We have seen that india rubber cannot be cast in moulds. Except in special cases deposition from solution is not available. It has to be shaped by a combination of heat and pressure. When gently heated it softens and can be pressed in a mould. As it cools it retains the shape thus given and is moulded. This applies to all unvulcanized india rubber. If mixed rubber is moulded and heated to a higher temperature without removal from the mould the curing process is brought about and the rubber may be not only moulded but cured and the product is moulded vulcanized india rubber. The mixed sheet whose manufacture is described in chapter IV. (page 42) is the starting point in rubber stamp making. It is made for this purpose by the manufacturers.

When the material is examined it looks like ordinary white india rubber, being firm in texture and quite strong. On heating to 280° F. to 290° F. (137° C. to 143° C.) it begins to become “cured,” and if in a thin sheet one to ten minutes are sufficient for the process. As the heat is applied the india rubber first softens and becomes much like putty. It can now be pressed through the smallest orifice and will fill up the finest details of anything it is pressed against. It is at this point that pressure must be applied to drive it into the interstices of the mould.

As the heat continues it begins to lose its doughy or putty-like consistency. This marks the reaction of the vulcanizing materials. They gradually combine with and change the nature of the caoutchouc. The rubber while still quite soft is elastic. If pressed by the point of a knife it yields, but springs back to its shape when released from pressure. The india rubber is vulcanized.

On removal from the mould it will be found to reproduce its smallest detail. The color and appearance have not changed much, but its nature and properties are now those of vulcanized rubber. It is unaffected by heat or cold within ordinary ranges of temperature, and if the india rubber is of good quality and made by a proper formula it will last for years.

The first thing to be described is the mould, which includes the arrangements for pressing the sheet of india rubber while heated. A small press is needed for this purpose. It may be of the simplest description, and as an example of a home-made but perfectly efficient one the illustration may be referred to. The base of the press is a piece of iron, if heat is to be directly employed. Where a chamber vulcanizer is used both base and platen may be of wood. But from every point of view iron is the best. It lasts forever, admits of direct heating, and does not split, warp, or char. Through two holes drilled near its opposite sides two ordinary bolts are thrust. It is best to use flat headed bolts, and to countersink a recess for the heads in order to keep the bottom level. The heads may need to be filed off so as to reduce their thickness, in order to secure this object. The bolts may be soldered in place. One thing should be carefully watched for—the bolts should be set true so as to rise vertically from the plane of the base.

The platen is best made of iron, cut of the shape shown. This is an excellent disposition of the screw-bolt slots, as by swinging the right end of the platen back it can be taken off without removing the nuts and lifting it over the ends of the screws. Besides the two nuts fitting the thread of the screws it is well to have half a dozen extra ones larger than the others, which will slip easily over the bolts, so as to act as washers. The object of these is to adapt the press to objects of different thickness. The thread upon an ordinary bolt does not extend clear to the head, but by slipping on some loose nuts the plates can be forced together if desired.

This press can be simplified. Both base and platen can be made of wood, the platen being simply bored for the bolts, and the latter driven tightly through the holes in the base so as to retain their place. Even this can be improved on as regards simplicity. Two blocks of wood screwed together by two or more long wood screws may be made to do efficient work.

One trouble is apparent with all these devices, and that is the want of parallelism of the opposed planes. The base and platen may be true and parallel or they may not. Perhaps the simplest way of securing this is the best. It consists in placing across the base two distance pieces, which may be slips of wood. These must have perfectly parallel faces. As the press is screwed up they will be gripped between the platen and base and will not only ensure their parallelism but will keep them at an exact distance apart. Such distance pieces are shown in the same cut. Pieces of printers’ “furniture,” spaces, or “quads,” may be used for this purpose. They should not be fastened in place if there is need to adapt the press to more than one thickness of material and matrix.

The above described apparatus is a vulcanizing press. A further improvement in it may be effected by the use of spring pressure. Two strong spiral springs may be dropped over the bolts, the nuts being screwed on above them, or a powerful spring of flat brass or steel ribband bent into the shape of a shallow letter V may intervene between nuts and platen, the centre of the bend bearing against the centre of the platen.

As regards the strength of the springs there is this to be said. The distance pieces will prevent a spring that would ordinarily be too powerful from doing any harm. Such distance pieces should be used, as the springs must be based upon giving a pressure of many pounds per square inch of surface to be acted on. They should have a range of an eighth of an inch or more. The greater the range the more evenly will they work.

The next cut shows an excellent little screw press, that is made for the purpose of pressing vulcanizing flasks. This is so simple that it will suggest to the mechanical reader how he can make a single-screw press, which is by far the most convenient to use. In the stationery stores very small model cast iron copying presses designed for use as paper weights are sold. They are excellent for a limited amount of small sized work.

A large sized gas heated press, such as made for the purpose of manufacturing rubber stamps, is shown in the next cut, p. 53. Its construction is obvious. It is termed by the trade a vulcanizer. Its manipulation will be given further on.

Type are generally the object to be copied. These are best set up with high quads and spaces. Naturally rather a large type is chosen, with extra wide spaces between the letters. Some advise rubbing the type faces full of hard soap, afterwards brushing off the face, leaving the hollows filled. Sometimes wax is recommended for the same purpose. This prevents the plaster of the matrix entering so deeply into the cavities of the letters.

The type forming the model to be reproduced, is locked in a frame. Two pieces of printers’ furniture or other wooden strips screwed together by wood screws at their ends will answer for a locking frame for small inscriptions.

The model to be copied need not be type, but any desired relief may be used, such as an electrotype, a stereotype, an engraving or another rubber stamp. In any case it is to be placed upon a flat surface, best an “imposing stone” or piece of marble, with the inscription upwards. On each side of it distance pieces reaching about one-eighth inch above its upper surface are to be placed.

The next shaping appliance is the matrix or mould, or reverse of the model which is to be copied. This in the case of rubber stamps is properly called the matrix. Those who have witnessed the stereotyping of a large daily newspaper have seen the matrices of the type made of paper and paste, the whole mixture being termed “flong.” Such a matrix is required for rubber type, but paper is rather too susceptible to heat although good work can be done with it. It also does not enter as deeply into the cavities of the type as is desirable. As a rule a fine quality of plaster of paris is to be recommended. What is sold as dental plaster is the best, but common plaster can be used. It is mixed with water or with a solution of gum arabic or dextrine in water. For the latter enough gum should be added to make the mixing solution as thick as thin syrup.

A piece of iron, perfectly flat and true, is now to be taken, large enough to more than cover the inscription to be copied. Upon its surface a putty made of the plaster and the liquid used in mixing is to be spread. This should be rather stiff. The surface of the iron should not be too smooth as it is desirable that the plaster should adhere well on setting. The plaster should be smoothly spread to a depth of three-sixteenths or a quarter of an inch. It is best applied with a palette knife or trowel, although a table knife will answer perfectly. If its surface does not become smooth it can be made so by applying a little of the solution with the knife or trowel.

Before this has been done the model must be oiled. Olive oil or other clear oil is applied to all parts of the type faces, and the excess is then wiped off and cleared out of the interstices with a piece of blotting paper.

Next the plate with the plaster is inverted and is pressed steadily down upon the model until it strikes the distance pieces. It is left to set. In about ten minutes it can be raised, when it will be found to give a beautiful impression true to the smallest detail of each letter.

It has been said that water may be used as the mixing fluid. If this is done it is well to strengthen the mould by saturating it with an alcoholic solution of shellac, after it has dried thoroughly, best for a few hours in an oven. This operates to strengthen the small projections that are liable to crumble or to break off in use.

The dealers in rubber stamp supplies sell a lever press for conducting the operation of producing the matrix. The type is locked in a special chase, which is carried on a bed that travels under and out from under the platen of the press upon rollers. From each corner of the chase in which the type model is locked, a pin rises which is encircled by a spiral spring. A square frame of flat iron with holes at the corners for the pins to pass through, rests upon these springs well above the type. The pins pass through holes in its corners. The matrix plate with its coating of plaster is placed upon this frame, which supports it above and not touching the type. The whole is now rolled under the press and the lever pulled to produce the impression. As the pressure is released the frame with the matrix is raised from the type by the action of the springs. This can be done immediately, and before the plaster has set. It is almost impossible to raise it by hand with the requisite steadiness. The same chase with corner pins and springs can be used in a screw press, the one press answering for making the matrix and for moulding and curing the stamps. The plaster matrix can also be made by casting from a thinner mixture of plaster and water. After the type has been set up, or the model has been selected and placed face up and horizontal, a little ridge or projection must be made all around it. Paper can be pasted around it, and wound with thread for this purpose. It is oiled and wiped off as before. The plaster is now mixed with water to the consistency of cream, and is poured upon the model until it lies even with the projecting ledges or paper border. In an hour or less it can be removed. If water is used the mould should before use be treated with shellac solution as already described. The plaster may also be mixed with gum arabic solution, or with three to ten per cent. of powdered marshmallow root. This increases its toughness.

What is known as the oxychloride of zinc cement appears to the author to be far preferable to common plaster of paris. It is a trifle more expensive, but it costs so little that it is well worth trying. It is made by mixing oxide of zinc with a solution of zinc chloride. No particular strength of solution or proportions are prescribed; the zinc chloride solution should be a strong one, and the mixture should be of about the consistency of soft putty.

Zinc chloride may be bought as a solid substance or in strong solution. The latter answers for the mixing directly. It may also be simply made by dissolving metallic zinc in strong hydrochloric acid. The manipulation is exactly the same as with plaster of paris.

The manufacture of papier maché and of other matrices is given in a special chapter. For all ordinary purposes the plaster or cement matrices are ample.

The stamp is made from the mixed uncured sheet rubber, whose preparation in the factory, including the operation of calendering it into sheets, has already been described. The best advice the reader can be given is not to attempt to make it except as a matter of interest and experiment. It can be purchased especially prepared for stamps from the dealers in india rubber.

A piece is cut from the sheet large enough to cover the face of the matrix. It should have a perfectly smooth surface, without cloth wrapper marks sometimes found impressed on it. The sheet as received from the maker is about one-eighth of an inch thick. It is thrown into a box of powdered soapstone or talc to secure a coating of the same on both sides. A little is dusted over the matrix and the excess is blown off. The matrix is now placed upon the base of the press, and heat is applied.

To carry out the process most simply the press if of metal may be placed upon a support over a gas burner or kerosene lamp, or even on a kitchen range or stove. It will in a few minutes become warm. The sheet of india rubber is now dusted off and is placed in the press upon the matrix. The platen of the press is screwed down upon it.

As the india rubber becomes hot it begins to soften and flow. By the action of the screw of the press it must be forced down from time to time as it softens. This drives the putty-like material into all the interstices of the mould. The excess escapes from the sides of the tympan in cases where the latter is of restricted area. The press theoretically should be heated to the vulcanizing temperature, which is 284° F. (140° C.). In practice the heat is not determined with a thermometer. The operator learns by experience how much heat to apply. The regulation type of gas heated press or stamp vulcanizer is shown in the illustration on page 53.

As some of the india rubber is sure to protrude, the progress of the work can be watched from its action. By pressing the point of a knife against it the period of vulcanization can be told. Before the material is heated it is elastic and resists the pressure of the knife; as heat is applied it becomes soft like putty; as the heat increases it again stiffens and becomes quite elastic. At this point the press can be opened and the sheet and matrix can be taken out or the platen swung aside. On pulling or stripping the sheet from the matrix it will be found to reproduce the model in elastic india rubber to the minutest detail.

As regards the minor details there is something to be said. Distance pieces to gauge the thickness have been recommended for the home-made press, page 48. Care must be taken to have these low enough to provide for enough excess of material to produce a good impression. For ordinary stamp work they should allow about one-sixteenth of an inch for the “squeeze.” It will be seen that by using the distance or gauge pieces both for making the matrix and for moulding and curing the stamp, absolute parallelism of surfaces will be secured.

The reader will have noticed in the description and will find at once in practice that the press has to be screwed up as the rubber softens. Where heavy iron presses are used the large mass of heated iron comprised in the platen of the press instantly heats the upper surface of the india rubber sheet and the heat immediately penetrates into it, while the heated matrix heats it from below. Thus it softens at once, and the press is directly turned down and the india rubber is driven into the mould and curing at once begins. But where small presses are used this manipulation is not so easy. For such the springs mentioned on page 51, are highly to be recommended. The matrix and india rubber can be put into the cold press, and the tympan with intervening springs can be screwed down so as to compress them. Then on applying heat the moulding takes place automatically.

With a hot press and good sheet a period of three to ten minutes is ample for moulding and curing.

Instead of sprinkling with talc the matrix may be oiled and sprinkled with plumbago and afterwards polished with a brush. This is not so clean a material as talc and is not to be recommended for general use, especially as oil is a bad substance to bring in contact with rubber.

The distance or gauge pieces whose use has been recommended are not necessary where presses working truly parallel as regards their opposing faces are used. But where home-made apparatus is used they will be found a valuable addition.

In describing the simple press it was said that it could be made of wood. It is evident that a wooden press could not be used for direct heating. Such a press must be used in a hot chamber or vulcanizer, properly so called. Originally rubber stamps were generally made in chamber vulcanizers.

The next cut shows a combined matrix making, moulding and vulcanizing apparatus of very convenient and compact form and adapted for rapid work. As the press stands in the cut the matrix press is seen in front. A box or chase is carried under its platen by two trunnions, so as to be free to oscillate to a limited extent. The type model is secured in this box. Above this box or chase is a cross-bar with screw and platen attached, connected at will to two standards or pillars, so as to constitute the matrix press.

A matrix plate swings on a hinge joint between the two presses. The hinge-pin is removable. Its ends can be seen projecting to right and left of the press columns. The hinge is at such a height that when the matrix plate is swung forward over the type box it will rest upon it in a nearly horizontal position. The pivoted box will adjust itself so as to come into parallelism with the plate.

When the matrix plate is swung back it falls upon the base plate of the vulcanizing press seen in the rear.

In use the composition used for the matrix is spread upon the matrix plate, which may for this purpose be removed from the apparatus. It is replaced and the hinge-pin is pushed home. This is done with the composition coated side facing the front of the apparatus as it stands in the cut. The plate is then swung forwards, the platen of the matrix press being turned forward out of the way, and is pressed down upon the type or other model that rests in the type box. If desired the press is used to force it home. The cross-bars of both the presses are arranged to swing each one on one of the pillars, so that the platens are turned to one side out of the way of the matrix plate as it is swung back and forth.

The pressure is released and the platens are turned aside. The matrix plate is swung over to the rear upon the bed-plate of the vulcanizing press. Here it lies with the composition-matrix upwards.

A lighted lamp, either alcohol or gas, is placed beneath the bed-plate of the vulcanizing press on which the matrix rests. This quickly dries it and brings it to a good curing temperature. The cross-bar and platen may be swung over it during the heating so as to be heated at the same time. The matrix is talced when dry and hot; the mixed sheet itself talced, is placed upon the matrix, the platen is screwed down upon it, and in a minute or two the moulding and curing is completed.

A vulcanizer, properly speaking, is a vessel arranged to heat to a definite degree any desired articles which are to be cured. The favorite type have been the steam vulcanizers. If steam is generated from water at a constant pressure, other things being equal a constant temperature will be produced. By raising or lowering the pressure the temperature can be made to rise or fall. A steam vulcanizer is a tightly sealed vessel which contains water and which is provided with a thermometer or a pressure gauge as well as a safety-valve, safety disc or safety plug. By keeping the gauge at constant pressure or by keeping the thermometer constant the temperature can be limited and kept steady. The following table gives some pressure in pounds per square inch with temperatures corresponding to steam of such pressures:

The illustration, p. 64, shows a vulcanizer of modern type made for rubber stamp work. In some recent vulcanizers the water and steam are excluded from the vulcanizing chamber, being contained within double walls forming a steam jacket and maintaining a constant heat within the chamber. These illustrate a point that has been much misapprehended, namely that curing is independent of pressure or atmosphere. Because vulcanizers have generally been filled with steam at high pressure many have supposed that the steam or pressure had something to do with their action. The fact is that it is only the heat due to the steam at such pressure that is instrumental. Steam is a very powerful radiator and absorber of so called radiant heat. For this reason an atmosphere of steam maintains all parts of the vulcanizer at an even temperature and is to that extent advantageous. Its presence and the pressure it generates are not by any means required for vulcanizing. Its pressure is entirely without effect.

To use a steam vulcanizer, water is introduced, the article in the press or mould is placed in it, and the top is secured. Heat is then applied, best if on the small scale, from a Bunsen gas burner gas, or oil stove. Either the pressure gauge or thermometer may be watched, and the flame turned up or down to keep it at the proper temperature.

Moulding cannot be executed in the ordinary closed chambers. The press must first be heated to the temperature of boiling water or thereabouts and the moulding is then effected by screwing down the mould screw, upon the sheet and matrix. It is then placed in the vulcanizer and cured.

The manufacturers supply gas regulators which automatically regulate the gas supply. These are worked by the steam pressure. If any one wishes to study the practical manipulation of small steam vulcanizers he can see them in use at any dentist’s office.

There is no need of a steam vulcanizer for ordinary stamp work. The hot press system already described answers every purpose and is in use by the most advanced manufacturers for thin sheet work. But if a wooden moulding press is used then it must be heated in a vulcanizer or some kind of oven or hot chamber.

A very simple and reasonably satisfactory oven or air bath can be made from a flower pot and a couple of tin plates. A plate larger in diameter than the mouth of the flower pot forms the base of the apparatus. This is supported on a stand over the gas lamp or other source of heat. A smokeless flame or one depositing no lampblack should be used. Alcohol or a kerosene oil stove illustrated on page 59 are excellent. On this plate a smaller plate is inverted, which latter must be so small as to be surrounded by the flower pot and to be included within it when the pot is placed over it like an extinguisher.

A chemical or round stemmed thermometer is arranged to go through the aperture in the upturned bottom of the pot. This may be hung from a support or it may be secured by passing through a hole in a cork or block of wood. Its bulb should be near the part of the chamber to be occupied by the mould or press.

The press with the article to be cured is placed upon the inner plate. The temperature is maintained at the proper point by regulating the heat, and all the conditions for excellent work are supplied. The disposition of the apparatus is shown in the cuts.

Another arrangement equally simple is given in the next cut. An iron kettle has a layer of type metal or lead poured an inch thick cast within it upon its bottom. A thermometer passing through a hole in the cover enters a cup of glycerine that stands upon the bottom. This gives the temperature.

The object of having a thick or a double bottom is to prevent excessive radiation of heat from any one part. The essential condition for good operation is to maintain an even temperature throughout the chamber.

The thermometer is not an absolute necessity. By removing the press from time to time and inspecting the overflow of india rubber the progress of the operation can be watched. An extra piece of india rubber may be placed on a piece of wood by the side of or upon the wooden portion of the press, and its condition can be taken as the criterion. Pressure with the point of a knife will tell the vulcanizing point.

By the press system of curing, a heat far above the vulcanizing temperature may be made to do good work by a very short application. There is however danger of burning the work if left in too long. If the air-bath with thermometer or the steam vulcanizer is used, and the heat is kept down to the proper curing temperature, there is no danger of burning the india rubber even if the curing is considerably prolonged.

As the flower pot has often to be lifted off for introduction or removal of the press, and as it gets quite hot, a holder of some kind is requisite. A piece of heavy blotting paper is very convenient for this purpose.

The flower pot system with thermometer can be further simplified by being used on a stove or range. A china saucer inverted, or some similar support, should be placed under the pot. A part of the stove at very low heat will suffice. The kettle vulcanizer, can also be placed on a stove so as to dispense with gas or oil.

Finally, as the last step in simplifying the work, a stamp can be made without any special apparatus beyond a hot flat iron. The matrix may be placed on a stove where the heat is rather low, the talc-coated mixed rubber sheet placed upon it, and on this a hot flat iron. In a few minutes if the heat is sufficient the stamp will be finished.

A few words may be said about the type. High spaces and quads between the letters should be used, such as will come up to the shoulder of the type, as has been said. But a very nice effect is produced by using low quads between words. This leaves each word elevated by itself, producing a good appearance.

Autograph stamps are made from a model cut in wood by a wood engraver. The autograph is written in some form of copying ink upon a piece of paper, and is transferred by moistening and pressure to a block of wood. With an engraver’s tool the wood is cut away from the lines, as the block is routed after the inscription has been “outlined.” The woodcut is used as a model for making a matrix.

It is evident that an autograph of fair quality could be obtained from a chalk plate. But in rubber stamp work to get good results certain essential parts should be of the best. These parts include the mixed rubber, model and matrix. A departure from excellence in any of these tends to the production of an inferior stamp. What is known as a “healthy cure” is above all essential to the appearance of the product.

The stamp thus made is attached to a wooden handle by common glue or by one of the rubber cements given in chapter XVI.