CHAPTER ELEVEN

RUBBER SHOE MANUFACTURE

Examine the rubbers we wear during the winter and stormy weather.

Rubber shoe coverings are made to protect the shoe from water and snow and may be in the form of either slippers or arctics. The covering is rendered waterproof by means of a compound rubber.

Rubber is the name given to a coagulated milky juice obtained from many different trees, vines, and shrubs that grow on that part of the earth’s surface which forms a band some three or four hundred miles on either side of the equator.

Rubber is graded commercially, according to the district where it is found. In the order of importance it may be divided into three general sorts, viz., American, African, and Asiatic. The best and largest quantities of rubber come from Brazil, along the banks of the Amazon River. The countries in the northern and western part of South America, and the Central American States and Mexico furnish considerable rubber. Eastern and western Africa also produce many kinds of rubber in large quantities, though somewhat inferior to the Brazilian product. The Asiatic rubbers are unimportant in quantity, and, excepting the rubber obtained from cultivated trees in Ceylon, are decidedly inferior in quality.

The fluid rubber obtained from Brazil is called Para and is used principally in the manufacture of rubber footwear. The method of gathering and coagulating the rubber juice (called latex) varies in the different countries. The native first clears a space under a number of trees and proceeds to tap the trees with a short-handled ax, having a small blade, by cutting gashes in the bark. A cup is fixed under each cut to catch the fluid as it flows out. As fast as the cups are filled, they are emptied into a large vessel and carried to the camp to be coagulated. A fire is started in a shallow hole in the ground, and palm nuts, which make a dense smoke, are thrown on. An earthen cover which has a small opening on top is placed over the fire, allowing the smoke to escape through the opening. A wooden paddle is first dipped in clay water and then into the latex and then held over the smoke. The heat coagulates a thin layer of rubber on the paddle. It is dipped again and again in the latex and smoked each time. After being dipped many times, a lump (called biscuit) of rubber is formed. A cut is made in the biscuit and the paddle removed. Then the rubber is ready for market. The world’s crop of rubber in 1911 was about ninety thousand tons.

Few people realize the number of operations necessary to produce from the crude biscuit of India rubber the highly finished rubber shoe of to-day. Briefly stated, the various steps are washing, drying, compounding, calendering, cutting the various parts, making or putting these parts together, varnishing, vulcanizing, and packing. Each of these processes requires a distinct and separate department, and many of these processes are subdivided into minor operations.

The huge stock of Para rubber, that is rubber obtained from the Amazon section, to be found in any of the leading rubber factories counts well up into the thousands of dollars. With rubber at or near $1.50 per pound, a stock of ten to fifty tons runs up into the five or six figures.

This crude rubber, as it comes from the Amazon, contains more or less dirt, pebbles, and other foreign substances, which must be removed.

The large cakes of crude rubber are first broken up by a cracker machine, consisting of two large, revolving steel cylinders, from which the product falls into pans or trays. It goes then to a machine known as a “washer” or “sheeter,” where it is run between revolving cylinders, upon which a continuous spray of clean water is maintained. After being rolled into rough sheets, it is put into a tank, from which it is taken to the “beater” machine, in which water runs continuously, and then it is washed again and “sheeted out.” It is then dried in one of two ways.

(1) The older way. The sheets are hung over rods in a large room, and allowed to dry in the air. To facilitate the same, a fan or blower is often used to cause a circulation and removal of the moisture-laden air. This requires a period of from one to two or three months.

(2) The second method is called vacuum drying. This process is gradually being introduced, so that now probably more rubber is dried in vacuum than by air. The vacuum drier consists of a large iron cylinder filled with plates, through which steam is allowed to circulate. The rubber is placed on the plates and the air is exhausted from the cylinder by means of an air pump until very nearly twenty-six degrees of vacuum are obtained. By this process only from two to three hours are required to produce perfectly dry rubber.

The making of a rubber shoe is not the simple matter which might at first be supposed. An ordinary rubber shoe consists of at least seven or eight different parts, sometimes twenty-one parts to a pair, while a high-button gaiter has seventeen distinct parts, and a rubber boot has twenty-three different pieces. There are insoles, outsoles, stays, piping, foxing, and a dozen other different pieces, each one of which is necessary to the proper construction of a rubber shoe or boot. The thinner sheets for the uppers are cut by hand, the deft work of the cutters in following the patterns outlined on the sheets being the result of years of practice. The sheets of rubber from which the uppers and soles are cut are at this stage of the work plastic and very sticky. It is necessary on this account to cut the various pieces one by one, and keep them separate. The soles and some of the heavier pieces are dried out by the machine, and the heels are made by a special machine, but by far the greater part is done by wonderfully skilled hands. All of these parts which go to make a shoe, or the twenty-three parts which go into a boot, are collected and sent to the making department, which, in most factories, is a large room containing several hundred operatives, each working by herself, and bringing the many separate parts into the fully finished footwear.

The sheets of rubber, after being dried, are taken to the “compound” room, where they are sprinkled with whiting, to prevent sticking, and weighed. Next they are taken into the calender room to a “mixer,” by means of which the rubber is combined with other substances, which include sulphur, litharge, whiting, lampblack, tar, resin, lime, palm oil, and linseed oil.

There are different calendering machines. The ones called the upper calenders form sheets of rubber stock for the upper part of the shoe. The soling calenders form the stock for the sole or bottom part of the shoe; other calender machines are used to coat a layer of gum on one side of the fabrics used for lining and various strips, fillers, toe, and heel pieces. The gum sheets are sent to the cutting room.

Generally, linings for nine pairs of shoes are cut at once. The linings are cut both by hand and by machine. Men who cut with dies, by hand, stand at the bench and use iron mallets, like those used in cutting heels. Inner soles, heel pieces, and linings are all cut by means of dies in the same manner.

The edges of the several parts are spread with cement, and then the parts are taken to the making room and distributed. In the making department the boots and shoes are put together. Women make the light overshoes; men make the heavy ones. Rubbers are made by women, but men put on the outer soles.

Linings are first applied smoothly to a wooden last and cemented together, the cement side out. The rubber parts are then stuck on and rolled firmly with a small hand roller. Young women become very skilled in this work, taking up the several parts in rapid succession, placing them accurately upon the last, and rolling and pounding them firmly together.

Perhaps the most interesting single process is that of putting the rubber boot together. This work is done by men, and requires, in addition to accurate eyesight, rapid and very deft movements of the hand and considerable strength. No nails, tacks, or stitching are required. The natural adhesiveness of the rubber, assisted by the use of rubber cement, holds the parts solidly together.

In the making of the shoe the last is covered with the various pieces which are so made as to adhere where they are placed. It is exact and nice work fitting all these pieces perfectly, each edge overlapping just so far and no farther. The lighter shoes are made by women, but the heavy lumbermen’s shoes, arctics, and especially the boots, are made by men, for this work needs strength as well as dexterity.

The goods which require varnishing are put on racks and treated with a mixture of boiled linseed oil, naphtha, and other materials, which are applied with brushes, and impart a gloss to the surface.

On vulcanizing boots and shoes, the shoes are placed on racks supported by iron cars, which are run over tracks into the vulcanizing chamber. This consists principally of a large room provided with a steam coil on the floor. The temperature rarely exceeds two hundred and sixty degrees Fahrenheit. In vulcanizing shoes, the heat is increased gradually from the beginning, about one hundred and eighty degrees Fahrenheit, otherwise the goods would be blistered, due to the rapid evaporation of moisture and other volatile constituents. They are kept in these heaters from six to seven hours. This causes a union of sulphur and rubber, which is not affected by heat or cold.

They are wheeled on another truck to the packing room, where they are inspected, taken from the lasts, tied together in pairs, or placed in cartons, as the case may be. They are then sent to the shipping room to be packed in cases ready to be delivered to the cars waiting at a side track of the railroad, or sent to the storehouse until they shall be called for by the jobbers or retail dealers.

An important branch of the rubber business is the manufacture of tennis shoes. This is a generic term, which is applied to all kinds of footwear having cloth tops and rubber soles. As the name indicates, they were first used in playing the game of tennis, but they have come into very general use as warm weather and vacation shoes, and every year shows an increased popularity. These shoes are made in a similar manner to the rubber shoes, the rubber soles being cemented to the cloth uppers and vulcanized the same as the rubber overshoes. Many different styles are made, and each year shows some improvements in the shapes, in the textiles which are used, in the colors and combinations of soles and uppers.

Rubber shoes should not be expected to give satisfactory service unless properly fitted. If too short, too narrow, or if worn over leathers with extra heavy taps, or unusually thick, wide soles, strains will be brought upon parts not designed to stand them and the rubber will give way. Rubber goods, particularly boots, if too large will wrinkle and a continued wrinkling and bending is liable to cause cracking.

Extreme heat or cold should be avoided. Rubber boots or shoes should never be dried by placing them near a heater of any kind. If left near a stove, register, or radiator, the rubber is liable to dry and crack. If left out of doors in winter, or in an extremely cold place, they will freeze. Then when the warm foot is put into them and the rubbers are worn, the rubber will crack.

Oil, grease, milk, or blood will cause rubber to decay in a very short time. If spattered with any of these, the rubber should be promptly and thoroughly cleaned with warm water and soap.

The oil in leather tops will rot rubber, so that care should be taken in storing and packing to prevent the leather and rubber from coming in contact.

Various heavy goods are advertised as proof against snagging. It should be remembered, however, that no rubber can be made strong enough to be absolutely proof against tearing or puncturing by extremely sharp edges, such as stiff stubble, sharp-edged rocks, broken glass, etc.

Mud, barnyard dirt, or filth of any kind should never be allowed to dry on rubbers. They should be cleaned as carefully as leather boots or shoes.

Exposure to strong sunlight for any length of time produces an effect on rubbers similar to that of putting them near a stove or radiator. Rubbers should not be left in the sun to dry. When not in use they should be kept in a cool, dark place.

RUBBER HEELS

Rubber heels are generally made for boots and shoes as follows. The compounded rubber is sheeted on a calender roll, on a drum, until several layers are obtained, thus making a sheet of about one inch in thickness. The heel is cut out from this sheet by means of a die and placed in a mold. It is there subjected to an extremely high pressure, generally obtained by hydraulic power. The plates of the press are heated with live steam. The heels are removed at the end of nine or ten minutes and the sheet which was formerly nearly an inch in thickness is now only about half an inch and has by pressure been molded into the shape of the heel desired, is semi or partially vulcanized, and also is imprinted upon the bottom with the name or other brand of the company.

The cup-shaped portion of the heel is now coated with a layer of rubber cement, and firmly placed on the boot ready to go to the vulcanizer, where vulcanizing of the heel is then completed.

Many articles of rubber are vulcanized by the use of chloride of sulphur, which process is sometimes known as “cold cure.” The action of sulphur chloride itself is so violent that it must be diluted, and for this purpose carbon bisulfide is often used. In some cases, as, for example, the manufacture of tobacco pouches, the articles are submerged for from one to two minutes in the liquid, then removed and washed thoroughly. In another case, as in the manufacture of some kinds of rubber cloth, such as hospital sheeting, the coated cloth is suspended in a suitable room and the chloride of sulphur and carbon bisulfide mixed and evaporated by action of heat so that the cloth is subjected to the action of vapor alone. Only articles with comparatively thin walls can be successfully vulcanized by the cold cure, as at best the vulcanizing action of the chloride is only superficial.

No account of vulcanization processes as employed in the manufacture of rubber goods is complete without the mention of “steam cure.” A great variety of rubber goods under the general term of mechanical sundries are cured by this method. This includes rubber matting, door mats, water bottles, druggists’ sundries, etc. This process consists in brief of submitting the articles to be vulcanized to the action of live steam for from half an hour to an hour, or until the goods are thoroughly vulcanized. The temperature and duration of time required depend to a considerable extent upon the thickness of the walls of the article. In order to prevent the goods from being pitted and damaged by the action of steam, they are wrapped with cloth or imbedded in pans of soapstone. A great variety of rubber tubing is cured by this method.

In rubber cloth making, the crude rubber is put through the washing process, dried and mixed with sulphur, litharge, coloring matter, etc., and then is taken to the cement room, where it is “cut” with naphtha, forming a thick paste or dough. This is taken to the spreading room in large tubs and fed into the roller machine, which is like a long table made of steam pipes placed horizontally in a single layer. Below one end is a roll of cloth, which is passed between two iron rollers on the end. The dough is fed in between these rollers and is spread smoothly over the cloth, which is rolled up and removed to a heating room, where it is unrolled and hung on racks, and then subjected to sufficient heat to cause the combination of the sulphur and rubber.

CHEMISTRY IN THE MANUFACTURE OF RUBBER GOODS

Too much stress cannot be laid upon the importance in all rubber factories of the chemical department. During the last two or three years there has been an unusual development along these lines, and to-day no factory for the manufacture of rubber goods is complete that does not possess a well-equipped laboratory. Not only does this department enable the manufacturer to control the purity and uniformity of his compounding ingredients and the innumerable grades of crude rubber, but, what is of even greater importance, it enables him to inaugurate research work as applied to his particular line of manufacture. This part of laboratory work is already producing results not only of scientific interest, but of very great practical and economic value. Still another rôle of the modern chemical laboratory is to exercise a control over the finished material, so that the manager of the works may be in possession daily of reasons for any variation detrimental to the standard of his products.

RUBBER TERMS

ANKLE PIECE. A large piece of light sheeted gum, which goes around the ankle and extends about halfway up the leg.

BACK STAY. A piece of frictioned sheeting similar to the side stay in shape and placed at the back of the heel and ankle.

GUM COUNTER. A piece cut out of sheeted gum, on the under side of which is placed a counter form or a piece of frictioned sheeting.

OUTER FILLER. A filling sole cut from rag-coated or frictioned sheeting, and designed to fill up the hollow on the bottom caused by bringing the edges of the gum vamp and counter underneath.

INNER SOLE. Usually made of felt or sheeting coated on one side with rag stock. In lasting up, the bottom edges of the lining (which have previously been cemented) are pulled under and adhere to the inner sole.

LEG COVER. A piece of sheeted gum rolled upon a piece of frictioned sheeting called the leg form.

LEG LINING. The lining, usually of felt or wool netting, for the leg.

PARA. A name given to rubber from Brazil.

PIPING. Strips of frictioned sheeting used to join the lining together over the instep and up the back, and also to hold the lining up on the tree by passing a strip over the top.

RAG COUNTER. Quarter stiff is a counter piece cut out of rag-coated or frictioned sheeting, which gives stiffness to the counter.

SIDE STAY. A spike-shaped piece of frictioned sheeting, placed on each side of the ankle.

RAG SOLE. A sole stiffening cut out of a sheet of rag stock, which covers the whole bottom. The edges are skived to make a perfect edge.

TOE FILLER. A rag-stock filling sole to fill up the hollow on the bottom caused by attaching the lining to the inner sole.

TOE LINING. The lining for the vamp, of the same material as the leg lining.

VAMP. A piece cut out of sheeted gum.

VAMP FORM. A piece of frictioned sheeting cut to the shape of the vamp, and put on over the toe lining.

WEB STRAPS. Straps put on with the joined ends between the leg lining and the leg cover, and forming a loop on the inside of the boot to pull it on with.