John Hesselberth on tue 27 feb 01
Paul Lewing wrote:
>John did, indeed, have some very good things to say about this subject,
>but I'd like to comment on his assertion that the weight percentage
>figure is useless.
>I don't find this to be true.
Hi Paul and Tony H,
Hmmmm. I think I'll stand my ground here on mole vs weight per cent.
Yes, unity and mole % just contain the same information in different
format. You can convert from unity to mole % just by summing all the
unity numbers and dividing by the total. That different format, though,
can be helpful, at times, to better understand what is going on.
Yes, Wt. % does give different information.
Yes, wt. % is commonly used by geologists and even by some glaze
formulators
All that still doesn't say it provides useful information for glaze
formulators except when they need to weigh out or buy stuff. It is
useful information for geologists because they sell things by the pound.
I don't think we value our glazes by how much one weighs; although that
will tell you how much it costs since we buy things by the pound. I
guess a manufacturing glaze formulator who had responsibility for cost
reduction would have to work with wt. %s. And a chemist needs weights
when she is going to weigh out stuff and run the reaction -- but not when
she figures out what reaction she is going to run.
Here is my basic problem with wt. % and why I favor mole % or unity.
Chemistry happens by the molecule. One molecule of this links up with 2
molecules of that, etc. I haven't ever seen a chemical equation written
in pounds. Those molecules don't much care how much each other weigh
except in a rare case when steric factors get in the way because one
molecule is just to big for the space others allow it have. So the type
of glass we get depends on how many molecules of each constituent we have
and what their physical and chemical properties are.
Here are 2 examples to illustrate my point.
-------
Glaze name: Example 1
Recipe: Percent
Frit 3124 26.50
EPK 28.50
Strontium Carbonate 23.00
Flint 22.00
Totals: 100.00 %
Unity Formula for Example 1:
0.012 K2O 0.514 Al2O3 3.251 SiO2
0.106 Na2O 0.204 B2O3 0.004 TiO2
0.267 CaO 0.004 Fe2O3 6.3:1 Si:Al Ratio
0.004 MgO
0.612 SrO
Percentage Analysis:
55.96 % SiO2
15.01 % Al2O3
4.07 % B2O3
0.32 % K2O
1.89 % Na2O
4.30 % CaO
0.06 % MgO
18.16 % SrO
0.17 % Fe2O3
0.09 % TiO2
--------
Glaze name: Example 2
Recipe: Percent
Frit 3124 31.00
EPK 31.70
Wollastonite 23.20
Flint 14.10
Totals: 100.00 %
Unity Formula for Example 2:
0.010 K2O 0.507 Al2O3 3.235 SiO2
0.107 Na2O 0.205 B2O3 0.003 TiO2
0.849 CaO 0.007 Fe2O3 6.4:1 Si:Al Ratio
0.034 MgO
Percentage Analysis:
61.08 % SiO2
16.25 % Al2O3
4.46 % B2O3
0.28 % K2O
2.07 % Na2O
14.96 % CaO
0.44 % MgO
0.35 % Fe2O3
0.06 % TiO2
One of these glazes is 56 wt. % silica and the other is 61 wt. % silica.
One is 15 wt.% alumina, the other 16.25 wt.%. One 4.07wt. % boron, the
other 4.46 wt.% boron. But in a pound of mixed glaze there are
essentially the same number of silica, alumina or boria molecules
floating around in each ready to mate with the others to make glass.
What has changed is that I substituted all the strontia molecules in the
first for an equal number of calcia molecules in the second--and that is
all that has changed. The unity formula (or mole %) tells me that
immediately. Wt. % doesn't tell me that at all. In fact it confuses me
by telling me I now have less calcia + strontia in the second recipe
(14.96 + 0 = 14.96%) than I had calcia + strontia in the first (4.3 +
18.96 = 23.26%). In fact I have essentially the same number of calcia +
strontia molecules in both--I have just shifted the balance from calcia +
strontia to all calcia. And that is what makes for the difference in
the way these two glazes will behave. Strontia is a slightly better
flux, has different color response, etc. than calcia.
Now it is true, in this case, that the 2nd recipe with a lower wt.%
silica will melt better than the one with the higher weight % silica.
That is because strontia is a better flux than calcia above 2000 degrees.
However if I had instead substituted K/Na for strontia (instead of
calcia) all that would have been reversed and the formula with the
highest wt. % silica would have melted best.
One of the things I most wanted
>calculation software for was to predict, if I could, which glaze recipes
>would work for me and which would not, without testing them. By "work"
>I just mean that they will melt to a useable surface, not that they
>would be beautiful glazes. Anyway, I find the weight % numbers to be as
>useful for this as the unity formula ones.
I don't disagree that it is sometimes difficult to tell whether a recipe
will melt by looking at the unity formula--particularly low fire recipes
where boron plays such a big role. But I bet if you will give it a try,
mole % will let you develop that sense better than wt. %. It is
unfortunate that mole % calculations haven't been available until
recently and are still not availble in some glaze calc programs. And, as
I said in the last post, it is also unfortunate that the glaze calc
programs don't even tell you that. A person who is not well studied in
chemistry more than likely didn't even realize it.
Yes I know I taking on conventional practice here. Wt. %s have been and
are still being used in the industry. But then I've tilted at windmills
before and probably will continue to do so in the future. Hey, fish that
swim downstream are a lot more likely to drown than those swimming
upstream--what else can I say?
What you have provoked me to do though is replot some data I have where I
have varied alumina level only (on a molecular or unity basis) vs.
leaching. I will go back and replot that vs. wt. % and see if it tells
me anything different. We can discuss it over a drink at NCECA. After
all that is just four more weeks!!
Regards, John
"The life so short, the craft so long to learn." Hippocrates, 5th cent.
B.C.
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