Natural Talc C-98

Alternate Names: C98 talc, C-98 talc

Oxide	Analysis	Formula
MgO	29.50%	0.92
Fe2O3	0.50%	-
CaO	3.50%	0.08
K2O	0.30%	-
TiO2	0.10%	-
Al2O3	0.30%	-
SiO2	53.50%	1.12
Oxide Weight		109.96
Formula Weight		109.96

Notes

American Talc Amtal C-98 became Natural Talc C-98 about 2018. No longer available as of March 2021.

A platy talc, mined and processed in Allamoore, Texas. A dark grey powder in the raw state but white-firing. It was used as a flux, filler and thermal expansion modifier for traditional, refractory and technical ceramic applications (wall and floor tile, tableware, sanitaryware, electrical Insulators, porcelain). It was popular in hobbyware and low temperature talc/ball clay bodies (because it was white burning and responded to deflocculation well). It was also used in technical ceramics where it functioned as the major silicate material (replacing clays and feldspars in lower grade alumina products such as steatite ceramics and low-expansion cordierite materials).

Talcs present production issues (the powders are hard to handle and wetting is difficult) and they have perceived safety issues. We are doing work substituting them for Nepheline Syenite (as of 2021) and it is working unexpectedly well.

They claimed a chemistry of 63.7% SiO₂, 28.4% MgO, 6.98% CaO (strangely not mentioning the iron).

Related Information

Natural Minerals C-98 Talc bag

American Talc C-98 bag

Natural Talc C-98 Certificate of Analysis

This was from a shipment received at Plainsman Clays in 2019.

Montana talc vs Texas talc - Powder fire at cone 6

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.

Texas talc (left) and Montana talc (right)

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.

Permeability demonstration of Texas and Montana talcs

Texas talc (left) quickly absorbs all the water poured on it. Montana talc (right) resists whetting of the particles much more, the water is just sitting on top and has not penetrated at all.

0.11g C-98 Talc residue on a 150 mesh screen from 50g

Close-up photo. This is only 1/10th of a gram on 50 grams total. Very clean. This was a wet sieve analysis.

4 Talcs in the same casting clay body at cone 04

The body is 50:50 talc and ball clay. These are Silverline (Montana), Natural Minerals (Texas) and Cimtalc (Indiana). The Texas talc is the whitest, the Montana is the darkest.

Cimtuff 9115 vs Natural Talc C-98 in a low fire white body

The clear glaze is G1916Q. Fired at cone 04. The body on the left is 50:50 Old Hickory M-23 ball clay and Cimtuff talc. The body on the right is a mix of KT1-4 ball clay and C-98 talc. It is firing whiter. But this is not only because of the talc, the KT1-4 ball clay is whiter-burning than the M23.

Talc:Ball Clay bodies have incredible casting properties

This bowl is 13cm across yet has a wall thickness of less than 2mm and weighs only 101g! It released from the mold with no problems and dried perfectly round. But it has a key advantage over stonewares and porcelains: When this is fired at cone 04-06 it will stay round!

Links

Materials	Cimtuff 9115 Talc
Materials	Amtalc-C98
Materials	Silverline 303 Talc
Materials	Sierralite Talc
Materials	Benwood Talc 2213
Materials	Magris Jetfil H 290 Talc
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.
URLs	https://insight-live.com/insight/share.php?z=P8uThTnFoY Replace Talc with Nepheline Syenite in low temperature white bodies

By Tony Hansen
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