Till Huntsman developed the crucible process in 1740, the only kinds of steel of commercial importance were blister steel made by carburizing wrought iron without fusion, and others which like it were greatly injured by the presence of particles of slag.
Plum-leaf blister is caused by Polystigma rubrum, a pyrenomycetous fungus which forms thick fleshy reddish patches on the leaves.
Locusts, green-fly, leaf-bugs, blister mites, and various other pests also damage cotton, in a similar way to that in which they injure other crops.
But now the chief use for blister steel is for remelting in the crucible process, yielding a product which is asserted so positively, so universally and by such competent witnesses to be not only better but very much better than that made from any other material, that we must believe that it is so, though no clear reason can yet be given why it should be.
The blister if unbroken was heated, pricked, and then rubbed level with a burnisher; if, as sometimes happened, the silver had flaked away it was replaced by coatings of pure leaf silver rubbed in with a burnisher.
The great extension of surface thus produced had the drawback of exaggerating any small defect in the union of the two metals, increasing it to a blister of an inch or more in diameter.
Similar spraying is recom- (From a specimen in the British Museum.) mended for pear-leaf blister Pear Scab (Fusicladium pyrinum).
For this reason a blister is placed upon the callous ulcer, which heals with the fresh inflammation thus excited.
In fine work the apex of the blister is ground off before the linal hammering.
In Great Britain the charge usually consists of blister steel, and is therefore high in carbon, so that the crucible process has very little to do except to melt the charge.