236 



NATURE 



[July 7, 1898 



GUTTA-PERCHA AND INDIA-RUBBER. 

 "DOTANISTS whoare interested in the cultivation oi Sapo- 

 ^ tacea: on a commercial scale, are beginning to realise the 

 consequences of the careless methods that have denuded the 

 Indo-Malayan regions of Taban trees. Cable-manufacturers 

 complain very seriously of the great felling-off in quality of 

 gutta-percha during the past few years, and the small hope of 

 obtaining better supplies in future. This degeneration of the 

 cultivating industry is beginning to make itself felt in the 

 Treasury Reports of the gutta-percha produciiig countries ; not 

 so much in the quantity annually shipped, as in the prices paid 

 for a given weight each year. 



For instance, the Sarawak (Borneo) Treasury Report of 

 revenue and expenditure for 1897 gives comparative figures 

 relating to the condition of supply and demand of gutta-percha 

 and india-rubber ; the following table, drawn up from the 

 Report, shows the fall in prices of gutta-percha during the four 

 years 1894-1897, inclusive, and indicates a corresponding 

 degradation of quality. (The "picul" is, for our purpose, 

 taken as 133^ lbs., and the Sarawak dollar as l^-. \\d.) 



Gutta-percha exported from Sarawak. 



With this may be compared the increased demand for, and 

 steady value of india-rubber throughout the same period. 

 India-rubber exported from Sarawak. 



From another source we are able to give the total weights of 

 gutta-percha landed in England, from all gutta-percha producing 

 countries, since 1895. 



Total Weight of Gutta-percha landed in England. 

 Year. Tons. 



1895 716 



1896 ... 318 



1897 396 



January to April 1898 626 



The present year shows a very marked rise in the demand for 

 gutta-percha ; this is more apparent when it is remembered that 

 the 626 tons was all landed between January i and April 30, 

 and that the quantity landed in April alone was 149 tons. 



We may sum up the condition of the gutta-percha cultivation 

 industry in a few words : there is an increasing demand, a 

 degeneration of quality, and an almost total disregard of the 

 future. Experimental efforts have, we believe, been made to 

 produce a steady supply of high-quality gutta-percha, but so 

 many years are required to establish the scheme on a profit- 

 earning basis, that it is almost beyond the powers of private 

 enterprise to make it a success. 



TREATMENT OF THE SURFACE OF 



MEDALS.^ 



Silver. 



TN this country medals have been issued for centuries with the 



tables or flat surfaces smooth and mirror-like, while a more 



or less frosted texture has been given to the portions in relief. 



This is especially the case in medals which have been struck as 



1 From a memorandum by Prof. Roberts- Austen, C.B., F.R.S., in the 



Twenty-eighth Annual Report of the Deputy Master and Comptroller of the 



Mint, 1897. 



NO. 1497, VOL. 58] 



specimen pieces, for after highly-polished dies have been used for 

 a certain time the difference between the appearance of the 

 tables and the parts in relief becomes less and less marked. 

 As is well known, medals with polished surfaces rapidly tarnish, 

 and even blacken, by exposure to the ordinary atmospheric in- 

 fluences. In France a different system has long been adopted 

 concurrently with the one just described. Unpolished dies are 

 employed, and care is taken to impart to the medals struck 

 from them a dead or frosted surface by rubbing them with fine 

 pumice. Recently, at the French Mint, medals have been sub- 

 jected to the process known as "sand blasting" by the aid of 

 an appliance which projects against the surface of the medal a 

 small jet of air, carrying with it fine sand, and having a velocity 

 of about 180 feet per second. When thus treated the surface of 

 the medal becomes minutely granular or frosted, and may then 

 be further treated in several ways. Sometimes the surface is 

 darkened by exposure to an aqueous solution of a sulphide, 

 followed by rubbing with very fine pumice, which removes the 

 dark layer of sulphide from the portions in high relief, and leaves 

 dark lines in the more deeply-cut recesses. It is, however, 

 preferable to cover the medal with a layer of platinum, and this 

 is effected by immersing it in an alcoholic solution of chloride of 

 platinum until a blackened surface is produced. Subsequent 

 rubbing with a brush and very fine pumice changes the blackened 

 surface to a delicate grey ; and if this operation is conducted 

 skilfully, graduated shadows may be left wherever the artist con- 

 siders their presence to be desirable. The beauty of medals so 

 treated, and the fidelity with which the details of the design are 

 revealed, are beyond question ; but it may be doubted whether 

 the surface of the medal is permanently protected. A medal with 

 a frosted platinised surface has, however, a great advantage 

 over one with a polished table, as the platinised medal is merely 

 deepened in tone by exposure to the atmosphere, and, unlike 

 medals which have been struck in the ordinary way, does not 

 become disfigured by blotches of tarnish. The frosted platinised 

 medal may be restored to almost its original freshness by careful 

 rubbing with a soft leather ; while a polished silver one cannot be 

 so renovated, as the tarnish attacks the surface and destroys the 

 polish. 



During the past year, for the first time in the history of the 

 Mint, medals have been issued with frosted and platinised 

 surfaces. More than 27,000 large silver medals were platinised 

 by a slight modification of the above method. It became 

 necessary, therefore, to provide an appliance for producing the 

 sand blast, and this, together with a small i H.P. motor for 

 driving it, has been fitted up in the basement of the Assay 

 Department. 



Bronze. 



Medals of bronze differ considerably from those of silver, as 

 their surfaces are far more liable to be influenced either by the 

 slow operation of the constituents of the atmosphere or by the 

 more rapid action of chemical agents. Ancient silver coins, for 

 instance, which have been long buried in the earth, do not show 

 anything like so wide a range of colour in their patina or crust, 

 as is revealed on coins of brass, bronze or copper, which have 

 been hidden in the same way. This is due to the fact that silver 

 is far less affected than copper by the chemical action of the 

 constituents of soils, or by atmospheric influences. The patina 

 acquired by an ancient coin or medal often constitutes no small 

 part of its value. " You would laugh at me," said Philander, 

 in Addison's charming dialogues upon the usefulness of ancient 

 medals, " should I make you a learned dissertation on the nature 

 of rusts ; I shall only tell you that there are two or three sorts 

 of them which are extremely beautiful in the eye of the 

 antiquary, and preserve a coin better than the best artificial 

 varnish." The object of the medallist is accurately expressed in 

 the above sentence, for he endeavours to protect the surface of 

 all medals in which copper is the main constituent, by a patina 

 or film of oxide, so as to preserve the medal from further change. 

 This may be effected in various ways. The medals of the 

 Italian Renaissance were not struck, but cast by the method of 

 cera perduta, aXxQudy descnhei in these Reports, 1 and much of 

 the beauty of the medal was due to the " skin" or pellicle of 

 oxide which the medal acquired during casting. The skill of 

 the artist in arranging the composition of the bronze, and fixing 

 the temperature at which it was cast, was revealed in the texture 

 of the medal's surface. 



In modern times most medals to which the name of bronze is 

 given are really of copper, " bronzed " or coloured artificially on 



1 Sixteenth Report (i886), pp. 24, 49 ; Seventeenth Report (.1886), p. 15. 



