US3537872A - Thermographic process - Google Patents

Description

United States Patent 3,537,872 THERMOGRAPHIC PROCESS Yoshio Kojima, Masaaki Yoshioka, and Isamu Fushlki, Tokyo, Japan, and Toshimi Kishida, deceased, late of Tokyo, Japan, by Hisako Kishida, administrator, Tokyo, Japan; said Kojima, Yoshioka, and Fushiki assignors to Konishiroku Photo Industry Co., Ltd., Tokyo, Japan No Drawing. Continuation-impart of application Ser. No. 345,652, Feb. 18, 1964. This application Apr. 26, 1968, Ser. No. 724,685 Claims priority, application Japan, Feb. 26, 1963, 38/9,560; June 22, 1963, 38/132,520 Int. Cl. B441? 1/10 US. Cl. 1171.7 12 Claims ABSTRACT OF THE DISCLOSURE A thermographic process in which finely powdered sulfur is exposed to infrared radiation while kept in a layer adjacent an original having an infrared-absorbing image area and a non-image area, until the image area of the original is brought to a high temperature so that the sulfur corresponding to its fuses and then allowing the image area to cool to below the melting point of the sulfur to form a latent image consisting of the fused sulfur in a supercooled state and then developing the latent image.

This invention relates to a new thermographic process, and further it relates to a composition and an element useful therein and the product obtained by the said process. This application is a continuation-in-part application of US. Ser. No. 345,652, filed Feb. 18, 1964 now abandoned.

In the prior art, there is known a method for obtaining a copy from an original bearing an infrared ray absorbing image, which method comprises placing on said original a heat-sensitive sheet bearing a heat-sensitive layer, which is normally non-tacky but can be softened by heating to leave a tacky mass even at room temperature for a certain period of time after heating, subjecting the resulting composite to infrared irradiation to form a dustable latent image in said layer, and then developing said latent image with a toner. There is also known a method in which an image-bearing original is placed on a heat-sensitive sheet having a heat-sensitive layer comprising a thermoplastic material or wax, and the composite is subjected to infrared irradiation to tackify the thermoplastic material or wax at a heated portion, the tackified portion being transferred, after a short period of time, onto another sheet to form a latent image thereon, and then the latent image is developed by means of a toner to obtain a copy, or the heat-sensitive layer has previously been incorporated with a spirit-soluble pigment and the composite is treated in the same manner as above to obtain a spirit master. Methods of the kind as mentioned above are disclosed in, for example, United States Pat. 3,259,061 patented July 5, 1966, and United States Pat. 3,260,612 patented July 12, 1966.

A principal object of the present invention is to provide a novel thermographic process in which, unlike in the above-mentioned known methods, sulfur is used as a heatsensitive material so as to utilize its marked supercooling property which is displayed when it is finely divided. Another object of the present invention is to provide a copying method by which a copy having good stability and high quality can be obtained in a simple and inexpensive manner. Still another object of the present invention is to provide a new recording method using a heating stylus or the like means. A still further object is to provide a simple method for preparing a spirit printing master or an 3,537,872 Patented Nov. 3, 1970 offset printing master. Other objects, features, characteristics and capabilities as comprehended by the present invention will be apparent from the description and claims which follow.

Primarily, this invention is based on the utilization of a supercooling (or undercooling) phenomenon occasionally observed in the solidification of fused substances. As the result of our study on the fusion and solidification of crystalline substances, it has been found that if the fused mass is in the form of very fine particles, it can be kept in the supercooled liquid state at a relatively lower temperature below its melting point for a considerably longer period of time than when the material is initially in a relatively coarse form.

In accordance with the process of the present invention, finely powdered sulfur which melts at about to C., leaving a liquid or semi-solid mass, is subjected to imagewise exposure to heat radiation while the sulfur is kept in a layer adjacent to an original bearing thereon an image to be reproduced. The image area of the original is brought to a high temperature, so that the sulfur corresponding to the image area fuses to form a latent image consisting of the fused material. By selection of an optimum particle size of the sulfur, the fused mass re sulted above can be maintained in the supercooled state for a period of from about ten seconds to about ten hours even at an ordinary temperatures. Therefore, the latent image while it is in the supercooled state having a dustadhering property can be readily developed by treating it with a finely powdered coloring material a (so-called toner) to give a visible image which may be fixed by after-treatment.

In addition to sulfur, there are other known substances which once melted or softened, display tackiness for a short period of time even at room temperature that is below the melting or softening points thereof, and thermographic processes using such substances are well known, as mentioned previously. However, thermoplastic resins, waxes and the like substances, which have been used in the prior art, are far inferior to sulfur in supercooling stability (that is, the period of time during which a dustable latent image can be formed by supercooling), the amount of dust adhered, the case of distribution over sheet surfaces, the stability in quality of the heat-sensitive sheet during storage, image resolution and contrast, tolerability the infrared irradiation. and cost.

That is, finely divided sulfur displays markedly high supercooling stability and, on the surface of tracing paper, sulfur particles of 0.1 mm. in diameter show a dustable supercooled state over a period of several hours at room temperature. The substances disclosed in the aforesaid patents cannot maintain tackiness for more than a few minutes. When fused, sulfur particles are high in surface tension, and therefore even when melted on the surface of paper or even when transferred onto the surface of paper, they do not penetrate into the tissues of the paper but exist in a spherical form on the upper surface. This results in such advantages that at the time of dusting, the toner adheres more strongly and the image is less blurred. Sulfur is low in self-adhesiveness, unlike waxes or resins, and hence can be relatively easily divided into fine particles. Further, in preparing a heat-sensitive sheet by rubbing an untreated paper with a heat-sensitive material having the aforesaid properties, sulfur can be uniformly distributed and results in no substantial generation of fog due to adhension of dust to other portions than the image portions. In addition, sulfur is diflicultly soluble in water and in most of organic solvents and hence is advantageously applied in the form of a dispersion, and a desired binder for this purpose can be freely selected. A method, in which a solution of a heat-sensitive material is applied onto the surface of a sheet, suffers from such a drawback that, in removing and drying the solvent, large crystals of the heat-sensitive material are liable to grow. If the amount of binder is increased, said crystal growth can be prevented, but the dust-adhering property is lessened. At those portions Where the large crystals were deposited large molten masses are liable to form, with the result that the supercooling property is undesirably lessened. Sheets over which sulfur is distributed do not deteriorate in properties even when stored in air for several years and do not cause such phenomenon as sticking to each other when stored in piles which occurs in the case of heatsensitive sheets prepared by the use of certain kinds of waxes or thermoplastic resins. The process of the present invention, more particularly the process comprising distributing sulfur on the infrared ray-absorbing image-bearing surface of an original placing the original on another sheet, subjecting the composite to infrared irradiation to form on the sheet a corresponding supercooled latent image of sulfur, and then developing said latent image with a. toner, ether as such or after thermal transfer of the image onto another sheet in order to change said image into a correct image, is entirely free from the thermal diffusion of any substance from the infrared ray absorbing image, so that the image of the resulting copy is extremely clear and far better than images formed according to the aforesaid known methods. That is, in effecting the copying of an original image printed with printing types of the Futura Bold type, for example, the process of the present invention can substantially faithfully copy even in the case of such small type as 105- point. According to the aforesaid known methods, however, copying of detailed portions becomes impossible in the case of printing types smaller than 14 point. Further, in effecting the copying of an original printed with both large types of the Futura Bold type and small types of the Baskerville type, the process of the present invention can faithfully reproduce both of the two printing types, whereas the aforesaid known methods result in inferior reproduction of either one of the two printing types even if the dose of infrared irradiation has been suitably selected. This results from the fact that the present process is far more tolerable to infrared irradiation. Further, a great ad vantage of the present process is that sulfur, which is used as the heat-sensitive material, is markedly inexpensive. Moreover, if, in the production of heat-sensitive material, sulfur is added in the sizing or top-coating step of paper, the heat-sensitive materials can be produced at markedly low costs without adding a separate step for the distribution of sulfur. Thus, the present process is far more advantageous than the aforesaid known methods which require the use of organic solvents.

As stated previously, the supercooling stability of sulfur becomes markedly greater with decreasing particle size. At a certain limit of particle size, the lifetime is expressed in parallel to the vertical axis, and this indicates that if the material having a particle size below the critical limit the supercooled state can be maintained as long as several hours.

The fusing conditions for the finely powdered sulfur used as the heat-sensitive material is another important factor by which a durability of the supercooled state is influenced. -If a substance is cooled immediately after completion of its fusion, the stability of the supercooled state obtained will be insufiicient because of the residual crystalline texture in the fused mass. In contrast to this, when the sulfur is heated to a temperature greatly exceeding its melting point or when the melt of said sulfur is cooled after said sulfur has been kept at or above its melting point for a substantial period of time, the stability of the supercooled state of the resulted melt will be remarkably increased. The latter case not only greatly differs in the time for nucleus formation but also in the velocity of crystallization following the nucleus formation. In a copying operation the precise temperature reached by the melt of the sulfur is not known. However, the experimental facts have taught us that said melt would be brought to a temperature considerably higher than its melting point.

A toner usable for development is prepared by mixing a pigment into a resin as a binder which either may melt at a comparatively low temperature or may be easily dissolved in an organic solvent and then pulverizing the resulted mixture to a particle size of Sl00 microns. Generally, a toner usable for an electrostatic photographic process may be employed for development. By selecting a pigment exhibiting a suitable color, a copy having a desired color tone may be obtained. Further, if a suitable resin is selected, a toner usable as an offset printing master can be obtained, and if a pigment soluble in a spirit type solvent is selected, a toner usable as a spirit printing master can be provided.

In carrying out the infrared radiation in the thermographic process of the present invention, it is convenient to use a quartz infrared lamp as a heat source. The radiation period should preferably be as short as possible in order to have an effective utilization of the heat taken up by the image area of an original, while avoiding heat loss due to transport to the nonimage area, and this is accomplished by employing a heat source having a high output. In general, when a quartz infrared lamp of 30-100 watts/ cm. output is used, appropriate infrared radiation can be made at a distance of 1 cm. from the radiation source for 0.2-1.0 second. A Thermofax copying machine (man ufactured by Minnesota Mining & Mfg. Co.) may be conveniently used for this purpose. In a specific instance in recording, heat can be directly applied imagewise, for example by using a heating stylus.

In one embodiment of the present invention, finely powdered sulfur may be used in a layer on a suitable support separate from an original. In other words, a thermographic element comprising a support and a layer of sulfur is used in the instant embodiment. In order to prepare such a thermographic element, the following general procedures will be in order: The sulfur, together with the small amount of a binder, is prepared as a solution or dispersion which is then applied or sprayed onto the surface of a support. Suitable supports are paper, or metallic film or plate, etc. Alternatiwely, if a support having a somewhat rough surface is used, finely powdered sulfur as fine as 1-100 microns in particle size can be rubbed onto the said surface to produce a useful thermographic element. Alternatively, the sulfur may be either incorporated into paper-making material or top-coated over the surface of paper, in order to have a thermographic copying paper comprising an internal layer of the heat-sensitive material. Still further, it is possible to have sulfur deposited on a paper web as a thin, uniform heat-sensible layer in vacuo. In applying of finely powdered sulfur onto a support, a small amount of finely powdered silica or titanium dioxide or the like white pigment can be used in combination in order to provide a copying sheet which can give a high contrast, clear copy.

In the practice of the present invention, the thermographic element which can be prepared by any of the above-mentioned procedures is brought into contact with an original and the composite is subjected to exposure of infrared radiation thereby to form on the element a latent image which can be developed with a toner and then fixed by heat or solvent vapor treatment. Depending on the type of support, either or both front-copying and backcopying are possible.

In another embodiment of the present invention, finely powdered sulfur is distributed directly onto the imagebearing surface of an original by lightly rubbing said surface several times with an absorbent cotton, felt, cloth or the like containing the fine sulfur powder which has been pulverized down to a particle size of 0.1- microns, whereby the surface of said original is furnished with the sulfur in a sufiicient amount to effect the copying of more than ten copies. An alternative method for forming a heat-sensitive layer directly on an original comprises dissolving finely powdered sulfur in a volatile solvent, spraying the resulted solution in an aerosol form onto the image-bearing surface of an original or coating said solution onto the image-bearing surface of an original with brush, cloth, felt, absorbent cotton or the like soaked with said solution, and drying the original which is then ready for copying. Particularly in the latter case, it is important to be careful that ink of the letters or figures on the surface of an original should not be dissolved and thus caused to blot the surface. In still another modi fication, a heat-sensitive material may be vaporized and the vapor applied to the surface of an original.

The original thus pretreated is brought into contact with a suitable receptor on to which a latent image of the supercooled sulfur is to be transferred, and then the composite is subjected to infrared radiation, whereby the sulfur corresponding to the image area is fused and transferred to the receptor so as to form thereon a latent image consisting of the sulfur in the supercooled state.

Suitable as receptor materials are paper and metal plates, and they may be selected according to the purpose. As a matter of course, the manner of infrared radiation will depend on the type and nature of receptor materials. For example, when an original bearing the sulfur and a receptor material are brought into face-to-face relation and the composite subjected to infrared radiation from the side of the receptor (reflex copying), said receptor should exhibit sufficient transmission for infrared radiation that the heat uptaken by the composite can melt or decompose the heat-sensitive material. As a receptor fit for such a purpose a thin sheet of white or colored paper, e.g., tracing paper or glassine paper, is generally used. As a receptor which will be used as an intermediate master for a toner image transfer process a material having a more smooth surface is preferred. Meanwhile, when the original and a receptor are brought into face-to-face relation and the composite is subjected to infrared radiation from the side of an original (front copying), the copying is free from the restriction depending upon the thick ness or heat transmission of said receptor.

I Generally in case of such a copying method as producing a mirror image, for instance where finely powdered sulfur on an original is transferred and deposited on the receptor surface by fusion the said mirror image can, when subjected to toner transfer procedures, be reproduced as a correct image on a separate receptor material. Otherwise, the mirror image may look a correct one if it is looked at from the back surface of said receptor. Alternatively, finely powdered sulfur is distributed onto the back surface of an original, and the original thus treated is placed with an appropriate receptor in the back-to-face relation and then the composite is treated to form a latent image. Then, the receptor onto which the image has been transferred is subjected to development by powder treatment to obtain a copy bearing a correct image.

In a specific embodiment of the invention, finely powdered sulfur is rubbed by means of a butt roll or added during a top coating or paper-making step into a thin sheet of paper, e.g., a tracing paper. The back surface of a paper thus treated, which back surface does not carry the finely powdered sulfur, is hand-written in black ink to prepare an original. Then, the original and a receptor are subjected to front copying to obtain a copy bearing a correct image. This embodiment is advantageous particularly in Asian countries where hand-written documents prevail.

In still another specific embodiment of the present invention, transfer of a latent image of finely powdered sulfur may be carried out in the following manner. A surface of an original, on which finely powdered sulfur has been distributed, is brought into face-to-face relation with a thin paper. The composite thus prepared is subjected to infrared radiation to form a latent image of the supercooled sulfur on the thin paper. Then, the thin paper is separated from the original and the latent imagebearing surface of the thin paper is brought into face-toface relation with another thin paper. This composite is heated to transfer a part of the latent image on the first thin paper onto the second thin paper. Both the thin papers are individually developed with a toner and fixed to obtain clear copies bearing a mirror image and a correct image, respectively.

In addition, reference will be made to another specific embodiment in which the composite comprising a heatsensitive sheet, a sheet of thin paper and an original is irradiated with infrared ray to obtain a mirror imagebearing copy and a correct-image-bearing one simultaneously. For example, on the surface of a heat-sensitive sheet over which finely powdered sulfur is distributed, another thin paper is placed in face-to-face realtion therewith. The resultant composite is further brought into faceto-back relation with an original and both are subjected to infrared radiation, to obtain latent images of the supercooled sulfur on the copying sheet and the thin paper, respectively. Then, the latent images as formed are individually developed with a toner and fixed to obtain a correct image on one and a mirror image on the other. The type of an image obtained on the heat-sensitive sheet or on the thin paper depends upon the position of the original relative to the composite at the time of the infrared radiation being eifected.

In still another specific embodiment of the invention, an image-bearing original is brought into contact with two or three superposed sheets of thin papers each of the said papers being treated on one side with finely powdered sulfur, and then the total composite is subjected to infrared radiation, whereby a portion of the sulfur which corresponds to the image area of the original is selectively fused and supercooled to form a latent image. This embodiment is convenient since it makes it possible to obtain several copies at once.

The present invention is an entirely novel process for copying, which succeeds in overcoming several disadvantages encountered in the prior art. Namely, according to the process of the present invention, a dark room is not necessary and, as it is a dry process, a dry copy may be rapidly obtained. Further, since in the present process a copying material of a special kind is not used and a copied image may be formed on a, paper or metallic sheet, merely if its surface is fiat, according to the purpose, the process of the present invention is applicable for various purposes and to great advantage. In case of copying a document, the most usual case of the application of the present process, the required expense is very low as compared with that of any of the conventional processes since the former does not necessitate use of a specific copying sheet which has to be prepared by complicated and expensive procedures, but only the use of unprocessed paper or the like as a receptor and a very little amount of finely powdered sulfur distributed on the surface of an original. If necessary, a heat-sensitive sheet may be used in the thermographic process of the invention, said heat-sensitive sheet being obtainable by distibution of finely powdered sulfur over a paper. Generally, a light-sensitive or heat-sensitive copying sheet as used in a conventional process is apt to deteriorate during storage and therefore care should be taken concerning storage. In contrast, the present process of the invention is quite free from such a care. Further, a copy fall from coloration or fading may be prepared according to the present process, since as visible image is formed by treatment with a toner containing a stable pigment. A further advantage of the present invention is that a thermographic process of the invention is effective for improvement in the contrast of an image to be reproduced that is to say, said process allows us to obtain, from an original with poor contrast, a high contrast copy which may, in turn, be used as a second original.

The following examples describe certain ways in which the principle of the invention has been applied, but the examples are not to be construed as limiting the scope of the invention.

EXAMPLE '1 Crystalline sulfur (M.P. 119 C.) is pulverized to an particle size of about 1-100 microns by means of a grinder. Five grams of the resulted powder is added to 100 cc. of a 1% gelatine solution and ground in a ball mill for 24 hours. The liquid dispersion thus prepared is coated on a thick tracing paper. The amount of sulfur used is about g./m. After the paper is dried, it is passed through a calendar roll to prepare a copying sheet. The copying sheet is brought into back-toface relation with the message-bearing surface of an original and the composite is then subjected to infrared radiation from the side of the copying sheet by means of a 1.5 kw.-output quartz infrared ray lamp with a length of 20 cm. at the distance of 1 cm. therefrom, while it is passed at the rate of 5 cm. per second. After radiation, the original and the copying sheet are separated. The heat-sensitive surface of the copying sheet is subjected to development with a Konifax toner (the trade name of a toner prepared by Konishiroku Photo Ind. Co., Ltd. in Japan) to produce a very clear image. Again, the developed image is fixed by infrared radiation by means of said infrared ray lamp at the distance of 1 cm. therefrom while it is passed at the rate of cm. per second or by treatment with trichloroethylene, thereby to obtain a copy. Otherwise, when a copying sheet and an original are brought into the face-to-face relation and subjected to infrared radiation to obtain a copy having a clear mirror image, this copy is conveniently used as an intermediate master in a diazo or blue printing process, because it is possible to place the imagebearing surface of this copy in a contact directly with the light-sensitive surface of a diazo or blue printing sheet so as to yield a print having an image of high resolution. If 2 g. of titanium dioxide be added in preparing the liquid dispersion of sulfur, the copy as obtained is improved in contrast, with the result that a better copy may be obtained.

EXAMPLE 2 Five grams of finely powdered sulfur, together with 1,000 cc. of a 1.0% methyl ethyl ketone solution of ethyl cellulose, is ground for 40 hours in a ball mill to prepare a liquid dispersion. This liquid dispersion is coated on a tracing paper. In this case, the amount of sulfur applied is about 2 g./m. The copying sheet and an original are brought into face-to-face relation, subjected to infrared radiation, development and fixation to give a clear copy. Otherwise, after placing an original and a heat-sensitive copying sheet in face-to-back relation, another ordinary typewriting paper is placed on the surface of the copying sheet. The resulting composite is subjected to infrared radiation to obtain a latent mirror image on the ordinary typewriting paper and a correct latent image on the copying sheet, respectively. These images are developed and fixed and the copy bearing the correct image may be used immediately and the copy bearing the mirror image may be used as an intermediate master in a diazo process to obtain many copies thereof.

EXAMPLE 3 Commercially available sublimed sulfur (M.P. 119 C.) is lightly rubbed onto an original by means of a buff roller. The original and a tracing paper are brought into face-to-face relation and the composite is subjected to infrared radiation from the side of the tracing paper by means of a 2.3 kw.-output quartz infrared ray lamp with a length of 50 cm. at a distance of 1 cm. while the composite is passed at the rate of 10 cm. per second. After radiation, the original and the tracing paper are separated. Then, the latent image formed on the surface of the tracing paper is developed with Konifax toner PP to obtain a very clear image. The developed image is again irradiated by means of said infrared ray lamp at the.distance of 1 cm. therefrom while it is passed at the rate of 15 cm. per second, or treated with trichloroethylene .vapor, thereby to fix said image. The image as prepared is a mirror one, but it can be observed as a correct image if looked at fromthe back surface. The obtained copy which bears a mirror image may be used as an intermediate master in a diazo or blue printing process, whereby many copies thereof having excellent resolving power can be obtained.

EXAMPLE 4 A type-written original is rubbed with finely powdered sulfur by means of a buff roll. The original is placed on a glassine paper, which is used as an intermediate, in face-to-face relation. Copying may be carried out according to the same manner as in Example 3, so that a latent image maybe formed and developed thereon. The image thus developed is brought into face-to-face relation with a thick opaque paper and the composite is subjected to infrared radiation. Transfer and subsequent fixation of a part of the melted toner are effected in a single stage, whereby the images as prepared are a correct image on the thick paper and a mirror image on the glassine paper, respectively.

EXAMPLE 5 Precipitated sulfur (M.P. 119 C. average particle diameter is about 1 micro) is rubbed onto the back surface of a transparent original as in Example 3 and said original and a spirit printing master paper are brought into back-to-face relation. The composite is irradiated with infrared rays to form a latent image. The paper is then dusted with a toner usable for spirit printing, comprising spirit-soluble pigment, thereby to obtain a mirror image on the spirit printing master paper and then fixed by heat. The spirit printing master thus obtained is usable on a spirit printing press to produce a number of copies thereof.

EXAMPLE 6 Commercially available colloidal sulfur (M.P. 119 C. average particle diameter is about 0.7 micron) is rubbed onto the image-bearing surface of an original, which is placed on a tracing paper in face-to-face relation, and the composite is exposed to infrared radiation from the tracing paper side to obtain a mirror latent image. The tracing paper bearing latent image thus obtained and a grained aluminum plate for an offset printing master are brought into face-to-face relation with a black paper placed on the back surface of said tracing paper. Then the composite is irradiated with infrared rays from the black paper side to obtain a correct latent image on the aluminum plate. The resulted image is developed with an offset toner comprising cumaron resin and fixed. The treated aluminum plate is mounted on an offset press, thereby to obtain a great number of copies thereof. On the other hand, on said tracing paper there is a latent image still having dust adhesion, so that, if it is developed with the toner and fixed, it serves as a copy bearing a mirror image.

EXAMPLE 7 On the back surface of the copying sheet of Example 1, letters or figures are handwritten in black ink to prepare an original. The surface of said paper, on which finely powdered sulfur is distributed, is placed on a suitable paper in face-to-face relation and the composite is irradiated with infrared rays from the message bearing surface side and the treatments for development and fixation are effected to obtain a correct image copy. By repetition of the above thermal transfer, dust development and fixing operation in sequence, 50 clear copies can be obtained from one original.

EXAMPLE 8 The copying sheet of Example 2 may be used for recording, since sulfur exhibits great stability during supercooling. For example, when the copying sheet is used as a recording sheet for a recording apparatus, e.g., an electrocardiograph, which uses a heating stylus, the finely powdered sulfur melts by the action of a heating stylus while it passes over the recording sheet, to form a latent image on the sheet. The latent image which is stable in the supercooled state event after a considerable period of time, can be developed and fixed to provide a recording bearing a clear and stable image.

What we claim is:

1. A thermographic process which comprises subjecting finely powdered sulfur having a particle size of from 0.1 to 100 microns to exposure to infrared radiation, while the said material is kept in a layer adjacent an original having an infrared-absorbing image area and non-image area, until the image area of the original is brought to a high temperature so that the sulfur corresponding to the image area fuses and then allowing said image area to cool to below the melting point of said sulfur to form a latent image consisting of the fused sulfur in the supercooled state, and then developing said latent image.

2. The thermographic process of claim 1 which comprises developing the formed latent image with a toner, and then subjecting the developed image to fixation.

3. A thermographic process as claimed in claim 2, wherein the said toner comprises a heat-fusible resin as the binder component and the said fixation is effected by treatment with heat.

4.. A thermographic process as claimed in claim 2 wherein the said toner comprises a solvent-soluble resin as the binder component and said fixation is effected by treatment with a volatile solvent vapor.

5. A thermographic process as claimed in claim 2, wherein said toner is an OESGt printing toner comprising a hydrophobic resin as the binder component.

6. A thermographic process as claimed in claim 2, wherein said toner is a spirit printing toner comprising spirit-soluble pigment as the coloring component.

7. A modification of the thermographic process of claim 1, which comprises subjecting the finely powdered sulfur to exposure to infrared radiation, while said sulfur is maintained as a layer on a separate sheet placed in contact with an original having an infrared-absorbing image area and a non-image area, until the image area of the originl is brought to a high temperature, so that the sulfur corresponding to the said image area fuses and then is allowed to cool to form a latent image on the said separate sheet, consisting of the fused sulfur material in the supercooled state, separating the said sheet from the original, contacting the latent image-bearing surface of the said sheet with a toner to develop said image, and then fixing the developed image.

8. A modification of the thermographic process of claim 1, which comprises subjecting a finely powdered sulfur to exposure of infrared radiation while the sulfur is kept as a layer distributed over the surface of an original having an infrared-absorbing image area and a non-image area, said original being in contact with a receptor, until the image area of the original is brought to a high temperature, so that sulfur corresponding to the image area fuses to form on cooling on the receptor a latent image consisting of the fused sulfur in the supercooled state, and then developing the latent image with a toner and fixing same.

9. A thermographic process as claimed in claim 1, further comprising placing the latent image-bearing surface into contact with a separate sheet, subjecting the resulted composite to heat treatment thereby to have the latent image transfer onto the separate sheet, developing the transferred latent image with a toner and fixing same.

10. A thermographic process as claimed in claim 1 further comprising developing the latent image with a toner, placing the developed toner image-bearing surface in contact with a separate sheet, subjecting the resulted composite to heat treatment thereby to have the toner image transfer onto the separate sheet and then fixing the image thus transferred.

11. The modification of the thermographic process of claim 1, which comprises bringing a supported, finely powdered sulfur layer into contact with a heat source, thereby to effect fusion of the sulfur to form a latent image which, while it is in the supercooled state, is de veloped with a toner and then fixed.

12. A thermographic process as claimed in claim 11, wherein said heat source is a heating stylus.

References Cited UNITED STATES PATENTS 2,503,758 4/ 1950 Murray. 2,629,671 2/ 1953 Murray. 2,880,110 3/1959 Miller. 2,999,035 9/ 1961 Sahler. 3,081,699 3/1963 GulkO. 3,196,029 7/1965 Lind. 3,259,061 7/1966 Wiswell. 3,260,612 7/1966 Dulmage et al. 3,364,858 1/l968 Kojima et a1.

MURRAY KATZ, Primary Examiner US. Cl. X.R.