A Low Cost Tester of Glaze Melt FluidityA One-speed Lab or Studio Slurry MixerA Textbook Cone 6 Matte Glaze With ProblemsAdjusting Glaze Expansion by Calculation to Solve ShiveringAlberta Slip, 20 Years of Substitution for Albany SlipAn Overview of Ceramic StainsAre You in Control of Your Production Process?Are Your Glazes Food Safe or are They Leachable?Attack on Glass: Corrosion Attack MechanismsBall Milling Glazes, Bodies, EngobesBinders for Ceramic BodiesBringing Out the Big Guns in Craze Control: MgO (G1215U)Ceramic Glazes TodayCeramic Material NomenclatureCeramic Tile Clay Body FormulationChanging Our View of GlazesChemistry vs. Matrix Blending to Create Glazes from Native MaterialsConcentrate on One Good GlazeCone 6 Floating Blue Glaze RecipeCopper Red GlazesCrazing and Bacteria: Is There a Hazard?Crazing in Stoneware Glazes: Treating the Causes, Not the SymptomsCreating a Non-Glaze Ceramic Slip or EngobeCreating Your Own Budget GlazeCrystal Glazes: Understanding the Process and MaterialsDeflocculants: A Detailed OverviewDemonstrating Glaze Fit Issues to StudentsDiagnosing a Casting Problem at a Sanitaryware PlantDrying Ceramics Without CracksDuplicating Albany SlipDuplicating AP Green FireclayElectric Hobby Kilns: What You Need to KnowFighting the Glaze DragonFiring Clay Test BarsFiring: What Happens to Ceramic Ware in a Firing KilnFirst You See It Then You Don't: Raku Glaze StabilityFixing a glaze that does not stay in suspensionFormulating a Clear Glaze Compatible with Chrome-Tin StainsFormulating a PorcelainFormulating Ash and Native-Material GlazesFormulating Your Own Clay BodyG1214M Cone 5-7 20x5 Glossy Base GlazeG1214W Cone 6 Transparent Base GlazeG1214Z Cone 6 Matte Base GlazeG1916M Cone 06-04 Base GlazeG1947U/G2571A Cone 10/10R Base Matte/Glossy GlazesGetting the Glaze Color You Want: Working With StainsGlaze and Body Pigments and Stains in the Ceramic Tile IndustryGlaze Chemistry Basics - Formula, Analysis, Mole%, Unity, LOIGlaze chemistry using a frit of approximate analysisGlaze Recipes: Formulate Your Own InsteadGlaze Types, Formulation and Application in the Tile IndustryHaving Your Glaze Tested for Toxic Metal ReleaseHigh Gloss GlazesHow a Material Chemical Analysis is DoneHow desktop INSIGHT Deals With Unity, LOI and Formula WeightHow to Find and Test Your Own Native ClaysHow to Liner-Glaze a MugI've Always Done It This Way!Inkjet Decoration of Ceramic TilesInterpreting Orton ConesIs Your Fired Ware Safe?Leaching Cone 6 Glaze Case StudyLimit Formulas and Target FormulasLow Budget Testing of the Raw and Fired Properties of a GlazeLow Fire White Talc Casting Body RecipeMake Your Own Ball Mill StandMaking Glaze Testing ConesMonoporosa or Single Fired Wall TilesOrganic Matter in Clays: Detailed OverviewOutdoor Weather Resistant CeramicsOverview of Paper ClayPainting Glazes Rather Than Dipping or SprayingParticle Size Distribution of Ceramic PowdersPorcelain Tile, Vitrified or Granito TileRationalizing Conflicting Opinions About PlasticityRavenscrag Slip is BornRecylcing Scrap ClayReducing the Firing Temperature of a Glaze From Cone 10 to 6Single Fire GlazingSoluble Salts in Minerals: Detailed OverviewSome Keys to Dealing With Firing CracksStoneware Casting Body RecipesSubstituting Cornwall StoneSuper-Refined Terra SigillataThe Chemistry, Physics and Manufacturing of Glaze FritsThe Effect of Glaze Fit on Fired Ware StrengthThe Four Levels on Which to View Ceramic GlazesThe Majolica Earthenware ProcessThe Physics of Clay BodiesThe Potter's PrayerThe Right Chemistry for a Cone 6 MgO MatteThe Trials of Being the Only Technical Person in the ClubThe Whining Stops Here: A Realistic Look at Clay BodiesThose Unlabelled Bags and BucketsTiles and Mosaics for PottersToxicity of Firebricks Used in OvensTrafficking in Glaze RecipesUnderstanding Ceramic MaterialsUnderstanding Ceramic OxidesUnderstanding Glaze Slurry PropertiesUnderstanding the Deflocculation Process in Slip CastingUnderstanding the Terra Cotta Slip Casting Recipes In North AmericaUnderstanding Thermal Expansion in Ceramic GlazesUnwanted Crystallization in a Cone 6 GlazeVariegating GlazesVolcanic AshWhat Determines a Glaze's Firing Temperature?What is a Mole, Checking Out the MoleWhat 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.
Gerstley borate 12
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:
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.