856
INDUSTRIAL AND ENGINEERING CHEMISTRY SU?II\I-IRY
A11 that can be said a t the present time regaiding this phenomenon of heating rate variation with particle size is t h a t it is believed to be a genuine electrical effect, independent of voltage applied and the exact electrical circuit employed, and dependent upon the size, shape, and physical and chemical structure of the particles. The frequency of the field is probably another important variable, since loss factor is known t o vary with frequency. Many more substances should be investigated before the formulation of any general theory is attempted. ACKNOWLEDGXIENT
The authors wish to express their indebtedness to the Kational Research Council for helping to sponsor this research through a fellowship grant t o R. V. Jelinek; t o the Induction Heating Corporation for kindly furnishing the generator and other electrical equipment; to W.W. Winship, of the Thermal Syndicate. Ltd., for supplving the silica samples; t o F. G. Foote and *J. IC.
HOT
Vol. 41, No. 4
Roros, of the Department of Metallurgy, Columbia University, for their kind cooperation in the x-ray work; and to A. W. Hixson, of the Department of Chemical Engineering, Columbia University, for general advice and assistance. LITERATURE CITED
(1) Dakin, T. S.V., and .4uxier, R. W., IND.ENG.CHEM., 37, 268-75
(1945). (2) Jelinek, It. V., “Dielectric Heating of Granular Silicon Dioxide,”
M.S. thesis, Dept. Chem. Engineering, Columbia Cniversity,
1947. (3) Perry, J. H., ed., “Chemical Engineers’ Handbook,” 2nd ed., section 15, New York, McGraw-Hill Book Co., 1941. (4) Schute, P. W., and McMahon, E. K., IND. ENG.CHEM.,38,
179-84 (1946). (5) Scott, G. N7., J r . , Elec. Eng., 64, 558-62 (1945). (6) White, W.P., Am. J . Sei., 47,1-43 (1919). (7) Worthing, A. G., and Geffner, J., “Treatment of Experimental Data,” New York, John Wiley & Sans, 1943. RECEIJ E D October 23, 1947.
SIT1
A New Photographic Medium S. D. STOOKEY Corning Glass F’orks, Corning, N . Y . Photosensitive glass, a new type of photographic medium recently developed to a commercial stage at Corning Glass Works, makes it possible to print colored photographic images within glass articles. The photographic process consists of two steps: exposure with ultraviolet light through conventional negatives, and development by heat treatment. The photographic image may be produced in a variet>- of colors. I t is threedimensional, and gives in some cases a stereoscopic illusion. Tw-o basic types exist: color transparencies, in which the image consists of sul~microscopicparticles of gold, silver, or copper; and photosensitive opals, in which the image is made up of microscopic nonmetallic crystals capable of diffusing light. The permanence of the image is believed to equal that of the glass article. A desrription is g i en ~ of the compositions, photographic process, and possible applications. Pertinent experimental data and a theoretical explanation of the photographic process are prcsented.
I
N JCKE 1947, Corning Glass K o r k s announced the develop-
ment of photosensitive glase. This designation refers to certain silicate glasses containing ingrcdients t h a t are capable of forming permanent photographic images in the otherwise clear glass Jvhen subjected to the successire action of actinic radiation and heat treatment. T w o basic types have been developed: photosensitive metal-colored glasses, in which the image is a color transparency consisting of submicroscopic metal part,icles within the glass, and photosensitive opal glasses, in which the image is translucent or opaque and consists of microscopic nonmetallic crystals. A wide variety of modifications exists, capable of producing images i n many colors and in ghsses Tvith a range of physical properties. Conventional glass-melting and forming methods are employed in its manufacture. Any type of glass art,icle may be made without impairing the sensitivity. The finished photographic design is believed to be as permanent as the glass art’icle itself, and is three-dimensional, giving in some ca,ses a stereoscopic illusion.
The research leading to development of photosensitive glass as initiated by the discowry of Dalton (9, 3) in 1937, that the color of copper ruby glass is caused to “warm in” more readilythat is, to develop on heatmg the originally colorless glass-when it is exposed to ultraTiolet light before the heat treatment. I n 1941 the writer began a search for glass compositions in TT hich this phenomenon could be enhanced sufficiently to make photography feasible. This iyas accompliqhed in the same year n i t h glasses colored by copper (IO). Later work disclosed that superior results were obtainable hy the use of gold, together with appropriate sensitizing agents and color modifiers (11). hfore recently it was discovered t h a t the photographically developed metal particles can act as nuclei for the formation and growth of nonmetallic crystals in certain glass compositions, resulting in the photosensitive opal glasses. Neanuhile, Armistead, who had been investigating the coloration of glass by silver, was able to make photosensitive silver glasses (1). Photosensitive glasses of both the color transparency and the opal types are now in commercial production as flat polished plate. The photographic work is done by licensed processors, except for louvered opal glass panels cuirently being introduced to the lighting fixture trade by Corning, under the name Fota-lite. This paper gives a description of the nature of photosensitive glass, the photographic process, the fundamental reactions involved, and possible applications. 77
GLASS COMPOSITIOKS
Photosensitive glasses are very similar to certain conventional glasses in composition, except for minute additions of constituents that may be classified as photosensitive metals, optical sensitizers, and thermoreducing agents. A general description is given below, and specific compositions may be found in the patents cited. Base Glass. Most conventional silicate glass compositions have been found capable of photosensitivity, provided a n appropriate combination of photosensitive ingredients is employed. Exceptions are those containing appreciable quantities of lead or other strong absorbers of ultraviolet light. Presence of a t least 5yc of alkali metal oxide appears necessary.
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857
PHOTOGRAPHIC PROCESS
Exposed Glass Bars F r o n t a n d side views of bars t h a t have had seven progressively longer exposures
Borate and phosphate glasses were not suitable because in them the metal developed coloration spontaneously when the melt was cooled or reheated. Glasses containing more than 5% of barium oxide have been found especially advantageous when used as base glasses for gold, because they permit a number of colors to be developed in addition to the usual gold ruby produced in other glasses. Photosensitive Metals. The more important photosensitive metals are gold, silver, and copper. Certain other metalspalladium, for example-were found capable of modifying the color when used in combination with one of the above-mentioned metals. It was determined that melting conditions must be oxidizing for gold- and silver-containing glasses, and mildly reducing for copper-containing glasses as described (12) for the metalcolored glasses. Sensitizers. THERMOREDUCING AGENTS. This type of sensitizer comprises certain members of the polyvalent group ( I $ ) , notably compounds of tin or antimony. The effect of adding traces of tin or antimony compounds to the batch is to increase the tendency of the metal to “warm-in” color on heat treatment. Excessive quantities cause spontaneous coloration. I n terms of the photographic effect these compounds were found to decrease the ultraviolet exposure required to produce a latent image, but to reduce the contrast of the developed image. OPTICAL SENSITIZERS.This type of sensitizer is distinguished from the first type described because the sensitizing effects were demonstrated t o result from absorption of the activating radiation by the sensitizer. Their influence is shown by sensitization of the metal to new wave lengths absorbed by the sensitizer, or by more rapid photographic effects with no loss in contrast. Cerium is the most important of the optical sensitizers known at present. Their effects are discussed below in the section on the latent image.
The processes of glass photography are fundamentally simple. Two steps are required, exposure and development. Exposurc is accomplished by irradiating the glass with either ionizing radiation or ultraviolet light in the band between 300 and 350 millimicrons. Use of the latter makes it practical to expose by the contact print method through ordinary film or glass plate negatives. Development consists of heating the glass at or above its annealing temperature until the image is sufficiently intense. A separate “fixing” treatment is unnecessary, because cooling the glass to room temperature freezes the image permanently, unless it is again heated to a high temperature. The whole process can be carried out in ordinary room light, and no chemicals are required. Four variables are controlled by the photographic process: color, intensity, contrast, and depth of penetration. Each can be varied almost independently of the others. This enables the photographer to produce a wide range of photographic effects in a single type of glass. Control of these effects depends on a few basic facts, which are given below. The discussion in this section is confined to the photosensitive barium-base glass colored by gold and sensitized by cerium, because i t is the most versatile in color and is now commercially available. The principles, however, apply to all the photosensitive glasses. Effective wave lengths for exposure and heat treatments for development are similar. Exposure. The character of the resultant image is a function of the quanta of effective radiation absorbed by the primary lightsensitive ingredient (a cerium compound in this case). The absorption by cerium a t any point within the glass is determined by the absorption coefficient of the cerium and by the effective radiation reaching that point. The lattcr depends in turn upon the intensity and spectral distribution of the incident radiation, external filters (including the negative), and the radiation transmitted to the point in question through the photosensitive glass itself. SPECTRAL SENSITIVITY. Figure 1shows the ultraviolet absorption band of cerium in the photosensitive glass, as determined from Beckman spectrophotometer curves on 1-mm. polished plates of glass with and without cerium (0.03y0 cerium oxide). This appears to coincide with the photosensitivity curve of the glass surface. I n the same figure is the ultraviolet transmittance curve of a 1-mm. section of the photosensitive glass. The absorption coefficient of the glass changes from a low value at the long wave-length end of the sensitivity ban8 to a very high one a t the short wave-length end. This means t h a t the effective sensitivity peak shifts t o longer wave lengths with increasing depth of glass, because the shorter wave lengths are increasingly filtered out by
Figure 1.
Spectral Sensitivity Curve of Photosensitive Ingredients % transmittance through 1-mm. glass thiokness
858
INDUSTRIAL AND ENGINEERING CHEMISTRY
White Louvers in a 3jlpInch Thick Sheet of Photosensitive Opal Glass Abone. Sign observed through plate a t an angle t h a t permits direct vision Below. Same object observed a t angle t h a t partially blocks direct vision
the glass. For practical purposes most photographic films also filter out the wave lengths shorter than 300 mp, so that t,he useful band for photography extends from 300 t o 350 mp. An exposure of the order of 2 milliwatt minutes per square cen& meter in the effective wave band is required to produce a medium density. Using an average photographic negative, the exposure is from three t o ten times t,his value because of t,he filtering action of t,he negative. EFFECT ON COLOR AND INTENSITY. Figure 2 shows the effects of varying exposure time on color and intensity. The samples ~ c r irradiated e with the longer effective wavc lengths to obtain uniform exposure throughout the cross section, then given an identical development treatment under normal conditions. The transmittance curves m-ere obtained with a General Electric recording spectrophotometer. An induction period exists in which esposure has no apparent effect. Then a blue coloration appears, which is intensified by further exposure. Still more exposure causes a shift in t,he band of low transmit,tance, or absorption peak, toward shorter wave lengths, so t h a t the visible color changes from blue through purple, ruby, and amber. This shift of the absorption peak leads t o an actual decrease of visual color density as the exposure is increased beyond a certain point, because the low transmittance band moves out of the range of maximum cyc sensitivity. For this reason, thc ruby and amber colors appear less intense than the blue and purple obtained ivith less exposure. The absorption peak corresponding to ext,reme esposure is a t about 480 mp. The apparent intensity of the image is a function of the color as described above, and of the depth of penetration, as well as of the actual intensity. EFFECT OF DEPTHOF IMAGE PESETRATIOS. The dcpth to which the image penetrates into the glass is a function both of exposurc (intensity timcs timc) and of the wave lengths of effective radiation employed. If only wave lengths below 315 mp are employed, the image is confined to a relatively shallow surface layer. If, a t the other extreme, all radiation of wave lengths shorter than 340 mp is eliminated, a uniform color is produced throughout a deep section of glass, ranging up to a maximum of 2 inches. The radiation characteristics of the light source control the effective energy limits, but these may be modified by using filters. Image penetration depends on the absorption of the effective radiation by the glass. This absorption, which includes that due do other constituents a1swell as that, of t'he optical sensitizer, progressively diminishes the activating energy as the depth of the glass is increased. This produces an exposure gradient between
Vol. 41, No. 4
the surface and the interior layers of glass. The penetration therefore increases progressively with time of exposure. A color gradient is produced between the surface and the interior for the same reasons, and it is possible with a single exposure to obtain the whole range of colors described above, in a crov section of glass. Development. The heat treatment influences all four of the photographic variables mentioned above. Rates of heating and cooling, within rcasonable limits (for effect of rapid heating see section on the latent image) are not significant in the photographic process. Development of thc image occurs very slowly (several hours) a t the annealing temperature of the glass, at a moderale rate (I hour) midway between the annealing and softening temperatures, and rapidly (8 minutes) a t the softening temperature of the glass. The development process may be halted a t any time by cooling the glass, and resumed by further heating. Figure 3 shows a time-temperature curve for equivalent normal dcvelopment, as compared with the viscosity-temperature curve. The development is accompanied by less deformation tendency a t lower temperatures. Before development the glass is substantialry colorleus. As heating a t developing tempwature takes place, a faint color gradually appears in the exposed portion and becomes progressively more intense. The color t h a t develops is dependent upon the exposure, as described above. Orange and red colors, characteristic of strong exposures, develop more rapidly than purplc and blue. Intensity of the developed color is progressively increased with temperature and time, up t o a maximum depending on thc concentration of coloring material, exposure, etc. The colors produced by strong exposure are developed more rapidly than those with less exposure. I n a thick piece of glass
w4 V € L ENG T U flp Figure 2.
Spectral Transmittances of Developed Colors
I n single plate of photosenritive gold glass 2 mm. thick. Curves are numbered acoording t o relative exposure time, except that 1 represents exposures of both 0 and 1
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INDUSTRIAL AND ENGINEERING CHEMISTRY
859
teriorate film, so t h a t i t has been found desirable to protect i t b y using filters which absor6 the harmful radiation while transmitting the useful light. This precaution is unnecessary with glass plate negatives.
z
Examples of Photographic Procedure. A few specific examples are given below, to show the methods of obtaining various effects. &
4
A11 these examples employ the photosensitive gold glass now in commercial production. A glass of different composition would require a different temperature range for development. Exposures are made with the Atlas Arc Type C3D, 60 amperes at 50 volts, a t 12-inch distance. Xegatives are on Ansco Commercial Ortho film. Development temperature is 620" C. I n examples 1 and 2, the negative used is of medium contrast and density.
EXAMPLE 1. Exposure time, 9 minutes, Development a t 620" C., 10 minutes. Pink image, low contrast, thin surface color. Development a t 620" C., 15 minutes. Purple image, medium low contrast, shallow peFetration. Development at 620 C., 30 minutes. Blue-purple image, moderate contrast, moderate penetration.
EXAMPLE 2. Exposure time, 5 minutes. Development at 620 C., 20 minutes. Blue image, moderately O
72/VPk-R.47Ufi€
Figure 3. Viscosity
"C
Time Required for Normal Development and of Photosensitive Glass as Function of Temperature
this means that the color develops more rapidly near the exposed surface than in deeper layers. Therefore the depth of penetration increases with time and temperature of heat treatment. Increased development increases the contrast, by preferentially deepening the penetration and intensity of strongly exposed areas over those weakly exposed. Photographic Equipment. LIGHT SOURCE. Any source of radiation in the band between 300 and 350 mp may be used. X-rays and @-rays have been shown t o produce a developable image. Because of the three-dimensional nature of the image, the light should preferably be parallel, or from a small source. Light penetrating the glass at various angles may produce a blurred image. Specifically, a carbon arc, with Kational C cored carbons, 60 amperes at 50 volts, h a s been found most nearly satisfactory t o date in producing high intensity from a small source, with reasonably constant output. Depending on the exposure speed required, arcs of higher or lower power may be better suited t o the needs of various operators. Several concerns are now developing new light sources especially adapted for photosensitive glass. Mercury vapor arcs are effective in the short wave-length region, cadmium-mercury arcs in the moderate to long wave lengths, and sunlight may be employed. HEATING UNIT. The type of heating unit t o be used depends upon the type of operation and the production scale. Fundamental rcquirements are the ability t o heat all parts of the glass uniformly from room temperature t o a maximum of 650 O C. and a n adequate temperature control. For small scale. experimentation a laboratory muffle furnace is perfectly satisfactory. Large volume production requires either large kilns or continuous lehrs. NEGATIVBS. Almost all types of film or glass plate negatives can be employed successfully. Only a few types absorb the ultraviolet light so strongly as t o be useless. The ultraviolet transmittance characteristics of the negatives must be taken into account in determining the exposure times required, either by test trials or by meters t h a t measure the effective radiation. Ultraviolet light of less than 300 mp wave length tends to de-
high contrast, thin surface color. Development a t 620 O C., 30 minutes. Blue image, contrasty, moderate penetration. Development a t 620" C., 60 minutes. Surface layer of red color, deeper layer of blue, contrasty, deeper penetration.
EXAMPLE 3. Using a positive transparency of higher contrast, t o obtain a range of colors with one exposure and development. Thickness of photosensitive glass plate, 0.125 inch. A positive is used because we wish thc shadowed areas t o be printed in blue, high-light areas in red. Exposure time, 18 minutes. Development at 660" C., 60 minutes. Image is printed in red high light2 purple intermediate intensities, and blue shadows. THE LATENT IMAGE
The latent image, which is formed within the glass by exposure to actinic radiation, is detected with certainty only by observation of the visible image subsequently developed. Formation of the latent image is accompanied by a slight change in the spectIal absorption of the glass in the ultraviolet or the visible spectrum, but this is not a reliable measure of the latent image, for iB some cases the same change is produced in the absence of t h e photosensitive metal; and in all cases the change virtually disand does not recur appears if the glass is heated above 400" on cooling. A number of properties of the latent image have been determined by subjecting i t t o various treatments and observing the visible image after development. Spectral Absorption. I n common with most glasses, photosensitive glasses become slightly discolored, or "solarized," by exposure to ultraviolet light or t o ionizing radiation. Photosensitive glasses containing silver or copper compounds have broad absorption bands in the middle ultraviolet (300 t o 370 mp) which are characteristic of these compounds, and develop faint "solarization color"-blue-gray for copper-containing glasses and yellow for silver-containing glasses-after exposure. Absorption measurements made with a Beclrman spectrophotometer on photosensitive gold glasses show no absorption band t h a t can be attributed to the presence of gold compounds. In photosensitive copper and silver glasses the latent image is formed by exposure to ultraviolet light in the bands absorbed by the copper and silver compounds. I n photosensitive gold glass containing no added optical sensitizer the latent image is formed only by short wave-length ultraviolet light (254 mp or shorter) which is absorbed by other ingredients of the glass, whereas addition of cerium compounds permits formation of a latent image by absorption of light wave lengths up to 350 mp, in the band absorbed by cerous ions.
c.,
860
INDUSTRIAL AND ENGINEERING CHEMISTRY
Effects of Exposure Temperature and Heating Rate. No essential change in exposure effect was found in the range from liquid air temperatures up t,o about 580' C. , the latter temperature (for the glass composition tested) being in the temperature range where development occurs. I t was possible to expose and develop color simultaneously at 580' C. Above 600" C., however, the glass was exposed to ultraviolet light up to fifty times the normal exposure time viithout producing a latent or visible image. When a photosensitive gold glass containing 0.01% of gold and 0.04% of cerium oxide was exposed t o ultraviolet light and subsequently heated almost,instant'aneously (as by immersing a I-mm. thick plate in molten salt) t'o above 600" C., the latent image was destroyed-that is, no visible image could be developed by subsequent heating a t the developing temperature, although a n identical piece n-hich was heated s l o d y t o 600 O C. or above readily developed a visible image. The image, once developed, did not disappear on further heating until the glass was molten (above 1150' C.). Re-exposure of the piece in which the latent image had been destroyed resulted in formation of a normal latent image, as evidenced by subsequent nornial development of a visible image. The destruction of the latent image a t high temperatures is accompanied by thermoluminescence. This fact is believed to be of major significance in developing a theory of the nature of the latent image. The above effects are much less marked in commercial glass containing a higher gold concentration. Aging of Latent Image. A plate of photosensitive gold glass which had been exposed t'o ultraviolet light through a negative so as to produce a latent image was kept in the dark for one year and then given a normal heat treatment'. A visible image was developed which appeared normal in all respects. Electrical Field. Application of high frequency (10- t o 100megacycle a t a potential gradient of 1 to 2 volts per mil) electrical fields t o a plate of photosensitive gold-containing glass during exposure to ultraviolet light produced no apparent effect on the latent image. Similarly, application of the field during development of a normally exposed plat,e produced no deviations lrom the normal behavior. THE DEVELOPED XMAGE
Development of the latent image to a visible photograph i6 accomplished by appropriate heat treatment. Development is a time-temperature function which depends on the viscosity of thP glass, the nature of the photosensitive metal, and the state of thir metal as determined by the thermoreducing agents and the degree of activation by ultraviolet light. Minimum developing temperatures rangc upward from approximately 100 C. below the annealing temperature in silver glasses to 20 O C. below the annealing temperature for copper and gold glasses. The maximum developing temperatures are limited in practice by deformation of the glass above the softening temperature [annealing and softming temperatures obtained by Littleton's method (5, S)]. Developing time decreases as approximately a n exponential function with increasing temperature (see Figure 3) Average development temperature is between 580' and 650' C. for a glass whose softness temperature is 710 ' C. and whose annealing temperature is 525' C. ilverage development time in this temperature range is between 10 and 60 minutes, and is shorter for a glass containing a thermoreducing sensitizer or a more strongly exposed latent image than for one in which the ronverse is true. The developed image in nearly all cases consists of colors identical with those of typical gold ruby, copper ruby, or silver yellow glasses. Spectral transmittance curves revealed no apparent differences, except in the special case of barium-base glasses containing gold. These latter glasses, while capable of developing the normal gold ruby color, can also develop other colors (see section on photographic process and Figure 2) T t has been shown by Gottfried (Q), Riedel and Zschimmw ( 8 ) ) a
Val. 41. No. 4
Rooksby (9), and others that the coloring agents in mctal-colorcd glasses consist of submicroscopic metal part,icles. This was confirmed in this laboratory for the case of the photosensitive gold ruby color by x-ray diffraction patterns. These patterns showed the presence of crystalline gold in a n area of glass that had been expoeed and developed t o a ruby color, while no evidence or crystalline gold was found in an adjacent colorlcss area t h a t had not been exposed t o ultraviolet light but, had undergone the sarno heat treatment. The sizes of the metal particles are in general too small to produce appreciable light scattering, and it appears possible that in cert'ain cases they may be atomically dispersed. Because the photographic image is essentially painless and exists in a transparent, optically homogeneous medium, resolving power and reproduction of detail are excellent. PHOTOSENSITlVE OPAL GL4SSES
I t has been found possible t o employ the photographically developed particles of gold, copper, or silver as nuclei for formation and growth of various types of crystals within the glass. This occurs in certain thermodynamically unstable glasses in which one crystalline phase has a strong tendency to precipitate but is prevented from doing so by the high viscosity of the glass. The controlled local introduction of inhomogeneities (metal particles) by the photographic process enables crystallization to occur in these areas. The glass viscosity prevents the crystallization from spreading through the piece, so t h a t a photographic image is produced. I n most cases the crystals are transpaient and colorless (except for the color imparted by the metal), but different in refractive index from the glass. It is possible to reduce the concentration of metal sufficiently to eliminate any coloration, and still retain the nucleation ability, in which case the imngc is a light-diffusing white design m-ithin the clear glass. THEORY
I n a recent paper (12) evidence was presented which appears t o prove t h a t in glasses colored by colloidal gold, silver, or copper the metal dissolves in a n oxidized state during the melting process. I n conventional gold and copper rubies and silver yellow glasses, the metal compound subsequently becomes reduced to the metallic state as the glass is cooled or reheated. The reducing agent is a polyvalent ion such as selenium, t,in, antimony, 01: arsenic, whose reduction potentials are greater a t low temperatures than at the melting temperature of glass (1400" to 1600" C.). The metal, being insolublc, f o r m discrete particles which impart a characterist'ic color to t'he glass. The photosensitive glasses described in the present work differ from the conventional metal-colored glasses in t h a t t,hey cont,ain little or none of the polyvalent thernioreducing agents, hence remain colorless on heat treatment because the metal compounds remain in a soluble oxidized state. Instead of thermoreducing agents these glasses contain optical sensitizers (among which the copper and silver compounds themselves are included) which become reducing agent's for the dissolved metal compounds on exposure t o actinic radiation. The initial state of photosensitive glass, before exposure to actinic radiation, is therefore believed to he as follows. Ions of gold, silver, or cuprous copper are homogeneously dispersed in a hard glass matrix, in very low concentration, along with ions of the optical sensitizer. The glass a t room temperature is rigid, permitting no mobility of ions or atoms and confining electronic mobility t o a few atom diameters. The functions of other constituents of the glass are in general unimportant, except as they influence the oxidation state of the metal and sensitizer, or competitively absorb the actinic radiation. The latent image is believed t o consist of ( a ) photoelectrons, emitted from light-sensitive ions such as silver, cuprous, ccrous, or thallium, and held in a metastable act,ivated state a t trapping cen-
April 1949
I N D U S T R I A L A N D E N G I: N E E R I N G C H E M I S T R Y
ters adjacent to the parent ions; and ( b ) metal ions capable of subsequently capturing the photoelectrons t o form neutral atoms. The trapping centers may be metal ions or some “lattice imperfection” in the glass network. Because of the rigidity of the glass structure at room temperature, neither the electrical forces nor the ionic structure can be rearranged to new equilibrium states, so t h a t the photochemical readion (reduction of metal ions t o the atoniic state by the photoelectrons) is not completed until higher temperatures reduce the viscosity of the glass. If the exposure is made at high temperature, or if after exposure the glass is heated instantaneously t o high temperature, violent therinal vibration destroys the latent image by returning the excited electrons instantaneously to their original equilibrium state in the parent ion before the process of diffusion permits reaction with other ions t o occur. This picture of the latent image is supported by the evidence of the presence of trapped photoelectrons which is provided by the thermoluminescence of the irradiated glass, as well as by the behavior of the latent image on rapid heating or with high temperature exposure. Development by heating is believed t o consist of two steps: capture of photoelectrons by metal ions to form atoms, and subsequent growth of metal particles, either by simple coalescence or by plating out of metal ions on contact with metal particles to which excess electrons have migrated. The photochemical reactions may be represented as follows: 1. A2
+ hu + A ( z + 1) + e*
where A2 = light-absorbing ion A of valence x, hv = a quantum of absorbed energy, and e* = an excited photoelectron. 2. Mu
+ ye*
(Mo)*
iM0
+*
where M v = metal ion of valence y (y equals unity for copper and silver, and perhaps for gold), (MO)* = metal atom with the excess energy contributed by the photoelectron, and * = excess energy, liberated as heat or light. Typical over-all reactions postulated are:
3. Gold sensitized by cerium Cef++
+ Au” + kv +r e + ” + + + Auo + *
4. Copper acting as its own sensitizer 2Cuf
+ hv --+
Cuff
+ Cuo + *
5 . Silver acting as its own senritizer 2Ag+
+ hv
Ag++
+ Ago -t *
I n its broader aspects the above theory is analogous to that given by Mees ( 7 ) for silver halide photography. I n both cases the essential feature is a photochemical reduction of metal ions t o free metal, with subsequent growth of metal particles. T h e differences in mechanism stem from the fact t h a t glass a t room temperature is a rigid nonconductor containing the photosensitive ingredients homogeneously dissolved, whereas silver halide grains permit migration of both electrons and ions and also contain inhomogeneities t h a t act as preferential centers for trapping electrons. APPLICATIONS
Photosensitive glass possesses a unique combination of valuable properties. Among these are: permanence, durability, transparency, and. other glass qualities; grainless image, exceptional fidelity of reproduction; wide range of tonal contrast; and threedimensional image. Photosensitive opal glass provides a three-dimensional lightscattering image rather than a grainless one. Photosensitive glass in the form of plate is expected to find wide use in portrait and scenic photographs, photographic murals, decorative windows, church windows, advertising displays, ornamental tile, and the like.
861
COURTESY BRADFORD BAOHRACH
Picture on Photosensitive Glass
Its lack of grain or turbidity makes i t suitable for applications requiring extremely high resolving power, such as scales or reticles in optical instruments, half-tone screens, and photographs requiring extreme enlargement by projection, such as microfilm or projection lantern slides. I t s heat resistance, durability, dimensional stability, and other characteristics fit it for industrial uses such as instrument dials, scales, patterns, push-buttons, signs, and signals. Photosensitive glass can take the form of tableware, jewelry, or lighting units. The three-dimensional image obtainable in photosensitive opal glasses should have practical applications in the lighting field. For special purposes glasses t h a t develop only a single color can be produced-blue, rod, brown, yellow, etc. It is possible to temper the glass after development of the picture, thereby increasing its strength severalfold. I n general, photosensitive glass is a new photographic medium combining the properties of glass and the uses of photography into a ncw many-sided tool for industry, art, and science. ACKNOWLEDGMENT
The writer wishes to express thanks t o his research directors, J. T. Littleton and 13. P. Hood, for encouragement and helpful advice during the investigation; to R. H. Dalton for the use of his data on copper ruby glasses; to W. H. Armistead for the use of his data on silver yellow glasses; and to the other members of the Corning Glass Works laboratory staff who have given valuable aid in the project. LITERATUFE CITED (1) Armistead, W. H. (to Corning Glass Works), Can. Patent 442,272 (June 1 7 , 1 9 4 7 ) . (2) Dalton, R. H. (to Corning Glass Works), U. S. Patent 2,326,012 (Aug. 27, 1945). (3) Ibid., 2,422,472 (June 17, 1947). (4) Gpttfried, C., Glastech. Ber., 6 , 177 (1928-29). (5) Littleton, J. T., Jr., J. Am. Ceram. Soc., 10 ( 4 ) , 259-63 (1927). ( 6 ) Littleton, J. T., Jr., J . Am. Optical Soc., 4 (4), 224 (1920). (7) Mees, C. E. K., “Theory of the Photographic Process,” New York, Macmillan Co., 1942. (8) Riedel, L., and Zsohimmer, E., Keram. Rundschau, 37, Nos. 12, 14, 16, 32, 34, 37 (1929). (9) Rooksby, H. P., J . SOC. Glass Technal., 16, 171-81 (1932). (10) Stookey, S. D. (to Corning Glass Works), Can. Patent 442,273 (June 17, 1947). (11) Ibid., 444,616 (Oct. 21, 1947). (12) Stookey, S. D., J . Am. Ceram. SOC.,32, No. 4 (1949).
RECEIVED April 16, 1948.
