CA1083183A

CA1083183A - Joint doping of porous glasses to produce materials with high modifier concentrations

Info

Publication number: CA1083183A
Application number: CA293,413A
Authority: CA
Inventors: Mrinmay Samanta; Joseph H. Simmons; Pedro B. Macedo
Original assignee: Individual
Current assignee: Individual
Priority date: 1976-12-30
Filing date: 1977-12-19
Publication date: 1980-08-05
Also published as: IT1092175B; FR2376084B1; NL7714580A; JPS62858B2; JPS53102324A; SE7713981L; FR2376084A1; DE2758897A1; SE430685B

Abstract

ABSTRACT OF THE DISCLOSURE:

A glass composition having at least 85 mole percent of Si02, where the improvement comprises at least 7 wt percent of at least one member selected from the group consisting of PbO
and Bi203 and at least 1.5 mole percent of at least one member selected from the group consisting of K20, Rb20 and Cs20. The method for producing the glass composition comprises immersing a porous glass matrix in a dopant impregnating solution containing a mixture of dopants to impregnate the porous glass with the solution and form an impregnated porous glass matrix having the above glass composition.

Description

~3~l~3 The use of porous glasses as su~strates for the molecu- ~ ;
lar deposition of selected materials has shown great promise in `
the production of materials with selected physico-chemical proper-ties and selected property variations. This process, called "Mo-lecular Stuffing" or doping has been described in detail in U.S.
Patent 3,938,97~ and Can. Patent Applications Serial Nos. 247,547 and 247,545 filed March 10, 1976. In addition to such porous glas~
ses, the present invention is applicable to porous glass produced by such other methods as chemical vapor deposition. (See U.S. ~ ~-Patent No. 3,859,093~.
By the molecular stuffing process, selected solutions containing materials which alter the physico-chemical properties of high silica glasses are diffused into the pores of a high sili-ca base glass preform to achieve a homogeneous concentration of the modifying agent or agents (solute~. For step concentration profiles these modifiers are subsequently precipitated and a cladding region is formed by their removal from the outer regions of the preform by a suitable solvent before drying and sintering ` ~ of the preform assembly. For graded concentration profiles, the ~ `
concentration of selected modifiers is altered to a desired varia- -tion by a second soaklng of the preform in selected solvent solu-tlons containing selected concentrations of modifiers. This is ` followed by precipitation of the modifiers and subsequent drying and sintering of the preform assembly. (See Patent Appl. Serial No. 247,545).
The change in physical property achieved by the addition of dopants is a function of the dopant concentration. Therefore i~ the addition of high dopant concentrations generally induces a ~ large change.
1 :
3,~ 30 Several products benefit from large variations in physi-cal properties and therefore large variatians in dopant concentra-tion. For example, in fiber optics, a large change in index of .,, ~ ~

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.

refraction between the core and cladding regions of a fiber yields a high numerical aperture, while in strengthening brittle mate-rials, a large change in thermal expansion coefficient and/or in glass transition temperature between the surface and the interior of an article allows the formation of large surface compressions (pre-stressing) and thus the achievement of correspondingly in-creased strengthening.
The numerical aperture, NA, of a light transmitting de-v~ce is a measure of its acceptance angle. In optical waveguides the numerical aperture is related to the difference in refractive index, n, between the axis or center of the waveguides and the off -axis elements. An increase in numerical aperture is obtained by increases in the index difference between these elements (for -example, in waveguides with step index profiles, the difference is between the refractive index of the core, nl, and the clad, n2, ~-regions; thus NA = ~nl - n22~.
Since numerical aperture is related to the angle of ac-ceptance of the incident light beam, high numerical apertures are desirable since this allows transmission of relatively more energy from a given light source. High numerical apertures are also de-sirable from the standpoint of reducing microbending losses in optical waveguide fibers, and for the preparation of lens elements and other optical elements.
The process described in Patent No. 3,938,974 and Patent . .
Application Serial No. 247,545~ demonstrates how molecular stuffing `j of porous glasses may be conducted using a series of dopants both individually and in groups to develop integrated optics components with tailored refractive index clistribution and .
strengthened articles with tailored thermal expansion coefficient and glass transition temperature distributions.
.
This invention employs glass compositions and dopants

- 2 -', ' ~ . . .

1083~83 .:
similar to those in Patent Applications Serial Nos. 247,547 and 247,545. ~owever, we have found that certain dopants, such as lead, induce noticeable scattering in the glass when large dopant concentrations are used. Though scattering has little effect on most uses such as integrated optics components and strengthened members, some uses such as the transmission of very high quality -images (e.g. in cystoscopes) or long, extremely low loss, (e.g.
below 20 dB/km) optical fibers with a high numerical aperture (e.g. N.A. greater than 0.35~ may be limited by the amount of light scattering present. For example, when lead is used as a do-pant oxide at doping concentrations above 40 grs. Pb (N03)2/100 cm3 of water, scattering is observed in the final giass. This is an important limitation because lead has a high atomic polariza-bility and is useful to obtain high index glasses. It would be of considerable advantage if glasses could be made with large con~
concentrations of Pb as a dopant in those situations where a com~
i bination of high NA and very low scattering loss were desired.
We have discovered that when the dopant in molecular stuffing is composed of certain combinations of lead and/or bis- ~ ;
muth with cesium, rubidium, and/or potassium, then a remarkable ~ ;
and unexpected decrease in the scattering loss occurs. Further, we have discovered that certain of these combinations which give low scattering losses may be used to obtain high numerical aper~
tures and/or high pre-stressing levels as well.
; The desired combination of dopants leads to glass arti-cles with the following final composition. The composition of these glasses consists of at least 85 mole percent SiO2 with im-provements which comprise at least 7 wt. percent of at least one member selected from Group (I) consisting of PbO and Bi203 and at least 1.5 mole percent of at least one member selected from Group (II~ consisting of K20, Rb20, and Cs20.

Even though the maximum dopant concentration is limited

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1~3~33 by the concentration of Si02, the broad limi-ts are a maximum of 25 wt. percent for Group (I) and a maximum of 9 mole percent for Group (II). Our preferred range covers at least 2 but not more than 9.5 mole percent B203 and at least 7 but not more than 20 wt.
percent of Group (I) and at least 1.5 but not more than 7 mole perc~nt of Group (II).
In another embodiment of this invention, we have disco-vered that a method which comprises adding a dopant to a porous matrix with interconnective pores, immersing the porous matrix in .. -a~solution of a dopant causing the dopant to precipitate in the ~ :
matrix, removing solvent and where necessary, decomposition pro-ducts, from the porous matrix and collapsing the porous matrix to a solid form, can be used to produce high silica glasses. These glasses can be produced with the following mixed dopant composi~
tions: Group I being Pb and/ox Bi and Group II being K, Rb, and/or .
; Cs in the form of nitrates, carbonates, acetates, borates, phos-~: phates, arsenates and/or silicates in either hydrated or unhydra~
ted form of mixtures therefrom used to pro~luce a glass having . ~:
` composition 7 to 25 wt.percent (.preferred range 7 to 20 wt. per-~` 20 cent) of the oxide equivalent of at least one member selected from ;~ .
Group I consisting of Pb and Bi and 1.5 to 9 mole percent (the .: ~ .
preferred range being 1.5 to 7 mole percent) of the oxide equiva-J lent of at least one member selected from Group II consisting of ..
K, Rb, and Cs.
:. Finally in another embodiment of this invention, letting I represent [Pb(N03)2~ and CBi(.N03)2~, and II represent the alkali nitrates of Cs, Rb and K, taken either singly or in combination the ranges of dopant which yield glasses with high numerical aper- :~
ture and low scattering loss are~

Broad 3 Preferre~
g/100 cm g/100 cm solution solution .
Group I 45-200 50-150

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~: ' ' . ' : , :

1~3~33 :

Group II 40-200 50-150 where the weight represents the weight of ~t least one member of the group in the form of a nitrate salt. $he solutions may be wa-ter, optionally with small amounts of low molecular weight alco- -hols such as methanol. The solvents used in precipitating the do-pants may be lo~ molecular weight alcohols such as methanol and ethanol.
In another embodiment of this invention when multiple dopants are used and precise control in the variation of dopant concentration near the surface of-the article, e.g., to produce a composition step profile, is desired, we have found that after the dopant has filled the pores of the article, it is necessary to im- , -~
; merse the articles in a solution of multiple solvents which are selected so that the solubility of each dopant compound is appro-.
ximately the same. Thus there will be approximately the same change in concentration of each dopant in the region of the arti-cle near the surface, preferably the dopant solubilities of 1/2 to 15 grs of dopant material per 100 ml of solvent solution. It :lS ~ ~.

,~ . .. : ' preferred to further wash the glass article in a soluti.on selected to control the solubility of each dopant compound within a range of 0 to 2 grs of dopant material per 100 ml of solution.

EXAMPLES
, . ~ .
:
Porous glasses are used as substrates for the deposition `, I of the selected dopant combinations. Any porous glass preform is ~ ~ satisfactory. In this example we prefer to describe a spec1flc -~ ~ method for forming the porous glass substrate by phase separation, j :
although other processes are just as useful. (See for example $ Schultz, Patent No. 3,859,073).

~n alkali borosilicate glass of composition 57% SiO2, -~

' 30 36% B203, 4~ Na20 and 3% K20 is melted in a Pt crucible in an electric furnace at temperatures between 1300 and 1450C. The - .
~1 melt is homogenized by stirring with a Pt stirrer, and is then . ~ , . . ~

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~83~33 pulled in the form of rods 5/16" diameter by 4' length. These ~ -rods are then cut into rods ~" in leng-th which are hea-t-treated at 550C for 1 1/2 hours to induce phase separation and subsequen-tly leached in a 3N HCl acid solution. During the phase separa--tion heat-treatment, the homogeneous glass decomposes in-to two phases, one with high silica content and one with high boron and alkali content and lower silica content. These phases are inter-connected sufficiently that exposure to the leaching solution com- j pletely removes the alkali rich phase leaving behind a high silica porous glass substrate. The porous glass is washed with deionized water.
The porous glass substrate is immersed in a solution containing the desired concentrations of dopants (see Table I) for 3 hours or longer to allow the solution to fill the pores comple-tely. The dopant compounds are then precipitated from solution. ;~
In following this process we have found it desirable to achieve precipitation of the dopants by thermal means; that is, lowering the temperature of preform and solvent to a point where the solution within the pores becomes supersaturated with the do-pant causing the dopant to precipitate in the pores. The sampleis then transferred to an unstuffing solution of sol~ent whereby some of the dopant is allowed to dif~use out of the pores yielding ; a sample with graded dopant concentration. This step is necessary in both fiber optics to achieve high numerical apertures and in strengthening to achieve high surface compressive stresses. When graded properly, the dopant concentration is nil near the surface of the object thus yielding a low refractive index and/or a low thermal expansion coefficient in the cladding region. The unstuf- ~ ~
fing step is often conducted sequentially in two different solu- i tions to insure maximum dopant removal from regions near the sur-face of the object (see Table I~ steps (a) and (b). The sample is kept at 0C where it is exposed to vacuum for 24 hours and : - 6 -: : . .
' ..
~: , , , ~83:~L8~3 then heatecl at 15C~hour up to 625C under vacuum and sintered between 830 and 850C.
EXAMPLES I, II ~ND III
Table I reports details (concentration of solution, tem-perature and times) of the preparation and measurements of refrac-tive index in the core (central) and cladding (outer) regions of the objects made by using lead nitrate and cesium nitrate as do-pants. Corresponding numerical apertures are also listed. Table II lists the compositions at the center of the final glass arti-cles. A fiber was pulled from Sample II and scattering losses were found to be less than 20 dB.km in each case.
EXAMPLES IV TO VIII
A porous glass preform prepared as described in Examples I to III is soaked in a dopant solution as described in Table III
at a temperature specified in Table III for 16 hours. This tempe-rature is chosen to be at or above the solubility temperature of ~ the designated dopant concentrations. This allows the dopant so- `
- lution to fill the pores of the preform completeIy and uniformly.
The preform is then removed from the dopant solution and is soaked 20 in a single solvent as specified in Table III for three hours in order to precipitate the dopant within the pores. No dopant remo-val was attempted since these glasses were made only to observe index change with concentration. The sample is then kept at 0C
` where it is exposed to vacuum for 24 hours and warmed up at 15C
per hour up to 625C also under vacuum. It is then heated to bet-~i ween 830 and 850C where sintering occurs. Refractive index mea~
surements conducted on these samples are reported in Table III.
Table IV details the compositions of the final glass articles lis-- ted for Examples VI and VII.

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0 X a~
~1 ~ 0 ~ ~r et' p; H U

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~ ~ O ~ ~ ~ O ~; ~
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O O ~ O O ~ O O ~1 0~ 0~ ~'~ O>1 o o ~ o O ~ a~ o ~ u' e ~ ~ Ei3 ~ '; .' H X ~1 0 ~-1 O 1-l O h ~1 : ::
o 1~ a) o\ ~- a) o\ ~ o d~
~ ~J O ; .' ',~
. 1 1 H O (d O 0 0 00 11~ 0 ~1 U) 3 ~ 3 ~ ~ 3 D ~ oC~ o o~ o~ o~) oU _ o o o o o o '. ;- ~
O . .
.` ~ ~ .C ~ ` . ' 0 Q 0 R ; ~:

: ~ ~ . . ~
O ' ': !
~)~rl ~ 3 0 ~I Q~ O O O
. ~o ~ a~ ~ O O ,~ , ,: ., '',,' ~ ~ o _ O ~ O ~ , " :' ,~ O t~l ~ ~1 ~ ~ ~rl ~)~,1 U~ :
.~.,, ,_ 0 41 .~J~ O~ ~ ~`1 o q-l ~ .,~ . ..
z o ~ ~ z o ~ ~ æ o ~
O -- ~1 O --~I H~) --~--I ~ : : . .
~.~~ æ u~ ~ 0 H æ u~.-, O H oU~ -1 0 ~ `~ . :. .
- 0 H ~ C,) ~ H-- ~ n H æ r~~ ~ rl .4 R -- IH .
o u~ O ~ ~ o u~ ) o u r ~ O ~ ~1 0 ~ 0 ~ ~ .
~: ~ ~ o ~1 0 ~ o ~1 o Q~ O -1 0 ~ ~ ~
~ 0 0 ~ h ~ 0 o ~5~ :~ 0 ~t~ S ` ~
o x ~ ~ X O O t:~ X O a) t~ ~
H + ~ 0 ~ r-l + ~ 4 0 ~1 C0 + 1 ~ 0 K ~:

- .

.. .. . . . .

TABLE II

Example SiO2 B203 Cs20 PbO PbO
mole % mole % mole % mole % wt %

TABLE III ~

Dopant solution Refractive -~
(.per 100 ml of Dopant solution Solvent index in EXAMPLE aqueous solution~ temperature temp. core IV 50 g KnO3 + o ethOnol 95 g Pb(N03)2 120 C O C 1.506 ;
- 63 g RbN03 + ethanol .
V 100 g Pb(N03)2 120C 0C :1.514 ~: 120 g CsN03 +
VI 100 g Bi(N03)3 + ethOnol :~
5H20 110C O C 1.514 .
136 g CsN03 +
: 57.1 g Pb(N03)2 +
;. VII 57.1 g Bi(N03)3 ethOnol : 20 5H20 110C O C 1.516 - :

TABLE IV
-` Example SiO2 ~ PbO PbO Bi23 Bi23 mole % mole % mole % mole % wt % mole % wt %
VI 91 3 4 0 0 2 9 ~ :
.~ .
~ VII 89 3 4.5 2.5 7 1 5 :
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~83183 In cases where multi~le dopants are used and precise control of the profile variation in the cross-section of the arti-cle is desired, we have found that the following uns-tuffing pro-cess yields good results.
The porous glass preform is prepared as discussed above and a combination of dopants are diffused in its pores. Let us denote these by dopant A and dopant B. Precipitation is achieved by either cooliny the glass article or immersing it into a solvent with extremely low solubility for both dopants A and B.
In the ensuing unstuffing process, where good control of the dopant concentration is necessary, we have found it desirable to use mixed solvents. It is useful to have the dopants A and B
both with the same solubility in the solvent mixture. This is ac~
complished by selecting a minimum of 2 solvents with the following properties.
The first set of solvents or single solvent has the de~
sired unstuffing solubility which in the case of step profiles is 0.5 to 15 grs/100 cc of solution for dopant A. The second solvent or set of solvents has the desired unstuffing solubility of 0.5 `~ 20 to 15 grs/100 cc of solution for dopant B at the same temperature The solvents are chosen so that when mixed together, the two sets of solvents have the desired unstuffing solubility of .5 to 15 grs/100 cc of solution for both dopants A and B. Best control of profile is afforded when the solubilities for dopants A and B are equal to within + 50%; that is, if the solubility of dopant A in solution is 5 grs/100 cc of solution, then the solubili-ty of B is ~ ~
between 2.5 grs/100 cc and 7.5 grs/100 cc of solution. ~ -In the stuffing step, dopant concentration is in the or- -der of 100 gm per 100 cc of solution. In contrast the first unstuffing solution solubilities given above are in the order of a factor of 10 less than during the stuffing step.

Preferably the glass article is further washed in a :' :, ' - 1 0 - , ,,~

,:~ .
-~: :: . , ~L~83~33 wash solution in which the dopants are even less soluble, e.g., a solubility of 0 to 2 grs of dopant per 100 cc of solution. This wash step ensures that substantially complete precipi-tation of do-pant has occurred before drying is commenced.
Solvents can be selected from the following group:
1. Water, in which the alkali me~als are very soluble.
In water, potassium, rubidium and cesium nltrates have a high temperature dependence oE solubility which is useful in thermal precipitation.
2. Lower molecular weight aliphatic alcohols (i.e., less than 6 carbons/molecule) in which the alkali metal and lead salts are progressively less soluble with increasing molecular weight. Alcohols with six or more carbons per molecule have such low solubilities for the dopants used here that they are not com-monly used.
3. Acids: the solubility of bismuth is strongly de-pendent on the pH of the solution. This solubility can be con~
~` trolled by introducing an acid. The acid and any resultant salts formed by reaction of the acid with the dopants of group I and II
must either evaporate or decompose into an oxide compound before - sintering occurs. We prefer to use nitric acid.
Ternary dopant systems are handled in the same way with several solvents in solution with each o-ther.
In both these cases of multidopant stuffing and con-trolled profiling, it is possible to develop high residual stres-ses in rods by selecting dopants which alter the thermal expan-sion coefficient or the glass transition temperature or both pro-perties of the porous glass. ~
The two examples below describe processes used for two ~;
and three joint dopants with controlled profiles to develop a re-sidual compressive stress in the outside layer of the consolida- ~ ;
ted glass articles.

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lS3 EXAMPLE IX
In this example we describe a procedure for 2 dopants which are jointl.y diffused into the pores of a porous preform.
Profiling of these dopants is accomplished by using a so].vent .
consisting of a solution of 3 solvents.
Porous glass articles in the shape of cylindrical rods were prepared as in Example 1 above. The porous preforms were : ~.
immersed in a stuffing solution containing 120 grs of CsN03 and 100 grs of Bi(N03)3 5H20 per 100 cc of solution, at 108C for a -:
period of 24 hours. The dopants within the pores were then pre- ~.
cipitated by transferring to an unstuffing solution consisting of three solvents at 0C in the following proportion: 6.4% methanol, 10.8~ of 70% nitric acid in water solution and 82.8% water by vo-lume. The rods remained in this solution for 5 hours at 0C. A
clear region appeared within the outside surface. The samples were washed for 48 hours in a solution of 82.2% methanol, 1.8% of ~.
70% nitric acid in water solution and 16% water at 0C, and then were dried at 0C for 7 hours. The samples were then heated in vacuum to decompose the nitrates and sinter the pores, reaching a . ~ -20 temperature of 825C. .~ ;~
: The consolidated samples were examined and consisted of a cladding region relatively free of dopant compounds adjacent to .
the outside surface of the sample, and a core region within the ~;
~ cladding which contained substantially all the dopants. Measure-ments with an optical microscope revealed a ratio of cladding .~ thickness to rod radius of 0.342 and a compressive stress in the cladding region of 14,000 psi. Measurements of light scattering : loss from these samples showed an intrinsic scattering loss of ;
3.4 dB/km at 1.05 ~m.
EXAMPLE X ~ , In this example we describe a procedure for 3 dopants which are jointly diffused into the pores of a porous preform.

- 12 ~

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.: . . .: . . .. . .: .. :
. .; ~ . . : . , :: ' ' '' , ' , : ; . '.''.. ; ' ' .

~83~l33 Profiling is accomplished by using a solven-t consisting of a solu-tion of 3 solvents.
Porous glass preforms were prepared as described above.
The porous preforrns were immersed in A stuEfing solution made by saturatincJ 50 ml of water with CsN03, then saturating the solution with KN03 and clissolving Bi(N0313 . 5H20 in the solution at a tem-perature of 100C. The inal composition of the stuffing solution consisted of 50 ml H20, 94 grs CsN03, 142 grs KN03 and 234 grs Bi(N03)3 . 5H20. The rods were left immersed in this solu-tion for 48 hours at 100C. The dopants within the rods were then precipi-tated by transferring the rods from the hot stuffing solution to methanol at 0C. The rods were kept in methanol for 10 minutes ; and then were transferred to an unstuffing solution consisting of three solvents at 0C in the following proportion: 6.5% methanol, 10.8% of 70~ nitric acid and 82.7% water by volume. The rods re-mained in this solution for 1.5 hours at 0C. A clear region ap-peared over the external surface of the samples. The samples were ;~
then washed in methanol at 0C for 24 hours and were dried in va- ~;
cuum at 4C for 24 hours. The rods were then heated in vacuum to decompose the nitrates and sinter the pores at 850C.
The consolidated rods were examined and consisted of a . .
cladding region relatively free of dopant compounds adjacent to - the outside surface of the sample and a core region within the :
cladding which contained substantially all the dopants. Measure~
i ments with a microscope revealed a ratio of cladding thickness to ; rod radius of 0.16 and a compressive stress in the cladding region of 25,000 psi. Measurements of light scattering loss from these samples showed an intrinsic scattering loss of 1.6 dB/km at 1.05 ~;;

~m.

'~ 30 :' .

- - . : .

' ' ': , ' , ' , '' .

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A glass article having a center and a surface, said article having a composition comprising at least 85 mole percent of Si02, from 7 to 25 weight percent of at least one dopant member selected from the group (I) consisting of PbO and Bi203 and from 1.5 to 9 mole percent of at least one dopant member selected from the group (II) consisting of K20, Rb20 and Cs20 in said center and decreasing amounts of said dopant mem-bers of groups (I) and (II) away from said center and an in-creasing Si02 concentration away from said center as said sur-face is approached.

2. The glass article of claim 1 wherein said compo-sition comprises from 7 to 20 weight percent of said dopant member of group (I) and from 1.5 to 7 mole percent of said do-pant member of group (II).

3. The glass article of claim 1 wherein said composi-tion further comprises at least 2 mole percent B203.

4. The glass article of claim 1 wherein said compo-sition further comprises from 2 to 9.5 mole percent B203.

5. The glass article of claim 1 wherein said dopant member of group (II) is Cs20.

6. The glass article of claim 1 wherein said dopant member of group (II) is Rb20.

7. The glass article of claim 1 wherein said article is an optical waveguide fiber.

8. In a method of producing a glass article compri-sing adding a dopant to a porous glass matrix with interconnec-ted pores by immersing the porous glass matrix in a solution of the dopant to impregnate the porous glass matrix with the solu-tion, precipitating the dopant from the solution within the po-rous glass matrix, removing solvent and when necessary decompo-sition products from the porous glass matrix and collapsing the porous glass matrix to a solid form, the improvement which com-prises immersing the porous glass matrix in a dopant impregnating solution containing a mixture of dopants to impregnate the po-rous glass matrix with the solution and form an impregnated po-rous glass matrix with the following composition:
(1) 7 to 25 weight % of the oxide equivalent of at least one member selected from the dopant group consisting of Pb and Bi, and (II) 1.5 to 9 mole % of the oxide equivalent of at least one member selected from the dopant group consisting of Cs, Rb, and K.

9. In a method of producing a glass article accor-ding to claim 8 where in the amount of Group (I) is 7 to 20 weight percent and Group (II), 1.5 to 7 mole percent.

10. In a method of producing a glass article accor-ding to claim 8 the improvement which comprises depositing a mixture of dopants by precipitating them from an aqueous solu-tion whose composition is 45 to 200 g of at least one member selected from the group consisting of Pb(N03)2 and Bi(N03)3 per 100 cm3 of solution and 40 to 200 g of at least one member selected from the group II consisting of CsN03, RbN03 and Kn03 per 100 cm3.

11. In a method of producing a glass article accor-ding to claim 10, the improvement which comprises having 50 to 150 g of Group I/100 cm3 of solution and 50 to 150 g of Group II/100 cm3 of solution.

12. In a method of producing a glass article accor-ding to claim 8, further comprising, prior to removing solvent and when necessary decomposition products from the porous glass matrix, immersing the impregnated porous glass matrix in a se-cond dopant removing solution containing a separate solvent for each dopant group selected so that the solubilities of the do-pants in the second solution are less than in the first solution and are approximately equal to within ? 50% to decrease the concentration of each dopant substantially the same amount in the region of the article near the surface.

13. The method according to claim 12 wherein the so-lubilities of each of the dopants in said second solution is 0.5 to 15 grams per 100 cc of solution.

14. The method according to claim 12 wherein said second solution is an acidified aqueous solution containing a lower aliphatic alcohol.

15. The method according to claim 12 and further com-prising precipitating the dopants within said impregnated porous glass matrix during immersion in said second solution.

16. The method according to claim 12 and further com-prising precipitating the dopants prior to immersing said impre-gnated porous glass matrix in said second solution.