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In′dia-rubber.

India-rubber in the crude state is prepared for manufacture by grinding the sheets in water, for the purpose of cleansing it. After this it is dried and subjected to the tearing action of toothed rollers, one of which revolves faster than the other. At this stage the vulcanizing material, consisting of 1/2 ounce of sulphur or more to the pound of rubber, according to the required hardness, is added and thoroughly incorporated. Various other ingredients, as sulphate of zinc, whiting, plaster of paris, lampblack, pitch, etc., in addition to the sulphur, are also employed for the purpose of hardening. The selection of these depends on the peculiar ideas of the manufacturer and the destined application of the material.

In making boots and shoes the rubber is passed. in contact with cotton cloth, between heated rolls which rotate at different speeds, and is forced between the interstices of the cloth, to which it becomes firmly united. In some cases the rubber is drawn, either with or without the cloth, through rolls of equal speed, to a thickness suitable for the soles or uppers of shoes. The sheets thus formed are cut up, by suitable dies, into the proper forms for the soles and uppers, which are afterward cemented on a last by a solution of rubber dissolved in benzine. At this stage a coating of linseed-oil varnish is applied with a brush, and the shoe subjected to a heat of about 280° Fah., when the operation is complete.

In making hose or tubing, the rubber or rubber cloth is wrapped around a tube until the desired thickness is attained, and placed in a steam-heater at a temperature of about 280° Fah., which partially fuses the compound, causing the layers to unite. It is then removed from the heater and the tube withdrawn.

Belting is formed by laying together a sufficient number of layers of the vulcanized gum or gumcloth and subjecting them to the action of a flat press at a heat of 280° Fah. Additional hardness is attained by the use of a larger proportion of sulphur and the application of greater heat.

The operation of uniting several layers of the material is, however, performed with much more expedition by the use of a steam cylinder-press.

Previous to 1821, it is said that india-rubber had only been brought to this country as an article of curiosity, molded into various fancy forms, as alligators and other reptiles. In that year a pair of what appeared to be solid models of shoes were brought home by a sea-captain, who presented them to an ingenious boy. Curiosity led him to cut into them at the instep, when he found a clay mold inside, this he removed, and finally, by dint of heating and stretching, adapted them for wear. This becoming known, led to the importation of the native shoes made upon clay lasts. See caoutchouc.

No process has yet been devised for restoring old rubber to the condition in which it was previous to vulcanization. It is, however, extensively used for mixing with new raw material in the manufacture of all kinds of rubber goods. By steaming and rolling it is reduced to a semi-plastic state, and in this condition it is applied to coarse fabrics and used for heel stiffening, a purpose for which it is well adapted. Pure rubber is but little used except for artists' purposes.

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1821 AD (1)
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