Dolomite
Double carbonate of magnesia/calcia
Formula: CaCO3.MgCO3 or CaMg(CO3)2
Dolomite as a ceramic material is a uniform calcium magnesium carbonate. 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. Dolomite by itself is refractory, but when combined with the typical oxides in a glaze (especially boron) it readily enters the melt.
Commercial dolomites are not able to achieve the theoretical 54:46 calcium carbonate:magnesium carbonate ratio, 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 (e.g. 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 900C. However, in many circumstances where a raw glaze employs both CaO and MgO, dolomite is an economic alternative to sourcing with a mix of calcium carbonate 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 darken 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 (therefore do not generate glaze bubbles) and melt far earlier than mineral sources of MgO and CaO. Using ceramic calculations it is quite easy to adjust a recipe to source MgO from a frit instead of raw materials.
Mechanisms
Glaze Matteness - Silky surface 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.
Out Bound Links
- (URLs)
Dolomite at Wikipedia
http://en.wikipedia.org/wiki/Dolomite - (Materials - Related)
Limestone - CaCO3
- (Minerals - Mineral equivalent)
Dolomitic Limestone, Dolostone
Dolomite as a mineral exists as layers of predomin... - (Typecodes)
1: GNM - Generic Material
- (Typecodes)
1: FLS - Flux Source
- (MDT - Member)
Crystal Glazes
These materials are specially defined for makers o... - (MDT - Member)
UK
We are working on this database and would apprecia... - (MDT - Member)
Ron Roy
This is the traditional Ron Roy materials file. He... - (MDT - Member)
North America
The decision about what materials to include in th... - (MDT - Member)
New Zealand
We are working on this database and would apprecia... - (MDT - Member)
Glass Industry
The materials included in this MDT were selected i... - (MDT - Member)
Latin and South America
Latin America and South America. We are working on... - (MDT - Member)
Generic
Generic for building a completely custom MDT. Only... - (MDT - Member)
Europe
Countries of Eastern Europe and former Soviet Unio... - (MDT - Member)
Australia
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Asia
All of Asia including Turkey, Russia, Indosnesia, ... - (MDT - Member)
Africa
All of continental Africa. We are working on this ...
In Bound Links
- (Materials - Related)
Magnesite - MgCO3
Mag Carb, MgCO3, Anhydrous Magnesium Carbonate
- (Materials - Related)
Talc - Mg3Si4O6 or 3MgO.4SiO2.H2O
Magnesium Silicate, Steatite, French Chalk, Hydrated talc
- (Materials - Parent)
Calcined Dolomite - CaO.MgO
Dead-burned Dolomite
- (Materials - Parent)
Camadil 95 Dolomite - Ground Dolomitic Limestone
- (Materials - Parent)
Dolocron 4013 - 325 mesh Dolomite, CaMg(CO3)2
- (Materials - Parent)
Kuncice Dolomite - Unknown
- (Materials - Parent)
Dolowhite - White burning Dolomite, CaMg(CO3)2
- (Materials - Parent)
KARIBIB Dolomite
- (Oxides - Material source)
CaO - Calcium Oxide, Calcia
- (Oxides - Material source)
MgO - Magnesium Oxide, Magnesia
- (Glossary - Unspecified)
Matte Glaze
A glaze that is not glossy. Of course, unmelted gl...
Pictures
Classic dolomite glaze at cone 10 reduction on a speckle producing clay body (10R). The magnesia flux in dolomite creates a silky matte surface.
Example of calcium carbonate (top) and dolomite (both mixed with 25% bentonite). They are fired to cone 9. Both bars are porous and refractory, even powdery.
XML for Import into INSIGHT
<?xml version="1.0" encoding="UTF-8"?>
<material name="Dolomite" descrip="Double carbonate of magnesia/calcia" searchkey="Calcium Magnesium Carbonate, Raw Limestone" loi="0.00" casnumber="69598-19-2">
<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=""/>
<oxide symbol="Al2O3" name="Aluminum Oxide, Alumina" status="" percent="0.000" tolerance=""/>
</oxides>
<volatiles>
<volatile symbol="CO2" name="Carbon Dioxide" percent="47.610" tolerance=""/>
</volatiles>
</material> | 
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