CHALK PLATES.
The base for this form of matrix is a metal plate whose surface is slightly roughened with sand-paper. It is next rubbed over with white of egg, and flooded with the chalk wash made as follows: Flong paste (described under Papier Maché Matrices, [page 80]), six ounces; whiting, twenty-four ounces; water, three pints. The whiting is softened by soaking for an hour or more. The whole must be intimately mixed. It should cover the plate to the depth of one-thirtieth to one-twentieth of an inch. The plate is dried in a perfectly horizontal position.
When dry the design or writing, etc., is made with a smooth steel point, the lines being carried clear through the white layer to the metal. The mould is now baked at a temperature well above boiling water; as high as 392° F. (200° C.) may be reached without harm.
If the coating seems too thin, an extra coat can be given between the lines especially over the larger areas. This must be done before the baking. A pipette may be used for putting on this coat. This deepening has the bad effect of increasing the chance of the coating stripping from the metal.
The matrix thus prepared is used in the press just as is the ordinary plaster matrix. It is suited for reproduction of autographs, scrip, diagrams, etc.
CHAPTER IX.
THE MAKING OF VARIOUS SMALL ARTICLES OF INDIA RUBBER.
India rubber can be so readily shaped in moulds and the latter are so readily made of plaster of paris that any one who is interested in such things will find endless amusement in working out different designs. Before suggesting any specific articles the following are the general points to be kept in mind.
The material may be uncured mixed sheet of any thickness. As we have seen this material when heated and pressed runs together. It can be forced into any shape by comparatively slight pressure. So exactly does it reproduce the smallest line or mark, that care must be taken to have the moulds very smooth and free from defect. Powdered soapstone is used to prevent adherence to the mould, but great care must be taken not to mix it among the pieces of the india rubber, where several are used in one article, as it will prevent their coalescing or running together.
Another point is to contrive to introduce the proper quantity of rubber. The aim must be to have a slight excess, but to avoid waste this should be as little as possible. Unless some rubber is squeezed out there is no certainty that the mould has been filled. Any projecting “fins” from the overflow are cut off with a knife or scissors after the article is removed from the mould.
Plaster of paris or dental plaster mixed with dextrine or gum arabic water or the zinc oxychloride cement, already described, is to be recommended for the moulds. They should be made, if deep, in frames or “flasks” of tin, as plaster if unsupported is liable to split open when the rubber is forced home.
For many articles the hot press can be used. Such articles are mats and other thin flat pieces. The rubber stamp sheet is a good material for them. For thicker articles a thicker sheet can be used, and sheet of any gauge can be procured from the maker. Much of what has been said about india rubber type applies to the making of miscellaneous shapes. It will also be understood where wooden moulds are spoken of that plaster, or, still better, metal can be substituted, and is to be recommended for nice work as the grain of the wood is very apt to show where the india rubber comes in contact with it.
Suction discs and similar small articles into which an extra thickness of india rubber enters are best cured in a vulcanizer. The flower pot arrangement is excellent for such. The time for curing may be somewhat extended on account of the greater thickness of material to be acted on.
Suction Discs.—For suction discs a mould is required which will produce a shallow cup with the edge feathered or reduced to a very slight thickness. Its outer surface should be raised in the centre so as to give a projection for attachment of the hook. The discs are generally made small, not over an inch in diameter, as they are not reliable for any heavy service. Their principal use is to suspend advertising cards and light articles to the glass of show windows. The following is a method of making a simple mould.
A hole to give the outside contour should be bored in a small piece of wood. A marble which will exactly fit the hole is next required. Some plaster of paris is mixed with water and put into the bottom of the hole, and the oiled marble is pressed down until the plaster rises and fills the entire space under the marble. After it has set the marble is removed. The proportions should be so arranged that the plaster will have risen at the sides within an eighth of an inch of the surface of the wood. This gives the exterior mould. For the cup or hollow a marble a shade too large to enter the hole may be used.
One or if necessary two thicknesses of mixed sheet rubber cut into disc shape so as to fit the hole are inserted in the block, and the larger marble is placed on top and screwed down by the press. Heat is now applied in the vulcanizer. When the thermometer indicates 212° F. (100° C.), or better a little more, the mould is withdrawn and the screws turned until the rubber is forced down and the excess begins to squeeze out between the marble and the wood, which two should now nearly touch. It is replaced and the heat is brought up to the curing temperature 284° F. (140° C.). It is possible that a second screwing up may be needed. The spring press is in such cases particularly convenient as it avoids the necessity for removing the press from the vulcanizing chamber. After half an hour it will be thoroughly cured. A hole is made through its centre from side to side thereof, but not penetrating the disc, and through this hole a brass nail is thrust and bent into hook form.
Mould for Suction Discs.
In the cut the correct shape for the mould and consequently for a suction disc is shown. This can be easily secured where a disc already made is procurable by casting in plaster, or, with a little ingenuity the template for the mould and the plunger to be used instead of the marble can be whittled out of wood. The lower body of the mould in such a case can be made of plaster of paris. To secure the alignment of the two parts of the mould, dowel pins, indicated in dotted lines, should be placed near the periphery. The gum should be introduced in a lump near the centre, in order that it may sink well downwards to the bottom of the mould before spreading laterally. Sometimes the tips have a recessed end. This is secured by the use of a mandrel, shown in dotted lines in the axis of the mould. Such discs are sometimes made to be cemented to arrows to be discharged against smooth surfaced targets, to which they adhere on impact by atmospheric pressure, giving rise to a very interesting game.
Another use of suction discs is as photographic negative holders. They can be fastened to a wooden handle and be attached by suction to the back of a negative under treatment. For this purpose they should be at least two inches in diameter.
Pencil Tips.—These are generally little cylinders of india rubber, which fit into a tube that slides over the end of the pencil. They can be thus simply made. A hole is bored in a piece of wood the diameter of and a little more than the depth of the pencil tip. A short cylinder that exactly fits the hole is required for plunger. The gum is put into the hole in little discs, or rolled up into a cylinder, the plunger is placed on top, and the mould put in the press. It is shaped by pressure and cured as described.
Sometimes the tips are cup shaped. For these the mould is made in two sections fastened by catches or by pins set in the plaster as shown in the [cut]. The hole is made larger at bottom than at top, and at the top is a little smaller than the shaft of the pencil. A plunger that nearly fits the small end is provided. The india rubber is placed in the mould and heated. When soft, the plunger is forced down to the proper distance in the press and the article is cured. Care must be taken to give the plunger a good coating of talc, and it must be made to sit vertically. The arrangement of a cylindrical hole shown in the [cut] secures this result perfectly. As distance piece a pin is passed through the plunger.
Mould for Pencil Tips.
Cane and Chair Leg Tips, etc.—By carrying out the process just described with larger moulds and of slightly different section very convenient tips for chair legs and walking canes can be made. Such tips can be modified in size and thickness to answer as covers for the mouths of bottles, test-tubes, etc.
Corks.—These may be made in moulds tapering from top to bottom. The india rubber must be packed in with great care to secure as solid filling as possible. A plunger is used that enters the larger end and is a very little smaller in diameter, so as to descend a little way into the mould. This distance determines the length of the cork. As the perimeter of the plunger strikes the walls of the mould it cuts off almost completely the excess of rubber that has squeezed up past it. An excellent modification of the mould is shown in the [cut]. The upper part with parallel sides serves as a guide for the plunger. It is a similar extension as the one recommended to be used for the plunger in the hollow pencil and chair leg tip moulds just spoken of.
Mould for Rubber Corks.
Mats.—These may generally be made in the hot press. Designs for them in great variety may be found in cut glass and pressed glass dishes. Many of these have patterns on their bottoms that can be moulded in plaster to serve as matrices.
Cord, Thread and Seamless Tube.—By placing the mixed india rubber in a cylindrical mould fitted with piston and with one or more round holes in the bottom, the material may be softened by heat and forced out of the holes by depressing the piston. This will form cylindrical thread or cord. As it descends it may be received in a box of powdered talc and be afterwards cured. By providing the hole with a mandrel seamless tubing may be thus made. In making such the mandrel usually remains in place during the curing. Plenty of powdered talc must be used.
Skeletonized Leaves as Models.—These would form interesting models from which matrices could be made in plaster. It would be possible to produce some very pretty stamps or mats from these and similar models.
After some experience inspection of any article will show how it was moulded. The fin will indicate the joint in the mould, and with this as a clew the mould can be almost certainly constructed like the original.
India Rubber Bulbs.—Bulbs and hollow articles generally, such as dolls, toys and the like, cannot be made without special high pressure hollow moulds. The general process consists in cutting out gores from mixed sheet as for a balloon. The edges are coated with cement (thick benzole or carbon disulphide india rubber solution) and while the rubber is warm the seams are pressed and knitted together with the fingers. A hole is left in one place through which some pure water or water of ammonia is introduced. The bulb is now blown up with the mouth or otherwise, and while inflated the hole is pressed shut. This is often done with the teeth. Any projections around the seams are cut off with curved scissors. The mould is of iron and in two halves. Powdered talc is applied, and the bulb is placed in and shut up in the mould which it should exactly fill. The mould is clamped together and the whole is put into a vulcanizer, and the rubber is cured. The steam and vapor formed by its liquid contents expand it and press it with great force against the sides of the mould. After curing the mould and bulb are removed from the vulcanizer, cooled by a shower bath of cold water, the mould is opened and the bulb is removed. Often an iron pin is left projecting through the side during the vulcanizing, which pin, when withdrawn, leaves the necessary aperture, or it is perforated. The bulbs are polished by tumbling in a revolving cylinder. Considerable skill and practice are needed to succeed in making hollow bulbs. Great accuracy is needed in cutting out the gores and in joining the seams.
CHAPTER X.
THE MANIPULATION OF MASTICATED SHEET RUBBER.
The manipulation of pure sheet rubber is simple, yet is liable to lead to disappointment. When two pieces are laid face to face and cut across with a sharp knife, or scissors, the edges will adhere with considerable tenacity. This may be increased by applying some thick solution of india rubber in a volatile solvent, and by manipulating the sheets so as to bring the entire surfaces of the cuts together. Finally the material may be charged with sulphur by absorption or by Parkes’ process, and cured in a glycerine or calcium chloride bath, all of which are described in chapter XI. The same treatment will affect the cement used in making the joint also, bringing about its vulcanization.
Such in a few words is the main process in the treatment of this class of goods. Where it is desired to prevent adherence, soapy water or powdered talc is used.
Adherence may be produced between the surfaces of the sheets if they are clean, by pressure and a little warmth. The method of making toy balloons will give an example of how the article is dealt with by the manufacturer.
A pile of pieces of masticated sheet rubber is made. Every piece has one side coated with powdered talc, and two talc-coated sides are placed in contact in each pair. As they are piled up, the outer surfaces of each pair are moistened with water. A steel punch or die, pear shaped in outline, is used to cut down through the pile, cutting all the pieces into that shape.
The pile is then taken apart in pairs. The separation takes place between the wet surfaces, the edges of each pair adhering slightly so as to enclose the talc-coated surfaces. The neck is opened if necessary. A rather weak or thin solution of india rubber in benzole is now brushed over the freshly cut edges. By pulling out the centre of each piece the edges are brought into contact, and adherence is produced.
If the Parkes process of vulcanizing, chapter XI., is employed they are cured to the slight extent necessary upon a tray coated with talc. The balloons are then ready for inflation.
They are rather delicate articles to make except for immediate use as the thin material is liable to become over vulcanized.
In the chemical laboratory sheet rubber can be used for covering the ends of glass stirring rods. These answer very nicely for cleaning out from beakers the last particles of a precipitate. The sheet is cut of proper size and is bent around the end of the rod and cut off close with a pair of scissors. It adheres where cut. It is then pinched with the fingers to bring the edges into better contact and the operation is complete. A slight heat makes it adhere better.
To connect glass tubes in setting up laboratory apparatus the same material was formerly used. It was wrapped around the joint, tied with thread and slightly warmed. At present this form of connection is wholly displaced by ready made rubber tubing.
It is interesting to observe in all articles made from this sheet the marks of the original cutting knife. These may be observed in inflated balloons, as parallel lines running all over the surface, and magnified by the expansion due to the inflation.
CHAPTER XI.
VARIOUS VULCANIZING AND CURING METHODS.
The regular methods of vulcanizing and curing can be departed from and good results obtained. A few excellent methods differing essentially from the ordinary ones are described which will be of service to workers on the small scale, as they enable one to dispense with vulcanizer and air bath entirely.
One type of curing process does away with the air or steam vulcanizer, and substitutes, as the curing agency, a hot bath of liquid. For this purpose a fluid is required that will not act injuriously upon the india rubber, and which will give a curing temperature without boiling away. One favorite liquid is glycerine. This can be heated to the necessary degree and is an excellent substitute for the expensive apparatus often used. For experimental work it is exceedingly convenient.
In use it is placed in a vessel of proper size and a thermometer is suspended so that its bulb dips into the liquid near one side and does not touch the bottom of the vessel. The heat is applied by a gas burner, alcohol lamp or oil stove. Of course the vessel may be placed on an ordinary cooking stove or range, and the heat may be graduated and adjusted by moving it about until it reaches a part of the stove where the proper heat will be maintained.
The mould with its contents is immersed in the glycerine, care being taken to see that it so placed as to assume the mean temperature of the liquid and not to be heated too hot. This might happen if it stood on the bottom of the vessel, so it is well to have it supported or suspended a little above it.
It is easy to see that the whole may be so arranged that the screw handle or pressure nuts of the mould will rise above the liquid. In this case the press can be screwed down while the article is heating.
Instead of glycerine a strong solution of some salt in water has been recommended. A solution of calcium chloride, or some other salt can be substituted. Either are very cheap and will be quite satisfactory.
Another treatment which applies also to the mixing operation is by the sulphur bath. Sulphur is melted in an iron vessel and brought to a temperature of 248° F. (120° C.). A piece of unmixed pure caoutchouc immersed in this bath will gradually absorb sulphur. The case is almost parallel with the absorption of water or benzole by the gum. The piece swells and thickens as it is acted on and eventually will contain enough sulphur for vulcanization. It may absorb as much as fifty per cent. The point of proper absorption must be settled more or less empirically or by successive trials.
After enough has been taken up the piece is removed and dipped into cold water, which cracks the adherent sulphur so that it can be brushed or rubbed off. This gives a piece of mixed rubber ready for moulding and curing. It can be heated and moulded and may be cured as desired, in a liquid bath, hot press or vulcanizer.
It will be observed that this provides for the admixture of sulphur only; no talc or other solid can be thus introduced. The addition of these solids tends to make the rubber of a more attractive color and their use is not to be deprecated in all cases. Hence the sulphur bath process is not to be considered a perfect one.
In the sulphur bath the mixing and curing processes can be combined. If the liquid sulphur is heated to the vulcanizing temperature, 284° F. (140° C.), a thin strip of gum immersed in it will be vulcanized completely in a few minutes. A heating of several hours at the lower temperature will effect the same result.
The sulphur bath processes must be regarded as unsatisfactory. It is not easy to feel that any dependence can be placed upon them as regards reliability or constancy of product. The sulphur also will mostly effect the surface. Thin pieces may be satisfactorily treated, but the same confidence cannot be felt as is experienced when specific amounts of ingredients have been mixed in with pure caoutchouc in a regular mixing machine.
The sulphur bath is of value to the experimenter, enabling him to do his own mixing without expensive apparatus.
Bromine, iodine, chlorine and nitric acid are vulcanizers. A piece of sheet rubber dipped into liquid bromine is instantly vulcanized. Iodine and nitric acid have also been used in commercial work.
Alkaline or alkaline earth sulphides can be employed in solution under pressure for vulcanizing. At a vulcanizing temperature their solutions will answer for thin sheet very well. Polysulphides of calcium have thus been employed.
By simply lying embedded in finely divided sulphur at a temperature of 233° F. (112° C.) as much as ten per cent. of sulphur may be absorbed by thin sheet rubber. This is one of the processes peculiarly suited for work on the small scale. It may be used instead of the Parkes process next to be described.
Chloride of sulphur is an orange red mobile liquid of a peculiar and disagreeable odor. It boils at 276° F. (136° C.). It dissolves both sulphur and chlorine so that it is not easy to obtain it in a pure state. If unmixed india rubber is exposed to its action it will quickly become vulcanized. At ordinary temperatures the mixing action takes place, though it is much accelerated by a slight application of heat.
It is quite possible that this action may be of use to the reader in his manipulation of india rubber. Thin sheet may be vulcanized by being immersed in a solution of this substance in bisulphide of carbon followed by slight heating. The thin layer of caoutchouc left by evaporation of the chloroform solution of india rubber may thus be vulcanized so as to become comparatively strong and elastic. Where the same solution has been used as a cement or for patching overshoes and finishing the patch, a vulcanization can thus be given to it.
The process is known as Parkes’ cold curing process.
A solution of one part of chloride of sulphur in forty parts of bisulphide of carbon is of good strength for rapid work. A thin article needs but an instant of immersion. It then is placed in a box or tray upon some talc powder and is heated to about 104° F., (40° C.). One minute of curing will suffice. It is advisable to wash off the articles afterwards in water or in weak lye to remove any traces of acid.
Petroleum naptha can be used as the solvent instead of bisulphide of carbon. The latter substance has an exceedingly disagreeable odor, and its vapors must be considered rather injurious especially to those who are not accustomed to them.
When thick articles are to be cured by this process a much more diluted solution is used. One per cent. or less of the chloride of sulphur is the proportion used. The object of this is to enable a longer immersion to be employed so that the interior will be affected before the outer layers become too much charged with the vulcanizing material.
In this short description of the Parkes curing process hints for a useful method may be found. The process is beyond doubt by far the simplest known for treatment of india rubber. Exactly what reaction takes place is unknown. Whether the sulphur or the chlorine is the acting vulcanizer has not as yet been determined.
Its defect is that it produces surface action, analogous to casehardening. One method of avoiding this is to remove the articles from the sulphur chloride bath and at once to immerse them in water. This prevents the rapid volatilization of the solvent and an equalizing of the absorption ensues.
CHAPTER XII.
THE SOLUTION OF INDIA RUBBER.
India rubber presents some difficulties in its solution. If a piece of pure gum just as received by the factory is placed in hot water it will swell and whiten after a while, but will not dissolve. If a similar piece is placed in benzole a similar but greatly exaggerated action takes place. The piece if left to soak for a day or more swells enormously, but very little solution is effected.
The swollen india rubber can be removed from the benzole in a single piece. It will display all the layers and marks of the original piece which was perhaps of not one hundredth part of its volume. Some parts will be a perfect transparent jelly.
It has been found that masticated india rubber dissolves with comparatively little difficulty. If the experimenter will place in a porcelain mortar, the jelly-like mass obtained as above detailed, and will rub it up thoroughly, it will be effectually masticated. This requires a little patience, as the slippery material seems to elude the pestle. Yet eventually it will all be reduced to a perfectly homogeneous mass. Its action while being rubbed up is very peculiar. At first no progress seems to be made. After a little the lumps yield to the friction. The rubber then begins to attach itself to the pestle and mortar, and begins to be drawn out into ever changing webs and threads. As the operation approaches completion the material makes a snapping, crackling noise familiar to all rubber workers. When complete there will be no lump left, and the whole will be a uniform pulp.
If benzole or a volatile solvent has been used, the rubber will easily be removed from the mortar with a spatula or palette knife. If turpentine was the solvent it will be impossible to remove the last traces except after long standing or by solution.
If replaced in the original solvent it will now come into nearly or quite perfect solution. This is the best way of masticating on the small scale. It is almost impossible to masticate untreated gum in an ordinary mortar.
The dealers sell a special india rubber for the manufacture of cement and solutions. This is so treated by mastication that it dissolves with great readiness. It is also said that heating under pressure is used to dissolve it in some factories.
Many solvents have been used and none work without some difficulty. Benzole, coal tar naptha, petroleum naptha, carbon disulphide, ether and chloroform, oil of turpentine and caoutchoucin are the best known. The naptha best suited for its solution is termed solvent naptha. It has a specific gravity of .850 at 60° F. (15½° C.); it boils at from 240° F. (115½° C.) to 250° F. (121° C.) and on evaporation should leave no more than ten per cent. of residue at 320° F. (160° C.)
Payen recommends a mixture of 95 parts bisulphide of carbon with 5 parts of absolute alcohol.
Commercial chloroform is apt to be too impure to act as a good solvent. It is apt to contain alcohol mixed with it as a preservative, which impairs its effectiveness.
Some of these solutions are better suited than others for the deposition of thin layers by evaporation. Turpentine gives a very sticky and unmanageable solution, which dries very slowly. Payen’s solution and the chloroform and the benzole solutions may be cited as especially adapted for this purpose. Careful vulcanization by the cold curing method can be applied to articles made by such deposition from evaporation.
In the case of all of them some form of mastication for the india rubber is needed. The simple mortar grinding of the gum swelled by the solvent is the only practical treatment without special apparatus.
When it is remembered that fixed oils are destroyers of vulcanized or unvulcanized india rubber it will be obvious how important it is to use pure solvents. Too great care cannot be taken to preserve the liquids pure and free from such matter.
A solid hydrocarbon may be used. Thus paraffin wax, such as candles are made of, when melted acts as a solvent. The resulting liquid solidifies when it cools, retaining an almost greasy feel.
Boiling oil of turpentine is recommended by some for the solution of vulcanized india rubber. Phenyle sulphide, it is stated, will soften it so as to render it workable. The latter discovery is credited to Dr. Stenhouse.
It is stated that a solution or pasty mixture of one part of caoutchouc in eleven parts of turpentine with one half part of a hot concentrated solution of sulphur (potassium sulphide) gives on evaporation a film neither tacky nor soft, a species of vulcanization taking place.
It is of much interest to note that an aqueous solution of india rubber has been proposed in which the vehicle is a solution of borax in water. This is well known to be a solvent for shellac and other resins. It has been recommended often as a vehicle for rubbing up india ink. The ink made by mixing lampblack with the shellac solution is nearly waterproof. A shellac varnish is given by the plain solution.
The experiments upon india rubber were published in a recent trade paper. One method of making the solution is as follows.
A solution of borax two fifths saturated is made by adding to two volumes of saturated solution three volumes of water. To this is added a solution of india rubber in benzole or other hydrocarbon of such strength and in such quantity as to contain from three and one-half to four and one-half per cent. of india rubber referred to the borax solution. It is now vigorously shaken and heated to 120°-140° F. (49°-60° C.) and the agitation, not too violent, is continued until it cools. Ceara or Madagascar rubber answers best; Para is not so good for this formula. This may be termed the indirect or emulsion method.
For direct solution from two to three volumes of water may be added to three volumes of saturated borax solution. The india rubber is added in extremely thin shavings and the solution is heated. For weak solutions the boiling point need not be reached. For strong solutions the heating should be done under pressure so as to bring up the pressure to one to three atmospheres.
Such solutions may contain as much as eight per cent. of the gum. The mixture is liable to coagulate or gelatinize just at the wrong time, but it may be of value as a vehicle or as a waterproofing agent. It deserves further investigation, which it is to be hoped it will duly receive.
Great care is necessary in working with naptha, benzole, carbon disulphide and similar liquids. Their vapor is given off at ordinary temperatures and may travel some distance to a lamp or fire and become ignited and carry the flame back to the vessel. Their vapors are also anæsthetic and should be avoided as regards inhalation.
CHAPTER XIII.
EBONITE, VULCANITE AND GUTTA-PERCHA.
Ebonite and Vulcanite.—These two well known substances are india rubber, in which the vulcanization process has been intensified. From twenty-five to fifty per cent. of sulphur is added in the mixing, and the curing is prolonged to several hours. A temperature of 275° F. (135° C.) for six to ten hours is sometimes recommended, but generally a shorter period at the regular temperature, 284° F. (140° C.), may be employed.
The mixed sheet is made and sold extensively for dentists’ use. It is soft and flexible and very easily moulded. It is treated like the regular mixed sheet in every respect, except that plumbago brushed on the slightly oiled surface of the mould is recommended instead of the light colored talc, to prevent adherence. Wax where available is better than oil.
Sometimes specimens are built up in sections. About an hour before full vulcanization in the fourth stage, new material can be added and will attach itself to the old. The stages of vulcanization are thus given by Bolas.
“Several distinct stages or steps may be traced during the curing of ebonite; and I wish to call your attention to some specimens illustrating these various stages.
“Here, in the first place, is the plain mixture of sulphur and rubber, this being nearly white, and capable of becoming quite plastic or soft by the application of a gentle heat.
“The second specimen illustrates the action of a very moderate degree of heat on the mixed material, this particular sample having been heated to 128° Centigrade for twenty minutes. It is, as you see, somewhat darkened, and has lost a little of its original softness; while a degree of heat which would have rendered the original mixture plastic, like putty, fails to make much impression upon it.
“The third specimen illustrates the effect of a more prolonged heating, this sample having been heated for an hour to 135° Centigrade. It is olive green in color, and has acquired a certain amount of elasticity, resembling that of a rather inferior quality of vulcanized caoutchouc.
“The fourth stage of curing is illustrated by this specimen, which you see is brown, and tolerably hard. Ebonite in this state refuses altogether to become plastic by heat, and a temperature of 150° maintained for half an hour or less would suffice to bring it to the fifth stage, or that of finished ebonite.
“The fifth stage, or that of properly cured ebonite, is the goal to be arrived at in manufacturing the material. There should be no places where the curing is imperfect, a kind of defect which is likely to happen when articles of unusual thickness are vulcanized, and no portion of the ebonite should be spongy or honeycombed by air bubbles.
“The sixth, or spongy state, is generally the result of over-heating, bubbles of gas forming in the material, and converting it into a kind of porous, cinder-like mass.
“A specimen will now be handed round, which illustrates the third, fourth, fifth and sixth stages, as already described. The specimen in question was cured on a hot plate, this having probably been heated to 160° or 170° Centigrade; and you will be able to trace all gradations in the curing operation, from the first setting of the plastic material to the destruction of the ebonite by overheating.”
Cement for uniting pieces of the partially cured material may be made by rubbing up some of the untreated scrap with benzole.
At the heat of boiling water, ebonite can be bent to a certain extent, which bend it retains on cooling. When warm an impression of a coin or relief die may be made on it by heavy pressure which it will retain. On heating the image disappears. If before heating the surface is planed off and the piece is heated the image formerly in intaglio will expand into relief.
By the exact process of rubber stamp making excellent stereotype plates may be made of ebonite.
It can be turned at high speed in a lathe and polished with fine 000 emery paper followed by a cloth bob with rotten stone, etc., and water or oil. Blotting paper, charged with the above or with tripoli, is excellent for polishing small surfaces by hand.
Ebonite is a good connecting material between softer rubber and iron, the whole being vulcanized together; the iron should be well roughened or cut into rasp-like or file-like projections.
Ebonite is properly the name for black hard rubber, and vulcanite for the colored products such as used by dentists and others.