Michael McDowell on thu 21 jan 99
Jon Pettyjohn sent in an analysis of Mt. Pinatubo's Volcanic Ash, and remarked
on the "similarity" of Pinatubo's ash analysis to that of Mt. Saint Helens Ash
given by Lee Love. This is interesting, because I recently sent in a post with
a different analysis of Mt. Saint Helens Ash that is much closer to Jon's
Pinatubo analysis, and my thrust was more remarking about how "different" our
two analyses were. I'm putting the three analyses together in one table below
so that each of you can decide for yourselves if the glass is "half empty or
half full".
> Oxide........Lee's #'s.............My #'s..........Pinatubo
> Ca0...........6.81%...................4.19..............5.66
> Mg0..........6.11......................1.78..............2.75
> K20 ..........0.5.......................1.65..............1.66
> Na20.........3.9.......................4.59............ ..4.48
> Fe203.......6.81......................4.19..............5.66
> Mn0..........0.2........................0.07...................
> Ti02..........0.3........................0.52..............0.84
> P205........0.2........................(trace).................
> Al203.....22.32.....................18.46............16.19
> Si02........51.85.....................66.05........... 61.9
Actually, Jon sent me this table in a private post a couple of days ago. He
remarked on the similarity of all three analyses in that message as well, but
he didn't add the closing caveat that I saw in last night's digest:
>BTW when it comes to comparing raw materials I think
>I'm more comfortable with percent than Seger ratios, I'm
>not sure why, I wonder if anyone else would agree.
This is an interesting remark, because after getting the private message from
Jon touting the "similarity" of all three analyses I was moved to get out my
Insight software and derive the figures for "Seger ratios" (flux-unity) for
each of the three analyses in hopes of pointing out to him how "different" Lee
Love's analysis was from Pinatubo's and the one I have for Mt. Saint Helens.
Here's an excerpt from that note to Jon.
>It could be useful to look at these three analyses expressed as "unit
>formula", since this is a presentation of the same information in a form more
>suited to comparison of materials as glaze components. This is because for
the
>sake of glazes we are interested in the relative proportions of molecules of
>different weights, very different from simple proportion by weight as given
in
>the analysis.
>Oxide......Pinatubo........McD.........Love
>*CaO........0.39..............0.32..........0.36
>*MgO.......0.26..............0.29..........0.44
>*K2O.......0.07..............0.07..........0.02
>*Na20......0.28..............0.32..........0.18
>Fe2O3......0.14..............0.11..........0.12
>Al203.......0.61...............0.77..........0.64
>SiO2........3.98...............4.69..........2.53
>Si/Al
>Ratio........6.50...............6.08..........3.95
Now Jon remarks that "The Pinatubo ash makes a good glaze (brown slightly
metallic) on it's own at ^9-10." That is also my experience with Mt. Saint
Helens ash as well, and indeed the two "Seger formulas" are fairly close. Both
slightly exceed Green & Cooper's recommended limits on K/Na20 for a cone 10
glaze, but are otherwise acceptable in that range. If there is a material that
conforms to the analysis Lee Love submitted, it is not likely that it is a
comparable glaze by itself at cone 10. K/Na2O contributes only a fifth of the
flux in Lee's analysis, while it contributes over a third to the other two.
Lee's material also shows Magnesia well in excess of the recommended limit of
0.35 molar parts, and the Silica at 2.53 falls below it's limit for a cone 10
glaze. The Silica/Alumina ratio less than 4 for Lee's analysis would predict a
very matte glaze in any case (as would the oversupply of Magnesia), while both
the other two analyses show Si/Al ratios above 6, which is in the region where
we could expect a "soft gloss".
Most of you who might be reading this are unlikely to have sources of any of
the materials under discussion. My point in going to this depth in comparing
them is simply to point out how easily one can misinterpret apparent
"similarity" between materials based on a straight analysis by weight. I would
suggest that those of you who are not comfortable using the "Unity formula"
approach to comparing glazes and glaze materials endeavor to become more
comfortable with it. It's an approach that will serve you well.
Michael McDowell
Whatcom County, WA USA
mmpots@memes.com
http://www2.memes.com/mmpots
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