All Articles 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 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 Formulating a Clear Glaze Compatible with Chrome-Tin Stains Formulating a Porcelain Formulating Ash and Native-Material Glazes 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 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 Black Art of Drying Ceramics Without Cracks 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 Using Plaster Bats 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 Leaching Cone 6 Glaze Case Study
An example of how we can use INSIGHT software to determine of a glaze is likely to leach
Following is an example of how we might look at a glaze and determine if it is
food safe. Here is a letter we received:
"I have a receipt for a matte turquoise glaze which I am using on your M350
clay and I was wondering if it was food safe. The following is the recipe. Turquoise Glaze
Potash feldspar 12
Nepheline syenite 23
Strontium carb 15
Copper carb 5
Everyone I ask about the strontium carb. gives me a different answer about it's safety. I would like to be sure it is food safe before I use the glaze on
functional pottery. I would appreciate an answer from you. Thanks."
It appears that strontium carbonate could be the least of this glazes problems. Here is the chemistry of the glaze as calculated by INSIGHT:
Na 2O 0.12*
Al 2O 3 0.26
B 2O 3 0.11
SiO 2 1.53
ZrO 2 0.14
Other articles on this site talk about balance in the chemistry of glazes and SiO
2 and Al 2O 3 content are of primary concern in this regard. This glaze is extremely low in SiO 2 (it is 1.5)(recommended minimum is 2.5), that leads to unstable glasses that leach. You could simply increase the silica in this glaze but it is so low that it needs to be increased to 30 parts in the glaze recipe to supply the minimum amount of the oxide SiO 2! That is certainly going to affect the appearance of the glaze.
Copper is a destabilizing influence in many glazes. Often non-leaching glazes will begin to leach after copper is added. This glaze has lots of copper. Leach testing is obviously needed.
The recommend maximum for SrO is around 0.2, this has 0.27, that is likely too high (INSIGHT has built-in limit formulas that assist in determining minimum and maximum amount for oxides).
Perhaps you would agree that the writer should go back to the people who said this glaze is safe and ask them what they were thinking. We cannot guarantee that it is not, but certainly a simple
leaching test is advisable. I am guessing overnight in vinegar will leach out the color at least. Links