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?

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

Description

An example of how we can use INSIGHT software to determine of a glaze is likely to leach

Article

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
Dolomite          12
Kaolin             5
Gerstley borate   12
Potash feldspar   12   
Nepheline syenite 23   
Silica           5.5   
Strontium carb    15   
Ziropax           10   
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:

 CaO       0.32*
 MgO       0.21*
 K2O       0.09*
 Na2O      0.12*
 SrO       0.27*
 Al2O3     0.26 
 B2O3      0.11 
 SiO2      1.53 
 ZrO2      0.14 

Other articles on this site talk about balance in the chemistry of glazes and SiO2 and Al2O3 content are of primary concern in this regard. This glaze is extremely low in SiO2 (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 SiO2! 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

Articles Are Your Glazes Food Safe or are They Leachable?
Many potters do not think about leaching, but times are changing. What is the chemistry of stability? There are simple ways to check for leaching, and fix crazing.
Articles How to Liner-Glaze a Mug
A step-by-step process to put a liner glaze in a mug that meets in a perfect line with the outside glaze at the rim.
Articles Is Your Fired Ware Safe?
Glazed ware can be a safety hazard to end users because it may leach metals into food and drink, it could harbor bacteria and it could flake of in knife-edged pieces.
Materials Copper Carbonate
Materials Copper Carbonate Basic
Materials Copper Oxide Black
Materials Copper Oxide Red
Glossary Leaching
Ceramic glazes can leach heavy metals into food and drink. This subject is not complex, there are many things anyone can do to deal with this issue

By Tony Hansen


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