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7.			 SODIUM AND POTASSIUM
On following the effect of 1 mole-% of monovalent alkali metal oxides on the
change in various properties (density, refractive index, thermal expansion, electrical
properties), one observes differences in that Li and Na lie at one extreme and K,
Rb and Cs at the other. From the point of view of glass chemistry, monovalent
alkali metals should therefore be divided into two groups, one being composed
of elements of the two small periods (Li-Na) and the other of elements of the
three large periods (K-Rb-Cs). Sodium and potassium therefore belong in two
different groups, which leads to some technologically significant characteristics
of these two elements in glass.
1. Potassium belongs among the largest ions, being similar in size to oxygen,
(6)rK = 1.33; (6)rO = 1.40, whereas sodium is one third smaller, (6)rNa = 1.02.
This is why potassium is less mobile in the glass structure, in particular in an
electrical field.
2. The relatively large effective radii of the two monovalent alkali metals reduce
the density and increase the specific volume, potassium more so because of the
larger size of its ion.
3. In view of the larger radius, the strength of the potassium bond to oxygen
in glass is one third lower than that of sodium. However, the field strengths of
both alkali metals are roughly ten times lower then that of silicon. The low bond
strength of monovalent alkali metals in the glass structure is the cause of the
lower chemical durability and increased thermal expansion imparted to glass by
both elements, again to a greater degree by K + owing to its lower bond strength.
4. Potassium is a more electropositive element than sodium. In the presence 
of the former in glass, Fell is more readily oxidized to FellI and potassium crystals
therefore lend themselves better to decolorization than pure sodium glasses.
5. The more basic nature of K + has the result that K2O increases the solubility
of SiO2 much more than Na2O does. For example, 1 mole of Na2O - 2 SiO2
dissolves 0.25 mole of SiO2 at 650°C in 120 min, whereas 1 mole of K2O - 2 SiO2
will dissolve 4.25 mole of SiO2 in the same period of time even at higher temperature
(725°C). This is why potassium and sodium-potassium crystals may have
a higher SiO2 content and thus also have superior properties (gloss stability, better
chemical durability) than sodium crystals, although equimolar potassium glasses
are inferior to equimolar sodium glasses.
 

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6. The deformability caused by the larger potassium ion is about double that
due to sodium, as indicated by the ionic refraction values, RNa = 4.75; RK = 8.25.
7. Because oxygen in the Si-O-K bond is more strongly deformed as a consequence
of the weaker field strength of potassium, and the K + ion proper is
more deformable than Na +, K2O will contribute more than Na2O to the formation
of glass. Most of the potassium silicates also have lower melting points than the
corresponding sodium silicates. However, this is not a general rule. Melts close
to disilicates in composition and alkali metal aluminosilicates containing K2O
have higher melting points than the corresponding sodium aluminosilicates. Mostly,
however, K2O promotes the formation of glass and suppresses the tendency for
separation into immiscible phases to occur, in particular in the presence of Na2O.
In soda-potash glasses it suppresses the tendency for crystalization to occur, and
reduces the liquidus temperature and the crystallization rate. It is even so effective
that in Pyrex-type glasses it almost completely suppresses the strong mineralizing
effect of fluorine, which otherwise causes a marked separation into immiscible
phases.
8. With ionically colouring elements, potassium produces different colours
than sodium, shifting the colour towards shorter wavelengths. The colours are
purer. Coloured commercial glasses therefore usually contain a certain proportion
of K2O in addition to Na2O

TABLE 11
SILICATES AND EUTECTICS IN THE BINARY
Na2O- SiO2SYSTEM
(after Fanderlik [1.3])

9. In contrast to sodium, potassium is radioactive (similarly to rubidium).
It emits ß-particles and is converted into argon. However, its radioactive efficiency
is only 1/I000th that of uranium.
Binary alkali metal silicate glasses show poor chemical durability. Their only 
technical significance lies in the manufacture of "water-glass", a colloidal solution
 

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corresponding approximately to sodium trisilicate. An analogous potassium glass
is manufactured on a more limited scale. The technologically most important
glasses are those of the system Na2O-nSiO2, where n is the glass modulus. This
system consists of three silicates and three eutectics, listed in Table 11.
In the more acidic region of the binary phase diagram between 0 and 50 wt.-%
of Na2O there arise two compounds, Na2O - SiO2 (m.p. 1089 °C) and Na2O -
2 SiO2 (m.p. 874°C), and two eutectics, 62.0 SiO2 - 38.0 Na2O (m.p. 837°C)
and 73.6 SiO2 - 26.4 Na2O (m.p. 789 °C).Beyond 73 % SiO2, the melt crystallization
temperature increases rapidly with increasing SiO2 content.

FIG. 64 - Binary Na2O -SiO2
system with regions of technical glasses for "water glass" and with separation regions.

The soluble glasses in this system are in the approximate modulus range 1.2-4,
and melt most readily between n = 2 and 3.6. All sodium silicate technical glasses
belong to the region of the two low eutectics. The most usual commercial glass
is situated in the narrow range between n = 3.2 and 3.6, belonging to the region
of the third lowest eutectic. Glasses in this region are commercially designated
"neutral" glasses, with the abbreviation NE. These glasses are more convenient
 

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to manufacture owing to the lower tendency for crystallization to occur than with
the most alkaline types, and show superior adhesive properties in colloidal solution.
Glasses of higher acidity than the neutral types are not produced owing to
their difficult melting and also because they are not so readily converted into
colloidal solutions.
Sodium disilicate divides the phase diagram into two sections (Fig. 64), differing
in the structural arrangement of silicon tetrahedra. With the disilicate Na2O.-
 2 SiO2, nSi = 0.2. When the Si : 0 ratio is reduced by adding sodium, the glass
has to contain some SiO4 tetrahedra with only two bridging oxygen atoms. Such
tetrahedra gradually produce a structure characterized by chains or "fibres"
rather than a spatial network. With sodium silicate glasses this change is indicated
by a distinct deflection in the curves of density, refractive index and electrical
properties of the disilicate. Technical "water-glasses" are selected from the acidic
region, to the right of the boundary of disilicate with moduli higher than n = 2.
Glasses in the Na2O - SiO2 system show a much lower tendency for distinct
separation than, e.g. the glasses of the Li20- SiO2 system. In spite of this, even
the former system involves microheterogeneities and separation into phases [1].
Regions richer in SiO2 and richer in Na2O arise. There is a devitrification region
between 8.7 and 13.6 mole-% of Na2O, where heat-treated samples exhibit microheterogeneities
under an electron microscope [2]. Devitrification can be promoted
still further by nucleating agents, e.g., NaF, which concentrate in the phase richer
in Nal. The SiO2-rich phase is present in the form of droplets [3]. The separation
effect disappears above 15 mole-% of Na2O. Following separation, glasses containing
13 mole-% of M2O are completely extractable, similarly to Vycor-type glasses,
producing porous glasses. Potash silicate glasses contain much larger pores than
the sodium silicate glasses.
Technology
Na2CO3 reacts with SiO2 in solid state at about 400°C [4]. The reaction depends
significantly on the ratio of the two components; however, the combination always
involves the following three main reactions, which take place to various extents
and in various sequences:

Na2CO3 + SiO2 ? Na2SiO3 + CO2 (proceeds up to 600°C)
Na2SiO3 + SiO2 ? Na2Si2O5 (at 700°C)
Na2Si2O5 + Na2CO3 ? 2 Na2SiO3 + CO2

The solid-state reaction thus yields metasilicate (m.p. 1090 0c) and disilicate
(m.p. 870°C), which react with soda and sand. In addition, technologically
significant eutectics arise, including the most important Na2Si2O5 + SiO2 with
a melting point of 790°C. The first melt is formed at this temperature. The main
reactions take place between 700 and 900°C. Although all of the soda may react
 

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with sand in the solid state, the reaction rates increase only above the melting
point of soda (852°C) even though the first melt forms at 790°C [5].
It is interesting that the reaction product of sand with soda is a transparent
substance even at low temperatures, and retains the shapes of the original sand
grains. The reaction temperatures are lower the smaller are the sand grains.
Volatilization of alkali metal compounds from the glass is of special technological
significance. The vapours condense on cooler points, resulting in corrosion
of refractories and the formation of stalagmites which may drip into the melt
and be a source of inhomogeneities. The vaporized substances may likewise settle
in the regenerator chambers, where they attack the lining.
The losses of alkali metals introduced into the batch vary between 0.1 and 0.2 %
[6] and do not exceed 0.3 % [7]. They occur even when the batch consists solely
of cullet. The losses increase especially when the atmosphere contains SO2 + O2,
The losses are high in particular with cations that reduce surface tension, that is,
in particular with potassium raw materials which reduce the surface tension
13-15 times more than the corresponding sodium compounds [8].
The alkali metal oxides proper volatilize comparatively slightly from molten
glass. Chlorides vaporize more than fluorides; in contrast to chlorine, fluorine
may become a component of the glass network as a substitute for oxygen and is
thus more strongly bound.
Alkali metal compounds vaporize most extensively at the melting temperature;
this is due to the reactions of glass with CO2, SO2 and O2 from the furnace
atmosphere and to the resulting secondary formation of carbonates and sulphates.
The partial pressures of CO2, SO2 and O2 in the furnace atmosphere are higher
than the pressures of decomposition of carbonates and sulphates, which at
1100 °C hardly exceed 1333 Pa. The first to condense among the vapours is Na2CO3, 
followed by Na2SO4, which settles in the regeneration chambers.
Alkali metal borate compounds show particularly strong volatilization. Among
these compounds, the highest heat of vaporization is shown by K2O- 2B2O3,
which is why its vapour pressure increases most strongly with temperature. At
890°C the vapour pressure of this borate is 100 times higher than that of sodium
borate.
On the other hand, K2SO4 has a lower vapour pressure and sublimes much
less than Na2SO4, Therefore, it remains in the glass where it is a source of blisters
and sulphate froth, in particular when molasses potash with a high K2SO4 content
is used in a strongly oxidizing atmosphere.
NaCl starts to vaporize already above its melting point (804°C). However,
at 865°C its vapour pressure is still very low, about 133.3 Pa. Atmospheric pressure
is attained by NaCI only at 1465°C, and by KCl at 1407°C. When chlorides
are used for refining, they vaporize from the melt almost completely so that only
trace amounts remain in the glass (usually about 0.1 %). However, this amount is
sufficient, in particular with borosilicate glasses (of the Pyrex type), to improve the
 

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wetability of raw materials by reducing the surface tension and to affect favourably
the refining process. NaCl boils at 1413 °C and KCl at 1500°C. In the regions
of lower temperature in the furnace the chlorides therefore condense in the form
of glazes or even stalagmites. This occurs, for instance, when refining cullet batches
with chloride. When boric acid is added to the chloride, however, this results
in the reaction 6 NaCI + 2 H3BO3 --+ 6 HCl + B2O3 + 3 Na20. Hydrogen chloride
attains the vapour pressure at - 85°C and evaporates rapidly without producing
any coatings. These could be formed with borosilicate glasses by volatilization
of Na2O - B2O3 and Na2O - 2 B2O3. However, whereas NaCl attains a vapour
pressure of 135 Pa at 865°C, the same low pressure is attained by sodium diborate
only at 1135 °C; the latter therefore shows substantially lower losses and does not
condense. Because Na2O - B2O3 boils at 1400 °C, that is at a temperature generally
exceeded at the furnace crown, with borosilicate glasses the extent of depositions
is limited.
Soda vapour may strongly attack exposed refractories; Na2CO3 corrodes
muIlite most readily, quartz less extensively and corundum least strongly.

Effect of sodium and potassium on the properties of glasses

Alkali metals generally reduce the overall bond strength of glasses, thus strongly
affecting all of their properties. They decrease density, refractive index, modulus
of elasticity, internal bonds, and thus also strength insofar as it depends on the
internal weak points in the structure, and hardness is also influenced. The entry
of alkali metals into glass strongly reduces its viscosity; in contrast to Na2O, K2O
reduces viscosity at low temperatures (about 740°C) and increases it at higher
temperatures (at about 1300 °C). Surface tension is little affected by Na2O and
strongly reduced by K2O. The chemical durability of glass is considerably impaired
by both sodium and potassium as a result of their weak bonds in the network.
After strong heating of glass, Na+ in particular migrates towards the glass surface
owing to the surface activity effect, where it may form efflorescence. This phenomenon
may occur, for instance, in ampoules as a result of the high shaping temperatures
used for sealing the bottom in ampoule machines. This efflorescence
is revealed by local red coloration after treatment with iodeosine or after application
of fine aluminium powder. Other efflorescence is formed after long-term
exposure to water vapour. This may enter thermometers from combustion gases
during their final sealing, and when unsuitable glass has been used dendritic
star-shaped crystals identified as NaHCO3 may be formed on the internal wall,
often only after several months. This surface hydrolysis can be suppressed by
ZnO and PbO, which are typical components of thermometer glasses. If the glass
is not in an electrical field, the cation may migrate without any barriers at elevated
temperature and the phenomenon is generally called diffusion. If, however, the
ion is under the effect of an external electrical field, the jumps are accelerated,
because the potential barriers in the direction of the electrical field are lowered;
 

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conditions are created for the jumping over of ions and for shifting the charges
in one direction, so that the glass transmits electrical current.
With glasses that contain alkali metals, the transmission of electrical current
is due solely to alkali metals, whereas with aluminate alkali metal-free glasses ions
of bivalent elements, especially calcium, also take part in the transmission.
Sodium and potassium have different effects on the colour of glasses. K2O is
responsible for clearer shades of ionically coloured glasses and also promotes the
fluorescence of uranium glasses. It changes the fluorescence of manganese glasses
from green to red. With increasing K2O content, glasses coloured with iron oxides
change from green to blue-green. Potash glasses containing CaO attain stronger
red colours. Amber glasses are likewise coloured by potassium to deeper and
redder shades .

Properties of NE glasses for "water - glass"·

Technical-grade NE glasses (solid sodium silicate) as a raw material for the
manufacture of water-glass have a density close to that of alkali-lime-silica glass,
that is, between 2.406 and 2.427 g /cm-3. According to composition, the refractive
index is in the range 1.49 -1.51, which is relatively low. The coefficient of thermal
expansion is high [a (20-300"C) = 10.8 * 10-6 K-1] whereas the transformation
temperature is low (tg = 435°C) and the softening point does not exceed 612 °C.
The glass crystallizes in the tridymite region at the liquidus temperature of
793°C [9].
Technical-grade NE glasses usually contain an insoluble residue in amounts
up to 1 wt.-%, containing CaO, MgO, Al2O3 and Fe2O3' Their chemical durability
is low. In spite of this, their solubility in cold water is poor. Even the powdered
material does not produce any lye test on the tongue. The glass loses its gloss
in air with time and is decomposed in boiling water, in particular in an autoclave
at 0.3 -0.4 MPa, which is the principle of the industrial production of "waterglass".
The process does not involve the formation of true solutions, being based
on hydrolysis, which holds generally for all glasses [10].

REFERENCES
7. 1. HUMMEL,J. J., Congres international du verre, Bruxelles, 1965, I, p. 36.
2. MORIYA,Y., Phys. Chern. Glasses 8,1967,19.
3. VOGEL,W.-REHFELD, A.-RlTSCHEL, H., Silicates industriels 32,1967, 161.
4. HOWARD,F. W. - THOMASSONC,. V., Symposium sur la fusion du verre. Bruxelles 1958, p. 373.
5. WILBURN, F. W. - THOMASSON, C. V., Symposium sur la fusion du verre.· Bruxelles 1958,
p.373.
6. LOFFLER,J., Symposium sur la fusion du verre. Charleroi 1958, p. 531.
7. DIETRICHS,W., Glast. Ber. 31, 1958, 38.
8. DIETZEL,A., Sprechsaal82, 1942, 35.
9. VOLP,M. B., Skldr a kerarnik 12, 1962, 252.
10. MAYER,H., Das Wasserglas. Braunschweig 1939.