US2269628A - Blackboard - Google Patents

Description

l Jan. 13, 1942. R. KoLovsKY BLACKBOARD Filed July s, 1940 :s 4sheets-shaet 1 Jan. 1.3,.1942. L.. R. KoLovsKY BLACKBOARD Filed July 3, 1940 ,3 Sheets-Sheet 2 Jam; 13, 1942. R. KoLovsKY BLACKBOARD Filed July 5, 1940 3 Sheets-Sheet 3 Z 2 ,2 y Z 2 J Z AVV/Mv@ k, 2 w 2 2 Patented Jan. 13, 1942 BLACKBOARD Leon R. Kolovsky, Chicago, Ill., assignor to Weber Costello Company, Chicago Heights, Ill., a corporation of Illinois Application July 3, 1940, Serial No. 343,825

9 Claims.

This invention relates to a composition writing board. The present invention relates to a writing board or panel adapted for use in school rooms and other places and which, while adapted to be made in colors in addition to black, such, for example, as buif and green, is of the general character of boards which have come to be known by the generic term blackboards An important object of the present invention is to provide a new and improved composition and so-called self-contained blackboard which overcomes difculties which may be experienced in connection with the prior art natural slate, composition glass, and other types of blackboards, and to provide a composition and so-called selfcontained iblackboard which is free of defects which may be experienced in the manufacture or use of prior art blackboards.

Another |object of the present invention is to provide a composition blackboard which has all of the advantages of natural slate blackboards, and of the prior art composition blackboards, but none of the disadvantages of the same while, at the same time, being relativelyl inexpensive to manufacture and lighter than such natural slate blackboards and prior art composition blackboards.

Other objects will appear hereinafter.

The accompanying drawings show typical applications for uses of the new b-lackboard, and therein:

Fig. 1 is a top plan view, partly in section, showing a typical embodiment of the new blackboard iixedly mounted in position of use upon the Wall lof a school or other building;

Fig. 2 is a fragmentary transverse vertical sectional view on line 2-2 of Fig. 1;

Fig. 3 is an enlarged fragmentary detail View of the embodiment of the new blackboard which is illustrated in Figs. 1 and 2 and illustrating a typical method of securing the same in position of use upon the wall of a school or other building;

Fig. 4 is a front elevational view `of another typical embodiment of the new blackboard showing the same mounted in a frame for use as a portable blackboard of the swinging or revolving type;

Fig. 5 is a transverse sectional view on line 5 5 in Fig. 4;

Fig. 6 is a fragmentary vertical sectional View of a modified form of the invention;

Fig. 7 is an enlarged fragmentary sectional View of the modified form of the invention which is shown in Fig. 6;

Fig. 8 is a vertical sectional view of another modified form of the invention; and

Fig. 9 is an enlarged fragmentary vertical sectional view of the form of the invention which is shown in Fig. 8.

A typical embodiment and use of the new blackboard is illustrated in Figs. 1, 2 and 3 of the drawings wherein the same blackboard is indicated generally at I and is shown as being xedly secured or fastened to a supporting wall II of a school or other building by means of suitable fastening elements su-ch as the nails I2 which are extended through the body l0 of the blackboard itself into suitable wooden anchoring or mounting strips I3 which are arranged upon or in the wall I I.

As shown, a frame or molding I4 extends around and conceals the marginal edge portions of the blackboard I, as well as the heads of the fastening elements Ior nails l2, and is secured to the supporting wall II by means of fastening strips; a chalk trough I being provided at the bottom of the blackboard I0, rand secured to the supporting wall Il, as is customary in the art.

Another typical embodiment and use of the new blackboard is illustrated in Figs. 4 and 5 wherein the same is indicated at Il and is shown as being mounted in a swinging or revolving frame I8 which is mounted in a supporting frame or pedestal I9; this form of the new blackboard being provided with Writing surfaces on both sides thereof, rather than on only one side, as in the fxedly or permanently mounted form of the new blackboard which is illustrated in Figs. 1, 2 and 3.

I have found that one of the principal reasons why a so-called self-contained blackboard has not heretofore been successfully made from rubber, mineral filler and abrasive particles, is that it has not been known what the percentage of rubber employed as an initial ingredient in the composition should be, nor the percentage of rubber relative to the mineral filler and `abrasive contents which should be employed in the composition.

I have found, however, that the ratio of rubber and mineral filler employed as initial ingredients in the composition is critical and should be not substantially more than six (6) nor less than two and one-half (2.5) parts of mineral filler to about one (l) part of rubber, by weight; the percentage of rubber employed being preferably somewhat higher (within the range stated) in the longer slabs such, for example, as those six feet in length, and being somewhat less (Within the range stated) in the shorter slabs. The aforesaid range is quite critical for the reason that if the ratio of rubber to mineral ller employed is substantially less thanthat specified the resulting composition is apt to contain insufficient rubber to bond the mineral filler content and the abrasive content. This causes the resulting composition to be difficult to mix, work and press and the resulting product or blackboard is weak and brittle. On the other hand,

if the ratio of rubber to mineral ller employed is substantially more than that specified the resulting product or blackboard has a higher degree of linear expansion and contraction than is desirable for use as a blackboard and shines easily and readily, cracks readily, causes or develops glare after repeated uses with chalk crayons, thus making the Writing thereon difficult to read, and does not have the proper bite I have found, in this connection, that the amount of linear expansion and contraction in a blackboard 0f the character which forms the sub ject matter of this application is directly dependent upon, and is a function of, the rubber content thereof, whereas the mineral filler content and the abrasive content of the blackboard aifect the linear expansion of the blackboard only Very slightly' since the amount of linear expansion and contraction in the abrasive content and in the mineral filler content of the blackboard is very slight in a temperature variation of about one hundred degrees (100 F.) which is approximately the range of atmospheric'temperature variation to which blackboards are sub-` y jected when in commercial use in schools and like places.

I have also found that if the ratio of rubber to mineral filler employed in making the new blackboard substantially exceeds the limits specified above the amount of sulphur required in the composition increases to a point where sulphuric acid or other sulphate is formed on the surface of the blackboard on exposure to light, causing a very undesirable whitish yellow discoloration or,l

nlm to form on the surface of the blackboard,

What has been said above with reference to live rubber applies also, in general, to the use of reclaimed rubber in the practice of the present invention, with some distinctions, among which are the fact that the amount of reclaimed rubber employed in making the new blackboard should not be substantially less than fifteen percent (15%) nor substantially more than twentythree percent (23%) cf actual rubber byweight of the initial ingredients entering into the composi" sition from which the new blackboard is made. I have also found that in order to make a desirable blackboard according to the practice oi the present invention it is essential that the par-,

ticle size of the mineral filler and abrasive' cn` tents entering into the composition from which the new blackboard is made should be as uniform and as close to about 325 mesh (standard Tyler screen) as it is possible to obtain the raw materials commercially and the particle size 0f"l the mineral filler and abrasive contents should not be substantially larger than about 200 mesh. I have found that if the particle size of the mineral filler and abrasive contents of the composition is substantially larger than about 200 mesh the Writing surface of the resulting blackboard is too coarse and has too much bite and if the particle size of the mineral ller and abrasive or crystalline silica contents of the composition is` substantially less than about 325 mesh the Writ` ing surface of the resulting blackboard isy too smooth. I have likewise found that if the Yparticle sizes of the mineral filler and abrasive contents of the composition from which the new blackboard is made vary, that is to say, if the A suitable formula for making the new blackboard, black in color, is illustrated by the following example in which all constituents indicated are expressed in terms of parts by weight:

Example No. 1

Forty-six (46) parts of reclaimed rubber having approximately the following composition:

ittclc-coored board) Parts (a) Rubber 23.0 (b) Clay '7.0 (c) Carbon black 12.0 (d) Whiting 4.0

(e) Plus other materials present in the the reclaimed rubber including cellulosic fiber, passive sulphur, reclaiming cils, plasticizer, etc.

It should be noted, in connection with the foregoing example. as well as other examples herein which call for the use of reclaimed rubber, that all of the initial ingredients of the reclaimed rubber vmay be allowed to remain therein when the reclaimed rubber is used in the practice of the present invention although clay and Whiting if originally present in the reclaimed rubber may, if desired, be taken out of the reclaimed rubber and other inert materials added before using the reclaimed rubber;` or, if desired, reclaimed rubber made from used or old rubber bands and like material in which no clay is employed may also be used as, for example, when it is desired to make a light or buif colored writing board.

Another suitable formula for making the new blackboard, black in color, is the following, in which all parts indicated are by weight:

Example No. 2. (Black-colored board) Forty-six (46) parts of reclaimed rubber having approximately the following composition:

Parts (a) Rubber 23.0 (b) Clay 7.0 (c) Carbon black 12.0 (d) Whiting 4,0 (e) Plus other materials present in the reclaimed rubber including cellulosic fiber, passive sulphur, reclaiming oils, plasticizer, etc.

Feldspar 23.0 Amorphous silica 34.0 Crystalline silica (nely divided) 19.5 Silicon carbide (finely divided) 4.0 Vulcanizing agent (sulphur) 11.0 Accelerator, hemamethylplenatramine) 0.15 Age resistor (aldolo-naphthylamine) (cptional) 0.20 Plasticer (pine tar oil) 2.0 Deodorant (vanillin) 2.0

In making the new blackboard according to either of the foregoing formulae reclaimed rubber has been found to be an inexpensive and satisfactory base raw material "and when the reclaimed rubber employed has been made `from old tires and the like, and contains clay, its clay content aids in working the reclaimed rubber and does not interfere with its function or usefulness in the new blackboard but, on the other hand, in such cases it serves as a desirable one of the constituents of the mineral filler which is embodied in the new blackboard, although, as pointed out above, reclaimed rubber made from old or used rubber bands and which contains no clay may also be employed. Likewise, pure or live rubber may also be used, if desired, as

the base material although its cost and longer curing time will not, in general, make this desirable, and in certain respects reclaimed rubber is preferable to live rubber in that it has lost its nerve or vitality. In addition, reclaimed rubber has the additional advantage which resides in the fact that it requires less time to cure than live rubber.

It is to be noted in this connection, however, that a mixture of reclaimed rubber and pure or live rubber may also be used as the base raw material in making the new blackboard without materially increasing the time required for curing the same or causing any necessity for changing the method employed in eifecting the curing operation.

Thus, a suitable formula for making the new blackboard, using a mixture of reclaimed rubber and live or pure rubber, as the base raw material, and without materially changing the method of, or time required in, curing the composition, as explained hereinafter, is represented by the following example in which all parts indicated are by weight:

Example No. 3: (Black-colored board) Parts Thirty (30) parts of reclaimed rubber having approximately the following composition:

(a) Rubber 15.0 (b) Clay 4.5 (c) Carbon black 7.8 (d) Whiting 2.6

(e) Plus other materials present in the reclaimed rubber including cellulosic fiber, passive sulphur, reclaiming oils, plasticizer, etc. u

Live rubber 8.0 Carbon black 4.2 Lime 3.0 Plasticizer (pine tar oil) 2.0 Vulcanizing agent (sulphur) 11.0 Accelerator (diorthotolyguanidine) 0.2 Slate dust 56.0 Crystalline silica (nely divided) 19.5 Silicon carbide (finely divided) 4.0 Deodorant (activated charcoal) 4.0

foregoing examples, the clay content (in those cases where reclaimed rubber embodying clay is used), the slate dust, the feldspar and the amorphous silica function as and provide the desired filler in the rubber matrix, and also contribute some abrasive characteristics to the new blackboard, whereas the finely divided crystalline silica and silicon carbide function as and provide the desired and controlled abrasiveness in the resulting blackboard, and in addition to or in the place of the finely divided crystalline silica and silicon carbide, ground glass, carborundum, alundum (calcined aluminum oxide), ground iron slag, aluminum oxide (not calcined), corundum, tripoli, pumice, Turkish emery, domestic emery, nely divided quartz, flint, sand and the like may also be used, if desired.

It is to be noted, in this connection, that it is preferred to use the so-called silicious abrasives in the manufacture of the new blackboard, although itis possible but less desirable to use other and non-silicious abrasives such, for example, as aluminum oxide, silicon carbide, iron silicates (slag), tin oxide and others.

In the manufacture of writing boards according to the practice of the present invention, it has been found desirable to employ a relatively higher ratio of rubber to mineral ller content employed (within the critical range specified herein) in making longer slabs such, for example, as

slabs six, eight or ten feet long and to employ al relatively lower ratio of rubber to mineral filler content (within the critical range specified herein) in the shorter slabs such, for example, as slabs four feet long. This is for the reason that the longer slabs require a somewhat higher percentage of rubber, within the critical limits specified, in order to give them greater flexibility than the shorter slabs, since if the same percentage of rubber were used in both the short and the long slabs the latter would have a tendency to crack, especially when being lifted during the operation of installing them. It will be understood, in this connection, that as the percentage of rubber in the new board decreases the tendency toward brittleness increases.

It has likewise been found desirable, in the practice of the method set forth herein, to employ a larger quantity of plasticizing oil such, for example, as Reagen, (Dow), petrolatum or pine tar oil, or any other compatible oil, when making short slabs such, for example, as slabs four feet long. This is for the reason that as the percentage of rubber employed in making the shorter slabs is relatively less than that employed in making the longer slabs, the workability or plasticity of the composition, which is necessary to insure adequate mixing of the ingredients therein is also decreased, and a relatively larger quantity of plasticizing oil is required in order to make up for the loss of plasticity or workability of the composition caused by the decrease in the rubber content thereof.

A typical and suitable composition for making the new writing board in longer and non-shining slabs such, for example, as those six, eight and ten feet long, may be made according to the following formula in which all parts indicated are by Weight:

Example No. 4

`There is at the present time a tendency towards the desirefor and use of colored writing boards in schools and like places, and green and buff are, in this respect, the most commonly preferred colors. Accordingly, as stated hereinbefore, one of the advantages of the new writing board is that it may be made in various colors, in addition to its natural black color, without loss of any of its desired or inherent physical or chemical properties or lowing example, in which all parts Vindicated are byweight:

Ercample No. 5: (Dark green-colored board) Thirty (30) parts of reclaimed rubber having approximately the following composition:

Partsv (a) Rubber 21.0 (b) Clay 7,0 (c) Carbon black 2.0

(d) Plus other materials present in the reclaimed rubber, including reclaiming oils, plasticizer, cellulosic fiber, passive sulphur,

It is to be noted in connection with the foregoing Example No. 5 that the pigment (calcined ,K

chromium oxide) should be calcined and ground with oil rst in a Banbury mixer before adding the lime, sulphur, etc., and after being thoroughly dispersed the pigment should be mixed for a period of about ve minutes before adding the sulphur, lime, etc., otherwise (u) the lime will react with the oils to saponify the latter and form a metallic soap, (b) a uniform color cannot be obtained, and (c) the sulphur will darken the pigment.

calcium stearate, so that it will not react too rapidly with the pigment when added to the mixture.

In the formula which is illustrated by the l foregoing Example No. 5 the Chrome Green pigment which is referred to therein is a mixture of lead chromate, barium sulphate and other materials which is sold under the trade name Chrome Green (Imperial). However, other green pigments, both inorganic andV organic, may be used in place of the particular pigment which is speciiied in Example No. 5, and among the organic green pigments which may be used are those of the copperphthalocyanine and phosphotungstic acid types.

If a board having a lighter tint of green than that which is imparted to a board made according to the oregoing Example'No. 5 is desired, it is necessary to employlive or pure rubber in place of reclaimed rubber so as to avoid the dark (green)A color imparted to the resulting board bythe carbon` black in the reclaimed rubber, and a `suitable formula for making such a light-colcharacteristics, and av A suitable formula for making the new writing 'board in`a green color is represented by the fol- Likewise, the sulphur should be kept in a passive condition, as by the use of ored board is represented by the following example `in which all parts indicated are by weight:

Example No. 6.' (Light green-colored board) Parts Live rubber (or reclaimed rubber made from light-colored stock) 18.0 Clay 6.0 Plasticizer (pine tar oil) 3.0 Lime 3.0 Vulcanizer (sulphur) 11.0 Accelerator (hexainethylplenatrarnine) 0.2

Pigment (Chrome Green) (Imperial No.

3664-Light) 25.0 Feldspar 23.0 Amorphous silica 34.0

`Alundum 4.0 Crystalline silica 19.5

A suitable formula for making a buff-colored board is represented by the following example in which all parts indicated are by weight:

Example No. 7 (Buff-colored board) Parts Live rubber (or reclaimed rubber made from light-colored stock) 20.0 Lime 3.() Plasticizer (pine tar oil) 2.0 Vulcanizer (sulphur) 12.0 Accelerator (triphenylguanidine) 0.15 Pigment (Hansa Yellow) 5.0 Feldspar 23.0 Amorphous silica 34.0 Alundum (or carborundum) 4.0 Crystalline silica 24.5

t per square inch and the sheets being kept in the mold for a period of about twenty (20) minutes so as to effect the cure of the rubber, whereupon the board or sheet thus formed is allowed to cool, while being held under pressure in the mold, for a period of about twenty (20) minutes.

It should be noted, in this connection, that by keeping the full pressure of the mold upon the sheet during the time the sheet is coolingV and shrinking, afterthe vulcanizing operation, greater uniformity in the thickness of the resulting writing board or sheet is assured in that in this manner the board is prevented from pulling away from the surface of the mold during the time the sheet is cooling and shrinking, which would cause undesirable irregularities in the resulting blackboard or sheet; and, in addition, this method prevents air pockets from being formed in the resulting blackboard or sheet.

However, when making the new blackboard according to any of theformulae which are represented by Examples Nos. 4, 6 and 'I and in which only live or pure rubber is employed, as the base raw material, and which do not employ any reclaimed rubber, the curing time is somewhat 'longer than in those cases where reclaimed rubber is employed alone or mixed with a certain amount of live or pure rubber, asin Example No. 3. Thus the curing time when only live or pure rubber is employed as the base raw material is preferably about forty (40) minutes at the temperatures and pressures which have been specified hereinbefore relative to those formulae in which reclaimed or a mixture of reclaimed rubber and a certain amount of live or pure rubber is employed as the base raw material.

It is to be noted in connection with the pr-actice of the present invention, that close control of temperature is necessary during the making of the new writing board and that the use of a pacifying agent such, for example, as calcium stearate, should be employed in the composition to prevent discoloration of the composition by reaction of the sulphur with the pigment, in cases where a pigment is employed.

It is also to be noted, in this connection, that it is possible in the practice of one form of the present invention, to pre-cure the live or reclaimed rubber employed in making the new writing board in a heater or kiln at a temperature of about 275 F. for a period of about three hours or at a somewhat lower temperature for a somewhat longer time or, conversely, at a somewhat higher temperature for a somewhat shorter period of time. Inthe practice of this phase of the invention, which is applicable to any and all of the compositions set forth in the foregoing examples, the porosity of the resulting sheet is reduced by -a final cure in the press. This method has several advantages, namely, (l) it aids in eliminating flow marks in the resulting sheet or board and (2) it materially reduces the cost of manufacture by shortening the time it is necessary to keep the sheet in the press; it being understood, however, that in the practice of this form of the invention the composition must be calendered.

After the vulcanizing and molding operation the sheet or panel so formed is removed from the mold and either one or both sides thereof, as desired, may be prepared for use as a writing surface or surfaces, this being accomplished by removing the mold scale or skin from either one or both sides of the board, as desired, in order to expose the abrasive particles which are embedded in the rubber matrix and which provide the desired bite for the chalk or crayon during the writing operation, when the new composition blackboard is intended to be used with chalk crayons rather than with crayons of the wax or unctuous type. This may be accomplished by grinding the said side or sides of the board with pumice stone, sand paper or other abrasive material (the abrasive being kept wet), or by sand blasting, after which (except in the case of Sandblasting) the side or sides of the board thus treated may be rubbed down with a paste of any suitable abrasive material such, for example, as silica, pumice, or the like, so as to complete this operation and thus provide the desired writing surface or surfaces on either one or both sides of the board, as desired. It is to be noted, in this connection, that for sand blasting the surface of the new blackboard it is preferred to use calcined aluminum oxide of about 180 mesh size since this material does not sink into the surface of the board as do iron shot, filings, etc.

However, if and when it is desired to use the new blackboard with an unctuous or wax type of crayon, either one or both sides of the same may be left smooth, that is, as they come from the mold, to provide the desired writing surface on either one or both sides of the board, as may be desired.

It is important to note that in the manufacture of the new writing board the rubber content and the mineral ller content thereof should be present therein within the critical ratio of not substantially more than six (6) nor substantially less than two and one-half (2.5) parts of mineral ller to about one (1) part of rubber, by weight, and that the composition from which the new writing board is made should also preferably include carbon black, a plasticizer and a vulcanizing agent, such as sulphur, the sulphur being present, as shown by the foregoing examples, in a quantity sufficient to render the resulting body or writing board hard and rigid. Moreover, the new writing board should include nely divided particles o-f crystalline silica substantially uniformly distributed throughout the entire internal structure of the said body and bonded together by the vulcanized hard rubber base, in order to assure in the resulting writing board the advantages and the desirable properties which are referred to herein.

A modified form of the new writing board, and one which is somewhat less expensive to manufacture, is shown in Figs. 6 and 7 and comprises a writing board, generally indicated at 20, and which is shown as being mounted, in a conventional manner, upon a supporting wall 2| to which is also attached, below the writing board 20, a chalk trough 22.

The new writing board 20 comprises a supporting base sheet or backing 23 which may be made of any relatively cheap or inexpensive material such; for example, as paper, asphaltic paper, asphaltum, vulcanized rubber or cement asbestos in sheet form, and the writing surface of the board 20 may consist of a sheet made according to any of the formulae hereinbefore set,

forth. In this form of the new writing board the sheet 24 is laminated to its supporting backing sheet or carrier 23 to provide the desired writing surface; it being noted that the sheet 24 is relatively thin compared to the thickness of its supporting base sheet or carrier 23.

It is to be noted, in this connection, that in cases where, as in the writing board shown in Figs. 6 and 7, a writing surface is provided on only one side of the supporting base sheet or carrier 23, it is necessary that the m-aterial of which the supporting base sheet or carrier 23 is composed have the same degree or coefficient of expansion and contraction as the material of which the writing surface 24 is composed and this requires the use, in the backing 23, of a highly filled composition having a coefcient of expansion and contraction similar to that of the Writing surface 24, otherwise the writing surface 24 Will tend to pull away from its supporting base sheet or carrier 23, or conversely, with variations of temperature.

However, in cases where, as in the modified form of the new writing board which is shown in Figs. 8 and 9, and generally indicated at 28, a writing surface, such as 26, is provided on each side of a supporting base sheet or carrier 21 it is not essential that the supporting base sheet or carrier and the sheets laminated thereon to provide the writing surfaces 26 have the same coefficient of expansion and contraction.

Except for the differences stated above the modified form of the new writing board which is shown in Figs. 8 and 9 is substantially the same as that shown in Figs. 6 and 7, and isshown as being mounted in a support 2S which enables the double surface writing board 25 to be swung or pivoted so as to present selectively either one of the two writing surfaces 25 to the user, as desired.

By way of recapitulation, it may, therefore, be said and it will be seen from the foregoing description that: efforts have been exerted heretofore to make composition writing boards due, among other reasons, to the fact that the supply of suitable natural slate for making writing boards is rapidly becoming exhausted and due, in part, to the fact that such natural slate as is now available for making Writing boards frequently contains naturally occurring imperfections which make it dimcult and expensive to work into Writing boards; that among such earlier efforts in connection with composition Writing boards may be mentioned those Which are disclosed in the Blood patent, No. 392,395, in the Keeran patent, No. 633,551, in the Snell Patent No. 1,755,393, in the Mayall patents, Nos. 27,818 and 2,087,111, and in the French Patent No. 622,958. The present invention accomplishes one of its primary objects, namely, of providing a composition writing board which overcomes and is free from the objectionable features and difficulties which have been experienced heretofore in either the manufacture, structure, composition or use of natural slate, composition, and other blackboards; that the present invention provides a self-contained rubber composition Writing board the rubber constituent of which may be composed either in part or entirely of a relatively inexpensive or cheap base raw material namely, reclaimed rubber; and that the composition Writing board which is provided by the present invention, in addition to being relatively cheap and inexpensive to manufacture, possesses desirable properties not possessed by either natural slate or composition writing boards including: (a) greater elasticity and greater strength and resistance to cracking and breaking; (b) greater latitude and facility in mounting in that it may be mounted in position of use by means of nails, screws or like fastening elements; (c) greater latitude in the use of pigments and in the selection of colors and shades or tints thereof in which it may be made; (al) greater latitude in the range of sizes in Which it may be made; (e) it retains a good writing surface permanently throughout the life of the blackboard itself and without developing shine or smooth or slippery areas or spots such as are found in natural slate blackboards; (f) it is highly resistant to the action of Water, air, and atmospheric moisture; (g) it may be made in sheets which are much lighter per square foot than the natural slate and composition glass and other coinposition writing boards which have been suggested or used heretofore; (lz) it may readily be used for writing purposes on either one or both sides thereof; and (i) in addition to the foregoing advantages, it possesses the other advantages and desirable properties, and accomplishes its intended objects, which have been set forth hereinbefore.

It is to be understood that the term mineral ller as used hereinbefore and as used hereinafter in the claims is a generic expression which includes Within its scope a mixture of non-abrasive material and abrasive material in any proportions Within the critical limits for the mineral filler specified hereinbefore and in the claims, or the mineral filler may be comprised entirely of abrasive material or largely of abrasive material with only a relatively small proportion of non-abrasive material, Within the aforesaid critical limits of the mineral ller.

While I have described typical formulae which may be used in making the new blackboard, and preferred methods of making the same, it is to be understood that the present invention is not limited to the exact details of the composition and formulae which I have described and that I desire to avail myself of such modications and variations as come Within the scope of the appended claims.

I claim:

1. A Writing board comprising a vulcanized hard rubber base containing finely divided particles of mineral filler containing crystalline silica substantially uniformly distributed throughout the entire internal structure of said body and bonded together by the said hard rubber base, said rubber and said mineral filler being present as ingredients in said body in a ratio of not substantially more than six (6) nor substantially less thantwo and one-half (2.5) parts of mineral filler to about one (l) part of rubber, by Weight.

2. The Writing board dened in claim one (l) inwhich the said body includes a plasticizer as an ingredient thereof.

3. The writing board dened in claim one (l) in which the said body includes carbon black as an ingredient thereof.

4. The writing board defined in claim one (1) in which the said body includes as ingredients thereof a plasticizer, carbon black, and sulphur.

5. The Writing board defined in claim one (1) in Which the said body includes as ingredients thereof a plasticizer and sulphur as a vulcanizing agent in a quantity sufficient to render the said Writing board both hard and rigid.

6. The writing board defined in claim one (11) in Which the rubber included as an ingredient in said body is live rubber.

7. The Writing board defined in claim one (1) in which the rubber included as an ingredient in said body is reclaimed rubber.

8. A Writing board comprising a body including as an ingredient thereof a vulcanizing agent and embodying a vulcanized hard rubber base containing finely divided particles of mineral filler composed of one or more substances selected from the group consisting of carborundum, alundum, (ealcined), ground iron slag, aluminum oxide, (uncalcined), ground glass, pumice stone, tripoli, Turkish einery and domestic emery, substantially uniformly distributed throughout the entire internal structure of said body and bonded together by the said hard rubber base, said rubber and said mineral ller being present as ingredients in said body in a ratio of not substantially more than six (5) nor substantially less than tivo and one-half (2.5) parts of mineral ller to about one (l) partof rubber, by Weight.

9. A Writing board comprising a vulcanized hard rubber base containing finely divided pari ticles of mineral ller comprising abrasive material substantially uniformly distributed throughout the entire internal structure of saidbody and bonded together by the said hard rubber base, said rubber and said mineral ller being present as ingredients in said body in a ratio of not substantially more than six (6) nor substantially less than tvvo and one-half (2.5) parts of mineral filler to about one (l) part of rubber, by weight.

LEON R. KOLOVSKY.

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