Glazes from Native Materials
Concentrate on One Good Glaze
Cone 6 Floating Blue Glaze Recipe
Copper Red Glazes
Crazing and Bacteria: Is There a Hazard?
Crazing in Stoneware Glazes: Treating the Causes, Not the Symptoms
Creating a Non-Glaze Ceramic Slip or Engobe
Creating Your Own Budget Glaze
Crystal Glazes: Understanding the Process and Materials
Deflocculants: A Detailed Overview
Demonstrating Glaze Fit Issues to Students
Diagnosing a Casting Problem at a Sanitaryware Plant
Drying Ceramics Without Cracks
Duplicating Albany Slip
Duplicating AP Green Fireclay
Electric Hobby Kilns: What You Need to Know
Fighting the Glaze Dragon
Firing Clay Test Bars
Firing: What Happens to Ceramic Ware in a Firing Kiln
First You See It Then You Don't: Raku Glaze Stability
Fixing a glaze that does not stay in suspension
Formulating a Clear Glaze Compatible with Chrome-Tin Stains
Formulating a Porcelain
Formulating Ash and Native-Material Glazes
Formulating Your Own Clay Body
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G1214W Cone 6 Transparent Base Glaze
G1214Z Cone 6 Matte Base Glaze
G1916M Cone 06-04 Base Glaze
G1947U/G2571A Cone 10/10R Base Matte/Glossy Glazes
Getting the Glaze Color You Want: Working With Stains
Glaze and Body Pigments and Stains in the Ceramic Tile Industry
Glaze Chemistry Basics - Formula, Analysis, Mole%, Unity, LOI
Glaze chemistry using a frit of approximate analysis
Glaze Recipes: Formulate Your Own Instead
Glaze Types, Formulation and Application in the Tile Industry
Having Your Glaze Tested for Toxic Metal Release
High Gloss Glazes
How a Material Chemical Analysis is Done
How desktop INSIGHT Deals With Unity, LOI and Formula Weight
How to Find and Test Your Own Native Clays
How to Liner-Glaze a Mug
I've Always Done It This Way!
Inkjet Decoration of Ceramic Tiles
Interpreting Orton Cones
Is Your Fired Ware Safe?
Leaching Cone 6 Glaze Case Study
Limit Formulas and Target Formulas
Low Budget Testing of the Raw and Fired Properties of a Glaze
Low Fire White Talc Casting Body Recipe
Make Your Own Ball Mill Stand
Making Glaze Testing Cones
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Organic Matter in Clays: Detailed Overview
Outdoor Weather Resistant Ceramics
Overview of Paper Clay
Painting Glazes Rather Than Dipping or Spraying
Particle Size Distribution of Ceramic Powders
Porcelain Tile, Vitrified or Granito Tile

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Recylcing Scrap Clay
Reducing the Firing Temperature of a Glaze From Cone 10 to 6
Single Fire Glazing
Soluble Salts in Minerals: Detailed Overview
Some Keys to Dealing With Firing Cracks
Stoneware Casting Body Recipes
Substituting Cornwall Stone
Super-Refined Terra Sigillata
The Chemistry, Physics and Manufacturing of Glaze Frits
The Effect of Glaze Fit on Fired Ware Strength
The Four Levels on Which to View Ceramic Glazes
The Majolica Earthenware Process
The Physics of Clay Bodies
The Potter's Prayer
The Right Chemistry for a Cone 6 MgO Matte
The Trials of Being the Only Technical Person in the Club
The Whining Stops Here: A Realistic Look at Clay Bodies
Those Unlabelled Bags and Buckets
Tiles and Mosaics for Potters
Toxicity of Firebricks Used in Ovens
Trafficking in Glaze Recipes
Understanding Ceramic Materials
Understanding Ceramic Oxides
Understanding Glaze Slurry Properties
Understanding the Deflocculation Process in Slip Casting
Understanding the Terra Cotta Slip Casting Recipes In North America
Understanding Thermal Expansion in Ceramic Glazes
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Variegating Glazes
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Rationalizing Conflicting Opinions About Plasticity

Description

How can two potters have completely different opinions about the plasticity and workability characteristics of the same clay body

Article

Those of us that have done pottery for a long time have pretty firm ideas about what works and does not in glazes and clay bodies. But it always amazes me how two experienced people can have opposite philosophies and methods and yet both make them work. I find trying to explain 'the whys' of specific instances of this very educational.

At Plainsman Clays (I work there part time) they employ a kaolinite/halloysite material from Troy, Idaho in a number of our clay bodies. It is inherently very fine so requires almost no grinding, however it has very high fired shrinkage so they don't use more than about 20%. Its plasticity is quite low and even a body containing 70% would still lack plasticity. However this same material is successfully processed and promoted by Michael Wendt as Helmer clay at www.wendtpottery.com. He uses only 40% Helmer as the exclusive source of clay in his buff stoneware body and claims that while it is very plastic he has zero drying loss. Strangely his total shrinkage figures matched what I would consider normal. This seemed impossible. I mixed test batches of his clay body and we emailed back and forth without being able to resolve the difference in opinions. Finally he sent me a sample that he had mixed using his wet process. It threw a little better but I still found plasticity much lower than I am used to. And no wonder, its drying shrinkage was only 4% (compared to a typical of 6.5% in bodies made from our raw materials). This is a huge difference (highly plastic clays employing ball clay and bentonite have high drying shrinkages while low plasticity kaolin-based bodies have low shrinkages). In harmony with this I found that the clay tended to split during wedging, generated a lot of slip during throwing and had a lower dry strength.

How do we resolve this? First, Michael's total shrinkage measurements are comparable to what we consider normal because its drying shrinkage is much lower and compensates for the higher fired shrinkage. We all tend to evaluate bodies in terms of the ones we were 'brought up' with and almost unknowingly we develop ways of working that mesh with these bodies. I think Michael's and my priorities and therefore observations are different for this reason. Our opinions about plasticity obviously conflict, neither of us are necessarily right. Plasticity is a hard thing to quantify. I am inclined to define plasticity as 'a readiness of soft clay to assume a new or return to an old shape while maintaining strength during the move'. Others think of plasticity as the ability to hold a shape once it is assumed but they are less inclined to talk about its strength during the move (well traveled potters can tell stories about bodies in Japan or Europe that they found impossible to throw yet the natives handle them easily)?

Life is tradeoffs. To enjoy the advantages of this type of body Mr. Wendt has developed techniques that address its disadvantages. Michael would point out that he cannot tolerate the disadvantages of the high-drying-shrinkage kinds of clays available in our area. Low firing shrinkages would not be as important to him as low drying shrinkage. He would be hesitant to employ ball clay and cut its plasticity with sandy or siltly materials and would say that the dry strength of his body is fine.

This is fascinating. The range of plasticity possibilities is amazing. Some people use 70%+ ball clay bodies (i.e. Fairey F97) on one end of the spectrum, whereas others employ kaolin-only bodies on the other. Fara Shimbo, the author of the book Crystal Glazes, is another example. She employs a porcelain body containing 31% Grolleg and 15% EPK kaolin as the only sources of clay. Neither of these are plastic kaolins in the same league as #6 Tile kaolin, and yet I regard a body of 50% #6 Tile as much too short! It seems that people have found ways to make both plasticity extremes work well and will defend them and articulate their merits tirelessly. When it comes to using bodies that we have developed ourselves we may be tempted to claim that a particular one breaks all the rules (e.g. it is both plastic and perfect drying, vitreous yet does not warp, low in silica yet does not craze glazes). These statements defy the logic of clay body physics, on further investigation there turns out to be a lot more to the story. It is usually a question of the potter having adapted to the material.

Related Information

Links

Articles The Whining Stops Here: A Realistic Look at Clay Bodies
Jonathan Kaplan overviews clay bodies, body materials and body types, how they are formulated and tested, how to protect yourself when buying prepared bodies, how to take responsibility.
Articles Formulating a Porcelain
The principles behind formulating a porcelain are quite simple. You just need to know the purpose of each material, a starting recipe and a testing regimen.
Materials Helmer Kaolin

By Tony Hansen


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