GLASS VERSUS SLATE BLACDOARDS* FOSTER DEESNELL AND BEATRICES. FOX.130 CLWTONST., BROOKLYN, NEWYORK
Because of the progressively poorer quality of blackboard slate as the best deposits are exhausted, an alternative i s desirable. The defects of natural slate are knots, irregular color due to ribbons, and scratches. Slate requires refinishing after five to fifteen years. Asbestos and pulp bases with emery finishes are unsatisfactory for long-continued use. Sand-blasted glass i s either too rough or wears smooth too quickly. A glass blackboard described consists of a Nae0:Ca0:6SiOz glass m i x colored with manganese and cobalt containing twenty-five to thirty per cent of black chromite suspended throughout the glass while in a molten condition. The abrasive, which can be varied, must be sufficiently harder than glass so that normal wear will not cause it to become smoother than the original. That this result has been obtained is shown by artificial wearing tests with a n eraser operated by reciprocating motion to give the equivalent of one hundred twentyjive years' wear without appreciable damage to the writing surface. I t i s concluded that since a board of this composition i s producible in any desired quantity thepoblem of depletion of the slate supply need not concern the school. The cost of manufacture permits its sale at a price competitive with natural slate. The product i s not a "substitute" in the usual use of the term but a material to replace slate which i s equal i n quality i n every way and superior in many.
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The tendency in all branches of science today is to substitute the synthetic and the artificial for natural products wherever possible. Synthetic products can be made exactly according to specification and are therefore more dependable. Where the supply of the natural product is restricted, its quality is steadily deteriorating or its price increasing as the best of it is used up. Slate, and particularly slate suitable for blackboards, is a natural product of this type for which an alternative is becoming more and more desirable. Substantially all the slate suitable for blackboard use is found in a restricted area in eastern Pennsylvania. While the supply is sufficient for present demands, i t is inevitable that the best slate will eventually be exhausted and a new material will be needed to take its place. One alternative already introduced for slate as blackboards is glass. This is a synthetic, factory-made product, similar to slate in many of its physical properties and one made from raw materials of which there is a plentiful supply. In the past the method of preparing glass for this use has not been sufficiently perfected so that i t could compete with natural slate. A method has now been developed which gives a product which may satis-
* Presented before the Division of Chemical Education at the 80th meeting of the American Chemical Society, Cincinnati. Ohio, Sept. S 1 2 . 1930. 320
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factorily replace slate in every respect and is superior in many characteristics. Blackboard Slate The characteristics of slate that make it peculiarly suitable for blackboards are its dense, compact structure and the absence of any absorption a t its surface. The average water absorption for clear slate stock after immersion for twenty-four hours is 0.196% (1). This dense and non-porous structure enables the slate to be polished to a fine, smooth surface which is particularly adapted to taking chalk marks. Its non-porous nature prevents particles of chalk from clinging to the surface so that i t may be easily erased. This material is not markedly affected by heat or coldso that i t shows no important expansion or contraction with changes in temperature. It is resistant to the action of all chemicals, as well, so that its wearing qualities are very satisfactory. Practically all of the slate that is used for blackboards is quarried in Lehigh and Northampton Counties of Pennsylvania (2). The particular qualities of this slate are its softness, its dark gray color, its high fissility and its plane or only slightly curved cleavage. The slate is elastic enough so that if the cleavage is not perfectly straight when the slabs are split i t can be sprung into position. The slate in this region is particularly dense and uniform in structure. Its appearance under the microscope has been compared with that of high-carbon steel. Layers of this clear gray slate are interspersed with layers of a darker carbonaceous slate. These dark layers are present as ribbons in the clear slate and must be avoided when quarrying for blackboard use. In certain sections of this so-called Blue Mountain district in Pennsylvania the clear slate is taken out in large pieces and is used exclusively for blackboards. The clear stock, however, comprises only about twenty per cent of the extracted slate. Due to its scarcity and the necessity for care in selection and cutting out, its price is from twenty-five to thirty per cent higher than the ribbon stock (3). The following process is carried out in making up the slate into blackboard (4). The slate is taken out in huge blocks and hauled to the mill. There i t is steel-sawed into two or three pieces. These pieces are trimmed and squared by steel saws to the required length and width. They are then split into about one-half inch sheets by a professional "splitter," who must be able to recognize the grain in slate as easily as a carpenter does in wood. The splitting is done with chisels or wedges. A sheet of slate so obtained is next treated to obtain a finished surface. It is sand rubbed on a revolving rubbing bed and then given a "honed surface, which is obtained by polishing with round, flat abrasive stones in the presence of a steady stream of water. This gives the "velvet" finish which is so desirable. The importance of the use of slate for blackboards may be shown by the followingfigures. In 1928 the total value of the slate sold by producers
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in the United States was $11,472,291. Of this the value of the slate sold for blackboards and bulletin boards was $1,079,452 (5). The latter amount was the value of 3,713,840 square feet of slate a t the quarry. From these figures, approximately one-tenth of the value of slate quarried is for blackboard purposes. The properties of slate outlined above had made it the standard material for blackboards and it is recommended by most architects and state authorities in building public schools. Since slate has been used for this purpose only a comparatively short time the very best quality of slate has been available. As this very fine slate is used up, however, and it becomes necessary to select pieces which are not so free from blemishes, the faults inherent in a natural product occurring in a restricted area become evident. Very few pieces of slate have a perfectly uniform surface. Even on the best pieces there is some irregularity in the grain and small spots or markings are evident on close inspection. The finishing process also leaves very fine scratches which become more noticeable after long use. These blemishes are, of course, present in greater quantity in the poorer slate that must be used after the best pieces have been selected. The cleavage of the slate cannot always be depended upon to be perfectly straight so that pieces are often obtained which vary in thickness from one-sixteenth to one-eighth inch from one end to the other. One of the qualities of slate that recommends itself most strongly is its permanence. It is, of course, extremely resistant to wear but in spite of this fact i t cannot be used continually in the schoolroom without further attention after i t has been set up. After a period of five to fifteen years, depending on the amount of wear, the surface requires refinishing. This is because it is stained or given an oil finish to make the color darker and blacker. This finish eventually wears off leaving the lighter gray color of the natural slate. Continual erasure and washing of the boards also tends to intensify the slight scratches that are present originally, so that the surface is considerably roughened in time. Alternative Blackboards Substitutes for slate have been developed for varying reasons. A number of cheaper products are sold which do not have the lasting qualities of slate nor as fine a surface. These consist principally of some cheaper material such as a wood pulp or asbestos composition which is given a surface coating of emery and adhesive to imitate slate. This surfacing is applied to cloth for very cheap blackboards that will not receive much wear. Alternatives have also been marketed which are designed to eliminate the disadvantages inherent in a natural product and yet retain the advantages of slate. Such a product should be uniform in composition and
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easily manufactured to meet any specifications. Economy would not be as important a consideration here as the board would be designed to have better lasting qualities than slate as well as other advantages of an artificial product. The material uniformly used for a blackboard of this nature has been plate glass. This is a siliceous material very similar to slate in weight, in hardness, in compact structure, and in porosity. It may be manufactured to any size or thickness desired and is of entirely uniform composition throughout. The surface of plate glass, of course, will not take a chalk mark and some treatment is necessary before i t can be used for writing purposes. Until recently no treatment had been devised which would give glass a writing surface comparable to that of slate in both smoothness and durability. Mud blasting gives a satisfactory surface but after five or ten years' wear this surface wears off due to the action of constant erasing and washing. The shiny, smooth surface of the glass is then unsatisfactory and the board must be refinished. If a more drastic treatment is used the writing surface lasts longer. Sand blasting bas been used in some cases. This gives a more durable surface but i t is correspondingly rougher so that i t is unpleasant for writing and very difficult to erase and keep clean. The dark color necessary for a blackboard is obtained with glass either by superimposing the colorless glass on a black background or by coloring the glass itself. A very deep black glass may be obtained, as demonstrated in black glass table tops and other articles made of that material. Development of a Permanent Glass Blackboard The problem to be overcome in developing an artificial slate blackboard was therefore one of further developing the use of glass by treating i t in such a way that i t could be given a surface as fine and velvety as that of slate, and which would a t the same time be perfectly durable. The method adopted was to incorporate a fine abrasive in the glass mix. When perfectly distributed throughout, the presence of this material would give a writing surface, the roughness of which would be dependent on the coarseness of the abrasive. Being uniformly distributed throughout the glass i t would also insure a permanent writing surface since no matter how far the glass may be worn the surface would always be the same. The abrasive added must not react with the glass in the melting process and must differ sufficiently in hardness from the glass so that the surface will not wear smooth. Abrasives that seemed promising for the purpose were tried out in laboratory mixes. The formula used for the glass base was sodium oxide, calcium oxide, and silica in a 1:1:6 ratio. Among the addition agents tried out were fine iron filings, powdered chromite, alundum, carbomdum, and pumice. These were unsatisfactory in the first experiments, either be-
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cause of reaction with the glass or inability to mix well, with the exception of the chromite. The latter seemed very promising both as to its inert nature toward the glass mix and its miscibility. A glass mix containing approximately twenty-six per cent chromite and four per cent pyrolusite was made up on a larger scale. The chromite used in these preliminary experiments was an 80-mesh grade or coarser. The samples produced showed even dispersion of the chromite throughout the glass on examination of the fracture. The rough surface of the fracture took chalk well and erased easily. This formula was therefore adopted as the basic composition for a glass blackboard. Further experimental batches were run on semi-plant and plant scales in order t o determine what materials were necessary to give the proper shade and degree of blackness and the desired velvety texture. The early batches, in which pyrolusite was used with chromite to give opacity, were a brownish black. Other materials of approximately the same hardness and density were tried as substitutes for the chromite in order to improve the color. Rutile and illmanite were not entirely satisfactory. The use of magnetite seemed promising in laboratory mixes but gave too light a color in larger scale batches. The difficulties were eventually overcome by use of a black chromite, a much finer grade than the 80-mesh grade giving a better texture. A grading of ninety-nine per cent through 100-mesh, ninety per cent on 200-mesh, was adopted as best. The color was further improved by the addition of a small percentage of cobalt oxide. In adopting this commercially, the ingredients are mixed in the glass melt and the procedure for making plate glass followedfrom that point on. The surface of glass of this composition after rolling is glossy. The concentration of chromite particles in the actual surface is very low, although immediately under the surface they are well dispersed. I t is therefore essential to remove this surface, both because of its imperfections and because of its smooth texture. The usual plate glass procedure of first "polishing" with coarse sand, then with fine sand, and last with rouge gives, as anticipated, instead of a smooth glossy surface, a velvety one similar to slate. The similarity of this procedure to that of finishing slate is also worthy of a passing mention. This velvety texture cannot be destroyed or a glassy surface obtained by further polishing with rouge. Wearing Tests and Comparison with Slate Since the desired feature of an artificial slate blackboard was durability, a test was devised t o show how the glass containing suspended abrasive would stand up under constant wear. The following procedure was designed to simulate as closely as possible the actual conditions of wear to which a blackboard is exposed. An ordinary blackboard eraser was mounted on a shaft operated by a
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Seloc Glass 9X
Slate 9X
FEBRUARY, 1931
Worn Seloc Glass 9X
Scloc Glass Slate 30 X 30 X Frcu~E3.-CHALKMARKSON SLATEAND SELOC GLASS
quarter horsepower motor with a reciprocating motion. The speed was controlled to give thirty cycles per minute. The eraser used was six inches long by two and one-quarter inches wide. The length of the .stroke was twelve inches. The wearing element, in this case the eraser, floated freely and could be loaded with any desired weight. For these tests a weight of two kilograms was used. The blackboard to be tested was mounted on a cloth pad and covered with a liberal supply of precipitated calcium carbonate or calcium sulfate to correspond to the abrasives which wear down the surface in actual use. The chalk was removed from time to time and a fresh supply added. Assuming that a blackboard in school use is erased ten times a day, gone ,over twice each time, and that the school is in operation five days a week for fifty weeks of the year, the board will receive eight hundred lineal feet .of wear per year. I n the tests made, any one spot on the blackboard was
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brushed by eighteen hundred lineal feet of eraser per hour. An hour's erasure in this manner therefore corresponded to something over two years of actual service. The machine was operated for a total of sixty-two hours in one test. This would compare with approximately one hundred twenty-five years of service. Such tests on the glass board, made commercially according to the composition developed as above, show the results expected. After approximately one hundred twenty-five years of service, this type of blackboard will still take chalk as readily as when new. In contrast with slate this long period of wearing does not lighten the color. Fading cannot occur since the color is uniform throughout. In obtaining a surface on glass that will not be destroyed by wear, i t was also desirable to obtain a surface that will be as fine and velvety as that of slate. By the use of a closely graded abrasive this is possible. In fact, the writing surface of this glass is superior to that of slate in that i t is perfectly even and free from irregularities, markings, or scratches. Photomicrographic comparisons of chalk marks are difficult because of the physical variations in the pressure used, depth of the marks when so viewed, and variations in lighting. With these expressed limitations Figures 1 and 2 (page 325) show chalk marks on the two a t six times magnification to give details of texture. The glass gives a mark of much finer texture and is more sharply defined. The slate shows scratches even a t that magnification, such scratches being due to the softness of the slate. The slate used was a sample piece selected as being unusually good. The scratches shown are macroscopic. Figure 3 (page 326) shows microscopic comparisons of smaller areas a t nine times magnification, including a worn sample of glass. Further detail is shown in thirty times magnification of the glass and slate. In these the difference in the marks is clearly brought out as the projections on the glass are small, taking small particles of chalk while the particles of chak on the slate are much larger. The glass board is approximately the same thickness, weighs the same per square foot, and is erected in exactly the same way as slate. The results presented i n this @per are published by permission of the New York Silicate Book Slate Co., 20 Vesey Street, New York City. Literature Cited ''Structural Slate." Structural Slate Ca., 1922, Chap. I, p. 5. CHAELESH. BEHRE,JR.. "Slate in Northampton County, Pa.," Pennsylvania Geological Survey, Fourth Series. Bull. M9. Ref. (I),p. 10. "Natural Slate for Blackbaxrds." Natural Slate Blackboard Co., 1920, p. 4. OLIVER BOWLES AND A. T. COONS, "Slate in 1928," Bureau of Mines Bull., Mineral Resources of the U. S.. 1928, Part 11. 1929, p. 45.
