US2077378A - Process of metal coating glass - Google Patents

Description

April 20, 1937. P. DEREN PROCESS OF METAL comma GLASS.

Filed Oct. 21, 1935 2 Sheets-Sheet 1 April 0, 1937- P. DEREN PROCESS OF METAL COATING GLASS Filed Oct. 21, 1935 2 Sheets-Sheet 2 Patented Apr. 20, 1937 UNITED STATES PATENT OFFICE I raocass or 56m Guess Pincus Deren; Magnolia, Mass., assignor, by

mesne assignments, to Birdseye, Electric Corporation, Dover, M, a corporation of Delaware Application October 21, 1935, Serial No. 45,947

9Claims.

I This invention consists in a novel process of applying a metallic reflecting coating to selected areas of glass articles. One important field of use for the invention is in connection with the 6 inside silvering of electriclamp bulbs, and for purposes of illustration it willbe described as it is carried out in this field.

It has been the practice heretofore in silvering lamp bulbs to apply a metallic coating to substan- 10 tially the entire inner surface of the bulb and s then in a subsequent operation to define the selected reflecting areas by removing portions of the coating by mechanicalmeans, as by a rotary scraping tool. Such a scraping operation, besides requiring considerable mechanical skill on the part of the operator and besides being relatively slow, is practically limited to bulbs of concen; trio and symmetrical shape, although a considerable proportion of the bulbs containing an internal reflector integral with the bulb are designed for particular purposes and have particular shapes that are not surfaces of revolution about the main axis of the bulb. Moreover, in dealing with the symmetrical bulbs there is always present a danger of scratching and even breaking thebulb by the scraping tool employed.

Accordingly an object of the present invention is to make available a chemical process for quickly and effectively removing the-coating from certain exposed areas of the bulb surface while protecting the coating in the selected areas where it is to be retained. In one aspect therefore, the

process of my invention consists in the steps of depositing a metallic coating on a larger area of the article than that eventually required, protecting. the included selectedvarea by covering it with a non-acid diluting or neutralizing medium and then treating the exposed surface with an acid which is diluted and rendered inactive at the boundaries of the selected area.

In carrying out the process .of my invention I prefer to utilize an acid which is in itself a strong acid and a strong oxidizing agent. Nitric acid happens to be both, and iswholly suited for the requirements of the case. For protecting the selected areas -of the coating I may use any alkaline solution having the capacity to neutrallze nitric acid at the rate it is supplied for dissolving the metallic coating. For this purpose atcaustic solutionfsuch-as sodium hydrate entirely satisfactory.

The nitric acid may be applied to the exposed silver surface in a variety of ways. Preferably it should be strong and used in the smallest quantities that will accomplish the result. In

no case should any large quantity be flowed or sprayed into the bulb, since there would be a tendency for rivulets to form and run down the sides of the bulb and into thev neutralizing solution with suflicient velocity to exist momentarily as nitric acid, below-the line of protecting solution, and an irregular line with discolored edges might result. The acid may be used'in small quantities-not more than 2 to cubic centimeters for say a 500 watt lamp bulb,-as a liquid, and should not be sprayed with any more force than isnecessary to cause it to spread over the entire exposed surface. The action of strong nitric acid is very rapid on such a thin coating, and much less than the above suggested amount is sufllcient to change the deposit to water soluble silver nitrate.

If desired, the nitric acid may be applied in vapor form or as mixed vapor of nitric acid and water, by leading the vapors from a boiling flask of the nitric acid into the space in the bulb above the. protecting solution. This may be most easily accomplished byhaving a small vapor tube pass up through the neck and main part of the bulb holding the protecting solution, and enclosed within a larger tube containing air only. This serves to insulate the small tube containing the hot vapors'from' the main body The vapor method is in some ways superior, in

that very small quantities can be used and it "is certain that all parts of the exposed silver will be reached and dissolved by this very small amount of acid. f Moreover, the rate of attack of the silver is more rapid, as well as more certain, because of the higher temperature employed. The safety is greater, and the amount of total acid and also of protecting causticemployed will be less, thus reducing costs.

The nature and scope of my invention will be best understood and appreciated from the following description of the manner in which it may be carried out in providing a lamp bulb with a reflecting surface disposed in a peripheral zone extending substantially from its line of maximum diameter to a line in the neck of the bulb.

In the accompanyingdrawinga- Fig. 1 is a view of aninvertedbulb in vertical cross section. in position on the apparatus. Fig. 2 is a view in side elevation, partly in section, of the completed bulb, and v Fig. 3 is a view similar to Fig. 1 showin a modified form of apparatus.

The bulb Iii is shown in Fig. 1 as being provided with a coating l I of metallic silver extending substantially over its entire inner surface. The bulb 'shown in Fig. 1, also acts as a stopper and comprises'a rubber washer l2 interposed between a main portion l3 and a downwardly movable head ll. when the head I 4 is drawn downwardly the washer l2 is expanded into firm engagement with the walls of the neck of the bulb, thus making a liquid-tight seal. A vertical tube l5 extends upwardly through the stopper from a two-way cock i6, which, in turn, is connected with a supply pipe I! and an overflow pipe l8. When the cock is turned into full-line position as shown in Fig. 1, the protecting solution, which in this case may be sodium hydrate, is admitted to the interior of the bulb until the latter is filled approximately to its line of maximum diameter, or to any other selected level.

A tube l9 also passes vertically through the stopper and into the upper part of the bulb to a point somewhat above the end of the tube ii. The tube I9 is preferably provided with a spray nozzle and may be arranged to act as a vent during the introduction of the caustic solution to the bulb, and then to supply an acid spray to the exposed walls of the bulb above the level of the caustic solution. when the bulb'has been filled to the desired level with the caustic solution, the two-way cock is turned to the dotted line position shown in Fig. 1 in which the overflow pipe [8 is connected to the tube I! so that the tube l5 itself now acts as an overflow pipe determining the liquid level of the caustic solution and as a vent for the bulb. Nitric acid is now introduced under pressure through the tube IS in such a manner as to thoroughly wash the exposed portion of the bulb, dissolving the metallic silver therefrom and leaving the glass entirely free of metallic silver upon its inner surface above the liquid level of the caustic solution. As the acid flows down the walls of the bulb it is diluted and neutralized by the caustic solution at the surface level of the latter, while the metallic coating below the line of the surface level is thus protected from the acid and maintained intact.

The operation of removing the surplus silver from the exposed surface of the bulb may be performed quickly and completely and the reflecting coating thus limited to a zone defined by the protecting caustic solution. The silver-containing 50 acid escapes from the bulb through the overflow pipe l8, where it will be collected for reclaiming or reuse as desired. At the conclusion of the operation, the caustic solution, which has now served its intended purpose, may be drained from 55 the bulb and the latter removed from its stopper.

The completed bulb is shown in Fig. 2 with the silvered zone ll defined on its upper edge in this case at a line coinciding substantially with the line of maximum bulb diameter. The lower edge 60 of the silvered surface, in the neck of the bulb,

may be conveniently determined in the, step of,

applying the silvered coating, or it may be formed by scraping the surplus silver from the neck or in any other convenient manner v t5 In Fig. 3 is shown a somewhat modified form of apparatus designed particularly for removing the metallic coating from the desired areas of the bulb by hot nitric acid vapor. It will be under- I stood that in practice the apparatus is set upin 70 multiple form and that perhaps one hundred bulbs are simultaneously treated. One unit only of the apparatus is herein illustrated.

The bulb ill, with its inner surface coating ll of metalliesilver, is showndn Fig. 3 as mounted in is inverted position upon a stopper 2!, Two tubes enter the bulb through its stopper, the tube 22 being substantially flush at its inner end with the end of the stopper and serving as a supply and discharge pipe for the caustic solution. The

larger tube 23 is an overflow pipe and determines the level to which the caustic solution may be admitted and it also acts as a casing for a smaller concentrically arranged tube 24 having a perforated nozzle 25 which extends above the end of the tube 23 into the open space in the bowl of the bulb. Through this tube 24 hot nitric acid vapor is admitted to the interior of the bulb and this acts at once to dissolve all metallic silver which is exposed above the liquid level of the caustic solution. A

The tube '22 is provided with a two-way cock 26 leading to a discharge pipe 21 and a caustic which is inserted a discharge pipe 33 and through which also passes the acid supply tube 24. Both the discharge pipes 33 and 21 may be arranged to discharge through a single funnel 34 to a reclaiming tank, not shown.

In using the apparatusof Fig. 3 the following steps are preferably carried out. Valve 3| .is closed and, with the cook 26 in the position shown, the valve 29 is opened to admit caustic solution 20 to the level of the overflow tube 23. The large tube 23 acts as an overflow for maintaining a constant level of the solution held in the bulb. When the bulb is filled to the proper level, small amounts of nitric acid vapor are admitted through the inner small tube 24, from a source not shown. The silver coating the bulb on the surface above the line of the caustic is rapidly attacked and changed to water soluble silver nitrate. The caustic protects the rest of'the silver coating on the bulb at a substantially even line around the bulb. I

Before drawing off the caustic solution, through tube 21, the nitric acid still adhering to the surface of the bulb above the desired line, must be has been withdrawn, .and so injure or remove some of the silver coating of the protected area.

This can be easily accomplished by gehtly open ing the valve which connects to a source of air under pressure oi a few pounds per square inch above atmospheric. A few bubbles of air passed through the body of the protecting caustic solution are efiectiveto splash the caustic over the surface of the top of the bulb, wash down into the caustic solution "the dissolved silver and neutralize any excess nitric acid.

The caustic solution may now be withdrawn from the bulb by turning the cock 26 so that it passes through pipe 21 to the collecting funnel 34 and to a suitable receiver. The caustic so removed can be used over a number of times, preferably by maintaining its caustic strength by additions of fresh caustic. Moreover, most ofthe dissolved silver will be found at the bottom of this reservoir of used caustic in the form of precipi- "tated silver hydrated oxides, from which it may flow down the'tube and through pipe 21 and can be collected in the storage tank for used caustic and silver residues.

Water for washing is now allowed to flow with considerable force through the small tube 2d and tip.25 from a source and connection not shown. This wash water leaves the bulb through the pipe 38, but it must be diverted from flowing to the container for used caustic and silver residues, by changing suitable connectionsnot shown-since otherwise the caustic would be too much diluted for further use, and silver recovery would be made 10 more difiicu'lt. When all chemicals havebeen removed, the bulb is rinsed with distilled water.

throughtube 24 and tip 25, by making other changes in the external connections-not shownand following the rinsing air, which can be heated if desired, is forced through the same tube 24, by

any suitable arrangement of connections, until the interior of the bulb is entirely dry. The air will leave the bulb by the same pipe 33 and will be dissipated into the atmosphere at the open connection at the funnel 34.

If it is desired to produce a bulb having a pcripheral silvered surface extending over its curved end, as is often required for indirect lighting, the process of my invention may be carried out with the bulb in, upright position by filling the bowl thereof with caustic solution and then spraying nitric acid into the neck of the bulb in sucha manner as to cause it to flow over thewalls of the bulb andremove the silver therefrom down to the line of maximum diameter or other predetermined level. The silver-containing acid is neutralized at the caustic level and pumped from the bulb as fast as it accumulates.

The bulb during this silver-removing operaample, for the purpose of removing silver from a predetermined area defined by the line assumed I of a surface of revolution around the axis of the bulb, could be readily treated as disclosed above, although it would be almost an impossibility to 50 remove the silver from such a bulb by mechanical means. The same can be said of a bulb havin v a frosted or any other irregular and roughened surface on the inside, over which the silver has been deposited.

I do not specifically illustrate further applications of my novel process, but contemplate its use suitably modified in the production of selected areas of any desired shape or location upon glass articles, and particularly upon electric lamp bulbs.

60 While I have specifically referred only to silver as a metallic coating to be treated, the process of my invention is not limited-in that respect but may be usefully employed in" connection with rhodium or other reflecting metals or alloys by suitable modification ofthe protecting solution, on the one hand, and the acid or dissolving solution on the other hand.

Having thus described my invention, what I claim as new and desire to secure by Letters Patent is,-

1. The process of coating with metal selected areas of lamp bulbs, which consists in depositing a metallic coating upon the inside of the bulb, filling the bulb to a predetermined level with an tion could be in a horizontal position, for ex Moreover it is apparent that a" alkaline solution, and treating the remainder of the bulb with an acid to remove the coating fr'om the exposed portions of the bulb.

2. The process oi inside silvering the bowl end surface of a lamp bulb, which consists in silver coating the interior of the bulb, filling the bulb to a predetermined level with sodium hydrate, and

then washing the remainder of the bulb with face of the lamp above the overflow level with an acidsolution,

4. The process of metallic coating selected areas of the innersuriace of lamp bulbs of unsymmetrical shape, whichis characterized by the steps of depositing metal upon a liarger area of the bulb surface than is eventually required, flooding with a caustic solution the selected areas, and flowing acid upon the remainder of the bulb surface, thus dissolving surplus-metal from the bulb except where it is protected by the caustic solution while the latter neutralizes the acid at the boundaries of the selected areas.

'5. The process of metallic coating selected areas of a lamp bulb, which consists in depositing metal upon a larger area than that eventually required, immersing the included selected area in a protective caustic solution, treating the exposed metallic areas with an acid to dissolve the metal, and then, before drawing'the caustic solution, neutralizing acid remaining upon the treated areas. s

6. The process of defining selected silvered areas in an, inside silvered'bulb, which consists in flooding the selected area with a liquid caustic solution, and exposing sllvered areas in the enclosed space above the liquidilevel of the caustic solution to nitric acid vapor which is thus neutralized at the boundaries of the selected area by the caustic so1ution,,..

7-. The process of making lamp bulbs with metal reflecting areas of predetermined shape, which consists in applying. a metallic coating to an area large enough to include the desired selected area, filling the bulb with a caustic solution up to a level defining the selected area, and then introducing nitric acid vapor into the space abovethe caustic solution.

tallic reflecting areas, which consists in silvering the bowl of the bulb, then filling the bulb to a predetermined llquid level with a caustic solution,

eventually required in the reflector, immersing the selected reflecting area in a liquid alkaline solution up to a predetermined liquid level line, and then washing the unimmersed walls of the-bulb with an acid solution effective to remove the deposited metal therefrom down to the level of the a PINCUS pm. I

alkaline solution.

55 8. The process of making lamp bulbs with mep

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