US 2967113 A
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Jan. 3, 1961 H. A. LIEBHAFSKY ET AL 13 COATING METHOD Filed Nov. 18, 1957 Inert Gas lnvenfors Herman A L/bhafsky Ari/7w M BueChve p Q by Their Attorney United States Patent COATING METHOD Herman A. Liebhafsky and Arthur M. Bueche, Schenectady, N .Y., assignors to General Electric Company, a corporation of New York Filed Nov. 18, 1957, Ser. No. 697,161
9 Claims. (Cl. 117-46) This invention relates to a method for coating the inner surface of hollow glassware for illuminating and irradiating purposes. More particularly, the invention relates to a method for causing the generation of a fume or smoke of light diffusing or irradiation modifying particles and for causing the desired deposition of said particles over the inner surface of hollow glassware. In addition, this invention relates to the preparation of hollow glassware having colored, light diffusing particles deposited over the inner surface of hollow glassware. In particular, our invention relates to the deposition of fume-produced diffusing and coloring coatings upon the inner surfaces of the neck and bulbous portions of the envelopes of incandescent lamps.
In the evolution of incandescent lamps to their present state. a great deal of attention has been paid to the appearance of incandescent lamps to the human eye. The
2,967,113 Patented Jan. 3 196 1 envelopes is to wash the inside of the bulb with a liquid containing coloring matter. This method is disadvantageous in that the inside of the bulb is wetted by this treatment and the bulb must subsequently be dried, thereby adding an additional step'in the manufacture of such bulbs.
One object of our invention is to provide an improved method for producing a diffusing coating on the inner surfaces of the neck and bulbous portions of the envelopes of incandescent lamps.
Another object of our invention is to provide an improved method for producing a colored, light diffusing coating upon the inner surfaces of the envelopes of incandescent lamps.
A still further object of our invention is to provide a one step method for producing a coating on the inner 1 surface of the envelopes of incandescent lamps which has both light diffusing characteristics and coloring characteristics.
These and other objects of our invention are accomplished by burning silicon tetrachloride and hydrogen with oxygen in the interior of an incandescent lamp envelope to produce a fume of finely divided silica particles which adhere to the inside surface of the envelope. The coloring material is applied at the same time as the silica coating by injecting an inorganic coloring material into the flame produced by the burning silicon tetraearliest incandescent lamps comprised a completely transparent bulbous envelope so that the glowing filament was visible to the eye and caused discomfort. To lessen the discomfort to the eye, the inside frosted incandescent lamp bulb was developed. The inside frosting was attained by etching the inside surface of the incandescent lamp envelope with a solution such as an aqueous hydrofluoric acid solution. With these inside frosted lamps, the glowing filament itself was no longer visible, but the center of the bulb appeared much brighter to the human eye than portions of the bulb near the envelope. Attempts to further decrease the light source intensity of incandescent lamp bulbs resulted in the development of the incandescent lamp envelopes described in Patent 2,545,896, Pipkin, which comprise glass envelopes which are fume-coated by burning hydrogen, oxygen and ethyl silicate inside of the lamp envelope. This results in a silica-like deposit on the inside surface of the bulb which almost completely diffuses the light from the filament so that the bulb appears to have a uniform light intensity at all points on its surface.
Although the method of forming the fume coating of the aforementioned Pipkin patent produces completely satisfactory fume-coated bulbs, the method of the Pipkin patent leaves something to be desired in that the silica producing material is organic in nature and the conditions required to form the silica coating must be carefully controlled to prevent the deposition of dark carbonaceous materials on the surface of the bulb.
The most recent advance in the incandescent lamp art is the large scale manufacture of such lamps in which the envelope is colored in some manner so as to produce illumination of the desired color. One common method of coloring the lamps is to coat the exterior of the bulb envelope with a colored resinous or ceramic material. This method offers certain disadvantages in that the colored coating tends to discolor from atmospheric contamination and is also subject to scratches because of the necessary mechanical contacts which the bulbs encounter. Another method of coloring incandescent lamp chloride and hydrogen.
Our invention, together with further objects and advantages thereof, may be best understood by reference to the following description taken in connection with the drawing which is a view, partly in section, of apparatus for applying the light difiusing and coloring coatings by the method of thepresent invention.
The method of the present invention is based on the chemical reaction between two moles of hydrogen, one
mole of silicon tetrachloride, and one mole of oxygen to yield one mole of silica in extremely finely divided form and four moles of hydrogen chloride.
The heat generated by the above reaction, which takes place inside of the bulb to be coated, is suflicient to insure that all of the material except the silica which comes into contact with the surface of the bulb is in the gaseous state, the silica is in the form of a fume or smoke of extremely finely divided particles which are extremely active and which come into contact with the glass surface of the lamp bulb and adhere firmly to this glass surface. The gaseous reaction products are swept from the inside of the bulb by the incoming combustion gases. When hydrogen, silicon tetrachloride, and oxygen are burned within a glass article to be coated, the
final product, in the case of a lamp bulb, is indistinguish-' -time as the silica coating is deposited. The resulting coating comprises silica together with a metal oxide which serves to color the inside of the bulb so that a diffuse colored bulb is obtained.
In Fig. 1 is shown apparatus which can be employed in producing both the silica coated glass articles of the present invention as the colored silica coated articles. This apparatus comprises a burner 10 having two concentric passageways 11 and 12. These passageways 11 and 12 extend into the interior portion of bulb 13 which is held in place by any type of holder such as the holder illustrated at 14. Passageway 11 is provided with means 3 for delivering oxygen to the interior of bulb 13, including conduit 15 which contains valve 16 for regulating the rate of oxygen supply. Generally, oxygen is supplied by means of an air stream whichfiows through conduit 15 and into passageway 11 to the upper end of burner 10. Both hydrogen and silicon tetrachloride are supplied to the end of the burner by means of passageway 12. Hydrogen is introduced into the system through conduit 17 containing valve 18 for regulating the rate of hydrogen flow. Conduit 17 extends through seal 19 into the base of a vessel 20 which contains liquid silicon tetrachloride 21. The base of conduit 17 is positioned below the surface of silicon tetrachloride 21 so as to cause the hydrogen to bubble through the silicon tetrachloride and become saturated with silicon tetrachloride. The hydrogen, which is then saturated with silicon tetra chloride, passes through seal 19 by means of conduit 22 into passageway 12 and thence to the tip of burner 10. The amount of silicon tetrachloride in the hydrogen passing through passageway 12 is controlled by controlling the temperature of the silicon tetrachloride 21 in vessel 20. This temperature is controlled by means of heater 23 which is illustrated as an electric heater with connector 24 for supplying the necessary electricity. However, it should be understood that any type of heating means may be employed to obtain the temperature de sired in vessel 20. Thus, instead of employing an electric heater of the type illustrated, the heating may be provided by a heating mantle or by a burner or the like.
-In order to supply the inorganic coloring material to the interior of bulb 13, a container 25 of inorganic coloring material 26 is provided. In the drawing the inorganic coloring material is illustrated as a liquid, but it is understood that material 26 may also be a solid, for example, a powdered material. Flask 25 is provided with heating means 27, which is illustrated as an electric hotplate having electrical connection means 2% but which heating means may be of any type desired, including various types of burners. Conduit'29 is provided between the tip of burner and container 25 so that vapors of the inorganic coloring material 26 may be delivered to the tip of burner 10. As illustrated, flask 25 is provided with a seal 30 through which conduit 29 extends. In order to prevent condensation of vapors of coloring material 26, heating means 31 are provided around conduit 29. Heating means 31 are illustrated in the drawing as a resistance heater, but any other type of heating means may be employed. To facilitate the delivery of inorganic coloring material 26 to the tip of burner 10, means are provided for bubbling an inert gas, such as nitrogen, through material 26. This inert gas becomes saturated with the vapors of material 26 and the saturated gas flows through conduit 29 to the tip of burner 10. Specifically, the inert gas is delivered through seal 30 by means of conduit 32, which contains regulating valve 33, to the bottom of material 26. The amount of material 26 delivered to the tip of burner 10 is controlled by regulating the rate of flow of inert gas by means of valve '33 and the temperature of material 26.
In carrying out the process of the present invention with the apparatus illustrated in the drawing, it is convenient to maintain a flow of air through passageway 11 at all times during the utilization of the process even though the interior surface of a bulb is being coated only during a portion of such time. The supply of hydrogen through conduit 17, vessel 20, conduit 22 and passageway 12 is provided only while a bulb 13 is in position about burner 10. The flow of hydrogen through conduit 17 and its associated parts is controlled by means of valve 18 so that the hydrogen (as well as the silicon tetrachloride vapors) flows through passageway 12 only while a bulb is in the position illustrated. Thus, it is seen that the air flow through the burner 10 is steady while the hydrogen and silicon tetrachloride flow through the burner is intermittent and occurs only while a "bulb is actually being coated. To insure combustion of the hy: drogen and silicon tetrachloride which flow through passageway 12 a pilot flame is provided adjacent the tip of burner 10. This pilot flame 34 is provided with a combustible gas such as hydrogen by means of conduit 35 which contains valve 36.
In the operation of the apparatus of the drawing to provide a white silica coating on the interior surface of bulb 13, it is unnecessary to provide coloring material 26 and the associated apparatus used to deliver it in the proper quantity to the tip of burner 10. For this coating operation the air supply to burner 19 is first set to the proper value by means of valve 16. Subsequently, the combustible gas for pilot flame 34 is provided in the desirable quantity by means of valve 36. After establishing pilot flame 34, and after heating the silicon tetrachloride 21 in vessel 20 to the proper temperature by means of heating element 23, a bulb 13 is positioned in support 14 around the tip of burner 10. Valve 18 is then opened the proper amount to provide the desired hydrogen and silicon tetrachloride flow to the tip of the burner. As soon as the hydrogen and silicon tetrachloride reach the tip of the burner through passageway 12, the hydrogen and silicon tetrachloride are ignited by pilot flame 34 and the heat of combustion of the reaction of the hydrogen and silicon tetrachloride with the oxygen of the air provides a fume or smoke of finely divided, heated, active silica. When this silica comes in contact with the relatively cool inner wall of bulb 13, it deposits in an adherent diffuse coating. As soon as the desired amount of silica is deposited on the interior of bulb walls 13, valve 18 is closed, thus preventing the combustion of further silicon tetrachloride and hydrogen. At this time, the pilot flame 34 still burns and the supply of air is still being delivered to the interior of the bulb through passageway 11. This relatively cool air tends to cool bulb 13 as well as to 'purge the interior of bulb 13 of any unreacted hydrogen and silicon tetrachloride and to remove from the interior portion of the bulb any remaining hydrogen chloride which has been formed by the reaction.
From the foregoing description of the process of the present invention, it is obvious that the method of this invention employing apparatus such as that shown and illustrated is readily adaptable to automatic operation, with the bulbs 13 being delivered automatically in their proper position about burner '10 and with valve 18 being opened and closed in response to the positioning of the bulb. The length of time during which hydrogen and silicon tetrachloride are delivered to the bulb is subject to ready control by means of suitable timing mechanisms.
In the practiceof the method of the present invention in which coloring material is also applied to the interior surface of bulb 13, thernethod of operation of the apparatus illustrated in the drawing is the same as previously described for the silicon tetrachloride coating alone except that container .25, coloring material 26 and the associated apparatus are provided. The only difference in the carrying out of the process is that the temperature of coloring material 26 and inert gas flow rate are maintained at values which deliver the proper amount of coloringmaterial to the tip of the burner. To conserve coloring .material 26, valve 33 is regulated manually or automatically to insure that inert gas passes through material 26 only when :a bulbis actually being coated. Therefore, coloring material 26 .is delivered to the tip of burner 10 only at times when hydrogen and silicon tetrachloride are being delivered to the tip of the burner. When depositing a coating of silica plus a coloring material to the interior of a bulb, the heat of the reaction between the hydrogen and silicon tetrachloride and oxygen as well as reaction which may take place between the coloring material and the other material supplied to the interior of the bulb is suflicient to insure that the coloring material is in the form of a metal oxide at the time of its deposition along with the silica on the interior surface of lamp bulb 13.
Although the stoichiometry of the reaction involved in the method of the present invention has been described as comprising two moles of hydrogen for one mole of silicon tetrachloride and one mole of oxygen, it should be understood that the hydrogen, silicon tetrachloride and oxygen need not be delivered to the interior of a bulb to be coated in stoichiometric proportions. Where an excess of one or two of the three reactants is employed, any unreacted material will 'not interfere with the proper coating of the interior of the bulb since the unreacted material will merely flow out of the bulb. Generally we deliver the hydrogen silicon tetrachloride and oxygen to the interior of the bulb in stoichiometric proportions. However, satisfactory results are obtained when one or two of the reactants is provided in excesses of a factor of or more. In the preferred embodiment of our invention we provide a small stoichiometric excess of hydrogen and an even larger stoichiometric excess of oxygen since this insures complete utilization of the silicon tetrachloride which is the most expensive reactant and also insures that all of the hydrogen is consumed so that no problem is presented by the escape of hydrogen to the atmosphere. One particularly useful ratio of feeds for our process is 2.8 moles of hydrogen and 4 moles of oxygen per mole of silicon tetrachloride.
When preparing a colored incandescent lamp bulb by the method of the present invention, the coloring material is added to the mixture which is burned in the interior of the bulb- The only requirement of this coloring material is that it be a compound of a metal whose oxide yields the desired color inside of the bulb and whose oxide is stable under the temperature at which the surface of the bulb operated, which is about 150 C. for a standard 100 watt incandescent lamp. This coloring material may be the oxide of the metal in question, .or may be any other derivative of the metal, but preferably is an inorganic derivative of the metal such as a nitrate, sulfate, nitride, nitrite, sulfite, cyanide, hydroxide, sulfide, ferricyanide, ferrocyanide, or preferably, a halide. In addition to using a single compound as the coloring material, a number of different compounds of the same metal or a number of different compounds of different metals may be employed. Preferably, however, we employ a halide, specifically a chloride, of a single metal as our coloring agent. Because of the high flame temperature inside of the incandescent lamp, the metal compound will be decomposed, or oxidized to form a metal oxide which deposits onto the surface of the glass lamp envelope along with the silica. Listed in the table beloware a number of the many metals whose compounds may be employed as coloring materials and the resultant color of the oxide.
Metal Color Gold Gray or brown. Bismuth Yellow or brown. Cadmlum Green or brown.
Yellow or red. Yellow or red or brown or green. Green.
Yellow or red. Red or brown. Yellow or red. Blue.
Green or Gray. Red
Yellow or green. Yellow.
Yellow or brown. Yellow or brown.
Red or yellow or blue. Yellow or brown.
. Illustrative of a few of the many specific compounds which may be employed as a coloring materials are cadmium nitrate, cobaltic chloride, ferric chloride, lead carbonate, nickel sulfate (morenosite), silver cyanide, chromium oxychloride, tungsten hexachloride, etc.
When the coloring material is added to the interior of the bulb by means of a metal halide, which is a preferred embodiment of our invention, the metal halide 26 is placed in vessel 25 and heated to the desired temperature. An inert gas is then bubbled through the coloring material 26 in flask 25. By varying the rate of flow of the inert gas through conduit 33 and the temperature of the coloring material 26 by means of heater 27, both the amount of inert gas entering the bulb and the ratio of the coloring material to the inert gas may be controlled. For the inert gas which acts as a carrier for the coloring material, We may employ any gas which is inert under the reaction. However, for economy reasons we prefer to employ nitrogen as this inert gas. In addition, it should be understood that other inert gases such as carbon dioxide, helium, argon, etc. may be employed. In addition to employing the coloring material 26 in its pure form, it is also possible to employ a solution of coloring material in a suitable solvent. Thus vessel 25 may contain a metal halide, for example, dissolved in water or a relatively inert solvent such as, for example, carbon tetrachloride.
When employing a coloring material in the process of the present invention the ratios of hydrogen, oxygen and silicon tetrachloride may remain in the same proportions as in the case of coatings employing no coloring ma-. terial. However, in the case of a use of a coloring ma? terial, it is desirable to insure that sulficient excess oxy gen is present at the tip of the burner so as to provide the stoichiometric amount-of oxygen required to convert the coloring material to the desired metal oxide.
When preparing colored bulbs by the process of the present invention, the ratio of deposited -silica .to deposited coloring metal oxide may vary within very wide limits. However, we prefer to have no more than one mole of the coloring metal oxide present in the coating layer per mole of silica in the coating layer. Suitable colored lamps have been prepared employing flow rates of the various ingredients in a ratio such that the ratio of coloring metal oxide to silica is from about 0.01 to 1 mole of the coloring metal oxide per mole of silica. The effect of varying the relative amount of the coloring metal oxide in the coating is a variation in the intensity of the colored coating, the higher the ratio of metal oxide, the higher the intensity of the color.
Although the process of the present invention is applicable to the coating of the inside of any type of hollow glassware, our preferred method relates to the coating of the interior surface of incandescent lamp housings. These lamp housings may be bulbous-type housings such as illustrated in the drawing or may be cylindrical housings or of any other conventional shape. The one requirement of the lamp housing is that its interior surface be clean prior to the deposition of the silica coating or the colored silica coating. The purpose of this cleaning operation is to insure that no film of dirt or grease remains on the interior surface of the bulb so as to interfere with the coating operation. The interior surface of the bulbs may be clear glass or the interior surface of the bulb may contain an inside frosting such as the inside frosting described in Pipkin Patent 1,687,510. Preferably, the inside frosted bulbs are employed, since the inside frosted bulbs tend to have a higher mechanical strength than clear glass bulbs.
In the table which follows is described the treatment of a number of watt incandescent lamp bulbs having a volume of about 11 cubic inches and a surface area of about 28 square inches by the method of the present invention. In carrying out the method, apparatus similar to that described in the drawing is employed.
The table indicates the bulb character, i.e., whether the bulb was clear or inside frosted (IF). In addition the table lists the flow rates of hydrogen, air, and silicon tetrachloride in cubic feet per minute and the tempera ture of the silicon tetrachloride through which the hydrogen was bubbled. The table also lists the length of time the hydrogen, silicon tetrachloride and oxygen were burned inside of the bulb. When colored bulbs were formed in the examples below, nitrogen was bubbled through the coloring material which was in liquid form. The table indicates the nitrogen flow rate in cubic feet per minute, the nature of the particularcoloring material employed, the flow rate of this coloring material, the temperature at which the coloring material was 10 percent because of the colored coating. In addition, these colored bulbs Nos. 11 through 16, when converted to incandescent lamps by the addition of filaments, gas, leads and bases exhibited the same mechanical strength and output versus life characteristics as observed in conventional inside frosted lamps.
The colored difi'use bulbs prepared by the method of the present invention have previously been described in connection with th simultaneous deposition of a layer of silica and a layer of metal oxide. However, we have also found that colored incandescent lamp bulbs may be formed by depositing the coloring metal oxide alone on the inside of the bulb surface. This is accomplished by burning a mixture of hydrogen, oxygen and a metallic maintained, and the color of the resulting coating. 15 compound whose oxide 1s characterized by the desired Coloring Material n2, An, SiCh, SiCh, Ni, Bulb No. Bulb cu. ft. on. it. on. it. Temp., cu. it. Time, Coating Character per per per C. per On. it. Temp., Seconds Color min. min. min. min. Nature per 0.
. 045 33 .016 25 T 3 White 045 33 016 25 4 White .045 33 016. 25 7 White .045 .18 .016 25 4 White 045 18 016 25 6 White 045 045 016 25 4 White 045 045 016 25 4 White 10 25 O5 35 3 Wh1te l0 25 05 35 9 White .10 .25 .05 20 4 White 1 0 025 20 10 FeOlz.6HzO 025 60 5 Red. 10 25 025 20 l0 FeClafiHaO 025 60 3 Red. 04 01 20 01 001 3 Green. ,04 .30 01 20 01 005 300 5 Yellow 04 20 01 20 03 01 8 0 2 Blue. .045 33 .016 25 01 t 005 2 6 Brown Examination of bulbs 1 through 10' described in the above table showed them to be completely comparable in properties to the bulbs described in Pipkin Patent 2,454,896. When these bulbs are formed into finished incandescent lamps with the conventional 100 watt filaments, leads and bases and with a gas inside the lamp comprising 88 percent argon and 12 percent nitrogen at a room temperature pressure of about 600 mm., it is found that the maximum brightness of the bulbs is about 35 to 40 percent of the maximum brightness of inside frosted bulb lamps in the case of clear lamps treated by the process of the present invention. With inside frosted bulb lamps coated by the process of the present invention, it is found that the maximum brightness is only about 5 to 7 percent of the maximum brightness of inside frosted bulb lamps. Despite this large decrease in maximum brightness obtained by the method of the present invention, it is found that there is no significant difference in the efficiency of the bulbs of the present invention as compared with bulbs which contain an inside frost but which do not contain a silica coating. It has been found, however, that the heavier the silica coating of the incandescent lamps, the lower the transmis sion through the bulb. However, with bulb No. 9 which had a coating of about 2 milligrams per square inch of bulb surface, the change in transmission between the silica coated bulb and the clear bulb is only about 5 percent. With a lighter silica coating, such as the coating applied to bulb No. 1, which was about 0.2 mg/in. of bulb surface, the loss in transmission efficiency between the coated bulb and the clear bulb is less than /2 percent. Bulbs Nos. 1 to 10 are comparable in efiiciency when converted to commercial incandescent lamps to conventional inside frosted incandescent lamps both as regards to mechanical strength and efficiency versus lifetime.
Bulbs Nos. 11 to 16 differ from bulbs Nos. 1 to 10 by virtue of the fact that the coating on the inside surface of the bulb was colored. This'resulted, of course, in a marked difference in appearance of the bulb. However, in ,no case is the efficiency of any of these bulbs, as measured by lumen output per watt, impaired more than color. Bulbs prepared by this method exhibit some degree of light diffusion so that the intensity of illumination is far more uniform than in the case of inside frosted bulbs alone. However, bulbs containing the coloring coating alone are inferior to bulbs containing both the coloring coating and the silica coating. Accordingly, in the practice of our invention we prefer to employ both a silica coating and a coloring metal oxide coating to produce coated bulbs.
What we claim as new and desire to secure by Letters Patent of the United States is: i
l. The method of coating the interiorsurface of a glass electric lamp bulb having an opening therein with a light-diffusing coating, which method comprises bubbling a stream of hydrogen at a preselected flow rate through liquid silicon tetrachloride maintained at a preselected temperature, whereby a mixture of hydrogen and silicon tetrachloride vapor is formed, said preselected hydrogen flow rate and said preselected silicon tetracbloride temperature being selected so as to provide a predetermined ratio of hydrogen to silicon tetrachloride in said mixture, passing said mixture through said opening into the interior of said bulb, simultaneously passing oxygen through said opening into the interior of said bulb, and igniting the gases in said bulb, whereby fumes of silica are formed which deposit on the interior surface of said bulb to form a light-diffusing coating thereon.
'2. The method of coating the interior surface of a glass electric lamp bulb having an opening therein with a colored light-diffusing coating, which method comprises bubbling a stream of hydrogen through liquid silicon tetrachloride whereby a mixture of hydrogen and silicon tetrachloride vapor is formed, passing said mixture through said opening into the interior of said-bulb, simultaneously passing oxygen through said opening into the interior of the bulb, simultaneously passing a coloring compound through said openinginto the interior of said bulb, and igniting the gases in said bulb, said coloring compound comprising a metal salt whose oxide is char acterized 'by the desired color other than the white color of the deposited silica, whereby fumes of silica and colored metal oxide are formed which deposit on the interior surface of said bulb to form a light-difiusing coating thereon.
3. The method of coating the interior surface of a glass electric lamp bulb having an opening therein with a colored light-diffusing coating, which method comprises bubbling a stream of hydrogen through liquid silicon tetrachloride whereby a mixture of hydrogen and silicon tetrachloride vapor is formed, passing said mixture through said opening into the interior of said bulb, simultaneously passing an inert gas stream through a liquid coloring compound whereby a mixture of inert gas and coloring compound vapors is formed, and passing said last-mentioned mixture through said opening into the in terior of said bulb, simultaneously passing oxygen through said opening into the interior of said bulb, and igniting the gases in said bulb, said coloring compound comprising a salt of a metal whose oxide i characterized by the desired color other than the white color of the deposited silica, whereby fumes of silica and colored metal oxide are formed which deposit on the interior surface of said bulb to form a colored light-diffusing coating thereon.
4. The method of coating the interior surface of a glass electric lamp bulb having an opening therein with a colored light-diffusing coating, which method comprises bubbling a stream of hydrogen at a preselected flow rate through liquid silicon tetrachloride maintained at a preselected temperature, whereby a first mixture of hydrogen and silicon tetrachloride vapor is formed, and preselected hydrogen flow rate and said preselected silicon tetrachloride temperature being selected to provide the desired ratio of hydrogen and silicon tetrachloride in said first mixture, bubbling a stream of inert gas at a preselected flow rate through a liquid solution of a metal salt maintained at a preselected temperature, whereby a second mixture of inert gas and metal salt vapor is formed, said inert gas flow rate and said liquid metal salt solution temperature being preselected to provide a predetermined amount of inert metal salt in vapor form, said metal salt being the salt of a metal whose oxide is characterized by the desired color other than the white color of the deposited silica, simultaneously passing oxygen and said first mixture and said second mixture through said opening into the interior of said bulb, and igniting the gases in said bulb, whereby fumes of silica and colored metal oxide are formed which deposit on the interior surface of said bulbs to form a colored lightdiifusing coating thereon.
5. The method of claim 4 in which the metal salt is ferric chloride hexahydrate.
6. The method of claim 4 in which the metal salt is chromyl chloride.
7. The method of claim 4 in which the metal salt is tungsten hexachloride.
8. The method of claim 4 in which the metal salt is vanadium tetrachloride.
9. The method of claim 4 in which the metal salt is silver nitrate.
References Cited in the file of this patent UNITED STATES PATENTS 2,272,342 Hyde Feb. 10, 1942 2,545,896 Piplin Mar. 20, 1951 2,626,874 Piplin Jan. 27, 1953 2,661,438 Shand Dec. 1, 1953 2,698,257 Kronouer Dec. 28, 1954 2,772,654 Herkart Dec. 4, 1956 2,806,444 Werner Sept. 17, 1957 2,844,492 Fitzer July 22, 1958