US2357913A - Photographic process for etching figures on glass - Google Patents

Description

Patented Sept. 12, 1944 PHOTOGRAPHIC PROCESS FOR ETCHING FIGURES ON GLASS John V. Sigford, Wabash, Ind., and Waldo H.

Kliever, Minneapolis, Minn.,

assignors to Minneapolis-Honeywell Regulator Company, Minneapolis, Minn., a corporation of Delaware No Drawing. Application August 29, 1941, Serial No. 408,818

1 Claim.

Our invention relates broadly to an improved process for etching whereby, in the etching of figures in material in. general, the differential efiects are obtained from differential exposure to light and the common methods of mechanically marking a resist are eliminated.

In particular, the invention relates to a new and novel photo-engraving process involving the etching of glass, quartz, vitreous enamel, plastics and other substances. A particular object of our invention is the provision of a process for the photo-engraving and etching of glass to be employed as reticles for use in telescopes, range finders, microscopes, surveying and other optical or aligning instruments. However, it will be obvious that our invention is of broad application in the fields of photo-etching and of photo-engraving and accordingly is to be limited only by the scope of the appended claim.

In well known photo-engraving processes such as those embodying the use of copper or zinc plates for printing, the configuration to be reproduced by etching is photographed for the making of negatives from which the etching resistpattern is produced on the metal. The metal is coated witha light sensitive colloid, exposed under the negative to a strong actinic light and treated to produce the enamel etching resist of a material such as insoluble glue or albumen. In the etching process the metal which is not protected is controllably dissolved by the action of the etching mordant while leaving the protected or enameled portions in relief. Well known etching mordants employed in connection with these metals may consist of ferric chloride for the copper and nitric acid for the zinc. In the usual methods for etching glass and optical material, an etching mordant such as hydrofluoric acid must be employed. It is found that such a mordant will attack light sensitive coatings even though it hasbeen exposed and treated to form an enamel etching resist of the type successfully employed with metals. It is an object of this invention to provide a photo-etching and a photo-engraving process for etching in fine detail materials in general. A particular object of the invention is the provision of a process for the photo-etching of glass and like materials that require the use of an etching mordant such as hydrofluoric acid which is harmful to resists of the type normally employed in connection with known photo-engraving processes.

A further object of the invention is to provide a process for the photo-etching of materials which permits the use of an etching mordant such as hydrofluoric acid which will attack a light sensitive resist by the use of a suitable resist in proper relation with the light sensitive material. For example, in accordance with our invention, the material to be etched may be coated with silver to form a suitable resist for ahydrofiuoric acid etching mordant. The well known photo-engraving processes referred to above may then be employed. Thus the portions of bichromated colloid coating for the material when exposed to light under a suitable negative will become insoluble to form a suitable resist for an etching solution for silver or other first coating. The silver in the image which is not protected by the enamel of the resist may accordingly be dissolved away by immersion in a solution of sodium thiosulphate and potassium ferricyanide known as Farmers reducer, or other mordant. When the image is thus obtained on the material proper, it is rinsed, dried and may then be etched by the hydrofluoric acid mordant. The material is then rinsed in concentrated nitric acid to remove any remaining enamel and silver to thus complete the etching process.

Other objects and advantages will become apparent to those skilled in the art and a more thorough understanding of our invention will be obtained by reference to the accompanying description of the application thereof to the production of reticles for use in telescopes and range finders.

In the practice of our invention as applied to the production of reticles in glass, the glass material must be treated in accordance with known processes to render-it chemically clean. The glass is then silvered and this may be accomplished by the use of any standard process such as used in the making of mirrors including chemical deposition, cathodic sputtering, or by evaporation processes. The silvering may be accomplished with very good results by the use of the well known Rochelle salts process, and in order to produce a perfectly resistant coating it is preferable that at least three successive coating-s be employed. It is found desirable to carry the silvering to a stage where it has a slightly white appearance which occurs after the solution has become sludgy. In this manner a surface is obtained to which the enamel will better adhere than is the case with a perfectly smooth coating, although if the surface is permitted to become too white the enamel described below would be porous and non-resistant.

In the handling of telescopic reticles a sheet of plastic containing recesses for holding the reticles in place may be employed. The cleaning and silvering may be accomplished by the use of trays such as used in photographic work. In this manner they may be handled in quantities of, for example, two hundred at a time. cleaning .and silvering operations maybe performed in a period of about thirty minutes.

The silvered reticles are then coated with a light sensitive bichromated colloid illm which r.

iecting the image to the coating. A strong actinic light which may be produced by an arc lamp may be employed with an exposure having a duration of about thirty seconds. In placing the image from the diapositive, or mask, with respect to thelight sensitive coating on the glass, we find it preferable to cut the diapositive to substantially the same diameter as the reticle and with the image properly oriented to employ a ring as a when exposed under the negative, forms the etch- 1 frame to hold the negative and retlcle in posiing resist for the silveretching process described below. The sensitizing solution may consist of sensitized resin which usually contains bichromateand is known in general as Cold Top enamel.

tion. .A considerable number of reticles may be exposed-at a time. The exposure must be of suiilcient duration and intensity to cause the .proper. reaction by the exposed light sensitive Other colloidal substances employed may be of ldcoating.

reticles and preferably made to flow evenly by a 20 centrifugal or spinning process. The reticle may be attached by a suction cup to a drill stock or motor which is rotated at a relatively low speed and the coating material is applied to the center of the reticle. It is desirable that the coating be as applied as evenly as possible and the thickness of the coat should be controlled because of its eifect on the exposure required. The thickness of the coatdepends on the temperature and the speed of rotation. A light is employed that is not go and, rendered insoluble in the developer fluid,

sumciently bright for exposure purposes. The

' coating must be thin enough to give th desired detail and thick enough to provide the required resists.

The coating thickness may be instantly gazed without stopping the spinner by observing the interference pattern on the surface. We have found that the formation of a red interference disc of the first order in the center of the rotating reticle, when dry, is indicative of a thickthe surface while spinning and before the coating is applied is lightly rubbed with a piece of cotton wet with alcohol. The coating proass requires about three minutes and one person may conveniently handle about ten spinners at once.

The network and markings of the ilgure to be reproduced in glass as required in the formation of reticles for telescopes and range finders may be drawn on paper at preferably many times actual size. In producing an accurate image in the negative it is necessary to so design the figure contained in the drawing as to compensate fol lens imperfections and him halaticn by revision of the drawing in an amount determined-by ex; perimentation. The figure may then be copied on microfilm or some high resolution film, for exjmple, by using a lens of short focal lcnsth possessing excellent resolving power to provide the necessary sharp and line detail. To minimize tain sumcient density in the imase. Our invention is not limited to the use of photographic 7 The diapositive of the figure is then placed in P oper relation with the silvered and light sensitive transparent coated glass for exposure to a light of high intensity for the purpose of in)? After-the light sensitive coating of the reticle has been exposed under the diapositive to light of strong actinic intensity the coated surface of the reticle is washed to develop the enamel resist. The developer may consist of alcohol particularly where a resin type colloid is employed, or of water where an albumen or glue colloid is used. Likewise.

a methylviolet dye. or other dye. may be mixed with the developer to dye the cold top image so sion of about ten seconds and permitted to drain. As soon as the image is clearly seen the reticles are rinsed in water and dried. The portion of the light sensitive coating that is exposed to the light forms an enamel resist to protect the under coat of silver in the subsequent silver" etching process. That portion of the coating protected from the lisht by the negative is dissolved awa leaving the image contained in the unprotected silver coating. In this development process if the protective coating is weak and tends to come off in the developer an underexpoe d condition is indicated. and if the mage does not raise an overexposed condition is indicated.

' The development is completed by washing in water andby lightly stroking the surface with a wad of cotton or a soft brush. A well known an tendency of the enamel coa to blister.

The application of heat in drying may likewise serve to harden the enamel for betterwithstanding the action of theetching mordant.

The silver in the image which is unprotected by the enamel is etched or.diseolved away by immersing the reticles in a solution of the 'well known Farmer's *reducer, containing sodium thiosulphate and potassium ferricyanide, al- M though other known agentsmay be employed.

This solution does not attack the Cold Top or other enamel resist and'this step of the process is terminated by complete transparency of the image on the exposed glass. Completion of I step may be accomplished in about one minute following which the reticles are rinsed and dried.

The etching oi-the glass is preferably accomplished by-exposing the reticles to. hydrofluoric "acid fumes. This etching'mordant'will attack halation, the exposure may be kept somewhat ,9." the Cold Top or other light sensitive enamel and lightv with copper intensification employed to n. elective resist is formed by the undercoab ing of silver. It is found that a four oriilvosecond exposure of the reticles to fumes of com-5 inertial fifty-two percent acid. heated in awater bath't'oa temperature of 120 1-. will produce the dcsired'etching-of theglas'e. To maintain uniform strength of the acid fumes a lead container may be employed, having a sliding cover on one 'sideofwhichia-arecessfcrholdingthereticie,

and an the image.

that is may more readily be seen. The reticlcs are lifted from the developer following an immerhardening solution may be employed to prevent.

The reticle is placed over the recess and the cover is moved over the container for the required time, whereupon the reticle is removed and placed in water to check the etching action.

A rinse of concentrated nitric acid, or other known solution, is applied to the reticle for the removal of the silver and remaining enamel.

The etched glass is then washed, dried and filled in any well known manner, to enable the image to properly stand out, and thus complete the production of the reticle in accordance with the practice of our invention.

Our process may also be applied to etchin of .plastic materials, using a suitable organic solvent in place of the hydrofluoric acid, and some minor modifications in cleaning, etc., which will be obvious to those skilled in the art. For example, a cellulose acetate sheet might be coated 'with silver and light sensitive colloid and ex-- posed and developed as described, leaving the silver film as a. resist, and the figure etched in the cellulose acetate surface with acetone.

The application of. our invention to the proconfiguration of the duction of reticles as disclosed above is intended to be merely illustrative. Other variations and applications of the invention will readily occur to those skilled in the art. Therefore, our invention should be limited only in accordance with the scope of the appended claim.

We claim as our invention:

A process for Dhotographically applying an image to an article of glass or similar material, comprising, successively applying a plurality of layers of silver coating to said article until the metallic surface has a slightly white appearance, successively applying a plurality of layers of light sensitive coating to said silver coating until said coating has assumed a thickness substantially equal to one-half the wave length of red light, projecting an image oi! the desired configuration upon said light sensitive coating, and then developing said coating to remove that portion of the light sensitive coating conforming to the image projected thereon.

JOHN V. SIGFORD.

WALDO H. KLIEVER.