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

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
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 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

Demonstrating Glaze Fit Issues to Students

Description

Glaze and body can both be adjusted to solve crazing and shivering problems. This describes a simple project to create body glaze combinations guaranteed to craze and shiver to demonstrate the principles involved.

Article

At the risk of sounding like a broken record, I want to flog the issue of crazing again. I continually meet students who have no idea about what it is. This is incredible to me. No piece of functional ware should be crazed. Period. This is our duty to customers.

Crazing is a technical issue and the buck stops with you. You cannot put the whole burden of dealing with it on your suppliers, you really need to understand what it is and what causes it to be able to exercise the necessary diligence. I won't review it all here, but suffice it to say that crazing happens when you have a size 5 glaze stretched on a size 6 pot. The glaze then cracks to relieve the stress. The stress develops during cooling (with its associated shrinkage) in the kiln after the molten glaze freezes.

Most solids expand when heated and contract when cooled. Glaze and body are glued together so obviously they need to expand and contract the same amount during heating and cooling or something has to fail.

Typical glazes naturally tend to have higher expansion (and thus higher contraction) than bodies. The vast majority of mineral mixtures that melt the proper amount at pottery temperatures will be crazed on typical pottery bodies. Crazing is thus 'nature's norm'. That means that special attention is needed to produce the abnormal result of proper glaze fit.

Bodies high in silica expand more and thus craze less (and vice versa). A good demonstration is to calcine some ball clay (it is very high in silica) and mix raw and calcine 50:50 (it is too plastic and sticky if you don't do this), slurry and dewater to make a body, then make, glaze and fire a small bowl. The glaze will shell off on cooling in the kiln (this is the opposite of crazing, it is a size 6 glaze on a size 5 body). Now make a body of 50% feldspar and 50% plastic kaolin (eg. Tile 6) and glaze and fire. The glaze will craze badly on this. The lesson: The latter body has no quartz, quartz content is the major factor in increasing body thermal expansion. That means more quartz makes glazes craze less. Unfortunately higher thermal expansion is a bad thing for other reasons (it means more susceptibility to cracking due to sudden temperature changes during ware use).

Oddly, glazes high in silica expand less when heated, this is the opposite of what bodies do. This is because the fluxes dissolve the quartz particles and they become part of a silicate glaze structure. In glazes chemistry is the key issue, in bodies the issue is mineralogy. To demonstrate, mix nepheline syenite with about 5% bentonite and make a glaze and apply to a pot and fire at cone 10. You will find a severe case of crazing on any body. Now blend silica and nepheline 50:50 (with 5% bentonite). Crazing will be dramatically less. Add some boron (from a frit or BORAQ) and the extra melting will enable you to add more silica which will reduce and eliminate the crazing.

The moral is that you are in control if you want to be. Body and glaze formulation are the major factors in crazing. Give me a glaze and I can adjust a body's mineralogy to fit it. Give me a body and I can adjust a glazes chemistry to fit it. Thus if your glazes always seem to craze on a body it means either that the body has less silica than is right for you or your glazes lack silica (or employ too much high expansion soda/potash contributing materials like feldspar).

Industry typically uses bodies of much lower silica since they are capable of formulating glazes of low expansion to fit. The benefit is ware of low thermal expansion that can withstand thermal shock. Pottery porcelains, on the other hand, have plenty of quartz so less technical customers have fewer problems fitting their glazes. Actually, few potters fit anything, it often just a matter of chance that their glazes fit the bodies. Bodies trend toward certain expansions in different parts of North America and often when potters move across the continent they find their glazes don't fit bodies available in the new area. This is especially true where clays native to an area are employed.

I'm going to bash crazing twice more: Crazing is not just unsanitary, but it weakens pieces up to 400% according to our tests. Why? Each minute glaze crack wants to extend itself into the body when stresses are applied. Ceramic propagates cracks very well because of its brittle nature. If you have a smooth crack and defect free surface cracks have nowhere to start and ware is much stronger and more durable over time. But crazing weakens it and supplies thousands of started cracks just waiting to extend themselves. Time and continued use are the catalyst to eventual chipping, cracking or premature breakage. Craze lines are also the point of entry of water into porous bodies (almost all stoneware bodies have some porosity). This results in further weakening and waterlogged pieces that can become 'bacteria farms'.

So take this whole issue seriously. Examine your colored glazes very closely to see if they are crazing. Stress your ware using boiling and ice water to bring out delayed crazing. For more information, search for 'crazing' at digitalfire.com.

Related Information

Links

Articles The Effect of Glaze Fit on Fired Ware Strength
The fit between body and glaze is like a marriage, if is is strong the marriage can survive problems. Likewise ceramic ware with well fitting glaze is much stronger than you think it might be, and vice versa.
Troubles Glaze Crazing
Ask the right questions to analyse the real cause of glaze crazing. Do not just treat the symptoms, the real cause is thermal expansion mismatch with the body.
Materials Nepheline Syenite

By Tony Hansen


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