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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)
Can We Help You Fix a Specific Problem?
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
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

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 body using clays native to your area
Formulating a Clear Glaze Compatible with Chrome-Tin Stains
Formulating a Porcelain
Formulating Ash and Native-Material Glazes
G1214M Cone 5-7 20x5 glossy transparent glaze
G1214W Cone 6 transparent glaze
G1214Z Cone 6 matte glaze
G1916M Cone 06-04 transparent glaze
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
Glaze chemistry using a frit of approximate analysis
Glaze Recipes: Formulate and Make Your Own Instead
Glaze Types, Formulation and Application in the Tile Industry
Having Your Glaze Tested for Toxic Metal Release
High Gloss Glazes
Hire Us for a 3D Printing Project
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
I have always done it this way!
Inkjet Decoration of Ceramic Tiles
Is Your Fired Ware Safe?
Leaching Cone 6 Glaze Case Study
Limit Formulas and Target Formulas
Low Budget Testing of Ceramic Glazes
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
Painting Glazes Rather Than Dipping or Spraying
Particle Size Distribution of Ceramic Powders
Porcelain Tile, Vitrified 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
Simple Physical Testing of Clays
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 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
Unwanted Crystallization in a Cone 6 Glaze
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 in understanding glazes?
Why Textbook Glazes Are So Difficult
Working with children

Duplicating AP Green Fireclay


Few people actually understood what AP Green fireclay really was (it is no longer available). By carefully ascertaining its physical properties we were able to formulate a substitute material mix.



AP Green Fireclay (APGFC) has been used for many years by potters to make stoneware clay bodies. It is a moderately refractory, light colored, slightly iron speckled, plastic clay. Its physical presence actually contrasts with the perception that most potters have of it. For example, it is not as plastic as many people assume. By itself, it is a little more plastic than an average clay body, thus it can exist in high percentages in the recipe. Its drying performance is better than expected given its plastic nature. In addition, unlike what many assume, its Cone 10 porosity is only a little higher than a typical stoneware. Likewise its fired shrinkage is quite high imparting a total shrinkage that is much more than a typical stoneware clay body. One misconception about AP Green fireclay relates to its particle size. While most people think of it as a coarse fireclay, it is actually a very fine particled clay material. It is perceived as being coarse because of the way its is powderized; the particles are agglomerates and slake readily in water to pass more than 90% through a 325 mesh screen. These particles however do not break down readily when being mixed into pugged bodies. This material also contains iron-bearing soluble salts that are left on the surface during drying. These impart a darker coloration to the clay surface. The fired color of APGFC bodies is therefore very much a product of maturity, iron content and surface solubles.

A buff burning stoneware clay body could be fashioned using 80% or more APGFC and a little feldspar to increase maturity and cut plasticity. The 'balance' and 'character' of this material compared to industrial minerals (like kaolin, ball clay, quartz and feldspar) are the reasons for its popularity.

However pottery uses are not a big segment of the APGFC market. Thus physical properties and consistency factors that are very important to potters are not well maintained. Impurities that are not a problem for other markets can be a headache for potters. The 'mystique' that has surrounded APGFC has been partly due to lack of pertinent information about its properties. Also, while the character imparted by the incomplete grinding of the material produces surface color variegation in dark bodies (lighter specks of fireclay in a darker dense clay matrix) the associated non-homogeneity is a source of fired weakness.


An exact match is impossible and we must stress that the key to a successful substitution is knowledge of the compromises made. This will enable you to take countermeasures. In many ways PFC is a superior material and ware quality should improve.

How We Duplicated It

As noted, the first step was to describe the physical and fired properties of APGFC (we do not consider the chemistry to be as important as physical properties). We then compared these to our traditional Buff Fireclay material blend and made substantial changes to its recipe to harmonize the key properties. This produced Plainsman Fire Clay (PFC). We then selected a clay recipe containing 50% fireclay and mixed it using APGFC and PFC and compared. Differences that appeared were rationalized and compromises made so that PFC would work as similarly as possible to APGFC in clay body recipes.

Fired Maturity

PFC by itself is a little more refractory than APGFC (which has about 2% cone 10 porosity). Thus the PFC can produce a slightly higher porosity when used in a clay body. However more feldspar may not be needed since vitreous bodies can actually appear more vitreous with the PFC in spite of the higher porosity.

Reduction Iron Speckle

They have a similar reduction speckle character, however PFC bodies may exhibit a higher population of more evenly sized specks. APGFC bodies tend to have some large scattered specks and iron blotches and this inconsistency has been eliminated in our PFC.

Soluble Salts

The APFG exhibits some unpredictable behavior in this area. Some of the fired character of pure APGFC is a product of its iron bearing solubles. These deposit differently depending on the potter's process and natural variations in the raw material. PFC also has solubles that tend to flux the surface imparting a brownish color.


Although AP Green is understood to be a coarse grained material, the opposite is actually the case. Although it is mechanically ground to around 20 mesh the individual particles are agglomerates that easily break down in water to produce a material that feels like porcelain on the wheel. While PFC also contains a large fraction of naturally fine clay particles, it also contains a wide range of larger mineral particles lacking in the APGFC.


Plasticity of the two materials is similar. If anything, PFC produces a body that is a little more plastic with a slightly more ball-clay-like character (as opposed to kaolin-like). Thus it may be easier to keep pieces on center and to neck them in during throwing.


PFC has a slightly higher drying shrinkage. That means you may need to be a little more careful to dry pieces evenly in plastic bodies with substantial amounts of APGFC.

Throwing Behavior

PFC contains 10% of a very fine white kaolinized sand. This additive gives us control of maturity and plasticity. Very sensitive throwers might feel this sand in high-fireclay bodies or notice it in the slip produced. This sand can provide a channel for water penetration into the plastic clay; if you experience water splitting during throwing (vertical cracks) then we recommend exercising more care to avoid leaving water on surfaces that are being stretched (e.g. bellies on vases). For a simple comparison test between the APGFC and PFC versions of your clay body, balance cigar shaped pieces of clay horizontally on your finger and put a few drops of water on top and compare the amount of time it takes for a split to begin.


BFC has more silica and less alumina than APGFC (see glaze fit, thermal expansion).

Thermal Expansion

Since PFC has a higher free silica content it will contribute a lower thermal expansion to bodies (crazing glazes will craze less, shivering glazes will shiver more). We see this as a big advantage of PFC since common high feldspar glazes typically craze on high APGFC bodies. However the higher expansion could increase dunting problems in bodies containing inadequate feldspar or recipes with high ball clay and silica additions.


Adjustability/Consistency: PFC is a blend of refractory materials (a ball clay, a stoneware, a kaolinized sand and a kaolin). Thus inconsistency in any one material will be dampened by a factor equal to its percentage in the mix. In addition, we will be able to make small adjustments in the mix to compensate for any inconsistencies that might arise and to fine-tune the properties to make it a closer match to APGFC. PFC is mixed and ground in a conscientious manner specifically for ceramic applications and each batch is thoroughly tested and compared with previous ones to reveal problems and isolate trends. Thus users will benefit from a material that is much more dependable than they have been used to.

Heat Duty

We have incorporated non-refractory ingredients into our blend to match firing characteristics of APGFC below cone 12. Thus you will need to test PFC to determine its suitability for non-ceramic refractory applications.

Physical Properties

 Drying Shrinkage: 6.5-7.5% (APGF is 6.0-7.0)

Sieve Analysis (Tyler mesh):

                      PFC        APGF
 +48 (300 microns): 0.1-0.5    0.0-0.5
   48-65 (300-210): 0.5-1.5    0.0-0.5
  65-100 (210-149): 1.0-3.0    0.0-0.5
 100-150 (149-106): 2.0-4.0    0.0-0.5
 150-200  (106-75): 4.0-6.0    0.0-1.0
 200-325   (75-45): 7.0-10.0   0.0-1.0

Fired Shrinkage:

             PFC        APGF
   Cone 6: 5.0-6.0%   6.5-7.5%
   Cone 8: 5.5-6.5    7.0-7.5
  Cone 10: 6.5-7.5    7.5-8.5
 Cone 10R: 6.0-7.0    7.5-8.5

Fired Absorption:

              PFC      APGF
   Cone 6: 5.0-6.0%   2.5-3.5
   Cone 8: 4.0-5.0    2.0-3.0
  Cone 10: 3.0-4.0    1.5-2.5

Chemical Analysis

 CaO       0.2
 K2O       2.1
 MgO       0.2
 Na2O      1.1
 TiO2      0.7
 Al2O3    27.0
 P2O5      0.0
 SiO2     58.1
 Fe2O3     2.0
 MnO       0.0
 LOI       8.7%

Compared to Buff Fireclay

Plainsman Fireclay (PFC) differs from Plainsman Buff Fireclay (BFC) as follows: PFC is a little smoother (less sand), not quite as plastic and not quite as refractory. PFC is actually BFC with some ball clay and sand replaced with a smooth stoneware clay.

Related Information


Materials APG Missouri Fireclay
Materials Plainsman Fireclay
About Plainsman Clays
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