"1. By the use of ores that are too easily fusible, or which is the same thing, by an excess of flux, by a want of density in the charcoal, and by too strong a blast, even when the working of the furnace is regular.
"2. By a surcharge of ore, which deranges the action of the furnace, and produces impure cinder, containing uncombined iron.
"3. By boshes of too rapid a slope, and a blast of too great a velocity; and this may occur even where the cinder is pure.
"4. By too low a temperature, even when the cinder is pure, and the furnace works regularly.
"5. By a derangement in the action of the furnace, arising not from a surcharge of ore, but from an irregularity in the descent of the charge.
"6. By the substances contained in the body of the furnace exercising too great a pressure upon those beneath; the heat in this case, concentrated in the hearth, cannot reach the boshes, and the upper part of the furnace; the working may be regular, the cinder and flame may in this case give no sign of derangement.
"7. By too great a breadth in the furnace.
"8. When coke is used, it may arise from too great a quantity of ashes, or of fossil charcoal, (anthracite,) being contained in it. The presence of these will keep down the heat of the furnace. An excess of ashes may be remedied, by using the ore and flux in proper proportions to fuse them, but a diminution in the charge must be made; the cinder becomes viscid, and likely to obstruct the descent of the charges.
"9. By an accidental cooling, arising from humidity, and other similar causes."
Among the last may be reckoned the presence of zinc in the ore. This metal, although volatile, is not separated at the temperature given in the process of roasting, nor does it sublime in the upper and cooler parts of the furnace. But, as the ore descends, it passes into the state of vapour, and requires for its conversion, great quantities of heat that becomes latent. It hence cools the lower part of the furnace far more rapidly than even wet coal, or moist ores. The cooling thus caused, may not be effected until the melted metal reach the hearth, and may there cause it to become solid. Thus the solid mass called a salamander, may, in some cases, be formed; and thus may be explained the fact, that ores of iron that contain the more easily fusible metal zinc, are more liable to interrupt the action of the furnace in this manner, than others. The volatilized zinc rises to the upper part of the furnace, where the heat is often insufficient to retain it in the state of vapour, and is then deposited on the sides. In this position, it will also disturb the action of the furnace.