476 



long, which is rapidly oscillating with a saw-like movement and 

 which is well lubricated with water or soap and water. Sheets cut 

 in this way show a fine striation due to the little ridges which mark 

 the progress of the knife at each stroke across the block or cylinder. 

 A good tobacco pouch is usually made from this cut-sheet and 

 shows the appearance described. The most interesting feature in 

 making cut-thread or sheet to the man interested in rubber is the 

 process of preparation of the rubber into blocks ready for the knife. 

 The utmost care must be taken in the preliminary washing, and if 

 any grit be in the raw rubber the washed sheet is subjected to a 

 final cleaning between smooth hardened steel rollers which crush 

 the grains of sand which are "then washed out. The rubber is then 

 well masticated and mixed with sulphur and whatever other ingre- 

 dients may be required. The rubber is then forced by hydraulic 

 pressure into huge iron moulds which will contain sometimes as 

 much as a ton of rubber and which are rectangular or cylindrical 

 according to the type of machine which is to cut sheet from them. 

 Special care has to be taken to prevent the inclusion of air bubbles 

 in this block of prepared rubber. When rubber is compressed in 

 this way into cylindrical moulds for manufacture of cut sheet an 

 axle of steel is forced through the centre of the mass while still in 

 the iron mould. The moulded mass has then been to be annealed 

 by gentle heating and maintaining it at a moderate temperature for 

 some little while. The next process is to harden the block by 

 freezing for a week in a refrigerating room, where it remains after 

 removal of the mould until wanted for use. The cutting edge of 

 the knife, and the surface of the rubber, are plentifully lubricated 

 with water during cutting, this also fulfils the further purpose of 

 thawing the immediate surface of the rubber and bringing the 

 rubber to a suitable condition of hardness for the operation. The 

 sheets when cut must be carefully handled, being still soft and self 

 adherent, unvulcanised, though perhaps containing mixed sulphur. 



DIPPED GOODS. 



33. There is still another mode of manufacture of hollow rubber 

 goods which may be called the dipping process, it is simple in 

 principle and very similar to the way in which the old-fashioned 

 tallow dip candles were made. A thick rubber solution is prepared, 

 usually of pure rubber and solvent, though pigments may be mixed 

 with it. A mould representing the internal shape of the required 

 article is dipped into this liquid and withdrawn. The solvent 

 evaporates leaving a film of rubber on the moulds, the operation is 

 repeated until the required thickness of rubber is accumulated. 

 Any manipulation or cleaning of the edges is now carried out and 

 the rubber still on the mould is vulcanised. Here the "cold cure" 

 has to be adopted, since the rubber contains no sulphur already 

 mixed, and the customary solution of 3 per cent, of sulphur chloride 

 in carbon bisulphide is employed as previously described under the 

 head of vulcanisation. Certain classes of surgical rubber goods 

 are made in this fashion and india-rubber teats for feeding bottles 

 are turned out by the thousand. A final dressing of rubber enamel 



