A Low Cost Tester of Glaze Melt Fluidity
A One-speed Lab or Studio Slurry Mixer
A Textbook Cone 6 Matte Glaze With Problems
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
G1214M Cone 5-7 20x5 Glossy Base Glaze
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
Monoporosa or Single Fired Wall Tiles
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
Rationalizing Conflicting Opinions About Plasticity
Ravenscrag Slip is Born
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 Thermal Expansion in Ceramic Glazes
Unwanted Crystallization in a Cone 6 Glaze
Variegating Glazes
Volcanic Ash
What Determines a Glaze's Firing Temperature?
What is a Mole, Checking Out the Mole
What is the Glaze Dragon?
Where Do I Start?
Why Textbook Glazes Are So Difficult

Understanding the Terra Cotta Slip Casting Recipes In North America

Description

This article helps you understand a good recipe for a red casting body so that you will have control and adjustability.

Article

In North America there is a raw material that makes the creation of a low fire red casting body very easy. It is called Redart. This clay is air floated, high in iron, low in soluble salts and matures at a low temperature (around cone 1). In addition Redart deflocculates very well. Adding iron oxide to a clay causes it to gel badly on dispersion however the natural form of iron in Redart does not do this at all. To use the casting process efficiently you must understand how deflocculation works. Believe me, if you are new to casting and have never mixed a heavy casting slip with low water content you will never go back to a simple clay water mix again after you learn the deflocculation process (see the link to an article on this).

Although it is possible, there are a number reasons why you would not normally use a 100% Redart formula.

Red Casting Powder Starting Recipe

Redart     50
Ball Clay  25
Talc       25

Recipe Notes

Terra Cotta Process Considerations

At cone 06 almost any mix not containing expensive fluxes is going to be non-vitreous and highly porous. However by 04 this figure drops dramatically. By cone 03-02 it is possible to have stoneware properties in most terra cottas. This is because natural red clays contain so much flux. Consider some of the considerations you must think about when formulating a terra cotta casting body. Body color is usually important, people want the deepest possible shade of red in their terra cotta. If you are using commercial glazes fit is a big issue and the body needs to have significant talc, up to 50%. This is not a problem for white bodies since you can couple it with white burning plastic ball clay. But for red bodies it is a challenge because there is not enough room in the recipe for flux given the amount of clay needed to achieve workability (the lower plasticity red plus the ball clay). This means you have a tug of war between color, fit of commercial glazes, maturity and workability. An obvious way to ease things is to formulate your own glazes, this enables removing the talc completely. As mentioned, you cannot just add iron to a white burning clay to get terra cotta color since iron causes slip to severely gel. In my experience you need to use at least 50% dark red clay and deal with the opposing issues of maturity and working properties in the remainder. In North America the primary red clay used in terra cotta bodies is Redart. It casts very well but it is just not plastic enough so you may need to augment it with ball clay to create a slip that has adequate dry strength and shrinkage to pull away from the mold. However to get a dark enough color you normally want a high proportion of red clay. If you can fit a glaze to the talcless version of the body, you may be able to work with a recipe of up to 80% red clay and 20% ball clay.

However it is not typical to fire earthenware this high since color and resistance to warp change dramatically with only slight over firing and the red color is lost under transparent glazes. The beautiful red colors depend on stopping well short of vitrification. Many commercial wares are 6-8% porosity and yet they are very strong, so do not worry about higher-than-stoneware porosity that is common with earthenware. Just make sure the glaze fits (can survive a hot water:ice water test) and strengthens the ware.

In terra cotta bodies the glaze is not nearly as well interfaced and adhered to the body as with stonewares. Glazes are dramatically more prone to crazing, shivering and chipping off. To make good quality earthenware requires much more technical glaze knowledge than for making stoneware.

Combining Casting a Thrown or Hand-build Elements

This is a very practical aspect of terra cotta. Some commercial terra cotta throwing bodies will deflocculate. However you will find that elements cast from the recipe given here can be joined onto leather hard objects using the casting slip itself as a glue. You may need to adjust the recipe to match color it ware is not to be glazed.

Deflocculation

Deflocculation refers to the magic process of creating a slurry with only half the amount of water that would normally be necessary (see the link to an article on this topic). Generally you measure the water and put it into a container, start your propeller mixer and add most but not all of the deflocculant. Then you add the powder body mix. The slurry becomes very heavy so you need a powerful mixer than can run for hours. Getting the last bit of powder to mix in can take some time so patience is required.

Powder Mix 100

Water

36.0% of dry amt

Soda Ash

0.05-.1% of dry amt

Sodium Silicate

about 0.2 to 0.3% of dry amt

or

Water

36% of dry amt

Darvan

0.5% of dry amt

Note that water and deflocculant amounts are examples, you need to know how to measure specific gravity and viscosity to adapt their amounts to your materials and water (see the article on deflocculation for more information).

Being Realistic

Terra cotta ware is never going to be anywhere near as strong as stoneware. Terra cotta is a matter of tradition and getting a nice warm red colored body. Terra cotta is much stronger than white burning earthenware and aesthetically it goes well with brightly decorated light colored opacified glazes. If you can make fine quality earthenware then you can be proud, it is not an easy job.

Related Information

Links

Articles Stoneware Casting Body Recipes
Some starting recipes for stoneware and porcelain with information on how to adjust and adapt them
Articles Understanding the Deflocculation Process in Slip Casting
Understanding the magic of deflocculation and how to measure specific gravity and viscosity, and how to interpret the results of these tests to adjust the slip, these are the key to controlling a casting process.
Articles Low Fire White Talc Casting Body Recipe
The classic white ball clay talc casting and modelling recipe has been used for many years. It is a dream to use as long as you are aware of the problems and risks.
Materials Redart
Materials Banta Red Clay
Materials Sodium Silicate
Glossary Deflocculation
The deflocculation process is the magic behind the ceramic casting process. It enables you to make a slurry of far lower water content and thus lower shrinkage.
Projects Troubles

By Tony Hansen


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