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Fighting Micro-Organisms in Ceramics

Edouard Bastarache, doctor of Occupational & Environmental Medicine in Quebec, and also ceramicist, translated this from Smart.Conseil's French site, le site Français dédié aux passionnés de céramique.

A quick survey of micro-organisms

Micro-organisms may vary in size from 1 micrometer (1µ = 10-6 meter) to several hundreds of micrometers.

Types Size
Viruses 10 to 300 µ
Bacteria 1 to 10 µ
Family of Fungi Yeasts 2 to 12 µ
Fungi 2 µ to x cm
Molds 2 µ to x cm
Lichens 2 µ to x cm
Algae 1 µ to x cm
Protozoa 2 to 200 µ

Micro-organisms are present everywhere in our environment

In air, water, on surfaces, and on organisms of all kinds. They are invisible and relatively unperceived. Micro-organisms are very numerous: 1 gram of earth may contain up to 25 billion micro-organisms (4 times the population of our planet).

Conditions for the development of micro-organisms in ceramic products are related to the presence of nutritive elements under the influence of temperature, moisture and pH of the medium. Under favourable conditions, one estimate is that the population of micro-organisms can double every 20 minutes! Therefore starting from a few micro-organisms placed in a favourable medium, a true contamination can occur in less than one day. The bacterial growth curve comprises a phase of logarithmic increase followed by a stationary phase, then by a phase of decline when food is suddenly lacking.

Needs Bacteria Molds and Yeasts
Light No No
Medium pH Alkaline Acid
Temperature 20 to 40 °C 20 to 35 °C
Nutrients Nitrogen, Hydrogen, Carbon Nitrogen, Hydrogen, Carbon
Presence of Oxygen O2 or inorganics: SOx, NOx O2

Signs of a microbial invasion in an aqueous product

In general, one observes changes in the color of the surface, viscosity, value of pH, by odors and gaseous emissions as well as variations in the behaviour of the product. One can determine the level and type of contamination by using indicators containing gelose, these make it possible to grow micro-organisms quickly (48 to 96 hours), to quantify and identify them according to appearance, bacteria, yeasts or mushrooms.

Hygiene of Manufacturing

The hygiene of manufacturing often takes place at the beginning of a bacterial contamination, on this matter it is necessary to take care regarding the following points:

  1. Biocides: Anti-bacterial and anti-fungal chemicals for preventive use. The aqueous medium necessary to the implementation of the majority of ceramic products must be protected by biocides chosen for their compatibility with these and introduced at the beginning of the aqueous phase, taking into account their pH.


    • Mixture of chlorinated and non-chlorinated isothiazolon compounds, pH 3.5 + / - 0.5
    • Aqueous solution of ortho-phenylphenol potassium salts, pH > 13
    • 10% solution derived from oxazin with sulphur and nitrogen heterocyclic compounds, pH 10-11
    • Aliphatic compound with sulphur heterocyclic compounds, pH 3-5

  2. Bleach: Note that bleach is an excellent disinfectant, but its high pH and its short duration effect make it a temporary cleaning material not very compatible with ceramic products, as a stable protective biocide.

In Ceramics

  1. In liquids

    The majority of disorders seen in liquid glazes are due to bacterial activity, it strongly modifies viscosity and produces odors.

    These disorders generally occur with natural products containing organic matters (e.g. clays), under the effect of heat and minerals concentration that produces high density in liquid glazes. Glazes containing glue and adhesives provide extra nutrients by bacteria.

    There are often two distinct phases in the deterioration of viscosity in the event of bacterial attack:

    • Fluidification due to the consumption of the components of the adhesive(glue) used in the glaze.
    • Flocculation or thickening due to the acid dejections of bacteria following the consumption of the organic elements of glazes. It then becomes necessary to correct the parameters of viscosity by adding adhesive or deflocculant in the glaze containers.
  2. In porous moulds

    The storage and maintenance of porous resin moulds for casting under high pressure require an anti-bacterial and anti-fungal action. The mould capillary network is significant (10 to 20 µ) and clay fines penetrate into it. Bacterial activity, in the mould layer impregnated with fines, leads to the filling of the pores in a way that is not easily curable by chemical action (when this filling is on the surface, the mechanical action of a high-pressure water spray makes it possible to cure this problem). The best solution consists in permanently having an anti-bacterial and anti-fungal preventive action from the start of using the mould, for its cleaning and its storage.

The Role of Biocides

Biocides are products with anti-bacterial and anti-fungic action. They are products for preventive action, generally used at very low doses (high cost, toxicity) and their action on the rheology of liquid glazes is not to be neglected (due to the biocide pH).

The most currently-used biocides are introduced at the beginning of the preparation, in the mixing water, and at a rate of 0.05% to 0.50% by weight.

In ceramics, bactericides-fungicides with broad spectrum activity, containing sulphureous aliphatic and heterocyclic compounds of low toxicity, are often used.

In too-low doses, these products will not have the expected effect and can give a false sense of security. Selective adaptation can occur with the micro-organisms. In too-large doses, they will be too expensive, their toxic effects will be enhanced and there are risks of environmental damage.

Biocide Suppliers

The vast majority of micro-organisms in nature are saprophytes, in the majority of situations they do not cause infectious unless host conditions already described exist; they are the opportunistic infections.

Opportunistic Infections


Infections among patients whose defence mechanisms are weakened making them susceptible to infections. These infections can be due to micro-organisms that are not highly pathogenic.

Defence mechanisms, physiological, immunological can be disturbed, or a breach made in them, by a disease, a traumatism, or by procedures or products used for diagnostic or therapeutic goals.

Following are a few examples.

Antibiotics and host defence mechanisms reduction.

A treatment by antibiotics modifies the normal flora of the skin, mucous membranes, digestive tract, and can lead to colonization of these organs by new micro-organisms.

This colonization is inoffensive if it is not followed by superinfection (invasion by indigenous germs, or micro-organisms from the environment).

Predisposing factors are:

Modifications of Anatomical Barriers

Examples where the normal anatomical barriers of these patients are broken and they can no longer fight infections.

Treatments by radiotherapy.
Intensive immunosuppressive therapy.

Cytotoxic Drugs

When opportunistic infections are due to a severe leucopenia (lowering of white blood cells).


Their systemic use disturbs many aspects of host defence mechanisms.
Those who suffer from Cushing's syndrome (increased secretion of an endogenous corticosteroid, cortisol) have an increased susceptibility to infections.


  1. The Biological and Clinical Basis of Infectious Diseases, Shulman, Phair, Peterson & Warren, last edition.
  2. Vademecum Clinique du Diagnostique et du Traitement, Fattorusso V. & Ritter O., last edition.

By Edouard Bastarache

Related Information


Typecodes Article by Edouard Bastarache
Edouard Bastarache is a well known doctor that has written many articles on the subject of toxicity of ceramic materials and books on technical aspects of ceramics. He writes in both English and French.
Glossary Micro Organisms
Ceramic glazes and clay bodies can host micro organisms. They can be just a nuisance, a source of worry or can render a product useless. What should you do?

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