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Alternate Names: Mag Carb, MgCO3

Oxide Analysis Formula
MgO 47.80% 1.00
CO2 52.20%n/a
Oxide Weight 40.30
Formula Weight 84.31


Magnesite is used in pottery bodies, glazes, and glass. Magnesite is used in low fire glazes to produce opacity and matteness.

In glaze melts it is an active flux at higher temperatures (2150C+), and it produces elasticity, a lower expansion coefficient and smooth buttery surfaces. At lower temperatures it is a refractory and will matte glazes and make them crawl due to its high shrinkage and contributions to the surface tension of the melt.

This material has a very high Loss on Ignition, this could cause glaze surface issues. Dolomite and talc more readily release their MgO to the glaze melt for a higher temperature glazes.

Magnesium carbonate by itself is very refractory, and is used to make bricks for the cement and metal industries. It is 'dead burned' in rotary kilns, then reground, sized, and dry pressed using organic binders.

This material has a much higher weight density than light magnesium carbonate, the latter is an incredibly fluffy and light white powder.

Related Information


Materials Talc
A source of MgO for ceramic glazes, a flux or thermal expansion additive in clay bodies, also used in the manufacture of cordierite.
Materials 95 Magnesite
Materials Dolomite
An inexpensive source of MgO and CaO for ceramic glazes, also a highly refractory material when fired in the absence of reactant fluxes.
Materials Light Magnesium Carbonate
A refractory feather-light white powder used as a source of MgO and matting agent in ceramic glazes
Materials Magnesium Oxide
Typecodes Generic Material
Generic materials are those with no brand name. Normally they are theoretical, the chemistry portrays what a specimen would be if it had no contamination. Generic materials are helpful in educational situations where students need to study material theory (later they graduate to dealing with real world materials). They are also helpful where the chemistry of an actual material is not known. Often the accuracy of calculations is sufficient using generic materials.
Typecodes Flux Source
Materials that source Na2O, K2O, Li2O, CaO, MgO and other fluxes but are not feldspars or frits. Remember that materials can be flux sources but also perform many other roles. For example, talc is a flux in high temperature glazes, but a matting agent in low temperatures ones. It can also be a flux, a filler and an expansion increaser in bodies.
Typecodes Refractory
Materials that melt at high temperatures. These are normally used for kiln bricks, furniture, etc. or for ceramics that must withstand high temperatures during service.
Magnesite at Wikipedia
Temperatures Magnesite decomposition (500-600)
Oxides MgO - Magnesium Oxide, Magnesia
Minerals Magnesite
A magnesium carbonate mineral. It is quarried in California, Washington, Austria, Russia, Manchuria,


Hardness (Moh) 4-4.5
Solubility Soluble in warm acids
Density (Specific Gravity) 3


Glaze MattenessIn low temperature glazes magnesium carbonate in amounts to 15% acts as a refractory, remaining in suspension in the glaze melt to produce a white opaque matte glaze.
Glaze Surface TextureMagnesium carbonate is commonly added to glazes, especially at low fire, to make them crawl (it shrinks and cracks the glaze layer at the low end of firing and then its high melt viscosity pulls the glaze melt into islands). This often produces dramatic visual effects, especially if the crawling glaze color contrasts with the underlying body or slip. Additions vary from 10-30% depending on the host glaze.
By Tony Hansen
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