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

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
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

Substituting Cornwall Stone


How to create a blend of materials to chemically substitute for another (Cornwall Stone is used as an example).


Cornwall Stone is used worldwide in ceramics as a source of fluxing oxides in both bodies and glazes. As a glaze material, Cornwall stone is akin to feldspar. It is sourced from granite rock and is, therefore, a combination of a number of minerals. Actually, there are several varieties depending on the state of alteration. The blue and purple colored types (less altered) contain more fluxes, while the whiter varieties contain less K2O, Na2O and almost no fluorine. For people who have used it in glazes like I have, it is hard to know whether the multiple personalities of the material are a result of grade confusion in the distribution channel or actual variability in the product. However, for many of us, it is the high cost of the material (to have it shipped half way across the world) that makes it well worth while to duplicate, that is, replace with a chemically equivalent mix of cheaper, more consistent, and easier to obtain materials. Really, it is somewhat like making the muffins your grandmother in England used to make. You will have to substitute a local flour (or mix of flours) and possibly even make adjustments to the rest of the recipe to duplicate those muffins in your locale.

Amazingly, many people have tried to simply replace Cornwall gram-for-gram with a feldspar. But this will not work, Cornwall Stone is too different from a feldspar. For example, consider Custer Feldspar: the amount of flux is much greater and it has a much lower silica content. Nepheline syenite also has an even higher flux content, much lower SiO2 and much higher alumina.

CaO MgO K2O Na2O TiO2 Al2O3 SiO2 Fe2O3LOI
Cornwall Stone 1.810.144.303.300.1516.3373.760.20  
Custer Feldspar 0.30 10.282.91 17.3569.000.12 0.04
Nepheline Syenite Unimin 0.350.034.8010.60 23.5060.200.08  

The first real challenge is to decide what analysis to duplicate. Since the whiter varieties are what I am accustomed to and they are the most pure, and contain almost no fluorspar mineral, I feel most comfortable zeroing in on this type. The first step was to locate all available analyses from data sheets and textbooks.

Following is the list I was able to find.
100 NUMBER 1 640.93 0.21 0.06 0.29 0.43 0.01 0.01 0.97 7.72
101 NUMBER 2 682.90 0.23 0.03 0.34 0.37 0.01 0.02 1.02 8.27
102 NUMBER 3 680.57 0.29 0.02 0.27 0.40 0.00 0.02 1.06 8.19
103 NUMBER 4 705.21 0.27 0.02 0.32 0.37 0.01 0.02 1.10 8.47
104 NUMBER 5 717.95 0.22 0.04 0.32 0.40 0.01 0.02 1.09 8.73
105 NUMBER 6 696.23 0.22 0.02 0.16 0.59 0.00 0.01 1.11 8.45
106 NUMBER 7 718.73 0.27 0.04 0.31 0.36 0.01 0.01 1.13 8.66
107 NUMBER 8 736.85 0.28 0.04 0.31 0.35 0.01 0.02 1.12 8.97
108 NUMBER 9 1118.35 0.18 - 0.56 0.22 0.01 0.03 1.85 13.68
109 NUMBER 10 963.42 0.15 - 0.39 0.42 0.01 0.02 1.61 11.70
110 NUMBER 11 685.63 0.26 0.01 0.48 0.25 0.01 - 1.13 8.10
111 NUMBER 12 738.13 0.18 0.02 0.35 0.43 0.01 0.01 1.15 8.82

Although each analysis contains some fluorine (not shown), I decided to attribute the amounts to other oxides. To get an average overall analysis, you can calculate the average of each column. I did it in Digitalfire desktop INSIGHT by entering each analysis (with LOI) as a recipe, converting to a formula and then inserting each into the MDT materials database. Then I keyed amounts of 500 of each of these into a new recipe (although I could have weighted it toward certain analyses that might be judged more reliable). Below is the resulting calculation to yield an average Cornwall stone analysis.

WEIGHT OF EACH OXIDE   56.10 40.30 94.20 62.00 160.00 79.70 102.00 60.00
EXPAN     0.15 0.03 0.33 0.39 0.13 0.14 0.06 0.04
NUMBER 1 500.00 640.93 .16 .05 .22 .33 .01 .01 .76 6.02
NUMBER 2 500.00 682.90 .17 .03 .25 .27 .01 .01 .74 6.06
NUMBER 3 500.00 680.57 .22 .01 .20 .29 .00 .01 .78 6.02
NUMBER 4 500.00 705.21 .19 .01 .23 .26 .01 .01 .78 6.00
NUMBER 5 500.00 717.95 .15 .03 .22 .28 .01 .01 .76 6.08
NUMBER 6 500.00 696.23 .16 .01 .11 .42 .00 .01 .79 6.07
NUMBER 7 500.00 718.73 .19 .02 .22 .25 .01 .01 .79 6.02
NUMBER 8 500.00 736.85 .19 .03 .21 .24 .01 .01 .76 6.09
NUMBER 9 500.00 1118.35 .08 - .25 .10 .01 .01 .83 6.12
NUMBER 10 500.00 963.42 .08 - .20 .22 .01 .01 .83 6.07
NUMBER 11 500.00 685.63 .19 .01 .35 .18 .00 - .82 5.90
NUMBER 12 500.00 738.13 .12 .01 .24 .29 .01 .00 .78 5.97
TOTAL 6000.00   1.90 .21 2.70 3.13 .07 .11 9.43 72.42
UNITY FORMULA   .23 .03 .33 .39 .01 .01 1.16 8.92
PER CENT BY WEIGHT   1.81 .14 4.31 3.30 .20 .15 16.33 73.76
WEIGHT 725.84

This sounds like a lot of trouble to go through since this collection of analyses are remarkably consistent. But for many materials it is necessary, one isolated analysis is unlikely to be representative. Many material substitution efforts have been less than successful merely because the original material, toward which formulation efforts were aimed, was not clearly defined.

I had excellent results working out recipes that substitute other materials for Cornwall stone. INSIGHT's cost calculation abilities make it easy to compare unit costs and it was a surprise to see the cost difference between the original glazes and the newly developed recipes (although I have not shown it here).

Following is a side-by-side report from INSIGHT. On the left is a cone 10R fatty matte glaze recipe. I am only using this recipe for demonstration since it has a high Cornwall Stone content (but it crazes on porcelain and has virtually no clay content so it will not suspend as a slurry nor will it have good application properties). The first column in the chemistry is the formula, the second the percentage analysis, the last the molar percent.

I began by entering this as recipe 1 in INSIGHT. Next, I deleted the Cornwall and recalculated. It was immediately evident that sodium oxide, potassium oxide, Al2O3 , and SiO2 were all lost with the Cornwall stone. Since INSIGHT calculates a formula so quickly, it only takes a minute to juggle the materials to supply the needed oxides. Below right is the glaze recipe I came up with. For me it calculates to a cost less than one third of the original.

  Cornwall Stone..............    65.00         Dolomite....................    20.71
  Dolomite....................    20.00         Silica......................    31.47
  Silica......................    10.00         Custer Feldspar.............    19.80
  Bentonite...................     3.00         Nepheline Syenite...........    16.91
                              =========         Whiting.....................     1.52
                                  98.00         Kaolin......................     9.59
          CaO       0.42*    8.29    9.20                                      100.00
          MgO       0.36*    5.14    7.95     
          K2O       0.10*    3.20    2.12               CaO       0.42*    8.32    9.23
          Na2O      0.12*    2.53    2.55               MgO       0.36*    5.16    7.96
          TiO2      0.00     0.11    0.09               K2O       0.10*    3.19    2.11
          Al2O3     0.36    12.72    7.77               Na2O      0.12*    2.54    2.55
          SiO2      3.22    67.74   70.23               Al2O3     0.36    12.76    7.78
          Fe2O3     0.00     0.26    0.10               SiO2      3.22    67.98   70.36
                                                        Fe2O3     0.00     0.05    0.02

              Calculated LOI:   10.02                                 
                 Imposed LOI:                               Calculated LOI:   11.75
                       Si:Al:    9.04                          Imposed LOI:
                      SiB:Al:    9.04                                Si:Al:    9.05
           Thermal Expansion:    6.62                               SiB:Al:    9.05
              Formula Weight:  285.63                    Thermal Expansion:    6.60
                                                            Formula Weight:  284.71

There is a second method. Let's consider an example to demonstrate the concept. Here is my recipe for the Cornwall stone substitute.

    Dolomite....................     0.37  
  Custer Feldspar.............    30.43  
  Wollastonite................     3.30  
  Kaolin......................    12.85  
  Silica......................    29.29  
  Nepheline Syenite...........    23.75  

This is the glaze recipe into which I want to substitute Cornwall stone, I derived it the same way as the recipe above. It is a transparent that works well with chrome-tin pink stains at cone 4-6.

WHITING............. 37.70
FERRO FRIT 3134..... 55.80
KAOLIN.............. 35.50
SILICA.............. 50.70
CORNWALL STONE...... 20.30

There are two ways to mix up a glaze using our substitute. First, you could weigh up a batch of the Cornwall substitute, mix it thoroughly, then treat it as a raw material. However, there is a problem with this. Powder mixing is very difficult and industry must use specially designed equipment to achieve a reasonable mix. A shaken batch in a plastic bag might look mixed, but it isn't.

A second method is to calculate a new recipe that is a combination of the two. INSIGHT can combine two recipes, or, more precisely add one to another. The two are merged so that common materials combine their amounts. In this case you could do it as follows:

We have demonstrated an effective approach to duplicating any material: Generate a final recipe for a substitute and save it. Any time the old material is needed, just perform the magic shown here, and you have a new recipe that is likely to work. Note that this recipe provided the basis for a long project I undertook (#1213) to produce one quality, documented, adjustable recipe for Orton oxidation cone 3-8. You will have different materials to source the oxides lost by removing Cornwall stone from a recipe and you may go about the substitution process a little differently, but the key to success is still the same: View things from the formula viewpoint.

Related Information

L3617 Cornwall Stone substitute vs. real Cornwall Stone

These flow tests demonstrate how similar the substitute recipe (left) is to the real material (right). 20% Frit 3134 has been added to each to enable better melting at cone 5 (they do not flow even at cone 11 without the frit). Links below provide the recipe for the substitute and outline the method of how it was derived using Digitalfire Insight software. This substitute is chemically equivalent to what we feel is the best average for the chemistry of Cornwall Stone.

Cornwall Stone off different dates compared to substitute L3617

This is a cone 11 oxidation melt flow test. Shown (left to right) are the new shipment of Cornwall Stone 2011, the L3617 calculated equivalent (a recipe, see link), the older Cornwall shipment we have been using and the H&G substitute 2011 (far right, mislabelled on the picture). These do not flow well here, a small frit addition is needed to better compare them. However they have melted enough to see some differences in whiteness and degree of melt. Notice the L3617 is more like the old Cornwall than the new Cornwall is.


Materials Cornwall Stone

By Tony Hansen

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