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The Nature of Glass
- by Karl Platt
A few issues back we started a discussion on the elemental nature
of glass. Our interest then was to define how glass comes about
and to look at those substances which readily lend to it's existence.
Here we want to review what we've covered before, and take-up
how these basic glasses are manipulated to conform to studio
Silica (SiO2) and Boric oxide (B2O3) form glasses readily all by
themselves and are the predominant glassformers used in Studio
glasses and glazes. SiO2 by itself will form an extremely viscous
melt, and then only at very high temperatures (+3000F). B2O3
melts at much lower temperatures, has higher fluidity, but as a
glass it reacts with atmospheric water to deteriorate.
The properties of these glasses is strongly tied strength
of the metal-oxygen bonds (Si-O or B-O) making them up.
Pure SiO2 glass is relatively hard and has an exceedingly low thermal
expansion (5.5x10-7). Although technically useful, SiO2 glass is
difficult to make and hardly practical in a day to day sense. B2O3
glass, by contrast, melts low, is fluid and makes a highly
expansive, easily corroded glass. In general, this indicates on the
atomic level that Si-O bonds are notably stronger than B-O bonds.
There are also structural differences between SiO2 and B2O3
glasses, but we'll take that up further later.
SiO2 and B2O3 glasses have extreme properties. If more broadly
useful glasses are to be had, pure oxide glasses need to be modified
by adding other elements, called, naturally, Network Modifiers.
The strong Si-O bonds, which account for the exceptional
properties of SiO2 glass, are perturbed by Network Modifiers.
Generally speaking, Network Modifiers affect the properties of a
pure SiO2 glass as follows:
A. Thermal Expansion Increases
B. Hardness Lowers
C. Chemical Durability Lowers
D. Density Increases
E. Surface Tension
F. Refractive Index Increases
It is exceptional to use B2O3 as a lone glassformer. Usually, B2O3
and SiO2 are combined to form what is known as Borosilicate
glasses. Borosilicates are a diverse and widely used class of
glasses. We will also take them up further along. So keep them in
mind. For now though we will focus on the effects network
modifiers have on SiO2 based glasses.
Since ancient times just about every element on the planet, and
some which weren't, have been added to silica host glasses. In
modern Studio setting, the range of compositions used has practical
limits. Of the entire palette presented by the Periodic Table of the
Elements, we use about 10% often and another 30% to a more
limited degree-- either as colorants or for special effects. The
remainder are either rare, dangerous or both.
Table 1 below lists the elements other than oxygen and fluorine
most frequently used in most of the glasses you'll meet. This list
is arranged according to the field strength each atom holds as an
ion. Field Strength? Ions? Glad you asked.
An ion is an atom which has either a net positive or net negative
charge. As such, an ion's interaction chemically is described taking
the point of view that each ion behaves like a little magnetic
sphere. A negatively charged ion, known as an Anion, such as
oxygen will draw to it positively charged ions, known as cations.
Conversely, either anions or cations alone will repel each other.
Again, Cations (+) and Anions (-) have an affinity for each other
(+)i(-), but will repel themselves (+) < > (+).
Ions become ions when electrons in the outermost region of an
atom are gained or lost to a neighboring atom. Electrons have a net
(-) charge and orbit about an atom's nucleus, which is composed
of protons (+) and neutrons (no charge).
The amount of charge borne by an electron or proton is the same,
thus for each proton there is an electron to balance it in orbit about
the nucleus. Well, most of the time sort of.
Anyway, electrons orbit about the nucleus
Electrons take up orbital positions in a very orderly way. We'll
leave the gory details to chemistry books, but those farthest from
the nucleus are held less firmly than those closer in. Outer
electrons, as a result, may be shed to, traded with or shared
with any neighboring atom. That is so long as the other atom
is not selfish with its outer electrons. What blend of these
circumstances occurs when two atoms meet depending on the elements
and their environment. Which atom sheds or yanks how many electrons
to or from whom and how is the substance of chemistry.
The outer electrons are known as Valence electrons. When one is missing, the
ion will be said to have a Valence of +1. When there's an extra available,
the ion is said to have a Valence of -1. Oxygen always has a Valence of
-2 and this dictates many of the reactions that make up the ceramic world;
like we'll see in an example with iron (Fe) below.
The process of absolutely losing an electron is known out in the world
as oxidation. When iron (Fe) in a glass or glaze is subjected to an
oxidizing fire electrons are stripped from the Fe, which
gives it a net (+) charge. As we noted before
any oxygen (-2) out loose would rather not be and will take advantage of
the electron deficiency to combine with the Fe in the ratio of 2:3;
which we know as Fe2O3. The Fe when stripped of an electron takes on a net
positive charge and assumes extra oxygen, which are negatively charged to
so that the system comes into balance create a balance.
Reduction is the reverse of this process. In the case of Fe, when recuced to
FeO it behaves as a network modifier and shares some rather low melting
compositions with SiO2. This is why reduced Fe glazes of tend to be
Only a few elements prefer to be anions (-) and chief among them,
for our purposes anyway, is Oxygen. Oxygen happily reacts with
anything presenting enough positive charge. Oxygen is Big. With
an ionic radius of 1.4 it is larger than any of the cations in
Table 1. Accordingly, our glasses, on the basis of volume, are an
oxygen bulk knitted together by cationic metals. Cationic, not
catatonic; wake up, this is important.
Alkaline metal oxides (alkalies) are foremost in altering the
properties of pure silica glass. In order of increasing atomic
weight, the alkaline metals Lithium (Li), Sodium (Na) and
Potassium (K) reside in the leftmost column of the Periodic Table.
FIGURE 2 PERIODIC TABLE
The alkalies have a valence of 1 and combine with oxygen in the
ratio 2:1 as Li2O,Na2O and K2O respectively. In an SiO2 melt
the alkalies perturb two Si-O bonds for each Li,Na,K-O bond
added to the melt. Each of the alkalies has a distinct impact on the
properties of the glass according to it's nature as an ion in the
glass. We should look at this more closely.
Lithium is a very small, with an ionic radius of 0.78è, and is
highly charged for its size. As such, it really perturbs Si-O bonds,
and being so small it can freely diffuse through the network of Si-
O. Li2O glasses have broad light absorption properties and thus
colors formed within them often seems lifeless.
Colors you see are the product of having the coloring ion (usually
transition elements) "squished" between the other components in the
glass. Li doesn't squish especially hard and the colors are subsequently
Sodium is the most widely used alkali. Abundant in nature, though
not always in a directly useful forms, sodium has played a huge
role in glasses since antiquity. Of the alkalies, it is of middling size
and has a moderate charge. It's ability to squishthe transition earths is
is middling and so are the colors produced.
Potassium is larger and more weakly charged than either Sodium
or Lithium. As such, the extent to which it lowers viscosity is
lower. The potassium ion, fattish and weakly charged, has
interesting effects on colorants, by "squishing" hard which
lends to sharper spectral absorption and purer percieved colors.
Almost immediately, alkali additions make an SiO2 melt more
fluid. By the time 10 mol% is added, viscosity will reach a
minimum for a given temperature. Figure 3 shows the effect the
alkalies have on viscosity. Note the steep slope of the curve
between 0 and 10 mol% and the relative position of Li to Na to K.
The effect of the alkalies is to interpose themselves within the
glassy Si-O network where they represent a weak spot through the
imposition of weakly held oxygens. Subsequently, all Alkali-Silica
glasses will react readily with atmospheric water, which happily
assumes a stronger affinity for the weakly held oxygen. The extent
to which the alkali-silica glass is soluble increases with the amount
Allied to low chemical durability and viscosity reductions is high
Sodium-Silica glasses (sodium-silicate) is widely exploited for its
solubility. When dissolved in water the Na+1 and Si+4 operate
to suspend dirt which leads to their wide use in laundry detergents.
For the same effect, sodium-silicates are used in ceramic slips as
a suspending agent. Li and K silicate glasses produce similar
effects, but neither as well or as economically as sodium silicates.
Solubility and excessive thermal expansions are, of course, highly
undesirable in studio applications. Fortunately there are many
substances which act to occupy the weakened oxygens present in
These are known as the alkaline earths, which are characterized by
a valence of +2. They combine with oxygen in the ratio of 1:1
and in alkali-silica glass they operate to tie up weakly held oxygen
internally, so that atmospheric water is thwarted from latching on
The behavior of the alkaline earths is a bit more complicated than
with the alkalies. Each alkaline earth has its own sort of
personality. Magnesium (Mg) and Barium (Pb) represent the
extremes in terms of size. Lead is of course the heaviest.
We will address the alkaline earths in the next paper.
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