US3513570A - Self-erase writing system - Google Patents

Description

United States Patent 3,513,570 SELF-ERASE WRITING SYSTEM Armand L. Dil Pare, 49 Alexander Ave., "Hicksville, N.Y. 11801 No Drawing. Filed June 25, 1968, Ser. No. 739,613 Int. Cl. B431 1/00 US. Cl. 35-66 23 Claims ABSTRACT OF THE DISCLOSURE A self-erase writing system is provided by the combination of a temperature controlled writing surface and a writing instrument. The writing instrument may comprise means for applying liquid or a solid to the writing surface, means for removing material from the writing surface, or means for holding and drawing a heated or cooled tip stylus across the surface. By control of the temperature of the writing surface at, below or above ambient temperature the writing can be made to appear, persist or to be erased. By way of example, a writing surface provided with a top porous layer is impregnated with an aqueous solution of cobalt chloride and dried to yield a blue colored writing surface. When a writing instrument provided with water or a dilute aqueous solution of cobalt chloride is employed there results pink markings on the writing surface. Erasure of the pin mark- 'ings is effected by increasing the temperature of the writing surface so as to vaporize the water or by permitting the water to evaporate at room or ambient temperature. By maintaining the writing surface at a temperature below ambient the Writing can be retained for a prolonged period of time. This self-erase writing system is based on the fact that cobalt chloride hexahydrate or an aqueous solution of cobalt chloride is pink in color Whereas anhydrous cobalt chloride is blue.

This invention relates to writing systems comprising a writing surface and a writing instrument suitable for producing a mark or writing on said writing surface. In accordance with one embodiment this invention is directed to a self-erase writing surface having a special top coating or surface and a writing instrument capable of being temperature controlled, e.g. heated or cooled, or capable of applying a special material to the writing surface such that when the writing instrument is applied to the Writing surface a mark or writing appears, the mark or Writing being temperature sensitive such. that by changing the temperature of the writing surface the writing can be made to disappear.

Writing systems having some self-erase capabilities are 'known. For example, Bolsey, US. 3,067,530 (1962) discloses a writing system comprising a silicone-treated writing surface, the writing surface being provided with heating elements. The writing material employed by Bolsey is a meltable greasy material. When the greasy writing material is applied to the silicone treated writing surface at ambient or room temperature a mark or writing appears. When it is desired to remove the mark or writing from the writing surface, the writing surface is heated by means of the heating elements incorporated in or associated with the writing surface to melt or render fluent the greasy material previously applied to the writing surface with the result that the applied greasy material flows off the writing surface and is removed. After a suitable cooling period the writing surface would again be in a condition to receive additional greasy material for writing thereon.

Writing systems having a self-erase capability known heretofore have not been completely satisfactory. Such systems for the most part have had rather limited usefulness and have been employed frequently as toys.

It is an object of this invention to provide a self-erase 3,513,570 Patented May 26, 1970 writing system having improved capability and usefulness.

It is another object of this invention to provide a temperature controlled self-erase writing system employing a special combination of a writing surface and a material for application to said writing surface to produce a marking or writing thereon, the resulting marking or writing on said writing surface being eraseable therefrom, such as by control of the temperature of the writing surface.

It is another object of this invention to provide a selferase writing system employing a special combination of a writing surface and a material for application to said writing surface to produce a marking or writing thereon wherein the writing surface in the absence of said Writing material has one color and another color in the presence of said writing material, said other color due to the application of said writing material to said writing surface be ing transient.

How these and other objects of this invention are ac complished will become apparent in the light of the accompanying disclosure. In at least one embodiment of the practice of this invention at least one of the foregoing objects will be achieved.

The self-erase writing system in accordance with this invention comprises a special combination of writing surface and writing instrument for the application of a material onto the writing surface to produce a marking or Writing on the writing surface. A characteristic of the special combination of writing surface and material to provide the writing thereon resides in the capability of the operator or writer to produce a writing which, in accordance with one embodiment of the practice of this invention, can be erased or retained on the writing surface by control of the temperature of the writing surface. The writing surface is selected in cooperation with the writing material and/ or writing instrument such that when the writing instrument and/or writing material is applied to the writing surface a readily distinguishable mark or writing appears thereon. Desirably, in accordance with this invention the color of the writing surface is different from the color of the writing material applied thereto. When it is desired to erase the writing the operator controls the temperature of the writing surface so as to effect removal or erasure of the writing. This erasure or removal can be effected by changing the temperature of the writing surface so as to alter the applied writing material or its color so that the writing originally applied to the writing surface is no longer distinguishable against the background color of the writing surface.

In accordance with one embodiment the self-erase capability of the writing system in accordance with this invention is due to temperature control of the writing surface. In accordance with this embodiment the writing surface may comprise or have associated therewith heating elements, such as electric heating elements, for increasing the temperature of the writing surface above that temperature, which may be ambient or room temperature or below ambient or room temperature, at which the writing material was initially applied. Also, in accordance with this embodiment of the invention the Writing surface may comprise or have associated therewith suitable thermo-electric elements capable of increasing or reducing the temperature of the writing surface depending upon the polarity employed in the operation of the thermo-electric elements to increase or to decrease the temperature of the writing surface above or below the temperature of the writing surface which may, in turn, be above or below ambient or room temperature.

Various techniques in accordance with this invention may be employed to effect the writing on the writing surface. For example, in accordance with one embodiment, hereinafter referred to as W1, the writing instrument may employ a fluid or liquid which is deposited upon a suitable writing surface to generate a visible mark or writing thereon. A suitable such instrument might be a felt nib pen or brush. The writing instrument in accordance with another embodiment, hereinafter referred to as W2, might comprise a solid material which is deposited or applied to the writing surface when the writing instrument is applied thereto so as to generate a visible mark or writing thereon. A suitable such writing instrument might comprise a piece of solid, chalk-like material or crayon-like material. In still another embodiment of this invention, hereinafter referred to as W3, the writing instrument might comprise a stylus which when applied to the writing surface is capable of removing a portion of the writing surface so as to create a visible mark or writing thereon or might comprise a heated or cooled tip stylus which when drawn across a thermo-sensitive writing surface creates a visible mark or writing thereon.

In the self-erase writing system in accordance with this invention temperature control in the erase mode can be employed to erase the writing or to cause the writing to disappear or not be observable on the writing surface. In accordance with one erase mode, hereinafter referred to as E1, in the practice of one embodiment of this invention temperature control or change in temperature serves to effect an actual removal of the writing, such as by change of state of the writing material employed in the writing. For example, by control of the temperature of the writing surface during the erase mode, depending upon the characteristic of the writing material, the writing can be erased or caused to disappear by sublimation or evaporation or volatilization of the writing or writing material from the writing surface. In accordance with another erase mode, hereinafter referred to as E2, temperature control effects erasure or removal of the writing by change of color of the writing to match that of the background color of the writing surface or to be transparent with the result that the writing is erased or disappears. This change of color may be effected by chemical reaction initiated or brought about by change in temperature of the writing surface to a temperature above or below the temperature at which the writing was initially applied.

In accordance with yet another mode, hereinafter referred to as E3, erasure or removal of the writing from the writing surface, such as a writing produced by drawing a stylus (W3) across the writing surface, involves either replenishment of the removed material due to chemical reaction or physical effect brought about or initiated by the change in temperature or removal of a surface layer from the writing surface such that the writing is effectively erased.

In the write-erase modes in a self-erase writing system in accordance with this invention employing temperature control or thermal cycling of the surface, six combinations of temperature control or thermal cycling modes of the writing surface are possible, as indicated in accompanying Table I.

TABLE I.-THERMAL CYCLING MODES Writing Surface Temperature T2 T3 T4 T5 T6 Cold Write-.. Erase. Write"? Erase .fimbient Erase-" Write Write... Erase.

0t Erase". Write". Erase". Write.

mode may be employed in the practice of this invention and are exemplary thereof.

A self-erase writing system involving the modes E1-W1 and T1 wherein the writing instrument may be a felt nib pen containing a volatile, non-toxic liquid which evaporates rapidly at room temperature, e.g. ethyl alcohol, and wherein the writing surface is maintained relatively cold, e.g. below room or ambient temperature, so that the rate of evaporation of the applied liquid is substantially reduced'or is negligible. When the liquid is applied to the writing surface by means of the felt nib pen there would be generated a visible wet line against a suitable writing surface. The temperature of the writing surface, if desired, could be maintained sufficiently below room or ambient temperature, e.g. below the freezing point of the applied liquid, so that there would be produced, when the liquid is applied to the relatively cold writing surface, a mark made up of the frozen, normally liquid material on the writing surface. Similar to this self-erase Writing system would be the combination of modes El-W1T3 but would have a more rapid erase capability than the self-erase writing system employing the combination of modes El-Wl-Tl. Another related combination of modes in the self-erase writing system in accordance with this invention would be the combination of modes El-Wl-TS. This special combination of modes would be useful wherein the liquid employed with the writing instrument, i.e. felt nib pen, would be a relatively less volatile liquid, e.g. Water.

Another special combination of modes in a self-erase writing system in accordance with this invention would be the combination of modes El-WZ-TS as exemplified by the self-erase Writing system wherein the writing instrument is a solid material, such as a crayon or chalk, comprising non-toxic sublimable solid material, e.g. a camphor-like material, having a capability of subliming at a moderately elevated temperature, e.g. about -175" F., more or less, but which would persist or evidence little or no volatility or sublimation at ambient or room temperature. In one variation the crayon is composed of a mixture of white chalk powder and camphor-like binder. Heating sublimates camphor-like binders, removing adhesion of residual chalk powder which then falls away from the surface. A related special combination of modes in a self-erase writing system in accordance with this invention would be the combination E1-W2-T3. This combination would provide a greater selection with respect to the writing material employed and/or provide greater persistency of the applied writing material in the write mode.

Another special combination of modes in a self-erase writing system in accordance with this invention would include the combination of modes E2-W1-T5 wherein the writing instrument is provided with a low volatility liquid whose color at ambient or room temperature contrasts with the color of the writing surface. The color, however, of the writing liquid at an elevated temperature would undergo an irreversible change, such as might be brought about by a chemical reaction due to the composition of the writing surface or due to the surrounding air, with the result that the applied liquid becomes either transparent or matches the color of the writing surface. A related combination of modes would be the combination EZ-Wl-T'a'. This special combination provides for an increased temperature range with respect to the selection of the components used in the self-erase writing system.

Another special combination of modes in the self-erase writing system in accordance with this invention is the combination of modes E2-W1-T 1 wherein the writing instrument comprises a liquid which at room or ambient temperature and/or in the presence of the ambient atmosphere (air) undergoes an irreversible chemical reaction resulting in a color change of the liquid to become transparent or colorless or a color matching the color of the writing surface. Prior to use this change in color of the writing liquid could be prevented by properly containing the liquid within an air-tight enclosure within the writing instrument and the color change of the liquid when actually applied to the writing surface could be prevented by maintaining the temperature of the writing surface at a temperature below ambient or room temperature. One writing liquid which could be employed comprises a mild stable alkaline solution mixed with an unstable red-toclear phenolphthalein-like indicator which, at room ambient temperature, rapidly oxidizes on contact with air to become colorless.

A related combination of modes would be the combination E2-W1-T2. In this combination of modes the writing liquid employed would be such that a relatively cold temperature or a temperature below ambient or room temperature would initiate the color change for the erase operation. The writing liquid would contain the liquidcrystal materials recently developed by National Cash Register Co. which comprise so-called liquid crystals in microscopic capsules and which are known to have such thermochromic properties.

Still another combination of modes in the self-erase writing system of this invention is an inverse combination of modes E2-Wl-T4, i.e. a direct combination of modes E3-W3-T4. In this special combination the writing surface could be coated with a thermo-sensitive material capable of undergoing a reversible color change which may be induced by a reversible chemical reaction in the write mode to change color when contacted by a heated writing instrument such as a heated stylus. The entire writing surface, including the writing thereon, could be restored and the writing erased by returning the writing surface to a relatively cold temperature in the erase mode. Another combination is E3-W3-T3; thermochromic ma terial, cooled-tip stylus for writing and heated surface for erase. Another combination of modes is E2-W1-T6, which is releated to E2-W1-T4, except that the thermo-sensitive material is erased at room ambient temperature.

The analogous relationship of the combination of modes E2-W2-T1 through T6 to the above-described combination of modes E2-Wl-T1 through T6 is apparent in the light of this disclosure.

Yet another special combination of modes E3-W3-T1 in a self-erase writing system in accordance with this invention would be a writing surface maintained at a relatively cold temperature or at a temperature below ambient or room temperature and provided with a readily removable top layer or coating. In this special combination the writing instrument could comprise a stylus suitable for removing a portion of the top coating from the writing surface. In the erase mode the temperature could be increased to effect removal of all the remaining top coating from the writing surface. When it is again desired to write, a new top coating would be applied or be generated by thermal cycling. This special combination of modes is similar to and related to the combination E3W3T3, which obtains a more rapid, elevated temperature erase capability.

The physical structures, especially the writing surface and the writing instruments, suitable for use in a selferase writing system in accordance with this invention are varied. For example, the writing surface may comprise a thin, conventional, opaque blackboard panel made up of slate, frosted glass, hard composition board, Masonite, etc. and which may contain associated therewith suitable heating and/or cooling means for temperature control of the writng surface. For example, a writing surface in accordance with this invention may contain associated therewith or embedded therein an array of electric heating elements and/or an array of thermoelectric elements which, depending upon polarity, would be effective to heat or to cool the writing surface. Likewise, the writing materials and the writing instruments containing or dispensing the writing materials are numerous. Substantially any solid or liquid, including water, which is non-toxic and non-irritating might be employed as the writing material in a self-erase writing system in accordance with this invention.

The following examples are exemplary of the practice of this invention:

EXAMPLE NO. 1

A writing surface comprising a sheet of porous material impregnated with a cobalt chloride solution and dried is particularly useful as the writing surface in a self-erase writing system in accordance with the combination of modes E2-Wl-Tl, -T3, or -T5 of this invention. Hydrated cobalt chloride, e.g. cobalt chloride hexahydrate, and an aqueous solution of cobalt chloride are pink in color, and solid, anhydrous cobalt chloride is blue in color. In accordance with this invention a writing surface provided with a porous top coating impregnated with anhydrous cobalt chloride would present a blue color. When an aqueous liquid or a dilute aqueous solution of cobalt chloride is employed as the writing material and is applied to this surface there appears thereon a pink mark or writing against the initially blue colored background of the writing surface. By maintaining the surface temperature of the writing surface below room or ambient temperature (modes-Tl or -T3) this writing can be retained for a substantial period of time. When it is desired to erase the writing, the temperature of the writing surface is increased, such as by means of electric heating elements associated with the writing surface to evaporate the water and to dry the writing surface (modes -T3 or -T5). When dried the writing surface is returned to its original blue color. It is possible in accordance with one embodiment of the practice of this invention to permit the writing surface to dry without employing heat from the heating elements associated with the Writing surface (mode -T1). By supplying heat from. the associated heating elements the erase time can be shortened.

EXAMPLE NO. 2

This self-erase writing system is similar to the selferase writing system described in Example No. 1 but differs therefrom in being based on cupric sulfate. Hydrated cupric sulfate, such as cupric sulfate pentahydrate, and aqueous solutions of cupric sulfate are blue whereas anhydrous cupric sulfate is white. By applying an aqueous liquid to a writing surface provided with a top coating impregnated with anhydrous cupric sulfate, the top coating having a substantially white color, there would be generated on the writing surface a blue mark or writing which could be erased by evaporating the water from the writing surface, thereby restoring the writing surface to its initial substantially White color.

EXAMPLE NO. 3

In a self-erase writing system in accordance with the combination of modes E2-Wl-Tl, -T3, T5, the writing material is a liquid made up of a dilute aqueous ammonium hydroxide solution containing phenolphthalein. The color of this solution is red. Accordingly, when the solution is applied by means of a suitable writing instrument to a white or other suitably colored writing surface a red line appears thereon. If desired, phenolphthalein may be incorporated in the writing surface with the result that when a dilute aqueous ammonium hydroxide solution is applied thereto a red line would appear. This red writing, in both instances, would be removed by heating the writing surface to a suitable temperature, e.g. above ambient temperature, to drive off ammonia from the ammonium hydroxide solution thereby returning the solution to a substantially neutral pH and rendering the solution colorless. If desired, reliance could be had upon the ambient temperature to eifect a gradual but more prolonged volatilization of ammonia from the applied solution, thereby in time leading to the erasure or the disappearance of the writing.

EXAMPLE NO. 4

In a self-erase writing system in accordance with this example the writing material is a solution comprising iodine and starch. At ambient or room temperature this solution is blue. Accordingly, when this solution is applied by means of a suitable writing instrument to a white or other suitably colored writing surface a. blue line appears. When, however, this solution is heated to an elevated temperature, such as about 160 F the solution loses its color. In an erase mode in accordance with this embodiment of the invention the blue writing is removed from the writing surface by heating the writing surface together with the blue writing appearing thereon to a suitable elevated temperature, e.g. about 160 F. to cause the writing to disappear.

EXAMPLE NO. 5

In a self-erase writing system in accordance With this embodiment of the invention the writing surface is provided by a thin sheet of iron which is maintained below ambient temperature during the writing mode. The writing material employed is an ink made up of dilute, acidified aqueous solution of potassium permanganate which when applied to the relatively cold or cooled sheet iron writing surface produces a deep purple marking or writing. When it is desired to erase this writing the sheet iron Writing surface is permitted to reach room or ambient temperature or is heated by suitable means to room temperature at which temperature the iron reduces the potassium permanganate and changes the color of the potassium permanganate solution from purple to a clear, transparent solution.

combination of modes E2-W2-T5 there is provided a writing surface with a dark gray color. The writing material is a chalk-like material made up of a mixture of cupric hydroxide which is a bluish-white gelatinous solid, and a white chalk binder. When this material is applied to the dark gray writing surface a substantially white marking or writing is produced. When the writing surface is heated the cupric hydroxide thereon decomposes to black cupric oxide. The remaining chalk deposited along with the cupric oxide then tends to blend into the background color with the result that the writing becomes indistinguishable or, in effect, disappears from the writing surface.

EXAMPLE NO. 7

A self-erase writing system in accordance with this embodiment is similar to that of Example No. 6. In this embodiment, however, the chalk-like material is made up of a mixture of readily thermally decomposable carbohydrate together with a white chalk binder. When this material is applied to the writing surface having adark gray background a white writing appears thereon. This writing can be erased by heating the writing surface to a suitable temperature to effect decomposition of the organic compound which, upon decomposition, changes to a substantially black color. When this occurs the chalk writing is effectively erased or disappears from the surface.

EXAMPLE NO. 8

In aself-erase writing spstem in accordance with the combination of modes E2-W2-TS there is provided a writing surface having a blue color. The writing material is a chalk-like material made up: of a mixture of ammonium chloride, calcium hydroxide and dry red litmus. When this chalk-like material is applied to the blue colored surface a red marking or writing is produced. When, however, the writing surface is heated the resulting induced chemical reaction between ammonium chloride and cal- 8 cium hydroxide produces moist ammonia which turns the litmus to a blue color thereby effectively erasing or causing the red line to disappear \from the writing surface.

EXAMPLE NO. 9

In a self-erase writing system in accordance with this embodiment of the invention there is employed a thermochromic compound having a normal color which is different from the color of the writing surface. When the thermochromic compound is applied to the Writing surface a marking or writing appears. When, however, the writing is to be erased, the temperature of the writing surface is increased to cause the thermochromic material to irreversibly change color to the color matching or blending with the color of the writing surface thereby causing the writing to disappear. Numerous organic thermochromic materials are known and are suitable for use in the practice of this invention, the selection of the thermochromic material being dependent upon the background color of the Writing surface.

As will be apparent to those skilled in the art in the light of the foregoing disclosure, many modifications, alterations and substitutions are possible in the practice of this invention without departing from the spirit or scope thereof.

I claim:

1. A self-erase writing system comprising a writing surface provided with a top coating of water sensitive material, said water sensitive material having one color in the anhydrous state and another color in the hydrated state or in the presence of Water, a writing instrument suitable for applying an aqueous liquid to the writing surface and temperature control means for controlling the temperature of the writing surface.

2. A self-erase writing system in accordance with claim 1 wherein said water sensitive material is cobalt chloride.

3. A self-erase writing system in accordance with claim 1 wherein said water sensitive material is cupric sulfate.

4. A self-erase writing system comprising a Writing surface, a writing instrument suitable for applying thermochromic material to said surface, said thermochromic material when applied at one temperature to said writing surface having a color different from that of said writing surface and when at another temperature above or below said one temperature having a color substantially the same as the color of said writing surface, the change of color said thermochromic material undergoes from said one temperature to said other temperature being irreversible, and temperature control means for controlling the temperature of said writing surface.

5. A self-erase Writing system in accordance with claim 4 wherein said one temperature is ambient temperature.

6. A self-erase writing. system in accordance with claim 4 wherein said one temperature is ambient temperature and wherein said other temperature is above ambient temperature.

7. A self-erase writing system comprising a writing surface, a writing instrument suitable for applying to said writing surface a sublimable material, said sublimable material when applied to said writing surface having a color different from or contrasting to the color of said writing surface and means for controlling the temperature of said writing surface and for heating said writing surface to a temperature at which said material applied thereto sublimes.

8. A self-erase writing system comprising a Writing surface, a writing instrument suitable for applying a liquid to said writing surface to produce writing thereon, said liquid when initially applied to said writing surface at a temperature T having a color different from or contrasting to the color of said writing surface and temperature control means for bringing said writing surface to a temperature T said liquid at said temperature T being characterized in that at said temperature T the writing made by said liquid disappears from said writing surface.

9. A self-erase writing system in accordance with claim 8 wherein said writing surface is provided by a thin sheet of iron, wherein said liquid is a dilute aqueous acidic solution of potassium permanganate, wherein the temperature T at which said material is initially applied to said writing surface is below ambient temperature and wherein the temperature T is ambient temperature.

10. A self-erase writing system comprising a writing surface, a writing instrument suitable for applying material to said surface, said material when initially applied to said writing surface having a color different from or contrasting to the color of said writing surface so that writing appears thereon and when at a temperature other than the temperature at which said material is initially applied to said writing surface said material irreversibly changes to a color substantially the same as the color of said writing surface and means for controlling the temperature of said writing surface.

11. A self-erase writing system in accordance with claim wherein said material is a heat sensitive liquid.

12. A self-erase writing system in accordance with claim 10 wherein said writing surface has a substantially dark gray color, wherein said material is a mixture of cu ric hydroxide with a white chalk binder thus providing a writing which appears as white colored writing on the Writing surface and wherein said other temperature is a temperature above ambient temperature to decompose said hydroxide to black cupric oxide, 13. A self-erase writing system in accordance with claim 10 wherein said writing surface has a dark gray color, wherein said material is a mixture of a thermally decomposable organic compound together with a white chalk binder thus providing a writing which appears as white colored writing on the writing surface and wherein said other temperature is a temperature above ambient temperature sufiicient to decompose said organic compound.

14. A self-erase writing system in accordance with claim 10 wherein said writing surface is blue and wherein said material is a mixture of ammonium chloride, calcium hydroxide and dry red litmus to provide red writing when said material is initially applied to said surface.

15. A self-erase Writing system in accordance with claim 10 wherein said material is tincture of iodine and a starch solution and wherein said other temperature is above the temperature at which said material is initially applied to said writing surface.

16. A self-erase writing system comprising a writing surface, a writing instrument suitable for applying material as a writing on said surface, the material applied as a writing on said surface being eraseable therefrom upon change of temperature of said surface due to the volatilization of said material from said surface and temperature control means for controlling the temperature of said writing surface.

17. A self-erase writing system in accordance with claim 16 wherein said material comprises a dilute ammoniacal solution of phenolphthalein.

18. A self-erase writing system comprising a writing surface provided with a top coating of a substantially anhydrous water sensitive material, said water sensitive material having one color in the substantially anhydrous state and another color in the substantially hydrated or water-wet state or in the presence of water and a writing instrument suitable for aplying an aqueous liquid to the writing surface.

.19. A self-erase writing system comprising a writing surface provided with a top coating of reagent sensitive material, said reagent sensitive material having one color in the absence of said reagent and another color in the presence of said reagent, said reagent being a liquid having a vapor pressure substantially the same as that of water at about room temperature so that the color imparted to said reagent sensitive material when said reagent is applied thereto is transient and a writing instruinent suitable for applying said reagent to the writing surace,

20. A self-erase writing system comprising a writing surface provided with a top coating of a reagent sensitive material, said reagent sensitive material having one color in the presence of said reagent and another color in the substantial absence of said reagent, said reagent being volatile at about room temperature when applied to said surface so that the color imparted to said reagent sensitive material upon the application of said reagent thereto is transient and a writing instrument suitable for applying said reagent to said writing surface. 21. A self-erase writing system comprising a writing surface provided with a top coating of a reagent sensithe material, said reagent sensitive material having one color in the presence of said reagent and another color in the absence of said reagent, the color imparted to said reagent sensitive material in the presence of said reagent being transient and a writing instrument suitable for applying said reagent to the writing surface.

22. A self-erase writing system in accordance with claim 21 wherein the change in color of said reagent sensitive material due to the presence or absence of said reagent is reversible.

23. A self-erase writing system comprising a writing surface provided with a top coating of thermochrornic material, said thermochrornic material having one color at one temperature and another color at another temperature, the color change of said thermochrornic material from said one color to said other color being reversible, a temperature controlled writing instrument suitable for application to said writing surface to change the temperature of said thermochrornic material at the location where said writing instrument is applied to said writing surface from said one temperature to. said other temperature to change the color of said thermochrornic material at the location where said writing instrument is applied thereto from said one color to said other color.

References Cited UNITED STATES PATENTS 2,750,575 6/1956 Doty et a1. 34621 X 3,067,530 1.2/ 1962 Bolsey 35-66 3,169,064 2/1965 Levy.

EUGENE R. CAPOZIO, Primary Examiner H. S. SKOGQUIST, Assistant Examiner US. Cl. X.R. 346-21