Dolomite

Double carbonate of magnesia/calcia

Chemistry

CaO	30.490
MgO	21.900

Volatiles

CO2

47.610

Links to Other Materials

Talc
Calcined Dolomite - Related
Dolomitic Lime - Related
Limestone - Related
Cache

Miscellaneous

Family: Flux Source
Region: North America
Mined At: Unspecified
Raw Mineral: No
Generic: Yes

Notes

Dolomite is ground magnesium limestone rock, it combines calcium and magnesium carbonates. In ceramic glazes it is used as a source of magnesia and calcia. Other than talc, dolomite is the principle source of MgO in high temperature raw glazes. 'Dolomite matte' stoneware glazes, for example, are highly prized for their pleasant 'silky' surface texture.

Commercial dolomites are not able to achieve the theoretical ideal of 54% calcium carbonate and 46% magnesium carbonate, they tend to have less magnesia. It is simple to do an LOI test by firing a specimen of powder in a thin bisqued bowl to confirm the consistency of dolomite shipments. The chemistry shown here is theoretical and many commercial materials approach this with much less than 1% of two or three other oxides like Al2O3, SiO2.

Dolomite is a carbonate (like whiting) in that it loses considerable weight during firing when it disassociates to form MgO, CaO and CO2, this process being complete by about 800C. However, in many circumstances where a raw glaze employs both CaO and MgO, dolomite is an economic alternative to sourcing with whiting and talc. However care needs to be taken to obtain a consistent grade since dolomites tend to vary more in mineralogy and can contain iron contamination that can contribute to a cream color in the fired glaze.

Synthetic substitutes to source MgO and CaO (e.g. frits) are worth considering, especially if glazes are not high temperature. Frits have no loss on ignition and melt far earlier than mineral sources of MgO and CaO. Using ceramic calclations it is quite easy to adjust a recipe to source MgO from a frit instead of raw materials.

(Richard Willis)

Mineral, of the ideal form CaCO₃, MgCO₃ with a typical empirical analysis of CaO 30.41%, MgO 21.86%, CO₂ 47.73%, with traces of Fe, Mn, Co, Pb and Zn.
Hardness: 3.5-4 Density: 2.85-2.95 Soluble in acids, slowly
A high-refractory compound, but easily fluxed by borons and/or a lead aluminum silicate frit to bring it down for under-1000ºC firings. Otherwise, it is used to increase a formula’s resistance to fusion and thus to enhance a glaze’s matteness, particularly at 1100ºC+ firings.

Properties

Surface Modifier - Matte
Dolomite can be used in glazes melting over 1170C to produce a silky matte surface. This occurs because high percentages of dolomite help to form diopside crystals (CaMg(SiO3)2) on cooling, and it is these that produce the popular butter-matte effect. This effect is most pronounced in reduction.

URLs

Wikipedia definition - http://en.wikipedia.org/wiki/Dolomite

Suppliers

Generic

Authors

Tony Hansen (Owner)

Pictures

Cinnabar On Dolomite
Dolomite Crystal Spheres
Dolomite Crystals

XML

<?xml version="1.0" encoding="UTF-8"?>
<material name="Dolomite" descrip="Double carbonate of magnesia/calcia" generic="1" rawmineral="0" searchkey="Calcium Magnesium Carbonate, Raw Limestone" loi="0.00">
<families>
<family name="Flux Source"/>
</families>
<regions>
<region name="North America"/>
</regions>
<oxides>
<oxide symbol="CaO" name="Calcium Oxide, Calcia" status="U" percent="30.490" tolerance=""/>
<oxide symbol="MgO" name="Magnesium Oxide, Magnesia" status="" percent="21.900" tolerance=""/>
</oxides>
<volatiles>
<volatile symbol="" name="" percent="47.610" tolerance=""/>
</volatiles>
<references>
<reference name="seealso" reason=""/>
<reference name="seealso" reason=""/>
<reference name="seealso" reason=""/>
<reference name="seealso" reason=""/>
</references>
<suppliers>
<supplier name="Generic" country="" url="" label=""/>
</suppliers>
<notes>
<note>Dolomite is ground magnesium limestone rock, it combines calcium and magnesium carbonates. In ceramic glazes it is used as a source of magnesia and calcia. Other than talc, dolomite is the principle source of MgO in high temperature raw glazes. \'Dolomite matte\' stoneware glazes, for example, are highly prized for their pleasant \'silky\' surface texture.

Commercial dolomites are not able to achieve the theoretical ideal of 54% calcium carbonate and 46% magnesium carbonate, they tend to have less magnesia. It is simple to do an LOI test by firing a specimen of powder in a thin bisqued bowl to confirm the consistency of dolomite shipments. The chemistry shown here is theoretical and many commercial materials approach this with much less than 1% of two or three other oxides like Al2O3, SiO2.

Dolomite is a carbonate (like whiting) in that it loses considerable weight during firing when it disassociates to form MgO, CaO and CO2, this process being complete by about 800C. However, in many circumstances where a raw glaze employs both CaO and MgO, dolomite is an economic alternative to sourcing with whiting and talc. However care needs to be taken to obtain a consistent grade since dolomites tend to vary more in mineralogy and can contain iron contamination that can contribute to a cream color in the fired glaze.

Synthetic substitutes to source MgO and CaO (e.g. frits) are worth considering, especially if glazes are not high temperature. Frits have no loss on ignition and melt far earlier than mineral sources of MgO and CaO. Using ceramic calclations it is quite easy to adjust a recipe to source MgO from a frit instead of raw materials.</note>
<note>Mineral, of the ideal form CaCO3, MgCO3 with a typical empirical analysis of CaO 30.41%, MgO 21.86%, CO2 47.73%, with traces of Fe, Mn, Co, Pb and Zn.
Hardness: 3.5-4 Density: 2.85-2.95 Soluble in acids, slowly
A high-refractory compound, but easily fluxed by borons and/or a lead aluminum silicate frit to bring it down for under-1000&ordm;C firings. Otherwise, it is used to increase a formula&#146;s resistance to fusion and thus to enhance a glaze&#146;s matteness, particularly at 1100&ordm;C+ firings.

</note>
</notes>
<pictures>
<picture description="Cinnabar On Dolomite" filename="cinnabar_on_dolomite.jpg"/>
<picture description="Dolomite Crystal Spheres" filename="dolomite_crystal_spheres.jpg"/>
<picture description="Dolomite Crystals" filename="dolomite_crystals.jpg"/>
</pictures>
</material>

Much more information with complete interlinking to many related
databases can be found by logging into the www.ceramicmaterials.info database

INSIGHT is ceramic chemistry
calculation software that runs on
Windows, Mac and Linux and talks
to this web site.

Ceramic Materials Database