CA1073213A

CA1073213A - Method and apparatus for the manufacture of glass

Info

Publication number: CA1073213A
Application number: CA233,203A
Authority: CA
Inventors: Francois E. Chevallier; Jean M.P. Fenouillet
Original assignee: Saint Gobain Industries SA
Current assignee: Saint Gobain Industries SA
Priority date: 1974-08-14
Filing date: 1975-08-11
Publication date: 1980-03-11
Also published as: GB1514317A; NO141749C; FR2281902A1; JPS5145113A; SE413397B; ES440243A1; LU73202A1; DE2535937C2; BE832408A; NO752828L; NO141749B; NL177682C; JPS5837255B2; SE7509072L; IT1041566B; DE2535937A1; NL7509629A; FR2281902B1; ATA629475A; AT366014B

Abstract

METHOD AND APPARATUS FOR THE MANUFACTURE OF GLASS

ABSTRACT
This invention relates to a process and an apparatus for the manufacture of fused glass, suitable for molding, wherein the total time of manufacture is reduced to about one hour. The process accelerates the homogenization and refining of glass by eliminating unfused particles and gas bubbles which are the main factors limiting the production rate of industrial glass.
This is accomplished by increasing the temperature of the molten glass to produce foaming throughout its mass while maintaining its viscosity below 1,000 poises.

Description

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1 ~IETI-~OD AND APPARATUS FOR T~IE ~NUFACTURE 0~ GLASS
. ~

2 ¦ 4I) r Tr~ACT
31 This invention relates to a process and an apparatus for ¦ the manufacture of fused glass, suitable for molding, wherein the 51 total time of manufacture is reduced to about one hour. The pro-6¦ cess accelerates the homogenization and refining of glass by eli-71 minating unfused particles and gas bubbles which are the main 8¦ factors limiting the production rate of industrial glass. This is 9¦ accomplished by increasing the temperature of the molten glass to lO¦ produce foaming throughout its mass.
ll ¦SU~IARY' OF THE INVE_TION
12 ¦ The vitreous material undergoing treatment is first melted ¦to form a molten mass. The molten material is then foamed through 14 ¦out its mass. This foaming results in an expansion of the molten 15 ¦vitreous material, by volume, of at least about 1.5 and prefer-16 lably between about 2 and 3. The foamed material is permitted to 17 ¦subside.
18 ¦BRIEF DES-C'RIPTI'ON OF'THE'D:RAWINGS
l9 ¦ FIGURE l is a schematic view, partially in longitudinal 20 ¦section, of the entire installation;
21 ¦ FIGURE 2 is a cross section along line II-II of Figure l;
22 ¦ FIGURE 3 is a top view of a variation of the refining 23 ¦channel;
24 FIGURE 4 is a longitudinal section along line IV-IV.of 25 Figure 3.
26 DETAILED DESCR'IPl`ION
,, 27 The vitrifiable mixtures of raw materials which can be 28 employed in the process of the present invention are of the type -' 29' commonly used in the manufacture of glass. Examples of a number 30 of these mixtures appear in Table II.

31The present invention requires that the molten material '; , ' . .
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~3Z~3 1 be foamed throughout its mass. To initiate the intense and com-2 plete foaming required a number of steps may be taken. For

3 example, foaming agents can be incorporated into the raw material .
.~ The foaming agents give rise, in the temperature range, corres 5 ponding to the desired viscosities, to the formation of gas 6¦ bubbles inside the glass.
7 It is also recommended that a refining agent be present, at least in the final phase, for the gases produced by these 9 refining agents are soluble in glass, and their solubility in the lo molten glass increases as its temperature decreases. Thus, after 11 the elimination of most of the gases, the refining agents aid in 12 the readsorption of the bubbles which remain on cooling.
13 The foaming agents are selected such that they do not in-1~ duce foaming of the vitreous material until that material has 15 reached a desired temperature, which temperature is maintained in 16 the refining channel. rhe following foaming agents are useful in 17 the process according to the present invention : arsenic com-18 pounds, such as arsenic trioxide ; antimony compounds such as anti 19 mony trioxide ; sulfur compounds, such as sodium sulfate ; and 20 halogen salts such as pottasium chloride. Other agents useful in 21 the process wi-ll be apparent to those skilled in the art.
22 Another method of ensuring the thorough foaming of the 23 molten mass is to subject the batch to rapid uniform heating 24 during the foaming operation of about 20~C per minute or more. -25 Such heating can bè obtained in various ways, possibly combined9 26 capable of acting within the batch, for example, submerged 27 burners~ submerged resistors, direct Joule effect or high-28 frequency inducation. If desired, this foaming can be in;tiated 29 or reinforced by mechanical action using an ultTasonic generator.
In a discontinuous melting instalIation, these heating ¦means are employed at a time when the ~itreous batch contains a .` I

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~3Z~3 l large number of solid or gaseous nuclei and a sufficient amount 2 of foaming agents to ensure an expansion of at least l.5, and pre-3 ~erably above 2 times the normal volume of the mass in the un-

4 foamed molten state.
51 In a continuous melting installation similar heating means 6 can be employed. The predefined time sequence corresponds to the 7 rate of treatment of the vitreous mass.
8 To aid the foaming process, it is also recommended that 9 the vitreous mass contains a large number of nuclei, such as un-melted particles or small gas bubbles, capable of inducing the ll foaming. When obtaining it through direct melting o-f raw 12 materials, the nuclei should be distributed throughout the molten 13 mass at a concentration of at least lO visible nuclei per cc.
14 Furthermore, it is desirable that the raw materials be agglom~
erated or sintered. The sintering ma~es it possible to preheat 16 the materials before actual melting. This melting is accomplish-17 ed by a brief and intense heat transfer ~less than lO minutes) 18 while simultaneously keeping the temperature of the materials 9 below the foaming temperature. This permits the maintainance of a high number of nuclei consisting of unmelted particles and gas 21 bubbles in the vitreous mass introduced into the total foaming 22 stage. The rapid melting of the sintered raw materials can be -~
23 accomplished in various ~ays, or example, by subjecting these 24 materials to hot gases at a controlled temperature, ~hich gases are driven at high speed and have a large exchange capacity.
26 The granules can be introduced directly into the stream of the 27 gas. The raw materials can take any number of forms, for example, 28 granules, balls, pellets or strips. The thickness of the layer 29 ¦of raw materials can also vary and can be the size of smallest 30 of the sintered materials undergoing melting.
31 To assure the presence of suf~icient nuclei, outside 32 nuclei, for example, cullet or colored cullet can be added to the ~'i'`' ~ .
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~ ~V~73'~3 1 raw materials. In relation to the usual glass refining processes, 2 it is important to note that the present invention, requiring the 3 presence of gas producing agents and foaming nuclei, can employ unrefined vitreous materials. It has been discovered that 1 to 2 mm grains originating from the limestone and dolomite in the 6 material introduced in the refining tank, are totally digested at 7 the end of the total foaming phase. The process according to the 8 invention is there-fore not dependent on the use o~ a vitreous 9 batch of high quality.
In terms of the vilscosity of the vitrifiable material, 11 it is preferably to maintain the viscosity of the material at 12 below about 1,000 poises while melting to form the molten mass.
13 This viscosity of 1,000 poises is also preferably maintained 14 during the foaming of the molten mass and during the time it takes the foamed mass to subside.
16 In continuous manufacturing installations it is important 17 to avoid upstream currents or currents which exist downstream of 18 the direction of flow of the glass through the refining vessel.
19 For example, currents of thermal origin often exist or are even deliberately created in the usual melting furnace. The currents 21 tend, in the process according to present invention, to mix 22 glasses in different stages of production. These undesirable 23 currents may be eliminated by using baf~les, dams, bottlenecks or 24 cascades stationed along the course followed by the vitreous mass undergoing treatment.
It is advantageous for the width of the channel in which 27 the molten stream flows to be narrow in relation to its length, -28 the ratio between the two being about 1:5 or less. Another para-29 meter that also affects the product is the thickness of the stream of flowing glass. In the example given below the height ~ , ''. . ~ 5~ ~ . ~
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lOq3~1~3 of the glass in the channel varies from ~ to 7 cm. In larger installations a height of 10 to 20 cm or more can be used provided the heiyht of the channel walls is sufficient to ensure total expansion and damaging curr~nts are'avoided.
In order to increase the maximum velocity of the gases in relation to the materials being heated, the materials should be maintained in a slow moving thin layer. In practice, this is ob-tained by directing the flow of the hot gases in a direckion ap-proximately perpendicular to the inclined surface on which the granules fall. A layer of granules is easily fixed on that surface and within a few minutes becomes a vitreous batch ready to undergo total foaming. The surface on which the thin-layer melting is accomplished can be the inner wall of a cyclone furnace, a rotary drum combined with a scraper to remove the vitreous batch or the in-clined surface on which the vitreous batch flows while being formed.
The rate of flow can be regulated by the surface's slope, by the temperature which affects the viscosity of the batch,and, con- ~-sequentiy, the adhesion of the granules to that surface, or by the direction and/or concentration of the gas jets. The example below describes both the process and the device o~ the present invention.
The installation represented in Figure 1 comprises a channel 1 in which the molten vitreous material circulates from right to left while -undergoing foaming. The refining channel is also shown in Figure 2. Channel 1 is formed from a .7 mm -thick sheet ~f 10% rhodium-alloyed platinum. Its lenyth i5 1.5 m.
Both the width and the depth are 15 cm. At both ends, the channel ;-contains connections 2 supplying it with electric current delivered by alternating current generator 3, the voltage of which is adjustable from 0 to 10 V for a power of up to 25 kVA (2500 A
maximum). Connec:tions 2 are rhodium-alloyed platinum plates 10 mm ,' thick, 20 cm long and 10 cm high. They are held between two copper '~' . ', ' ' ' , , _ 5 _ :~.; - . . .
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~ 73~3 jaws 4, cooled by water circulation (not shown) and to which axe attached current lead-ins 5. At lts lower end, the channel con-tains a draw pipe 6~ The draw pipe is welded to the bottom of the channel and heated by a rhodium alloyed platinum resistor 7 wound on an insulating tube surrounding pipe 6. A cock 8 contain-ing a rhodium-alloyed platinum needle valve allows for the gradual closing of pipe 6. Above the drawing hole, the channel is pro-vided with rhodium-plated platinum dam 9 which is welded to the walls of the channel and leaves a free passage 9a only 20 mm high at the bottom of the channel. ~he molten material flows under dam 9 before exiting through draw pipe 6. At the opposite end of the channel pl~mging resistor 10 is provided. The re-sistor consists of a U-shaped rhodium-alloyed platinum plate 0.7 mm thick and 20 cm long. Resistor 10 corresponds to the shape of the interior section of channel 1. The lower part of plunging resistor 10 is drilled with evenly distributed holes, the dimen-sions of which are designed to reduce by approximately 25~ the area available for passage of electric current. The purpose of this is to localize the dissipation of electric power and to improve the stirring of the vitreous mass in the course of foaming. Plun~ing resistor 10 is supplied with electric current by alternating current generator 11 (Figure 2) with adjustable voltage from 2 to 3V and a power of 5 kVA~ Refining channel 1 is completely ~urrounded by heat insulating cover 12-12a consist-:
ing of alumina bricks lined wi~h unsealed insulation bricks.

~ By the contrGlled removal of insulation, one is able to determine - the temperature curve of the material along the channel.

The refining channel is fed at its upper end with a vitreous batch Eormed in melting furnace 13 by means of junction --14 containing inclined hearth 15. Hearth 16 of mel~ing furnace ~-....... ................................... ................................. .......... ~ .

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~7~Z:~3 13 is also inc].incd. Steel pipes 17 cros~ hearths 15 and 16 perpendicular to the plane of symmetry o~ the system. In order to regulate the temperature of the hear.ths cooling fluids are passed through these pipes. Arches 18 and 19 of junction 1~ and furnace 13 respectively are also covered with insula-ting bricks.
~urnace 13 and junction 14 are heatecl, on one side, by burners 20 which cross the arch and are directed perpendicular to the hearths to which they correspond. On the other side, they are heated by burners 21 crossing the base of stack 22 of the furnace and stationed so that their flames converge in the area of hearth 16 where the granular material is introduced. These burners are of the type commonly called "intensive," i.e., the rate of ejection of the gases is greater than the rate of fuel combustion.
The flame is caught in the combustion chamber created in the arch, These burners can be fed with a mixture of propane, air and/or oxygen from a mixer (not shown) with a capacity of 600,000 calories per hour. The flames escape through stack 22 crossing heat exchanger 23 in which gravity causes the pre-sintered vitrifi-able mixture to flow backward. The gases exhausted in heat ex changer 23 as well as those coming directly from stack 22 (through bypass 24) enter dust-separating cyclone 25. The circulation and discharge of the gases are assured by fan 26. Heat exchanger 23 is made of refractory steel and contains a double wall in which is , placed a powdery heat-insulating material such as ~ieselguhr. The - introduction into the furnace of vitrifiable raw materials, sintered and preheated in exchanger 23, is assured by distribu-ting drum 27. The rate of rotation of drum 27 regulates the feed to the furnace.
In the melting operation the vitrifiable raw material used is a material sintered in an extrusio~ press which supplies .

_ 7 _ ' ' , ~ . .
. ' ' a~ 3 compacted bars 7 mm in diameter. A suitable composition o~ the vitrifiable materials for produciny 90 kg o~ yl~s is:
Sand (250~ m) 60 kg Limestone (100~ m) 8.5 kg Dolomite cl mm 14.5 kg Feldspar (500~ m) 5.5 kg Dense sodium carbonate6.8 kg Caustic soda with 50~ NaOH20.2 ky Fine sodium sulfate 0.9 kg The granules can be dried in a ventilated electric oven at 250C, and stored away from moisture without other precautions.
Exchanger 23 is fed at the top with cold granules which are progressively heated to a temperature ranging between 500 and 600C at distributing drum 27. Simultaneously, the gases entsring the exchanger at 750C are mixed with cold air admitted through hole 2S and are sucked toward cyclone 25 at a temperature of about 200~C. The granules delivered by distributor 27 fall directly on hiearth 16 in the zone of covergence of burners 21. They are rapidly converted into a vitreous mass which ~lows over hearth 16 at an average rate of 10 cm per minute. Upon arrival at hearth 15, the temperature of the batch is 1300C. Hearth 15 transfers the material very rapidly, due to its steeper slope and without notable heatiny, to the inlet of refinin~ channel 1. Corrosion of hearths 15 and 16 is rendered negligible by limiting the-tempera-ture of their surface to approximately 800C. This is accomplished by the cooling fluid in pipes 17. The temperature in the arches of these regions, however, is about 1450~C.
On falling into refining channel 1, the material is sub~ected to rapid heating by contact with the bottom and side 30 walls of the channel and with submerged resistor 10, the .
.

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temperature of which is main~ained at abou-t 1530C. ~or a flow of 52 kg of glass per hour, the electric power dissipated i5 28 kVA in the channel proper and ~ kVA in the submerged resistor.
Due to the intense heatin~ of the glass, upon crossiny the submerged resistor 10, a swelling of the mass occurs so that the thickness of the batch about 4 cm above the submerged resistor is 13 to 14 cm.
A probe inserted at the bottom of the channel, immediate-ly below submerged resistor 10, shows that the vitreous mass has passed totally to the foam state. At a temperature of about lS20C
downstream of resistor 10, a constant rate of swelling by foaminy is obtained over approximately a 1 m length. This corresponds to a sojourn of about 15 minutes. Over the next 10 to 15 cm, the foam subsides very rapidly and the vitreous mass becomes perfectly refined glass at dam 9, where the temperature is no more than about 1450C. The refined glass which has passed under dam 9 is drawn off through pipe 6. The level of the material in the channel is kept constant by regulating its delivery through pipe 6 using cock 8.
In the example just described, from the time a preheated granule falls on hearth 16 of the melting furnace and the time when the refined glass corresponding to that granule is drawn off through pipe 6 only 30 minutes elapses. The device is capable, without changing its dimensions, of supplying greater ~` flows of refined glass, for example, 100 kg per hour, provided the rate of foaming is reduced. For an identical vitrifiable composition, the quantity of fine sodium sulfate introduced ~n the vitrifiable mixture is reduced to 0.7 kg per 100 kg of glass produced. Under those conditions, the initial height of the batch above resistor 10 is 7 cm, and expands to about 14 cm for .
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1 ~n expansion of 2. Regardless the method employed (discontinuous 2 ~r continuous), Eor a given increase in temperature and a given 3 ~itrifiable mix~ure, having an identical sodiu~ sui~ate content, 41 the refining time remains constant.

5 ¦ Foaming of the vitr~eous batch throughout its mass, which 61 constitutes the essential characteristic of this invèntion, has 71 ever been heretofdre proposed as making it possible to accelerate 81 the process of melting, refining and homogenization of fused glass.
9¦ The ~ollowing tables give examples of the manufacture 10¦ of five glasses of common type by the process according to the 11¦ invention. Parts are by weight unless otherwise indicated.
12¦ Table I furnishes an analysis of those glasses 131 expressed in percentages by weight of oxides. The fusion 141 described in the foregoing example was glass No. 1.
15¦ Table II furnishes the composition by weight of five 1~¦ vitrifiable mixtures suitable for manufacture of the glass in I . . ..
171 uestion.

181 - Table III indicates the characteristics of the lg¦ rocess as applied to the five glasses.

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1 ¦ TABLE I
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2 ¦ COMPOSITION OF THE GL.~SSES
3 ~i~-- ~ ' ~ oxi~ 1 ~ 3 ~ ~ ' ~, 5 1` sio2 70.7 73. 7 29.5 56.o ~3.o I

6 ¦ A123 1~3 1. 2 2n4 0~0~ 2~95

7 1 Fe23 00 25 81 CaO 10.3 00l5 0~17 O.OS 7.35 ~ MgO 3.3 0.25 3.1 10 ¦ Ba O 0 . 15 2 ~
11 ¦ Na20 14 . û 4 A 8 1~ 45 ~ . 2 14 .1 I K20 0.3 2.55 11.0 0~8 14 ¦ PbO ~48.9 27.4 15 ¦ B203 1703 16055 18¦ Sb23 . - 007 li I AS23 ~ 7 18 I ~
¦ TABLE I I .
20 ¦ ~7IT~ F I AE~L~ T KTDI~
21 o. or Glasses comPonents 1 2 3 4 and 67. 0 72. 2 26. 65 56 ~ 3 ~6.1 imestone 9. 47 2~
olomite 16 . 2 1. 45 - 13. 6 eldspar 6.13 Phonol i te 12 . 4 aslin 3.2. 6.35 28 odium ca~bonate 70 58 1. 5 6 ,, 65 lg.. 6S
29 otassium carbonate 2. 35 16.15 3ar i um c ar bona .e 0 v 2 3 . 25 - ~ . . . .

~ 3~13 ;.ld oxid~ (PbO) 49.0 28.0 ~oric acid 12.7 30.0 Borax 15.65 Rasorite 5.6 Calcined coiemanite 8.55 50~ Caustic soda22.5 Sodium sulfate 1~0 1.3 Sodium nitrate 0.5 1.5 1.0 Potassium chloride 1.5 10 Antimony trioxide 1.0 : Arsenic trioxide 2.0 - -- .
TABLE I I I
CHARP~CTERISTICS OF TREATMENT

No. of Glasses 1 2 3 4 5 Preliminary melting 1350 1400 1050 1250 1300 temperature (C) Rate of expansion 25 25 30 35 25 heating ~C/min) Expansion starting 1400 1450 1100 1300 1430 20 temperature (C) Expansion . 3 2-3 2-3 2-3 2-3 Time of expansion : until clarification 10 15 8 S 4 ~in minutes) . Clarification temp-erature (C) 1520 1550 1260 14~0 1480 `; Figures 3 and 4 describe an alternate device having a ~ refining crucible in which the glass is heated by direct Joule . :
.. ~ effect. This device is not useful in the manufacture of the lead glasses of Examples 3 and 4~ but is more economical than the preVious method, because it uses molybdenum eIectrodes.
`~ ~ The cruci.ble consists o~ a channel of refractory material 30, the interior rectangular cross section of which iæ
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about 25 centimeters. Its length iB ~bout ~ meters. The lower part contains a narrow funnel-type portion 31 about 5 cen-timeters above hearth 32 and reducing the width to a few cen-timeters in order to conduct the glass to outlet 33 while avoiding any blind angles likely to create stagnation.
The hearth and wall of channel 31 as well as its arch (not shown) are of a material commonly employed in co~ventional glass melting furnaces, an alumina and zircon-base electrofused material, Cover 34 consisting of bricks of a liyht refractory material provides heat lnsulation. The heating of the glass passing through the chan~el and the regulation of its temperature are assured by six pairs of electrodes E1 to E6. These electrodes, distributed along the edges of the channel, are made of 3-centimeter plates and are arranged symmetrically in relation to the axis of the channel. They are distributed along the edges of this channel, Each pair of electrodes is connected to an independent adjustable electric power source. The current lead-ins of the electrodes horizontally cross the walls of the channel and make ; possible crosswise placement of the electrodes. The lead-ins are made of molybdenum.
The glass thickness above outlet 33 is sufficient to entirely submerge the electrodes and protect them from oxidation.
The current lead-ins are protected by bathing their hot parts in a reducing atmosphere consisting, for example, town gas.
The glass has free passage around the electrodes along the hearth and side walls. Passage of the current from ;' one electrode to the other produces active thermal convection which favors the crosswise homogenization of the ~Olten mass and eliminates parasitic longitudinal currents. The result approa-ches a uniform flow of glass called "piston" flow. Di~ferent temperature readings are taken at the points Tl to T7.

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.. . .. . . ..

~t~3Z~3 Table IV shows the oharacteristics of the electric power supply used ln a refining operati~ similar to that of the foregolng example, i.e., in which the batch of glass, resulting from preliminary me3ting of composition No. 1, is introduced into the tank at point Tl at a temperature of about 1250 to 1300C and at a flow of approximately 50 kg/h.
TABLE IV

.. .
Supply devices El E2 E3 E~ E5 6 .. .. . .. ..
Rated characteristics:

Power (kVA) 20 20 6 6 6 6 Voltage (V) 80 80 60 60 60 60 Intensity (A) 250 250 100 100 100 100 Conditions for a delivery ~ of 50 kg/h (glass No.l):

; Power supplied (kVA) 1010 3 3 1 0 Temperature:

Measuring points Tl T2 T3 T4 T5 Values (C) 1400 1550 1550 1520 1380 1250 ~ , '~

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Claims

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

1. A process for the manufacture of glass from a vitrifiable material which comprises:
(a) melting the vitrifiable material to form a molten mass having a viscosity below about 1000 poises;
(b) heating at an increasing rate to rapidly foam the molten mass throughout its volume to at least 1.5 times its initial molten volume while maintaining its viscosity below about 1000 poises;
(c) controlling the heating until the foamed mass subsides to dissipate the foam while maintaining it at a viscosity below about 1000 poises; and (d) recovering the fused liquid glass.

2. The process of claim 1 wherein:
(a) the expansion of the molten mass is between about 2 and 3 times its normal volume in the unfoamed molten state.

3. The process of claim 1 wherein:
(a) the progress of the molten mass occurs without return flow.

4. The process of claim 1 wherein:
(a) the vitrifiable material is sintered.

5. The process of claim 4 wherein:
(a) the vitrifiable material is prepared by subjecting it to rapid melting.

6. The process of claim S wherein:
(a) the melting takes less than 10 minutes.

7. The process of claim 6 wherein:
(a) the vitrifiable material is in the form of granules, balls, pellets or strips.

8. The process of claim 7 wherein:
(a) the thickness of the vitrifiable material during melting is the size of the smallest of the sintered elements undergoing melting.

9. A process for the manufacture of glass from a vitrifiable material containing at least one foaming and re-fining agent which comprises:
(a) melting the vitrifiable material to form a molten mass having a viscosity below about 1000 poises;
(b) foaming the molten mass throughout its volume while maintaining its viscosity below about 1000 poises by heating the molten mass to increase its temperature at the rate of at least about 20°C per minute from a temperature just below the foaming temperature;
(c) continuing the foaming until the material has been expanded to at least 1.5 times its initial molten volume;
(d) controlling the heating until the foamed mass subsides to dissipate the foam while maintaining it at a viscosity below about 1000 poises; and (e) recovering the fused liquid glass.

10. The process of claim 9 wherein:
(a) the solubility of the gas forming the foam in the molten glass increases as the temperature decreases.

11. The process of claim 10 wherein:

(a) at least one of the foaming agents is a refining compound soluble in glass.

12. The process of claim 11 wherein:
(a) the foaming agent is selected from the group consisting of sodium sulfate, potassium chloride, antimony trioxide and arsenic trioxide.

13. A process for the manufacture of glass from a vitrifiable material which comprises:
(a) melting the vitrifiable material to form a molten mass having a viscosity below about 1000 poises and wherein said molten mass contains a number of solid or gaseous nuclei favoring foaming;
(b) heating at an increased rate to rapidly foam the molten mass throughout its volume to at least 1.5 times its initial molten volume while maintaining its viscosity below about 1000 poises;
(c) controlling the heating until the foamed mass subsides to dissipate the foam while maintaining it at a viscosity below about 1000 poises; and (d) recovering the fused liquid glass.

14. The process of claim 13 wherein:
(a) the nuclei are distributed throughout the vitreous mass at a concentration of at least about 10 visible nuclei per cc.

15. The process of claim 14 wherein:
(a) cullet or colored cullet is added to the vitrifiable material.

16. A process for the manufacture of glass from a vitrifiable material containing a foaming and refining agent which comprises:
(a) melting the vitrifiable material in less than 10 minutes at a temperature below the foaming temperature to form a molten mass having a viscosity below about 1000 poises;
(b) foaming the molten mass throughout its volume by rapidly heating the molten mass to increase its temperature at a rate of at least about 20°C
per minute while maintaining its viscosity below about 1000 poises and until the material has been expanded to at least 1.5 times its initial molten volume;
(c) controlling the heating until the foamed mass subsides to dissipate the foam while maintaining it at a viscosity below about 1000 poises; and (d) recovering the fused liquid glass.

17. The process of claim 16 wherein:
(a) the material is heated to 1300°C in about 6 minutes and then from 1300 to 1500°C at the rate of 30°C per minute, the temperature being kept at 1500°C
for 10 minutes before separation of the fused glass.

18. A process for the manufacture of glass from a vitrifiable material which comprises:
(a) melting the vitrifiable material to form a molten mass at a temperature just below the foaming temperature;
(b) heating at an increased rate to rapidly foam the molten mass throughout its volume to at least 1.5 times its initial molten volume;
(c) controlling the heating until the foamed mass subsides to dissipate the foam; and (d) recovering the fused liquid glass.

19. A process for the manufacture of glass comprising:
(a) forming a vitrifiable mixture of materials having distributed therethrough a foaming and refining agent;

(b) rapidly melting the vitrifiable material at a temperature below the foaming temperature thereof to form a molten vitreous mass having visible nuclei distributed therethrough and a viscosity below about 1000 poises;
(c) foaming the molten mass throughout its volume by rapidly heating the molten mass to increase the temperature above the foaming temperature at a rate sufficient to produce rapid thermal convection and expansion throughout the molten mass to at least 1.5 times its initial molten volume;
(d) controlling the heating until the foamed mass subsides to dissipate the foam while maintaining it at a viscosity below about 1000 poises; and (e) recovering the fused liquid glass.

20. A process for the manufacture of glass from vitri-fiable material which comprises:
(a) melting the vitrifiable material to form a molten mass having a temperature just below the foaming temperature;
(b) continuously flowing the molten mass along a predetermined path;
(c) heating at an increased rate to rapidly foam the flowing molten mass throughout its volume until the material has been expanded to at least about 1.5 times its initial molten volume;
(d) controlling the heating until the foamed mass subsides to dissipate the foam; and (e) recovering the fused liquid glass.

21. A continuous process for the manufacture of glass from a granular vitrifiable material having distributed there-through a foaming and refining agent which comprises:

(a) rapidly melting the vitrifiable material to form a flowing molten mass having foam producing nuclei distributed therethrough;
(b) foaming the flowing molten mass throughout its volume by rapidly heating it to about 1500°C and above to produce rapid thermal convection and expansion throughout the flowing molten mass to at least 1.5 times its initial molten volume;
(c) controlling the heating until the foamed mass subsides to dissipate the foam while maintaining it at a viscosity below about 1000 poises; and (d) recovering the fused liquid glass.

22. The process of claims 21 wherein:
(a) the molten mass is heated at a rate of at least ] per minute to produce said foaming and thermal convection.