188 



DISCOVERY 



mould permanently, being now much more elastic than 

 the raw gum, and i)erfcctly soluble in the usual solvents. 

 Rubber possesses great adaptability for mixing with 

 other bodies, a fact which manufacturers are perfectly 

 aware of, and articles solely of the pure vulcanised 

 gum are only made for special purposes. Among the 

 many substances which lend themselves to com- 

 bination with rubber arc, zinc sulphide, white lead, 

 asbestos, litharge, chalk, hemp and boiled linseed 

 oil. B}' means of judicious admixtures of these, 

 rubber can be brought to any degree of hardness 

 and the cost can be manipulated to the makers' 

 if not to the users' satisfaction. Low-priced rubber 

 articles which are required to have the life and elasticity 

 of good rubber cannot be recommended, for the very 

 excellent reasons that good material is expensive, and 

 aJl makers must have a profit. 



A popular but utterly erroneous impression is that 

 old articles of rubber, like old brass and copper, can 

 be " melted again." The only possible waj^ of melting 

 rubber is to heat it, and this entails absolute destruc- 

 tion, leaving only a stickj', evil-smelling oil behind it, 

 which never again sets into the old solid form. Of 

 course, second-hand rubber is put to many purposes, 

 but this can only be done by rasping it into fine particles 

 and cementing these together into a solid form with 

 either boiled linseed oil or a little new rubber solution. 

 Once vulcanised, rubber is proof against all known 

 solvents ; many will soften and cause it to swell up 

 in size, but none wiU bring it back to a workable form ; 

 and although thousands of attempts have been made to 

 bring it back into the same condition as before the 

 heating with sulphur took place, they have all failed. 

 Buyers of rubber articles will have noticed a big 

 difference in the durability between low-priced, not 

 cheap, goods and the standard makes, which is easily 

 accounted for by the following facts. An unscrupulous 

 manufacturer will buy up old scrap, say, at 2id. 

 per pound, grind it up and mLx it intimately with 

 oxidised linseed oil, or make a better quality by 

 substituting a small portion of new rubber for the oil. 

 This gives him a material which he is able to put on 

 the market much below the price of the genuine 

 stuf?, leaving him in addition a very handsome profit. 

 Provided the shoddy loibber and the binding material 

 arc both of the same quality of rubber, chemical 

 analysis will not detect the fake ; the only test is by 

 actual wear, in which the difference is very speedily 

 found. The reason why remanufacturcd rubber will 

 not stand hard wear is the lack of coherence between 

 the particles of old material, these being only held 

 together by the merest film of new rubber, so that, 

 instead of being a tough, homogeneous substance like 

 new rubber, it consists of particles which readily 

 crumble and break away. When such a binding 



substance as linseed oil is used, the strength is much 

 less than rubber solution, and, owing to this oil setting 

 into a flexible substance verj' like rubber in appearance, 

 it is a favourite constituent of rubber compositions. 

 For such purposes as door-mats, cheap shoes, rubber 

 heels, toys, etc., the use of the resuscitated material 

 is legitimate ; but when applied to such purposes as 

 rings for the packing of high-pressure steam-pipes, it 

 cannot be so lightly looked upon, since in this case it 

 constitutes a serious danger. Hard \ailcanised rubber 

 was formerly much used for insulating purposes in 

 electrical trades, and for such purposes many of the 

 compositions made from old or reclaimed rubber are 

 quite effective enough, provided they do not deteriorate 

 with time and lose their insulating property. At the 

 present day, however, rubber for electrical purposes 

 has been largely replaced by paper, cotton, mica, 

 glass, etc., so that its importance to this industry has 

 grown very much less. 



Like water, rubber is " incompressible " — that is to 

 say, its form is readily modified by pressing or pulling — 

 but the actual bulk remains the same, and if compressed 

 in one direction it expands equally in another. Oil 

 or grease of any kind is a deadly enemy to rubber, 

 which softens under its action and quickly loses its 

 elasticity and strength. Sunlight is also destructive to 

 it, and sets up a gradual decomposition which nothing 

 can prevent ; in fact rubber articles of every kind seem 

 to undergo a slow change in their composition, becoming 

 more brittle with age, and with mackintoshes the 

 protective layers become friable and fall away. 

 Immersion under water is the best preservative for 

 rubber, and, as most people are aware, one day's 

 exposure to sunlight works more mischief with a rubber- 

 proofed garment than three months' wear in wet 

 weather. What the chemical compound of sulphur 

 and raw rubber is no one as yet knows ; but once 

 vulcanised the sulphur is held tenaciously, so that it 

 cannot be eliminated by any of the agents so far 

 available, and the reactions between the two seem 

 to be deep-seated and difficult to follow. A fortune 

 awaits the man luckj* enough to invent a method of 

 reclaiming old rubber or to find a substitute having 

 all the properties of natural caoutchouc. This field 

 of research has engaged the intellects of many distm- 

 guished chemists in the past, and is still doing so to-day- 

 Some time ago a patent for obtaining a rubber-like 

 body from cereals was brought out ; this was based 

 upon the chewing of com into a sticky gum which 

 many of us are familiar with as children, and the 

 process was very similar to that of mastication — 

 namely, washing away all the starchy granules until 

 nothing was left but a grey glutinous substance. 

 This, like many other ideas, does not appear to have 

 met with success, as nothing further has been heard 



