Karl P. Platt on mon 24 nov 97
The terms Oxidation and Reduction taken to mean that the process they
describe is taking place.
The terms Oxidized and Reduced infer that these processes are completed.
Ceramics are predominantly formed of metals and/or semi-metals
chemically combined with oxygen. These are referred to as oxides. The
nature of the chemical bond between the metals (and semi-metals) with
oxygen is mainly based on electrostatic attraction between the atoms.
That is, they behave mostly like little magnets attracting opposites
and repelling likes. Oxygen is negatively charged and the metals are
positively charged.
The amount of negative charge on the oxygen atom is -2. This means that
the atom has two extra electrons. While nature has its statistical
quirks, it s safe to consider oxygen as always having a charge of 2.
The metals lose electrons and become positively charged. Some atoms have
very stable outer electrons. Si, Al and Zr would be good examples
SiO2, Al2O3 and ZrO2. High melting temperatures and high chemical
durability indicating the great strength and stability of the bonds
within them characterize all of these oxides. Likewise, when added to a
glass or glaze we find that any of these oxides will raise viscosity,
lower thermal expansion, improve chemical durability, etc also signs
of great stability.
Other metals, such as Fe or Mn, which form FeO-Fe2O3 or MnO-Mn2O3 can
combine in several combinations with oxygen. Given that we ve agreed
that oxygen always has a charge of 2, and that the atoms behave like
little magnets, it stands to reason that to react with oxygen the
element combining must satisfy this 2 charge. This means the number of
that the number of metal and oxygen atoms depend on the number of
electrons missing from the metal, leaving positive charge of the atom s
nucleus unbalanced. Thus giving the atom a net positive charge in
proportion to the number of missing electrons.
The larger the number of missing electrons from the metal atoms, the
larger the number of oxygen atoms they can react (combine) with. Those
forms with larger amounts of oxygen are referred to as being more highly
oxidized.
Since these missing electrons can be replaced from the metal atom s
environment, it is possible to decrease the number of oxygen atoms such
metals as Fe or Mn are combined with. By adding electrons we reduce the
number of oxygens in combination with the metal. This is referred to as
reduction.
So, oxidation is when an electron is lost and reduction is when an
electron is gained.
Further, reduction can t happen without oxidation and oxidation can t
happen without reduction. The two are not mutually exclusive. If
something gets reduced, something else gets oxidized. We re just
shifting oxygen around, not destroying it.
I don t think this leaves a lot of room for ambiguity.
It was said here that:
You can t oxidize in an electric kiln
I say:
Horse puckey.
More than that you can not only oxidize, but also reduce.
The oxides that are susceptible to having the number of their outer
electrons manipulated are called Redox Oxides for their ready capacity
to be REduced or OXidized as described above.
When present in a glaze they are always present in some balance between
the oxidized and reduced form. This balance is referred to as the redox
equilibrium. It is practically impossible, for example, to reduce Fe
entirely to FeO there s always some Fe2O3 present. FeO causes a glaze
to be blue and Fe2O3 makes a glaze yellow. Glazes containing Fe will be
bluer, green (yellow and blue) or yellower depending on which form of
the oxide predominates. While a their development is a little more
complicated than mere redox equilibrium, we should recognize the role
played by this redox in the development of nice celadon type glazes.
We also need to consider that when several redox oxides are present
together they ll scrap with each other to balance the number of
electrons about between them in a manner independent of the electrons
available. This can have real drastic effects. A nicer one is that the
presence of Cr in an Mn bearing glaze strongly encourages the formation
of Mn2O3 (purple). I ll just state this as a fact and let the interested
reader fish-up any reasonable text on thermodynamics to find the proof.
Oxides will become reduced with increasing temperature. If you add Fe to
any clear silicate glaze (Bristol type, I suppose) and fire it to
progressively higher temperatures in an electric kiln, it ll become
progressively more blue. As this indicates reduction it is plain that we
can now say that you not only can, but also do reduce in an electric
kiln.
If there s oxygen available in the atmosphere of the kiln, and a reduced
form of an oxide is added to a glaze, we find that on firing it will
oxidize to establish a balance between the various forms of the redox
oxide. The easy test to observe this is to add FeO to a glaze and note
that through firing the glaze will not be FeO blue, but some combination
of FeO-Fe2O3 green. Therefore, we can also say that it is in fact
possible to oxidize in an electric kiln it happens all the time.
(I should also point out that the balance between FeO-Fe2O3 and any
redox pair is also dependent on the quantity present in the glaze, too.
)
Ensuring an abundance of electrons in the kiln accelerates the gain of
electrons at high temperature by a redox susceptible metal. Carbon (C)
as it happens is an excellent provider of electrons. Hydrogen, likewise,
is also. In fuel fired kilns adding in fuel in excess of what can be
consumed by the available oxygen has long been practiced to make
electrons available to the metals thereby reducing the number of
oxygen atoms combined with them. At temperatures above 1,000 F or so
natural gas and propane will decompose (in the absence of oxygen) to
liberate free hydrogen and carbon. This process is called cracking .
The extent to which the metals involved can be reduced depends on
heavily on temperature for two reasons. As the atoms in hotter things
are vibrating more furiously the statistical opportunity they have to
combine with anything in their presence is greatly increased. The
lightly held outer electrons are already perturbed the amount of
energy needed to make them move is less. As such their capacity to react
is increased (the whole process of firing depends on this behavior).
Conversely, it s real hard to reduce a glaze at room temperature.
Beyond this, the redox oxide is embedded in a glassy material, the
degree to which electrons can enter and react is greatly improved when
the viscosity of the glass is lower. Conversely, when the excess of
electrons is removed (i.e. reducing is stopped) there is also a
greater opportunity for the ingress of oxygen while viscosity is low.
This defines the behavior of reoxidation seen in some glazes. Reducing
conditions during cooling and fast cooling to temperatures at which the
glaze is highly viscous facilitate the avoidance of reoxidation.
Saying, nothing is changing in a firing is like saying nothing changes
when the sun goes down. Let s not even discuss this idea. It s plain
weird.
It is not reasonable to say that the atmosphere present in an electric
kiln is anything at all like that present in a gas kiln at all. An
electric kiln is essentially full of hot air 80% N2 and 20% O2. A gas
kiln should be full of hot N2, CO2 and H2O (water). However, if a burner
incapable of providing for complete combustion at the burner is used
there can be all sorts of intermediate (and reducing) combustion
products in the kiln formaldehyde being a prominent figure in this.
In either a gas or electric kiln it is necessary to ensure the presence
of plenty of oxygen in the kiln up to 1,500 F or so to ensure that
organic material and sulfur are burned off before the surface of the
ware seals-up as a result of shrinkage, sintering and/or vitrification.
Failure to accomplish this results in a host of defects from bloating,
to black coring to sulfur blisters coming from high-temperature
decomposition of sulfates in the clay.
Hot CO2 isn t reactive with much. Water, however, in the form of (OH)
groupings can enter the glaze and affect the redox equilibrium. It also
acts to reduce the viscosity of the glassy material in the glazes this
explains some of the difference seen between glazes given the same heat
treatment in gas and electric kilns respectively. Moreover, the
atmosphere in a gas kiln is frequently moving at a higher velocity than
that in a tight electric kiln. This lends to increases the evaporation
of susceptible elements from the glaze surface Na and B are frequent
victims of this. Carried to extremes impoverishment of these elements at
the glaze surface can have strong effects well beyond appearance. This
is to say that a so-called neutral atmosphere isn t neutral. The only
time you d have a truly neutral kiln atmosphere (regardless of the heat
source) would be if the kiln were full of an inert gas like Argon or if
it were under vacuum. The redox equilibrium in a glaze or ceramic body,
as a function of kiln atmosphere, when the atmosphere is stable, is
constantly changing, but statistically balanced it is, however, always
changing -- Always.
Neutral is just another way to say your running at chemically correct
combustion. A Kiln s atmosphere is anything but neutral. The regular
atmosphere is anything but neutral you should see how my neighbor s
steel windows have rusted after only 2 years.
Let me say that the above only intends to hit the highlights. Redox
equilibrium is connected to many things beyond temperature and
atmosphere things like the relative acidity of the glaze and other
such esoterica that matters. Amal Paul s book Chemistry of Glasses,
Chapman and Hall, 1992 or Paul s papers on redox in Advances in Ceramics
Vol. 27 (I think, I left the books in the lab and am writing this from
the hip) ACerS, 1991 (?) both have really tidy discussions about redox
equilibrium. Weyl s Coloured Glasses also discusses the effects of redox
equilibrium on color development. Frankly, these two books are
indispensable to anyone serious about making colored glazes.
OK, this has gone on enough.......... I need to go regain my own
equilibrium after a long and exciting day at the tile factory yet
another thing to write about someday, I suppose.
KPP
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