ON THE ACTION OF AIR AND WATER UPON IRON. 245 
entire difference in preservative power observed between coal- 
tar so used and Swedish tar laid on cold; had the latter been 
heated also until decomposition commenced and naphthaline was 
formed, less difference probably would have been found between 
them. 
202. In connexion with this may properly be mentioned the 
method stated to be used for giving the beautiful jet black var- 
nish coating to the Silesian or Berlin castings in iron. These 
beautiful specimens of art are covered externally with an ex- 
cessively thin coat of a black, shining, and remarkably hard 
varnish, which is not acted on for a considerable time by strong 
sulphuric or nitric acid, or for some hours by caustic potass, 
and which resists oxidaticn of the metal beneath for a long 
time; but, when continually exposed to air and water, at length 
forms isolated patches of rust, which gradually spread. The 
varnish is said to be thus produced :—the article of iron is sus- 
pended from a wire and covered with a very thin coat of linseed 
vil, it is then hung over a smoky wood fire within about a foot 
of the faggots, and exposed for some time to the smoke and 
flame—generally about thirty minutes ; it is then to be lowered 
to within three or four inches of the fire, now become clear, 
and heated more strongly for a few minutes, and immediately 
immersed in oil of turpentine, from which it is removed to be 
polished with woollen cloths. A second application is some- 
times requisite to give sufficient blackness and brilliancy, which 
is always more readily obtained with cast than with wrought 
iron. It is difficult to discern the precise nature of the changes 
which the oil undergoes in the process: watery vapour, Kupion 
and carburetted hydrogen are probably given off, and some of 
the volatile products of the wood, in imperfect combustion, 
may enter into combination. A different composition is recom- 
mended in the Dictionnaire Technologique, vol. xxii. p. 164, 
for producing this black varnish, viz.— 
Pntnentof Andra Leneihiais eT 2) sah. Fir 
Resins rot, pg te RPL Bea tts 
rai ih: lo lreely era ames es POTD BO 
Copal or Amber varnish . . . . . . 10 
with enough of oil of turpentine to make it spread, laid on the 
iron hot and baked. 
203. Amongst the mechanical coverings of iron for prevent- 
ing oxidation, may here be properly noticed the fusible enamel 
patented by Mariott, of London, and since by others. These 
are very fusible glasses, having, by the addition of large quan- 
tities of oxides, about the same expansion as the cast-iron culi- 
nary vessels, to which they were chiefly proposed being applied. 
