Karl P. Platt on sun 15 jun 97
We need to have a dialogue about the Glazebase. There are discrepancies
between the Seger Formula and "percentage" analysis in more than a few
cases. It would also appear that Wt% is being used in some cases and
Mol% in others. At any rate, discrepancies exist and we need to clean
them out. Also, is it possible to use a program such as Cold Fusion to
extract from and analyze the database? This would be interesting for a
number of reasons -- after a cursory glance, I think it would show that
the actual ranges of compositions being used are much more narrow than
imagined.
Back to the Ba-Sr thing.......
First we should consider why BaO gets used in potter's glazes in the
first place. I come up with the following and am open to hear other
perspectives:
A. Color Development
B. The particular matte texture BaO gives
C. High index of refraction glazes -- they're more "brilliant".
D. The high temperature fluxing effects of BaO
Color development is, perhaps, the most important and, I'd suggest, the
most informative in respect to observing the profound differences
between BaO and SrO.
I know no-one is goint to make this test, but I'll use the example
anyway hoping that maybe someone will.
Make-up the following compositions:
54.9 Ferro 3195 55.9 57.6
27.4 Custer Spar 27.9 28.8
7.6 BaCO3 5.9 SrCO3 3.2 MgCO3
4.4 OM4 Ball Clay 4.5 4.6
2.8 Whiting 2.8 2.9
2.8 Flint 2.8 2.9
Add -- 0.8% CuO -- "black"copper oxide.
You could also use Cr or Vn or Ni.
All of these glazes are identical except that all of the Ba is swapped
out for SrO or MgO. These should mature at cone 6-ish. You can fiddle a
little with the CuO to adjust the density of the color to your taste.
The finished texture you see will differ markedly depending on the RO
component added in. More importantly, you'll see marked differences in
the color produced by the Cu. The Ba version will be more blue than the
others.
Why?
The details are thick, but suffice it to say that the presence of Ba
lends to a more "basic" (alkaline) environment in the melt. The color
produced by the Cu (or Cr or Vn or Ni) in an oxidizing firing will
depend on this chemical environment. Blue (Cu02) Cu is only seen in
highly alkaline environments. One can actually rank the alkalies and
alkaline earths in order of the "alkalinity" they tend to impart to the
fusion as follows:
CaO < SrO < Li2O < BaO < Na2O < K2O
This order is also reflected when one looks at the ions themselves in
terms of their "field strength" -- how big of a sphere their electronic
charge makes. Again, it can be seen that Sr has more in common with Ca
than Ba, and Ba has more in common with Pb than Sr. This is not a
reflection of the chemical properties of these materials, which are
quite different and these contrasts should not be a source of confusion
or alarm.
While it's not something one sees much in common glazes owing to their
complexity, there is also a matter of what is known as phase-separation.
This is when a glass/glaze separates into 2 (or more) liquids -- the
visual effect may be compared to glycerine in water. Such effects are
seen in a variety of art glazes with mottling, runny areas, etc. In
phase separation one region is richer in some component than another.
SrO tends to encourage such effects more so than BaO. This can have a
number of interesting implications. Coloring materials tend to
selectively gather in one phase or another. An extreme example of this
is observed in crystalline glazes with two (or more) coloring oxides
added -- the crystal will be one color and the glassy material another.
Ron noted Parmelee's notes on Sr additions to glazes. Essentially all of
the references to Sr in glazes date from WW II when alternative
materials were sought to replace those which were appropriated to the
war effort. Sr was a screwball thing with few other applications -- this
has changed substantially. The behavior of Sr in the glazes referenced
depend (not surprisingly) on the other components present and are also
not surprising. However, I can't find any systematic (peer reviewed)
studies on the effects of replacement of BaO by SrO. Yet there is a lot
of allied literature which should offer some clues as to what one might
expect to find -- most of which suggests that Sr is a high dollar
substitute for Ca.
This should interest the leaching fanatics. Studies have been done on
glasses in which complete substitutions of the alkaline earths have been
made. It turns out that at low temperatures one can expect Sr to leach
more readily than Ba. At higher temperatures, Sr leached only slightly
less than Ba.
A glaze which replaces Ba with Sr must be more closely fit to the body
on which it is to be applied. This is because Sr will produce glassy
material much less elastic than that formed by Ba. In short, crazing or
shivering are more probable with this substitution.
In looking over the Glazebase, one finds that in a majority of low
temperature compositions (cone 6-ish) BaO serves to produce a matte
texture and it seems that about 7--10 wt% does the trick in most cases.
Of course, just how much needs to be added depends on the remainder of
the glaze components. Why use BaO for a matte? It has nice texture --
often described as "satiny". This is in contrast to the coarser nature
of mattes given by, say, CaO.
High index of refraction in the glassy material lends to more beautiful
(I think) colors with the 3d elements (V, Ni, Co,....etc) BaO, which is
much heavier than SrO lends to a high index and a more attractive
appearance.
........Know what....this could go on for a long time, and I'm out of
time for the moment. See, I have something of a date with a ballerina
tonight. Hmmmmmmm. So we'll get back to this later.
KPP -- off to the creperia
David Hendley on mon 16 jun 97
Karl:
I always enjoy your posts, even if they are so long my screen saver kicks in
before I can finish reading a page!
Concerning the Bao - SrO thing, can you explain this further:
One can actually rank the alkalies and
>alkaline earths in order of the "alkalinity" they tend to impart to the
>fusion as follows:
>
>CaO < SrO < Li2O < BaO < Na2O < K2O
If this is the case, and we want "alkalinity" for color development, why not
just use more Sodium or potasium in place of the barium for a high alkaline
enviroment? High expansion and crazing a problem? Use lithium in place of
the barium, as it's next in line?
In "Clay & Glazes" (I know, its not very through, but its what I learned
with and still go back to), Rhodes essentially agrees with your statement
that" SrO is the rich man's CaO", and "is not used much".....
David Hendley
Maydelle, Texas
asdrgerg on tue 17 jun 97
>If this is the case, and we want "alkalinity" for color development, >why not j
The strongest Cu blues always contain lots (12+wt%) of the alkalies --
as such they are also prone to crazing. Ba, as a replacement for the
alkalies, will lower the expansion of the glaze. Sometimes enough to
prevent crazing on higher expansion bodies. It also makes the glaze more
elastic (as noted) and this also serves to prevent crazing/shivering. Li
can serve, too -- but it's real expensive. In 1991 I made a series of
glazes using Cu to have a look at the relative alkalinity of the
elements above. The Li was greenish and the color real flat.
Apart from the chemistry, there are also a couple other things at work
here. If one has a look at the spectrum produced by the transition
metals (Vn, Mn, Fe, Co,.....etc) in glazes in which Li, Na and K are
substituted one for the other, it is seen that the K rich glaze/glass
will have the most pure color -- the absorption is stronger and more
narrowly defined. The same thing is seen when one replaces Mg, Ca, Sr,
Ba.... one for the other. In general increasing atomic weight gives
better color. There are a number of reasons for this, but I haven'ttime
to go over them-- and most would find it boring and/or more than
necessary. The best case for color is K2O-PbO-SiO2 glass/glaze. The
worst case for color would be a Li-Mg-borosilicate glaze. My eyes hurt
thinking about it.
Regards,
KPP
Jim Horvitz on thu 19 jun 97
KKP states that increasing atomic weight produces better color. I would
appreciate an explanation of the reasons for this. Jim Horvitz
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