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2,413,549‘
Patented Dec. 31, 1946
UNITED STATES PATENT~ OFFICE
2,413,549
VITREOUS COMPOSITION
Alden J. Deyrup, West?eld, N. J ., assignor to E. I.
du Pont de Nemours & Company, Wilmington,
DeL, a corporation of Delaware
‘
No Drawing. Application ‘September 23, 1944,
Serial No. 555,597
10 Claims.
1
(Cl. 106-53)
2 .
This invention relates to certain novel vitreous
compositions or glasses which are particularly
characterized by extremely low electrical loss
propertiesj My novel vitreous compositions are
unique in that they possess, to an exceptional
jor losses are in the dielectric substance, it is
usual to refer to the “Q" of a dielectric such as
that vofv my invention without specifying which
of the two terms is meant.
degree, those properties which render them useful"
fused quartz has sometimes been used to replace
for dielectric purposes.
The application is a continuation-impart of
these vitreous compositions or glasses.
possesses a much higher electrical e?iciency, more
my copending application Serial No. 504,888, ?led
particularly, a higher Q-value, than the lower
melting glasses‘and vitreous compositions previ—
October 4, 1943.
'
Vitreous compositions or glasses have previous
ly been used as insulating media and supportingr
structures for conductive members in electrical
and radio apparatus. ' Vitreous materials have
In an effort to ?nd a more satisfactory material
Quartz
ously available. The usefulness of fused quartz
is offset, however, by its extremely high melting
point, which e?iectively limits its utilization to
specialized uses where its low power factor is suf
ficiently important to offset the difficulty and
alsorbeen utilized as the dielectric elements in the 15
fabrication of electrical capacitors or electrical ' expense of fabrication.
I have now discovered certain novel vitreous
condensers such as those disclosed, for example,
compositions or glasses, melting within the range
in the copending application of Ballard, Serial
of melting points usual in enamel compositions
No. 494,627, ?led July 14, 1943. When utilized
for such purposes vitreous compositions or glasses 20 and'ordinary glasses, which vitreous compositions
are characterized by lower dielectric losses and
possess‘ outstanding advantages as regards
higherQ-values than previously obtainable with
strength, ease of fabrication, stability or con-.
vitreous compositions. In fact, the new vitreous
stancy of the dimensions and relatively low elec~
compositions of relatively low melting points
trical losses when subjected to direct current
electrical potentials. However, when these previ 25 when utilized in electrical apparatus have a Q
value which either compares very favorably with
ously known‘ vitreous compositions or enamels
that of fused quartz or, in many cases, is superior
are subjected to alternating electrical potentials
to that of fused quartz. The possibility of pro
of high frequency, such as those occurring in
ducing vitreous compositions or glasses of rela
radio circuits and similar apparatus, their use as '
tively low melting point having electrical proper
glass insulators, glass supporting bodies, or as
ties of the order indicated has neverbeen previ
dielectric-media, dielectric losses occur which re
ously considered possible in the art of preparing
sult in reducing the e?iciency cf the entire circuit.
such compositions for use in the manufacture of
In many cases the electrical losses introduced by
electrical apparatus such as capacitors and in
the use of these vitreous compositions are so
'
great that glass cannot be used for the insulators, 35 sulators.
In most instances, in the past, when vitreous
supports, or for the dielectric layers or media in
materials were utilized in preparing ceramic ca
electrical capacitors.
pacitors, the dielectric medium being formed of a
The electrical losses of a capacitor may be ex
pressed either in terms of power factor or of
Q-value. The power factor is de?ned as the
sine of the dielectric loss angle, and Q-value is
vitreous composition, the capacitors were of rela- '
tivelylow electrical efficiency, this ef?ciency gen
erally not exceeding that equivalent to a Q-value
of 1000 at a frequency of a million cycles per
the reciprocal of the tangent of this angle. Since
for small angles the sine and tangent are identi
Accordingly, it is an object of this invention to
cal within negligible percent error, the Q ?gure
is for practical purposes the reciprocal of the 45 provide the electrical industry with vitreous com
positions of relatively low melting point, compo
power factor. As Q-values for low-loss dielectrics
sitions melting within the ranges usual for enam
lie in the range ZOO-10,000, the Q-value is a con
els or glasses of usual character, which novel
venient integral index ?gure, increase of which
vitreous compositions possess Q-values of an order
denotes an improvement in electrical merit'of a
capacitor, and is used in this speci?cation to de 50 never previously considered attainable. It is an
other object of this invention to provide vitreous
?ne my invention. In strict accuracy, the effi
compositions or glasses of'relatively low melting
ciency 'of a dielectric substance is expressed by
point, ‘these novel compositions being character
“Q-factor” and the efficiency of a capacitor made
ized by lowv dielectric'losses and high Q-value,
from the same substance as “Q-value.” How~
evenas under most common conditions, the ma 55 being either equal to or superior in ‘these proper¢
second.
I
~
2,413,549
4
3
ties to fused quartz now sometimes employed for
insulators and dielectrics. Still another object of
in Table II are by weight, based on the total
weight of the batch.
this invention is to furnish the art with certain
When melted, these batches will yield vitreous
novel vitreous compositions particularly adapta
compositions having the above described im
ble for use as dielectric media in electrical ca
P31 proved electrical properties, the calculated com
pacitors or electrical condensers, the use of these
positions of these vitreous products being given
vitreous compositions permitting the preparation
in Table III on a mole percentage basis, For
of capacitors of extremely satisfactory character
purposes of comparison, these compositions have
and high electrical efficiency. These and still
also been calculated on a weight percentage basis
other objects of my invention will be apparent 0 and are so expressed in Table IV. In Tables III
from the ensuing description of certain preferred
and IV vitreous material A is produced by melt
embodiments thereof.
ing batch A of Table II, vitreous composition B
My novel vitreous compositions, characterized
by melting batch B of Table II, etc.
by exceptionally high Q-values and other desira
In practicing my invention the batch compo
ble electrical properties, comprise: lead oxide;
sition to be utilized is ?rst prepared and then
silicon oxide; one or more of the oxides of the
melted to yield a substantially homogeneous ?uid
alkali metals selected from the group consisting
glass. The temperature maintained during melt
of potassium, sodium and lithium; one or more
ing is not critical but is usually within the range
of the oxides of the bivalent metals selected from
1580-1300“ C. in order that rapid homogeniza
the group consisting of magnesium, strontium,
tion of the melt may be obtained.
zinc, barium, calcium and beryllium; and ?uor
After a homogeneous ?uid product is secured,
ine in combination as a metallic ?uoride.
More
it may be further processed or fabricated by any
process well-known in the art. It may, for ex
ample, be drawn or blown, or pressed into the
particularly, these ingredients must be present
within the composition ranges of mole percent
ages de?ned below in Table vI; these mole per
centages being the number of gram molecular
weights of an ingredient in relation to the total
gram molecular weights of all ingredients in the
composition. This means of expression of com
position, commonly used in the chemical arts, is
used in Table I in preference to weight percent
form of desired objects.
age because it serves to de?ne more accurately
In preparing insulators, capacitors, or other ar
ticles from my novel vitreous compositions it is
desirable to anneal the products'thoroughly, as
I have found that not only is the mechanical
powdered form may then be utilized, as by ?ring,
in order to sinter or fuse it into any massive form
or any desired shape.
and narrowly the composition range of my novel
vitreous compositions.
,
,
'
TABLE I
I
,
‘
‘
Or, if desired, it may
be poured into water or other liquid to form a
frit which may then be subsequently ground or
comminuted to av powder. The product in this
strength increased thereby, but that annealing
also results in an improvement in the electrical
efiiciency of the material.
In practicing my invention the batch mixtures
Mole percent
Lead oxide, PbO ______________________ __. 15—25
Silica, SiOz ______________________ __'_.____. 34-47
Alkali metal oxide, R20 ________________ __ 3-11 40 given in Table II, or any other suitable batch
compositions may be employed in producing the
vitreous compositions of Tables III and IV which
may then be utilized to produce glasses of high
In this table R20 denotes an alkali metal oxide
electrical efficiency and a high Q-value, these
selected from the group which consists of potas 45 glasses being characterized by varying nature ac
sium oxide, K20; sodium oxide, NazO; and lith
cording to the particular choice from the table
ium oxide, LizO. Either a single alkali metal ox
in properties such as ?uidity, softening point,
Alkali metal fluoride, RF ___________ __'___
5-11
Bivalent metal oxide. R0 _______ Q. ____ __ 11-30
ide, or any mixture or combination thereof, may
stability against devitri?cation, and those other
be utilized, the total amount constituting at least
properties characteristic of vitreous compositions.
3 mole‘ percent but not exceeding 11 mole per 50 It is posslble to depart somewhat from the spe
cent based ‘on the total vitreous composition. In
ci?c examples tabulated provided compositions
this table R0 denotes an oxide of a bivalent metal
having the constituents present within the mole
selected from the group which consists of mag
percentage ranges given are utilized. However,
nesium oxide, MgO; strontium oxide, SrO; zinc
for maximum electrical efficiency, I have found
oxide, ZnO; barium oxide, BaO; calcium oxide,
it desirableto utilize not one but several of the
CaO; and beryllium oxide, BeO. Either a single
alkali metal oxides together in the composition.
one of these bivalent metal oxides, or any mix
Further I ?nd it preferable to include more than
. ture or combination thereof, may be utilized, the
total amount constituting at least 11 mole per- -
cent but not exceeding 30 mole percent of the
total vitreous composition. In this table RF de
notes the fluorides of sodium, potassium or lith
ium separately or in combination. To obtain the
best results it is, furthermore, preferred that the
ratio of the mole percentage of silica (S102) to
the sum of the mole percentages of the lead oxide
(PbO) and the bivalent metal oxide or oxides
(MgO, SrO, ZnO, BaO, CaO, BeO) in the com
60
one bivalent metal oxide from the group of mag
nesium oxide, strontium oxide, zinc oxide, barium
oxide, calcium oxide and beryllium oxide. The
most preferred oxides in this group are combina
tions of magnesium oxide, strontiumoxide and
zinc oxide. In some cases I may prefer, as in Ex
ample H in Tables II, III and IV, to include 1 % to
4%, by weight, of antimony oxide in the batch
composition as I ?nd that this serves to stabilize
these glasses, when used in the form of powdered
enamel, against the harmful reducing or carbon
position'should lie between 0.65:1 and 15:1.
My novel vitreous compositions may be pro 70 izing effects of organicvehicles which may be
used for their dispersion. I have generally found
duced by melting any suitable batch composi
tion'and in Table II there. are listed several batch
it undesirable to include boron oxide in the vit
reous compositions, as the presence of this con~
compositions which, when melted, will result in
vitreousglass compositions falling within the
stituent tends to reduce electricalv efficiency to
mole percentage ranges noted. The percentages
an appreciable degree. Thus, the vitreous com
2,413,549
5
6
positions disclosed in my copending application
vitreous compositions overlap the ranges in which
Serial No. 421,245, ?led December 1, 1941, were
they are present in my improved vitreous‘ com
positions of high electrical e?iciency. This 11
lustrates the speci?c nature of the compositions
the ranges of some of the constituents in those 5 prepared in accordance with this invention.
found to have electrical characteristics no bet
ter than those of ordinary glasses, even though
TABLE II
Batch compositions suitable for producing calculated compositions of Tables III and IV, weight
percent
-
‘
'
Batch
Ingredients
Red lead (PbzOd _________________________________ _ _
Flint (SiOz) ______________________________ __
A
B
C
D
E
48. 5
24. 2
50. 9
25. 4
49. 6
22. 3
49. 2
24. 6
50. 4
25. 2
. F
G
H
I
.T
K
L
41. 7
20. 7
49. 2
24. 6
50.0
25. 0
39. l
19. 6'
37. 4
18. 8
36. 9
18.4
47. 3
23. 6
3.1
Potassium carbonate (ZKzCOs 31120)
4.
.1
' 3. 5
.1
3. 3
3.1
3. 9
Soda ash (NagCO3) ______ __
Lithium carbonate (LizCOz .
2.
1.
.5
.7
.1
.3
.5
.6
.0
.3
1.9 ' 1.8
1.2
1. 2
2.4
1. 6
.3
3.1
3.1
3.9
2415
..... _.
Sodium ?uoride
TaF)__
.5
Potassium ?uori
Magnesium
oxide e(Mg0)_
(KF)
Strontium nitrate (Sr(NOs§;)_.
Zinc oxide (ZnO)
Barium csrbonaté'iiiéiiéBiI _________ “I ............................................................................... _.
Whi‘in
Berylliu
(C8003) .......................................................................................... ._
6.5
............... _.
oxide (BeO) _______________________________________________________________________________________________________ ..
Antimony oxide (SbzOs) ___________________ ___ ____ _.'......... _;
. _ _ . _ _
_ _ _ _ _ .
. . . . _ _
. _ . . _.
Sodium zirconium silicate (55% ZrOz, 29% S102, 14% NazO) _; . _ . . . .
\
1. 7
_ _ . _ __
5. 5
______________________ ._
2. 0
2.0
1. 8
____ ..
TABLE III
Calculated composition, mole percentage
Composition of vitreous material
Ingredient
‘
AV
PbO _______________________________________________________ -_ 23. 2
SiO; _______________________________________________________ __ 44. 0
B
_C
D
E
F
G
H
I
J
K
22. 5
42. 6
'24. 2
41. 3
23. 2
44. 1
22.73
42. 5
21. 4
40. 3
24. 2
46. 0
22. 3
42. 5
20. 6
40. 5
21. 0
4i. 4
i9. 0
36. 9
2.
2.5
.8
2.4
.4
2.2
2.3
.
. 3
. 8
. 9
2. 4
2.2
.1
1
. 1
i. 9
2. 1
. 8
. 5
.8
TABLE IV
Calculated composition, weight percentage
Composition of vitreous material
Ingredient
. 7
L
20. 0
37. 8
2,413,549
7
As an illustration of the preparation of my
novel vitreous compositions and of their'improved
properties, vitreous material A of Tables III and
IV was prepared in frit form by melting Batch A
of Table II and pouring the homogeneous: melt
into water. The fritted product was then
ground to yield a. ?ne powder. The powder was
then baked at a temperature of 450° C. for one
occur reversibly during melting, vitreous compo
sitions madein this manner are, I ?nd, equivalent
to those made with; alkali metal ?uoride added
to the batch. Thus the expression of the ?uorine
content as alkali metal fluoride in Table I does
not limit the scope of this invention to those
vitreous compositions in which alkali ‘metal
hour to facilitate subsequent electrical capacitor
manufacturing operations, this step being in ac 10 ?uoride was used in the batch, but serves to de
cordance With the procedure disclosed and
claimed in my copending application, Serial No.
504,889, ?led October 4, 1943. The so-treated
vitreous composition was then fabricated into an
electrical capacitor unit of the novel type dis~
closed in the copending application of Deyrup
and Ballard, Serial No. 504,882, ?led October 4,
1943.
A ?ring temperature of 740° C. was utiiized
in the fabrication. The resulting capacitor,
when tested, was found to have a Q-value of ~
5000 at 1000 kilocycles per second.
This is an
electrical e?lciency higher than that of any pre
viously known vitreous composition melting be—
low the melting range of fused quartz.
It is to be noted that many of the composi- i
fine the stoichiometric composition range of these
novel compositions of high electrical efficiency,
all ?uorine being expressed as alkali metal
fluorides.
As many variations may be made in my im
proved vitreous compositions, certain preferred
embodiments of which are herein disclosed, with
out departing from the scope of the invention, it
is my intention that such variations shall be in
cluded within its scope to the extent that they‘are
comprehended wihin the appended claims.
I claim:
1. A vitreous composition having a Q-value
of over 2000 and comprising between 15 and 25
mole percent of lead oxide, between 34 and 4'1
mole percent of silica, between 3 and 11 mole
percent of at least one alkali metal oxide taken
‘the group consisting of potassium: oxide,
um oxide and lithium oxide, between 5 and 11
‘lo percent of at least one alkali metal ?uoride
some fabrication methods and generally avoided 30
from the group consisting of sodium
in the enameling arts, is a distinct advantage
tions of Tables II, III and IV devitrify quite
readily. Surprisingly this devitri?cation does not
impair the electrical e?icienoy. This tendency
to devitri?cation, while disadvantageous for
i
in fabrication of capacitors or' insulators from '
powder by sintering because compositions selected
for facile devitri?cation are not prone to warp
or deform from their desired shape when sub‘
?uoride, potassium ?uoride and lithium fluoride,
'vween l1 and 30 mole percent of at least one
i. “lent metal oxide taken from the group con
r siting of magnesium oxide, strontium oxide, zinc
oxide, barium oxide, calcium oxide and beryllium
jected to moderate over?ring. I ?nd that the
oxide, said mole percentages being based onthe
tendency to devitrify can be decreased by choos
total molecular weight of the vitreous composi~
ing compositions with a higher ratio of mole
percent silica (SlOz) to total sum of mole per 40
2. A vitreous composition having a Q-value of
cent lead oxide (PbO) and mole percents of
over 2000 and comprising between 15 and 25 mole
divalent metal oxides (MgO, SrO, ZnO, B10,
percent of lead oxide, betweenBé and 4.7 mole
CaO, and BeO) , this ratio preferably remaining‘
percent of silica, a total of between 3 and 11
between the limits of 0.65:1 and 1521. On the
mole percent of a plurality of alkali metal oxides
other hand, choice of com-positions with the afore
taken from the group consisting of potassium
said ratio at lower values within the range cited
oxide, sodium oxide and lithium oxide, between
generally results in securing the best electrical
5 and 11 mole percent of at least one alkali metal
properties obtainable.
?uoride taken from the group consisting of sodium
tion.
By preparing vitreous compositions within the‘
above-de?ned ranges of components, I have found
it possible to consistently produce capacitors hrv~ Y
mg a Q-value of over 2000.
rThe unusual electri- ~
cal efficiency of vitreous compositions prepared in
accordance with my invention is indeed surprise
ing, since it had previously been considered es
sential that the alkali metal oxides be excluded
insofar as possible from glass compositions which
'
'
?uoride, potassium fluoride and lithium ?uoride,
between 11 and 30 mole percent of atleast one
bivalent metal oxide taken from the group con
sisting of magnesium oxide, strontium oxide, zinc
oxide, barium oxide, calcium oxide and beryllium
oxide, said mole percentages being based on the
a total molecular weight of the vitreous composi
tion.
3. A vitreous composition having a Q-value
were intended for utilization in the manufac- 1
of over 2000 and comprising between 15 and 25
mole percent of lead oxide, between 3-4: and 47
ture of electrical apparatus. I have found that
the remarkable electrical properties of my novel 60 mole percent of silica, between 3 and 11 mole
vitreous compositions do not result from the pres
percent of at least one alkali metal oxide taken
ence or absence of any one constituent in the glass
from the group consisting of potassium oxide,
products, but are the result of the presence of
sodium oxide and lithium-oxide, between 5 and 11
the constituents noted and their utilization in
mole percent of at least one alkali metal ?uoride
the particular composition ranges disclosed.
65 taken from the group consisting of sodium
It will be understood that many varied raw
?uoride, potassium fluoride and lithium ?uoride,
materials may be used which will decompose in '
a total of between 11 and 30 mole percent or"
melting to produce the oxides required in the
a plurality of bivalent metal oxides taken rom
the group consisting of magnesium oxide, stron
compositions of‘Ta-bles I, IIand III; The ex~
pression of the fluorine content as alkali metal
tium oxide, zinc oxide, barium oxide, calcium
oxide and beryllium oxide, said mole'percentages
fluoride in Table I does not preclude its introduc
beingbased on the total molecular weight of the
tion to the batch in other forms, such as ?uor
spar CaFz, together with su?icient alkali metal
oxide compounds also added to the batch. Since
reactions such as:
vitreous composition.
_
_
‘
4. A vitreous composition having a Q-value
of over 2000 and comprising between 15 and 25
2,413,549
10
mole percent of lead oxide, between 34 and 47
mole percent of silica, a total of between 3 and 11
mole percent of a plurality of alkali metal oxides
taken from the group consisting of potassium
oxide, sodium oxide and lithium oxide, between
5 and 11 mole percent of at least one alkali
metal ?uoride taken from the group consisting
of sodium ?uoride, potassium ?uoride and lithium
?uoride, a total of between 11 and 30 mole
percent of a plurality of bivalent metal oxides
taken from the group consisting of magnesium
oxide, strontium oxide, zinc oxide, barium oxide,
calcium oxide and beryllium oxide, said mole per
centages being based on the total molecular
weight of the vitreous composition.
5. A vitreous composition having a Q-value
of over 2000 and comprising between 15 and 25
mole percent of lead oxide, between 34 and 47
mole percent of silica, between 3 and 11 mole
percent of at least one alkali metal oxide taken
from the group consisting of potassium oxide,
sodium oxide‘and lithium oxide, between 5 and 11
mole percent of at least one alkali metal ?uoride
taken from the group consisting of sodium
?uoride, potassium ?uoride and lithium ?uoride.
between 11 and 30 mole percent of at least one
bivalent metal oxide taken from the group con
sisting of magnesium oxide, strontium oxide, zinc
oxide, barium oxide, calcium oxide and beryllium
oxide, the ratio of the mole percent of silica to
the mole percent of the sum of lead oxide and
bivalent metal oxide in the composition being
between 0.65:1 and 15:1, said mole percentages
being based on the total molecular weight of the
vitreous composition.
6. A vitreous composition having a Q-value of
over 2000 and comprising between 15 and 25 mole
percent of lead oxide, between 34 and 4'? mole
percent of silica, a total of between 3 and 11 mole
percent of a plurality of alkali metal oxides taken 40
from the group consisting of potassium oxide,
sodium oxide and lithium oxide, between 5 and 11
mole percent of at least one alkali metal ?uoride
taken from the group consisting of sodium ?uo
* ride, potassium ?uoride and lithium ?uoride, be
tween ll and 30 mole percent of at least one
bivalent metal oxide taken from the group con
sisting of magnesium oxidastrontium oxide, zinc
oxide, barium oxide, calcium oxide and beryllium
oxide, the ratio of the mole percent of silica to
sium ?uoride and lithium ?uoride, a total of be
tween 11 and 30 mole percent of a plurality of
bivalent metal oxides taken from the group con
sisting of magnesium oxide, strontium oxide, zinc
oxide, barium oxide, calcium oxide and beryllium
oxide, the ratio of the mole percent of silica to
the mole percent of the sum of lead oxide and
bivalent metal oxide in the composition being
between 0.65:1 and 15:1, said mole percentages
being based on the total molecular weight of the
vitreous composition.
8. A vitreous composition having a Q-value of
over 2000 comprising between 15 and 25 mole
percent or" lead oxide, between 34 and 47 mole
percent of silica, a total of between 3 and 11 mole
percent of a plurality of alkali metal oxides taken
from the group consisting of potassium oxide,
sodium oxide and lithium oxide, between 5 and
11 mole percent of at least one alkali metal ?uo
ride taken from the group consisting of sodium
?uoride, potassium ?uoride and lithium ?uoride,
a total of between 11 and 30 mole percent of a
plurality of bivalent metal oxides taken from the
group consisting of magnesium oxide, strontium
oxide, zinc oxide, barium oxide, calcium oxide
and beryllium oxide, the ratio of the mole per
cent of silica to the mole percent of the sum
of lead oxide and bivalent metal oxide in the
composition being between 0.65:1 and 1511, said
mole percentages being based on the total molec
ular weight of the vitreous composition.
0. A vitreous composition having a Q-value of
over 2000 and comprising between 15 and 25 mole
percent of lead oxide, between 34 and 47 mole
percent of silica, between 3 and 11 mole percent
of at least one alkali metal oxide taken from the
group consisting of potassium oxide, sodium oxide
and lithium oxide, between 5 and 11 mole per
cent of at least one alkali metal ?uoride taken
from the group consisting of sodium ?uoride,
potassium ?uoride and lithium ?uoride, between
11 and 30 mole-percent of at least one bivalent
metal oxide taken from the group consisting of
magnesium oxide, strontium oxide, zinc oxide,
barium oxide, calcium oxide and beryllium oxide,
and between 1% and 4% antimony oxide, said
mole percentages being based on the total molec
ular weight of the vitreous composition.
10. A vitreous composition having a Q-value of
over 2000 and comprising between 15 and 25 mole
percent of lead oxide, between 34 and. 47 mole
the mole percent of the sum of lead oxide and
bivalent metal oxide in the composition being
percent of silica, between 3 and 11 mole percent
between 0.65:1 and 1.5:1, said mole percentages
of at least one alkali metal oxide taken from the
being based on the total molecular weight of the
group consisting of potassium oxide, sodium oxide
vitreous composition,
and lithium oxide, between 5 and 11 mole percent
7. A vitreous composition having a Q-value of
of at least one alkali metal ?uoride taken from
over 2000 and comprising between 15 and 25 mole
the group consisting of sodium ?uoride, potas-v
percent of lead oxide, between 34 and 4'7 mole
sium ?uoride and lithium ?uoride, a tota1 of be
percent of silica, between 3 and 11 mole percent
tween 11 and 30 mole percent of magnesium oxide,
of at least one alkali metal oxide taken from the 60 strontium oxide and zinc oxide, said mole per
group consisting of potassium oxide, sodium oxide
centages being based on the total molecular
weight of the vitreous composition.
and lithium oxide, between 5 and 11 mole percent
of at least one alkali metal ?uoride taken from
the group consisting of sodium ?uoride, potas
ALDEN J. DEYRUP.
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