1834.] On the Preservation of Sheet Iron from Rust. 191 



VI. — Experiments on the Preservation of Sheet Iron from Rust in India. 

 By James Prinsep, Sec. 8(C. 



The proposed enteimve employment of iron steam boats for the navi- 

 gation of the Ganges, rendered it a desideratum to ascertain what 

 varnish or composition would best preserve the exterior surface of such 

 vessels from the rapid corrosion to which iron is so peculiai-ly subject in 

 a hot climate. A series of experiments was undertaken with this view 

 by myself at the requisition of Government ; and it may perhaps be 

 useful to record the principal results in a journal of science. 



Two sets of six wrought-iron plates, each measuring three feet by two 

 feet, were fixed to two iron triangles, the plates being prevented by studs 

 from coming into contact with each other. The same varnishes were 

 applied to both sets, one being intended for entire submersion under water, 

 the other to be only half immerged, in order to feel the united influence 

 of air and water. 



The following were the coatings applied : 



1. Common coal tar, laid on hot, and the plate heated. 



2. Theetsee varnish of Ava, one coat. This took a very considerable time 

 (two months) to dry, kept first in a cool-room, and afterwards in a room heated by 

 furnaces*. 



3. Native Dhuna, applied to the iron hot, in a thick uneven coat. 



4. Best white-lead paint, three coats; allowed to dry and harden for nearly 

 three months. 



5. Coach-makers' varnish, two coats ; dried rapidly. 



6. Spirit varnish, several coats ; warmed. 



7. White wax, melted on the surface. 



8. White wash, of pure lime water. 



9. The surface of the iron plate cleaned and guarded with an edging of zinc 

 soldered on. 



10. The natural surface of the rolled iron sheets, covered with its usual hardened 

 grey oxide. 



Many of the foregoing were employed from curiosity only, especially 

 No. 6, the spirit varnish, which had on many occasions proved quite 

 ineffectual in preserving the surface of polished iron and steel from rust 

 in the atmosphere of Calcutta. 



The two frames were suspended as above described, one under water, 

 the other half immersed, from one of the unused dredging boats near the 

 Chitpur lock gates of the Circular canal, where they were left undisturbed 

 for three months, during a period of the year, when the water of the canal 

 was only slightly salt. 



* Major Burney states, that three or four days are sufficient for the varnish to 

 dry when laid on wood, (Journal, Vol. I. p 172.) I had not a damp vault in which to 

 expose the plate as recommended by that officer, and that may partly account for the 

 delay in drying ; but all varnish and paint takes longer to dry on metal than on wood, 

 from its non-absorbent nature. 



