i6o 



NATURE 



[June i8, 1908 



does not exhibit the artistic cnstalline structure produced 

 in hot galvanising. It is very even/ and takes on a 

 beautiful polish, so that actually it can be made more 

 artistic than zinc deposited by hot galvanising, which, 

 owing to its crystalline structure, cannot very well be 

 polished. 



But beside the actual cold gah'anising and the galvan- 

 ising by means of molten metal, there is a low temperature 

 dry process which was accidentally discovered by Mr. 

 Sherard Cowper-Coles some little time ago. In order to 

 temper certain articles, Mr. Cowper-Coles imbedded them 

 in zinc powder, and they were then heated to a temperature 

 which was considerably below the fusing temperature of 

 zinc. After cooling the articles were taken out, and 

 were, to the surprise of the operator, found to be homo- 

 geneously coated with metallic zinc. The zinc was firm 

 and adherent, and on cutting a rod of metal through, it 

 was found that the zinc had penetrated an appreciable 

 depth into the other metal. E.xperinients w'ere then under- 

 taken to ascertain \vhat was the lowest temperature which 

 could be employed, and whether all metals would thus 

 react with the zinc. It was found that metals could be 

 evenlv and completely coated with zinc bv heating to a 

 temperature of from 250° C. to 330° C. (zinc melts at 

 418° C). The process is now called Sherardising, after the 

 name of the inventor, and is worked essentially 

 as follows : — 



Zinc dust, which is obtained as a by-product 

 in the metallurgy of zinc, is the raw produi t 

 employed. This substance, which amounts to 

 about 10 per cent, of the zinc produced, and 

 can therefore readily be obtained in large 

 quantities, consists of very finely divided zinc 

 mixed with considerable quantities of oxide. 

 Scale and oxide are removed from the articles 

 to be coated ; this, of course, is also necessary 

 in all other galvanising processes. In other 

 processes, however, it is also necessary to re- 

 move grease, but in this dry galvanising the 

 removal of grease is not necessary, and its 

 presence, at any rate in small quantities, 

 appears to be a distinct advantage. After the 

 scale has been removed the articles are placed 

 in a closed iron receptacle, and the zinc dust 

 charged in ; the containing box mav be 

 arranged either to work stationarily or rotated. 

 The temperature is then raised to 250° C. tn 

 330° C. by means of gas firing, and maintained 

 so for from half an hour to several hours, de- 

 pending upon the coating required and thr 

 nature of the article to be coated. The drum 

 is then allowed to cool, and discharged. 

 Owing probably to the individual grains of zinc 

 in zinc dust being coated with oxide, it is a 

 verv difficult matter to cause it to melt 

 together even when the temperature is raised far above 

 its melting point. This is one of the troubles met with 

 in the metallurgy of zinc, that is, there is a tendency 

 for the metal to condense as dust and not to run together 

 and liquefy. But in the Sherardising process this is a dis- 

 tinct advantage, because, should the drum become over- 

 heated, there will be no danger in its melting together 

 and spoiling the articles to be galvanised. 



In order, so far as possible, to prevent increase in 

 oxide during heating, the receptacle is preferably made 

 air-tight, but when this is not possible about 3 per cent, 

 of fine charcoal is added to the zinc. Otherwise, as the 

 tjuantity of oxide increases, the deposit lacks in lustre. 

 The fact that small quantities of grease do not prevent 

 the production of a good deposit is of importance, because 

 it enables machined work, such as bolts, screws, &c., 

 to be placed directly into the galvanising drum without 

 previous cleaning. For small articles, such as screws, 

 rings, nuts, &c., a closed iron cylinder which can be 

 rotated or oscillated is the most suitable form of furnace. 

 For tubes, oblong or cylindrical work, it is most satis- 

 factory to have the door at the end of the furnace, and 

 oscillate it on its axis. .A number of Bunsen burners 

 are arranged below the drum, and the whole is enclosed 

 in an iron shell lined with asbestos. For larger work, 

 stationarv iron boxes are emploved, as illustrated in 

 Fig- 3- 



NO. 2016, VOL. 78] 



A plant has been erected near London containing four 

 furnaces capable of taking drums 6 feet by 2 feet, with a 

 cubic capacity of two tons of material at one charge. The 

 furnaces are heated with Dowson gas. It is essential 

 I that the zinc dust bo dry before being added to the 

 furnace, otherwise the zinc is oxidised, and hydrogen is 

 I given off on healing. 



It is stated that in practice dry galvanised or Sherardised 

 steel and iron will withstand the ordinarv corrosive agents 

 to which galvanised ware is usually e.xposed to a remark- 

 able degree. Kven afier the apparent removal of the zinc 

 by abrasion, the iron will retain its resistivity. 



This resistivity is probably due to the protective action 

 of the zinc-iron alloy formed below the actual zinc coat- 

 ing. The temper of steel wire is not reduced by dry 

 galvanising, owing to the low temperature at which the 

 process is carried out. In Sherardising, the zinc does not 

 form a more or less thick coating or skin, but actually 

 j sinks into the metal. .\s a consequence, the fine lines of 

 screws and other articles which have been machined are 

 not blurred. For example, the minute screws which are 

 employed in making watches can be Sherardised, and fit 

 the holes tapped to receive them quite as well as before 

 being zinced. 



In the ordinarv method of galvanising it has not been 



found possible to produce patterns, because, of course, the 

 whole article has to be dipped, and there are also difficul- 

 ties in doing this with electrolytic galvanising owing to 

 the electrolyte getting beneath the stopping material which 

 may be used ; but with the Sherardising process it has 

 been found possible to produce some very .artistic effects. 

 In order to do this, the article is coated with a stopping- 

 off composition. The composition is about the consistency 

 of soap, so that it can be easily cut with a knife. The 

 design is traced with an edged tool, and the portions to 

 be removed are lifted awav, so that a clean surface is 

 exposed to the action of the zinc. The object so prepared 

 is placed in an iron drum containing the metal which is 

 to be inlaid in the powdered form, for example, the zinc 

 dust. The galvanising is then carried out as already 

 described. 



The temperature and the time which the heating has 

 to be continued are regulated according to the thickness 

 which is required for the inlaying, and may vary from 

 a few minutes to several hours. .After inlaying it is 

 found that the part inlaid is much harder than the brass 

 or copper into which it is inlaid. 



Furthermore, it is possible to obtain a variety of colours 

 bv means of the process. Suppose, for example, one wishes 

 to inlay a copper vase, and convert some parts of it into 

 brass ; this can be done by variations in the thickness of 

 the stopping-off composition and by heating to a higher 



