US 2247058 A
Description (OCR text may contain errors)
June 24, 1941. w, BY I 2,247,058
REFLECTOR Filed July 27, 1938 Fvebruwzmna S/L v5? Inventor: William lrb His Attorney.
' the reflector. I
. in accordance .with my invention, are prepared v Patented June 24,1941- REFLECTQR William Irby,'Saugus,' Mass, assignor to General Electric-Company, a corporation of New York Application July 27, 19st, set-a1 No. 221,648
My invention relates to reflectors and more particularly to metal coated glass reflectors intended for use in light projectors.
One object of my invention is to provide a protective coating for the metal coating which will protect the metal against oxidation and consequent loss of reflectivity due to oxidation at operating conditions. I Another object of my invention is to provide a protective coating for the metal coating against all weather conditions.
For a better understanding of my invention together with other and further objects thereof, reference is had tothe following description taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claim.
The reflectors used for light projection are generally silver-coated glass. The glass is .a conoid of uniform thickness the convex surface of which is carefully prepared before a solution of silver nitrate is poured thereon. solution is poured on this surface a metallic layer is built up on the surface which is then reinforced with. another layer of metal electrolytically deposited. This reinforcing metal layer may be another layer of silver or, may be another suitable metal. I find copper suitable in this instance because of its low cost and ease of application by the electrolytic method. Silver is used for the first layer of metal because of its high reflection factor. Silver also has, however,
. a high affinity for oxygen and sulfur and therefore readilyoxidizes, or tarnishes, and correspondingly loses its reflectivity. Various protective coatings have heretofore been used to prevent this oxidatlon. These have met with varying degrees of success for operating temperatures below 200 C. For temperatures above 200 C. and up to 300 C. the coatings hitherto used have notbeen satisfactory.
In accordance with my invention-I provide a protective coating for metal coated glass reflectors, and especially for silvered' glass reflectors, which prevents an oxidation and consequent loss of reflectivity throughout the life of The surfaces of reflectors, made in the conventional manner. The glass form is thoroughly washed with a strong caustic soda solution and nitrieacid. This process is repeated until a thoroughly cleanand smooth surface is obtained. A solution, of silver nitrate and'a reducing agent, is then poured over this surface until a layer of metallic silver having an aver- When this a age thickness of approximately three hundredthousandths of an inch (.00003") is obtained. This process may be continued until a layer of any desired thickness is obtained.- LIt is more practical, however, to apply a second layer of metal by electro deposition. This second metal may be copper but is preferably another layer of silver. This second layer reinforces the first layer and actually alloys therewith so that the two .layers become, in substance, a single coat or layer. This second layer is applied by electrolytically depositing the silver by the passage ,of
an electric current for a time period suficient to buildup the total thickness of the deposited silver to a thickness of between 'six and eight hundred-thousandths of an inch (00006."- .00008"'). After this silver layer is applied a protective coating is sprayed or painted over the metal surface.
The protective coating of my invention is a soft porcelain, or vitreous, fenamel mixture made of litharge, boric acid, zinc oxide, silica, manganese dioxide, and cobalt oxide. Of these ingredients, litharge or lead oxide is the main ingredient, forming the body of the enamel. The other ingredients are refractories, fluxes and coloring oxides, the presence of which controls the characteristics and color of the final coating. Silica, for example, controls the firing, or
'maturing, temperature of the mixture which must be accurately controlled so that it will be high enough to produce a good weather resisting coating and yet will not be so high that the reflecting surface will be injured or the glass reflector sagged in the firing operation. Mixtures having a maturing or firing temperature range between 500 C. and 600 C. or 900 F. to. 1100 F.
ingly lower firing or maturing temperature.
An example of an enamel mix which is suit able contains: a
. Per cent Litharge 67 Boric acid 15 Zinc oxide 5 Silica; 11 Manganese dioxide 1 Cobalt /2.
In the preparation of this mixture the powdered ingredients are melted and in this molten state are poured into cold water. This process produces a friable glass or product generally known as a frit. This product, or frit, is then ground in a mill until finely powdered and is simultaneously mixed with water to produce a spraying mixture.
The above mixture is colored black and is suitable for most purposes. By omitting the cobalt and manganese dioxide and increasing the silica content, the frit produced becomes white. The litharge or'lead oxide (PbO) may be varied between 55 and 70% and the silica. content varied correspondingly between 10 and 20% to produce satisfactory mixtures. The boric acid content may be accordingly varied between 10 and 20% while the zinc oxide content is maintained at approximately the same proportion or 5% of the total mixture.
The above mixture is sprayed on the metal surface of the reflector in a layer having a thickness of approximately .002" to .005. The sprayed reflector is then'placed in an oven of which the temperature is accurately controlled.
When the reflector is placed into the oven the temperature is adjusted to 200 F. or slightly below. This is raised gradually over a period of one hour to a temperature of 1050 F. This latter temperature is then maintained for one to two hours and then is permitted to fall to room temperature over a period of two to three hours. A second coat may be sprayed on and fired as above for high quality work. Y
Reflectors made by this process must .be given a deposit of silver which is substantially above three hundred-thousandths (.00003") of an inch processes hitherto known.- The resistance to oxidation, for example, under an operating tem-v perature of 350 C., is of the order of five thousandor more hours, whereas the best coating hitherto used, so far as I am aware, provided a life of the order of five hundred hours. celerated test under salt spray disclosed for my improved construction a life of the order of one thousand hours. The novel protective coating and its method of application, therefore, produce a reflector having greatly improved weather and moisture resisting characteristics as well as a I greatly increased life under high temperature operation.
In the accompanying drawing, Fig. l, I have illustrated, in cross section, aconventional reflector of the type used in light projectors. The respective layers of which this reflector is made, such as the glass body, and the respective layers of sil ver and porcelain, or 'vitreous, enamel coatings, are respectively labeled.
In Fig. 2 I have illustrated a'modiflcation of this reflector in which the first silver layer, which is applied to the glass surface, is reinforced by Copper has hitherto been injurious to the silver reflecting coating under in thickness because the enamel absorbs, during high temperature operation. With a final coating of vitreous enamel however, copper may be used safely.
What I claim as new and desire to secure by Letters Patent of the United States is:
A reflector for light projectors operable at temperatures up to 350 C. comprising a glass body, a silver coating deposited thereon having a thickness of not less than .00003", and a protective coating of vitreous enamel fired into said reflecting coating, said enamel partially alloying with said silver and comprising to litharge, 10% to 20% boric acid, 5% zinc oxide and'10% to 20% silica. WILLIAM IRBY.