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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)
Can We Help You Fix a Specific Problem?
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 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
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 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 Ceramic Glazes
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

Crazing and Bacteria: Is There a Hazard?


A post to a discussion on the clayart group by Gavin Stairs regarding the food safety of crazed ware.


This is a post to a discussion on Clayart May 1998 by Gavin Stairs

This discussion about the safety of crazed ware for food service is a bit of a puzzle. We have been down a long country lane on the problem of infectious disease over the last century or so. From the time of Jenner (who introduced antisepsis in medical practice) until today, we have passed from the early, heroic struggles of people like Pasteur and Koch, through the miracles of sulfa, penicillin and the antibiotics, to this point. Certainly, elementary hygiene, including washing food ware in hot, soapy water, must be considered to be one of the most consistent successes. Bacteria and other infectious agents seem to be able to adapt to antibiotics and other agents, but a good, hot wash is still more than most can handle, and I've not heard of any cases of adaptation. There are organisms that can survive in hot baths (hot tubs, public baths, hot springs, etc.), some of which are pathogenic, but these do not grow and prosper on dishes.

In the end, we rely on our immune systems to protect us from the everyday risks of infection. Many of the disease causing agents that we fear are endemic in our environment, meaning that we come into contact with them every day. A completely sterile environment is not good for this immunity, since it gets no reminders of what to fend off, and can essentially go to sleep. Our daily contact is a sort of inoculation against the diseases, which we therefore only contract if we come into contact with an infectious agent in large doses.

If this analysis is correct, we should not fear low doses of bacteria such as what might hide in a crack in a glaze. However, if we then place into such a pot a nice culture medium, like some food, and keep it there for a while, we may find that the small colony in a crack has become a large culture in the food. Then we are in trouble.

This indicates to me that we should be sensitive to a range of risk.

Greatest risk is in food storage containers, especially for wet foods and liquids. My feeling is that these should be entirely free of crazing and surface blemishes on the food contact side. This includes the lip. And this should be the case in both pots for sale, and for individual use.

Next are food preparation surfaces and pots. I think these should also be craze free, certainly on articles for sale, and for the food service industry. For personal use, I would not necessarily throw out a favorite pot that developed a glaze crack or two.

Finally, individual plates, cups and bowls. These are the least sensitive, provided they are consistently washed in hot, soapy water, and are not used for food storage. So one might easily accept a crazed glaze on these for one's personal use. But can we control their use by purchasers? I would still be on the side of an unblemished glaze surface on pots for sale as food ware.

What about vases, flower pots, etc.? We know that these are not intended for use as food service vessels, and we might suppose that others would as well. But we have all seen pots intended for one thing being used for another. Government authorities tend to err on the side of caution (some might say extreme caution), and assume that anything that can hold food, will. We might do the same, and assume that any pot without a hole in the bottom is a food pot.

All of that being said, I come back to the argument that we have been eating off crazed plates for a very long time, and we are still here. The risk is not enormous, or there would be no room for discussion. I'm sure some people have become ill from eating food from crazed pots, but I have no statistics or anecdotes to prove it: just a slight understanding of how bacteria work. I would guess that most cases of infection are sub-clinical, meaning that they clear up by themselves without health professional intervention, and maybe without the person knowing it at all. We do hear of cases of widespread food poisoning from time to time, most of which are traceable to poor food hygiene practice in preparation. I would not countenance using crazed glaze pots in a cafeteria, for instance. Nevertheless, as I recounted to the list a short while ago, in a restaurant I once ate soup from a cracked pot and survived to tell about it.

I suppose that the case is about parallel to that of pasteurization of milk. If you are drinking milk from a cow which is known to you and is well, and which was milked the same day (or equivalently, the cheese or yogurt was made the same day), the risk of drinking raw milk is slight. If you are drinking milk which has been collected from many large farms, mixed and processed together, and stored for a time, the risk is far greater, effectively by a factor of the number of cows involved, etc. Pasteurization is warranted. In the case of pottery safety, if we are dealing with a case of an individual dish for our own use, carefully washed
and used appropriately, the risk is slight. If we are dealing with a line of dishes for sale, possibly to the food service industry, then the risks are very much greater. Or at least our liability is potentially much greater.

This discussion began with an exchange between a concerned potter and a large producer of food ware for public sale about a line of crackle glaze dinnerware. I presume the large manufacturer has not yet been sued for damages resulting from a case of infection. The possibility that they will be, or that someone may get ill or die from the use (or misuse) of their ware is for anyone to judge, perhaps particularly their legal department or some regulatory authorities. Other contributors to the list have pointed out that a) stoneware is not 100% non-porous, and b) bacteria can indeed lodge in the cracks, and c) there are techniques to produce the aesthetic effect of a crackle without the crazes on the final glaze. If I were in the position of the large manufacturer, I would certainly opt for a craze-free surface.

Finally, however, I can't help but note that our ancestors ate, prepared and stored food in porous, unglazed, earthenware pots for millennia before we came around to discuss the matter. Safety issues have few blacks and whites, and much grey.

Related Information


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 Attack on Glass: Corrosion Attack Mechanisms
Max Richens outlines the various mechanisms by which acids and bases can dissolve glass and glazes. He provides some information on stabilizing glazes against attack.
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.
Projects Hazards
Tests Boiling Water:Ice Water Glaze Fit Test
Ceramic glazes that do not fit the body often do not craze until later. This test stresses the fit, thus revealing if it is likely to craze later.
Tests 300F:Ice Water Crazing Test
Ceramic glazes that do not fit the body often do not craze until later. This progressively stresses the fit until failure point, thus giving it a score
Glossary Toxicity
Common sense can be applied to the safe use of ceramic materials. The obvious dangers are breathing the dust and inhaling the fumes they produce during firing. Here is a round-up of various materials and their obvious hazards.
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.

By Gavin Stairs

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