|Monthly Tech-Tip |
Description: MONTANA TALC
Silverline 325 is a 325 mesh, asbestos free, platy Montana talc. It is intended for use as a general purpose talc filler where high brightness is not required so their current data sheet does not show the chemistry (however we have it from a prior one). It certainly does have suitable properties for ceramic applications, though it is not actively promoted for ceramics.
It is useful in ceramics as a flux for middle temperature bodies, 2-3% can reduce maturing temperature by 2-3 cones. The iron content is high enough that this talc will yellow the color of white earthenware casting bodies (these typically employ 50% talc).
% Passing 200 mesh (74 micron): 100
325 mesh (44 micron): 98
-20 micron: 79%
-10 micron: 57%
-5 micron: 38%
-1 micron: 7%
Median particle size: 7.5 micron
Oil Absorption g/100g talc (ASTM D281): 29
Bulk Density (lbs per cubic ft) Loose: 16 Tapped: 32
Specific Gravity: 2.8
Asbestiform Mineral Test: CTFA J 4-1
Here is a grade comparison between the various Montana Talc products made by Imerys Talc:
Yellowstone: Microcrystalline 200 mesh. Excellent for steatite insulators and as a flux for high alumina ceramics.
Steawhite 2: 200 mesh steatite talc that fires off-white. Low alkali and low iron content. Excellent for electrical porcelain / glazes and color sensitive steatite applications.
Luzenac 2C: Unique talc-chlorite mineral that contains relatively high amounts of aluminum in the crystal lattice. Used in cordierite applications, sanitaryware flux, white-body floor tile and vitreous china.
Sierralite 252HS:Chlorite with high alumina content and low iron oxide content. Used in ceramics and refractory materials requiring low thermal expansion and good thermal shock resistance.
The company was bought by Imerys.
Texas talc contains some amorphous carbon. The carbon is not stand-alone, but as CO2 in the dolomitic part of the ore. It produces ~7% LOI between 750-850C. Even though the powder color is so much darker in the raw form, it fires whiter! But there is more going on here. On paper, both contain about 0.5% Fe2O3. But the iron species in the two talcs are different. In Texas talcs, the iron is part of the crystal lattice. But, in the Montana material, that 0.5% Fe2O3 is an external iron oxide mineral species, a physical contaminant. While the Montana material fires much darker because of this that iron seems to have little affect on the color of the raw white powder.
Although this Montana talc (Silverline 303) is white as a raw material (vs. dark grey for the Texas material, Natural Minerals C-98), it does not fire nearly as white (even though their data sheets both claim 0.5% Fe2O3). The Montana material is often suitable as a substitute in glazes where percentages are low. If higher percentages are employed (e.g. 10% to achieve a silky matte) then color will darken if Montana talc is substituted for Texas talc. In bodies, where talc is used as a flux in low percentages (e.g. 1-3%) the color should be OK. In low temperature clay bodies, where talc was employed to adjust thermal expansion, as much as 50% is used - of course in these cases the color will be far too dark.
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.
Natural Talc C-98
A source of MgO for ceramic glazes, a flux or thermal expansion additive in clay bodies, also used in the manufacture of cordierite.
|% Passing 325 Mesh Wet||99.5%|
|Density (Specific Gravity)||2.8|
|By Tony Hansen|
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