Manufacturer and builder 8 tai 9, 1869
If any hydro-carbon, or substance consisting chiefly of carbon and hydrogen, is burned with a great excess of oxygen, or when plentifully supplied with air by means of a blast or equivalent arrangement, no that the combustion is not only perfect but very rapid, much heat and very little light will be developed. When the supply of oxygen or air is less but still sufficient to make the combustion perfect without making it rapid, the development of heat is less, but in return more light is produced, and the amount of light will be at its maximum when the supply of air is so limited that the combustion takes place near the smoking-point, and the flame attains a yellowish color. When the supply of air is still less, so that the combustion takes place not only very slowly but very imperfectly - in other words, when many unburnt particles escape - the flame smokes, and the more so in proportion as the air-supply is deficient. The maximum amount of smoke is produced when so little air is supplied that the flame is just sustained, and is continually at the point of becoming extinguished.
When a substance of the above class is burned, the smoke consists chiefly of pure carbon very finely divided. Indeed, no fine is it, that there are no other known means of obtaining it in such a minute state of division. It is a most striking illustration of the diverse allotropic condition of bodies, that carbon when crystallized in the form of diamond is perfectly transparent and possesses a specific gravity 3.5 times that of water; that carbon, as graphite, is dense and not crystallized, but opaque and black, having a specific gravity of from 2 to 2.5, while as charcoal its specific gravity varies from 2 to 0.1, according to the source from which the charcoal has been derived. As lamp-black, it appears as an exceedingly light powder, of which eight tenths is pure carbon, one tenth water, a little less than one tenth hydro-carbon, while the balance is made up of ammonia, with traces of potash and lime.
In order to manufacture lamp-black, all that is necessary is to impede the combustion of a hydro-carbon, either by cooling the flame by the introduction of a cold body, or limiting the supply of air so as to have a very imperfect combustion. The first plan may be carried out by holding metallic plates in the flame, provided they are not allowed to become too hot, in which case no more soot will condense on their surface. Prechtl, therefore, proposes a roller of tin, which revolves ia the flame, and rubs laterally against a brush that continually removes the soot. By this arrangement, fresh cool parts of the metallic surface are constantly brought into the flame. Millanchau, of New-York, uses such a cylinder, cooling its inside with water, and has in this way provided very fine specimens of lamp-black for printing-ink. Dr. Van der Weyde, of New-York, has devised a process for diminishing the amount of carbon consumed during the combustion, and consequently for largely increasing the amount of carbon deposited, by confining the combustion more to the hydrogen of the hydro-carbon used. This is accomplished by the introduction of a small amount of chlorine gas into the flame, which gas, as is well known, has such a weak affinity for carbon that even a wax candle burns with a smoky flame in it, this gas being a strong supporter of combustion for hydrogen only and not for carbon. It is clear that by this modification of the process, hydro-chloric acid is formed in place of water, a fact which slightly affects the quality of the black formed, but in a favorable manner. These are the most recent processes for manufacturing lamp-black, the oldest method being the following:
Under a linen or canvas tent, well closed, a fire is made of some resinous wood, fir-leaves, chips, etc., the access of air being limited so as to produce as much smoke as possible. The product of the combustion is strained through the linen of the tent, the black remaining on its inside, from which it is removed from time to time. More recently there have been added to this tent canvas cylinders, into which the smoke is passed. These cylinders were intended to increase the extent of surface which receives the deposit; but this method has been long since abandoned, and the process is now carried on in stone-furnaces and large chambers, into which the smoke is passed and its carbon deposited.
This primitive arrangement for making lamp-black has, at present, been revived for the purpose of making zinc-white, an object to which it is very well adapted. Zinc-white is nothing but the product of the combustion of zinc - the oxide of zinc; but unlike the products of the combustion of carbon, carbonic oxide, and carbonic acid, which are gaseous, it is solid, and therefore, in place of requiring an imperfect combus-tion to obtain a solid deposit, as is the case with carbon, a perfect combustion is needed.
It has been found that metallic zinc is not necessary for this purpose. An impure zinc ore, consisting chiefly of sulphide of zinc, is mixed with coal-dust, and burned in a furnace; in which, by means of blowers, a very intense draught is kept up. The products of the combustion are, of course, carbonic acid, sulphurous acid, and oxide of zinc. The first two products are gaseous, and carry forward with them the last in the state of a fine dust. After passing through a stone-chamber, in which the coarsest particles of zinc-white and the grosser impurities are deposited, the gases pass through a system of large, iron cooling-tubes, in which another deposit takes place, and they finally enter a large building, containing several hundred linen or woolen bags, some twelve feet long and twenty inches diameter, suspended in rows, and all so connected that the blast of the gases from the furnace enters them and keeps them expanded. These bags filter the gases, retaining the zinc-white in their interior. This is from time to time removed by shaking the bags and emptying the contents collected at the bottom. As is to be expected, the finest qualities are found at the greatest distance from the place where the gases enter the series of bags, as the smallest and lightest particles are carried the furthest by the current of air.
The best qualities collected at the extreme distances are so fine that they do not require any grinding; the coarser ones, collected in the first cooling-chamber and tubes, are not only inferior in fineness, but some-times contain pieces of coal cinders, impurities from incombustible parts of the ore, which, by the strength of the blast, are carried forward, but are soon dropped. It sometimes happens that sparks from the fire reach the linen bags, and set them on fire, and to prevent this requires constant watchfulness, the bags being always kept very hot by the currents of heated air which pass into them. Fortunately, the zinc-white, being a product of combustion, and, consequently, not combustible, protects the bags to a certain extent. Woolen bags, also, are safer than those made of linen or cotton.