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Alumina Toxicology | Ammonia and Latex Toxicity | Antimony Oxide | Are colored porcelains hazardous? | Arsenic Oxide | Asbestos: A Difficult-to-Repace Material | Ball Clay | BARIUM and COMPOUNDS / Toxicology | Barium Carbonate | Bentonite Toxicity | Beryllium Monoxide Toxicology | Bismuth Trioxide Toxicology | Boron Compounds and Their Toxicity | Brown Stain | Cadmium Toxicity | Calcium Carbonate Toxicology | Carbon Monoxide Toxicity | Cesium Toxicology | Chromium Compounds Toxicology | Clay Toxicity | Cobalt Oxide and Carbonate | Cobalt Toxicology | Copper Compounds Toxicology | Copper Oxide and Carbonate | Cristobalite Toxicity | Cryolite and Ceramics | Dealing With Dust in Ceramics | Diatomaceous Earth Toxicology | Dioxins in Clays | Epsom Salts | Eye Injuries Due to Radiation | Feldspar | Fighting Micro-Organisms in Ceramics | Fluorine Gas | Gallium Oxide Toxicology | Hafnium Oxide Toxicty | Hydrofluoric Acid Toxicity | Iron oxide and Hematite | Lead Chromate | Lead in Ceramic Glazes | Lead Toxicology | Lithium Carbonate Toxicity | Lithium Toxicology | Man-Made Vitreous Fibers (MMVF) Toxicology | Man-Made Vitreous Fibers Safety Update | Manganese and Parkinsons by Jane Watkins | | Manganese Inorganic Compounds Toxicology | Manganese Toxicity by Elke Blodgett | Manganese: Creativity and Illness by Dierdre O'Reilly | Molybdenum Compounds Toxicology | Nickel Compounds Toxicity | Niobium Oxide Toxicity | Occupational Dermatoses | Overview of Material Safety by Gavin Stairs | Paraffin Toxicology | Perlite Toxicity | Plant Ash Toxicity | Potassium Carbonate Toxicity | Pregnancy and Ceramics | Propane Toxicology | Quartz Toxicity | Quartz Toxicity on Clayart | Rare Earth Compounds Toxicity | Rubidium and Cesium Toxicology | Rutile Toxicology | Silicosis and Screening | Silver Compounds Toxicology | Sodium Azide Toxicology | Sodium Carbonate Toxicology | Sodium Silicate Powder Toxicology | Stannous Chloride Toxicity | Strontium Carbonate Toxicity Note | Sulfur Dioxide Toxicity | Talc Hazards Overview | Talc Toxicology | Thallium Oxide Toxicology | The Use of Barium in Clay Bodies | Thorium Dioxide Toxicity | Tin Inorganic Compounds | Titanium Dioxide Toxicology | Toxicological Assessment of Zeolites | Tungsten Compounds Toxicology | Understanding Acronyms on MSDS's | Uranium and Ceramics | Vanadium and Compounds Toxicology | Vermiculite | Zinc Compounds Toxicology | Zirconium Compounds Toxicity | Zirconium Encapsulated Stains Toxicity

Manganese in Clay Bodies

A number of minerals source manganese in very high percentages, these can be used to manufacture pure manganese dioxide which is used in the manufacture of stains and as a raw colorant in glazes. But clay body manufacturers do not need to use pure manganese dioxide because the element manganese can be found in the materials umber and ochre, a benefit is that both are clay-like. It takes about 10% raw umber to make a buff burning clay body fire black. The umber does not add any handling hazards beyond those common to clays, the main of which is the presence of quartz.

What about the MnO in black clay bodies generating fumes during firing or leaching into food and drink? Obviously, if a producer adds 5-10% pure manganese powder to a body that is not a good idea (why even do it since a body can be stained to black using umber, it contains only 10% MnO). But put this in perspective. First, if a black body is covered by a safe glaze then it presents no food contact issue. Pure manganese is employed in thousands of glaze recipes in common use, often at levels far above 5%, that is where the attention should be. Consider also that manganese begins to volatilize just beyond cone 6 (black bodies will start bloating and then melt making that abundantly clear). If you are firing kilns full of MnO-sourcing materials (in the bodies and/or glazes) it only makes sense to have a ventilation system attached.

A way to completely avoid the above-mentioned problem is the use an engobe that is colored using a black ceramic stain. Our L3954B is a good base for cone 6 and L3954N for high temperatures.

Among potters and hobbyists, another significant use of manganese is in metallic raku-fired glazes (20% or more is common in recipes). Standing downwind or close to outdoor raku kilns laden with manganese-saturated glazes is an obvious hazard. However, adequate temperatures must be reached for the fuming to occur.

What about granular manganese used to add speckle to bodies? Before classifying these as dangerous the situation must be put into perspective. Such bodies contain only about 0.2% of 60-80 mesh manganese granular. We have found that at cone 6 oxidation, granular manganese can be tolerated without body bloating in vitreous (near zero porosity) bodies, this indicates that decomposition is not occurring, otherwise bloating would occur. Also, the vast majority of manganese particles are encapsulated within the clay matrix. Most of the tiny percentage of particles exposed at the body surface are engulfed by the glaze. All of the particles that actually bleed up into the glaze to either near or at the surface have been significantly diluted and stabilized against leaching by the glaze melt that surrounds them (they bleed and diffuse into it). Thus the total area of leachable manganese glass on a functional surface is extremely small. Notwithstanding this, there is a perception that must be considered. Some potters have used engobes and slips containing manganese granular so that these can be applied, under the glaze, to non-food surfaces only.

By Tony Hansen
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Related Information

Decomposing manganese granular particles are causing this stoneware to bloat

This is a cone 6 stoneware with 0.3% 60/80 mesh manganese granular (Plainsman M340). Fired from cone 4 (bottom) to cone 8 (top). This body is normally stable to cone 8, but with the manganese it begins to bloat at cone 7! This is evidence that particles of manganese are generating gases as they decompose and melt at the same time as the body is vitrifying, these produce volumes and pressures sufficiently suddenly that closing channels within the maturing body are unable to vent them out.

A body containing manganese bubbles the glaze

Laguna Barnard Slip substitute fired at cone 03 with a Ferro Frit 3195 clear glaze. The very high bubble content is likely because they are adding manganese dioxide to match the MnO in the chemistry of Barnard (it gases alot during firing).

An original container of manganese dioxide

This bag will give you a clue as to what manganese dioxide, MnO2, is mainly used for. Staining bricks.

This cone 6 black-burning stoneware is safe. Why?

A black stoneware mug

Black burning bodies are popular with many potters. This one is stained by adding 10% raw umber to a buff-burning stoneware. Umbers are powerful natural clay colorants, they have high iron and also contain manganese oxide. This mug fired perfectly. For both aesthetic and safety reasons. A white engobe, L3954B, was applied during leather hard stage, on the inside and partway down the outside. After bisque, transparent G2926B glaze was applied inside and GA6-B outside (over the engobe it fires amber but over the black clay it produces a deep brown). What about safety? Notice the clay has not bloated, the glaze has no bubble clouds or blisters, that means no manganese fumes were being generated. This happened because the top temperature on the firing was closely guarded to not exceed cone 6. And the body is a cone 10 stoneware, not cone 6, there is insufficient feldspar to trigger release of the MnO from the umber (and its subsequent decomposition to metallic state).

High-manganese black-burning body at cone 6. Practical and safe to use?

This type of clay is made by a number of North American manufacturers. Generally, raw or burnt umber are used as the stain (10% or more). The umber is a clay that matures at low temperatures, so it acts as a flux in the body. Some manufacturers make the mistake of adding it to a cone 6 body and this is the result: bubbling of the glaze (in this case it is a transparent) and bloating and warping of the body. The umber is decomposing and potentially gassing metal fumes. However, if the umber is added as a stain to a cone 10 clay, it fluxes it to mature the resultant body at around cone 6 and these gassing problems do not occur.


Materials Manganese Granular
In ceramics, it is used primarily in clays and glazes to achieve fired speckle (including the brick industry).
Materials Manganese Oxide
Materials Manganese Dioxide
A source of MnO used in ceramic glazes and the production of ceramic stains. Commonly made by grinding pyrolusite rock.
Materials Manganese Carbonate
Hazards Manganese Inorganic Compounds Toxicology
Hazards Manganese and Parkinsons by Jane Watkins
A story of one person and manganese poisoning.
Hazards Manganese Toxicity by Elke Blodgett
A story of the struggle of one person to identify and deal with manganese toxicity
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.
Glossary Engobe
Engobes are high-clay slurries that are applied to leather hard or dry ceramics. They fire opaque and are used for functional or decorative purposes. They are formulated to match the firing shrinkage and thermal expansion of the body.
Temperatures Manganese compounds may begin to fume (932-)

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