Joseph Herbert on thu 1 nov 07
Pat Wrote: My question is, this company in Texas sent me talc, which is a
dark gray, and volcanic ash, which is white. As I don't trust the company
because of all their other mistakes, could you tell me if this is the
correct colors for these items? Pat
Identifying geologic materials on the basis powder color is really
difficult. Almost everything is gray to white as a powder, regardless of
its color in larger pieces. There are few exceptions to this and the color
of the powder is then used to identify the material. Hematite (an iron
oxide mineral) can have many different looks in a hand specimen but when
scraped across an unglazed ceramic tile, it leaves a distinctive brick red
powder streak on the tile.
Back to the problem here, I have seen talc powder supplied for ceramic
purposes that is really dark gray. Actually it usually is a pretty dark
gray. Volcanic ash could be any color from really dark grey to a near
white. I feel a little reticent to mention this but the most prominent
characteristic of the mineral talc is its softness. Talc is number one on
the Moh's hardness scale. Volcanic ash is glass and has a hardness in the 5
to 6 range. When a field geologist is trying to determine if a rock is a
shale (composed of clay particles) or a siltstone (composed of very fine
quartz particles) he will take a small sample of the rock between his teeth
and see how it feels. If the material is gritty and gives a grinding
sensation on the teeth, it is silt; if there is no such sensation, it is
clay. The recommended action once the determination is made is to spit out
the sample and then have a beer.
One could, of course, try this test on your materials. One problem with
that is the possibility that some other minerals are included with the talc.
That could give a gritty feel even when most of the sample is talc. Another
problem is the possibility that the material is question is neither of the
mentioned possibilities. Were that to be true, it could then be something
that you shouldn't put in your mouth. Now, almost all ceramic glaze
materials that cost a dollar a pound or less are not harmful if injected by
mouth. They are, after all, mostly ground rocks. However, if the mouth
test is unappealing, grinding a small amount of each in a mortar could give
a definitive feel difference.
Density could be a way to tell the difference but determining the density of
a powder is very difficult. In general, the weight of a similar volume of
talc should be less than the weight of the volume of volcanic ash, if all
other things are equal. All other things are not equal. The shape of the
particles, how they are compacted, the relative dryness of the particles all
play a role in the weight of a volume of powdered materials.
Good luck,
Joe
Joseph Herbert
Technical Writer
Irving, Texas
214-725-8305 (Cell)
| |
|
