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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
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

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 body using clays native to your area
Formulating a Clear Glaze Compatible with Chrome-Tin Stains
Formulating a Porcelain
Formulating Ash and Native-Material Glazes
G1214M Cone 5-7 20x5 glossy transparent glaze
G1214W Cone 6 transparent glaze
G1214Z Cone 6 matte glaze
G1916M Cone 06-04 transparent glaze
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
Glaze chemistry using a frit of approximate analysis
Glaze Recipes: Formulate and Make Your Own Instead
Glaze Types, Formulation and Application in the Tile Industry
Having Your Glaze Tested for Toxic Metal Release
High Gloss Glazes
Hire Me to Fix a Specific Problem
Hire Us for a 3D Printing Project
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
I have always done it this way!
Inkjet Decoration of Ceramic Tiles
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
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
Painting Glazes Rather Than Dipping or Spraying
Particle Size Distribution of Ceramic Powders
Porcelain Tile, Vitrified 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
Simple Physical Testing of Clays
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 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
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 in understanding glazes?
Why Textbook Glazes Are So Difficult
Working with children

Diagnosing a Casting Problem at a Sanitaryware Plant


A company using a rubbery casting slip and depending on outside technical support discovers how simple it is to fix a slip that is drastically wrong using simple testing.


I joined a company that manufactured sanitaryware. It had been built and run by an overseas company who then sold it to a South African holding company. The plant was a conventional one, the slip being prepared in a sliphouse, the clays turned into slops using blungers, and a body slip made up and then filter pressed. The filtercakes were then reconstituted into a casting slip. This was the best way, rather than direct slip making, as the clays were crude and needed to be cleaned up first.

White glaze was produced at the plant, coloured glazes were bought in. Casting was by hand using casting benches and firing was carried out in electric tophat kilns.

When I arrived the slip was like porridge and casting times were long. It was something to watch casters handling separate cast rims, they flexed like rubber in their hands! I immediately had an idea what was wrong but needed to prove it. The original owners had used their overseas technical facilities and so the plant did not have a Lab to speak of. One test was being done - particle size by hydrometer, and that was wrong. So to investigate the slip problem I bought a torsion viscometer, a burette and stand, a small lab stirrer and a stopclock. Then I proceeded to do deflocculation curves on each clay. Of the four clays it became obvious that there were problems with two of them. One of them was a china clay obtained from a small company in the area. While it should have deflocculated easily, instead it gelled to the extent that I could invert the container of clay slip (with added deflocculant) and it stayed in place, not even shaking could remove it! The other clay was supplied by a brickworks and it also thickened rather than thinned as deflocculant was added, but not to the extent of the China clay. By now I definitely knew the problem was that of Montmorillonite in the clays, which causes gelling. I sent the clays for XRD analysis, reminding the laboratory to add glycerol to the them to cause the Monmorillonite to swell on its C-axis (otherwise it might not be detected). It was detected in both of the 2 clays.

The offending clays were replaced and the slip became fluid and the casting rate was much faster and the cast items were hard. A shorter body resulted but the casters soon adapted and the formulation continued to be used for years.

Monmorillonite contains Magnesium in the Brucite layer, I wondered how both of the errant clays, which were essentially kaolinitic, came to have Montmorillonite in them. I am not a Geologist but I reckoned that perhaps there was Magnesium containing rocks near to these clays. This turned out to be true, in the Cape there were such rocks exposed above the China clay deposit and only the clay near to the surface contained Montmorillonite. In addition, the brick clay was in a Dolomitic area.

Related Information


By Peter Clowes

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