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, used 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).
Natural Talc C-98 Certificate of Analysis
This was from a shipment received at Plainsman Clays in 2019.
Natural Minerals C-98 Talc bag
American Talc C-98 bag
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.
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.
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!
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 thought their data sheets both claim 0.5% Fe2O3). The Montana material is still suitable in glazes (since low percentages are common) and in bodies as a flux (where 1-3% is all that is used). But in bodies where talc was employed to adjust thermal expansion, as much as 50% is used, this is not suitable for that.
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.
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.
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 color is so much darker in the raw form, it fires whiter!