Karl P. Platt on sun 20 oct 96
Blue glazes are pretty direct -- except for Ti blues and I'm not going
to go through all of that again, but you can find a description of what
happens within these on the Ceramicwebsite Richard hosts.
The following is a highly generalized perspective on some of the things
that make glaze look blue.
Fe Blue
Iron (Fe) exists in glassy glazes in forms usually referred to as FeO
or Fe2O3. The situation is actually not quite so simple as that. While
the Fe may be in a state where it has an affinity for 1 or 1.5 oxygens,
how it actually sits amid those oxygens makes a substantial difference
as to how its presence is revealed in living color. When Fe resides as
"FeO" it generally causes a blue coloration and "Fe2O3" yellow. It is
essentially impossible to obtain a glaze in which all of the Fe is
entirely in one form or the other. This is why Fe glazes typically
appear green --- there's some balance between the two states depending
on the firing atmosphere and the nature of the glaze. The celadon type
glazes fired in reduction are an example of how one might urge the blue
FeO to predominate over Fe2O3.
Cu blue
Cu will only show its distinctive tourquoise blue in highly alkaline
environments. In that the alkalies tend to raise thermal expansion
markedly, this is why one sees so many of the nicest Cu blues crazed.
There is a tendency to add Li2O in efforts to develop Cu blue. Actually,
K2O is by far preferred in this role for a number of reasons. Mainly it
is more alkaline in chemical character that Li2O, but also owing to the
fact that K is a fat ion and deforms the Cu a little bit more than the
other alkalies. The weaker chemical bond formed by K over Li also
influences this behavior. Generally
speaking, K2O rich glazes result in a more sharply defined color for any
of the colors based on the transition elements, 3d elements, solution
colors or whatever you're comfortable calling them -- it's all the same.
Ba is found in many Cu blue compositions. It lends a couple of things.
Compared to any of it's cousins -- MgO, CaO or SrO -- it is the most
alkaline in character. As well it is poorly soluble in most glazes --
especially if chilly ^6 firings are practiced. While what BaO does get
dissolved aids in the development of Cu blue in any of the glassy
material found in the glaze, it also urges modifications of this color
in the crystalline material present. Some of these are really quite
striking and generally
extend into longer wavelengths --- they tend toward being purplish. I
saw an extreme example of this in a Ceramics Monthly last year and
believe that this glaze was also the subject of some controversy here on
Clayart.
Here I'll note that with Cu as a colorant it becomes very obvious that
BaO is not at all similar to SrO and replacing it in any proportions
does not lead to similar results in almost all cases. In fact,
equimolecular replacement with SrO will typically kill the blue, turning
it green. We'll save this for later. However, this might be a good place
to note that Cu is a very sensitive indicator as how acid or basic a
glaze is. The usefulness of this is something we might also take up some
other time.
CoO
Co blue is dumb easy to make. Or is it? The color is more complex than
meets the eye -- if you will. In Co blues there are actually 2 color
centers -- one is blue as blue can be and the other is pinkish red.
Red!? you say. Yes, red. Co, like Fe exists in the glaze in 2 forms.
Co2+ (CoO) is blue and Co3+ (Co2O3) is red. The blue Co2+ is a much
stronger colorant that the Co3+ and tends to predominate in the hue one
ends up seeing in a SiO2 rich glassy glaze. They are almost always both
present in any Co blue glaze.
Try making up two glazes similar in all respects except in one make all
of the alkali Na2O and the other all K2O. You will find that the K2O
rich version is distinctively more reddish -- and I think more
attractive. There are a number of chemical factors at play that we'll
consider in greater length if anyone here shows interest. Seen here
again is the beneficial effect K2O has on the solution colors -- 3d,
transition element....etc -- as not only will the color be more reddish,
it will also be more pure in general. Also at work is the nature of the
bond between Co-O-alkali as a consequence of the strength of the bonds
formed and the different sizes between the alkalies -- K is fat and
weakly charged, Li is small and highly charged -- sounds like some
people I know.
If one would bother to make a highly oxidized frit and to fire that frit
with Co in strong oxidation, this would further encourage the reddish
aspect of the Co color over that seen in other types of host glaze.
If the parent glaze is heavy in borosilicate frit, adding a % or two of
Li2O can also urge the formation of the reddish Co3+. The general
color, however, will be more, shall we say, dilute. B2O3 rich glazes
give mushy colors with the transition metals.
In ZnO rich glazes one can observe that CoO produces colors which are
decidedly more greenish. This is especially so if the glaze has much
crystalline material in it after firing. The green has to do with the
dissimilar way Zn and Co participate in the crystals present.
Al2O3 matte glazes produce yet a little different modification of the Co
blue -- I think of this as a Prussian blue. It is owing to the
participation of CoO in crystals with the Al2O3.
It has been seen by many ceramists that Co rich glazes tend to be runny.
This is owing to two things. One is the role of CoO as a modifier to the
glassy material in the glaze and also to the fact the the Co blue color
is much more effective at absorbing IR than just about anything else --
except Fe. That is, it aquires and gives off heat with good efficiency.
Both of these factors tend to render the Co rich regions apparently more
fluid.
Nd2O3 in flourescent light also produces a blue -- however, it'll be
pink in light more rich in red wavelengths -- like incandescent light or
sunlight. Nd2O3 is a weak colorant. A glaze would require some 10 wt%
(or more) to get a decent effect if it was opacified. 4% would be a
minimum if applied on a white body.
The color produced by Nd2O3 is pretty immune to what sort of glaze it
gets placed into.
A very cool effect can be had by adding 3-4 wt% Nd2O3 to a light Cu red
and firing that on a white body. The color seen is simply lush.
Cu-Co mixtures in alkaline glazes make what you'd expect from mixtures
of these colorants. This is a pretty linear thing. If the glaze is more
basic - adding B2O3, CaO, MgO, SrO, etc, Cu will not be blue, but green
and the habit of the color will change accordingly.Thus, the colors seen
by the Cu-Co combination are altered. The nicest Co blues in glass
typically contain some Cu.
Cu and Co are widely available as water soluble salts -- CuSO4 and
CoSO4, for example. Copper sulphate can be had at any pool supply house
and cobalt sulphate comes from chemical suppliers. These can be
dissolved in water and then used to paint on either the clay body or a
glaze covering it for different effects which depend on the parent or
cover glaze depending on what you're doing. The color effects seen are
consistent with what is described above.
Since we're on solutions that that can be ceramically colorful. While
not blue, marvellous effects can be had by dissolving K2CrO4 --
potassium di-chromate -- into water, painting this solution onto CaO
rich tin opacified white glazes and then firing these in oxidation. Of
course, this solution should not be imbibed, used as salad dressing or
mistaken for a form of contraceptive. You probably shouldn't dip your
fingers in it before you smoke, either.
Ni Blue
I did some fooling around with this color a few years back, got mixed
results and ran out of time to go after it further. Ceramists seldom
think of NiO as producing anything but hideous yellows, greys or browns.
In garden variety glazes, this is true. However, if you make up a glaze
using ZnO to the exclusion of any other RO component and allow K2O to
predominate as the alkali -- and it's best here to make your own frit --
Amazing blue and purple hues can be produced. They're nice on their own
merits, but because they're so seldom, if ever, seen they have a sort of
special charm all their
own. There's some literature on developing this coloration in old ACerS
journals from, I think, the 30's by a fellow named Pence. I found that
his experience was useful in guiding my experiments.
Subsequently I made several glasses which used K2O exclusively and the
purple was quite easily obtained -- and it is quite beautiful, if a bit
pricey.
Let's see, what else....... Later
KPP -- getting hungry
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