In such analytical work everything is based upon weight; i.e., constituents are determined and reported in percentages by weight. In chemical laboratory work everywhere the metric system is used, the cumbersome English system of weights and measures being practically impossible. Thus, the metric system is the international scientific standard. The unit taken is the gram, which is equivalent to ¹⁄₄₅₃ part of an avoirdupois pound. One gram of pig iron drillings is such an amount as could be held on an ordinary ten-cent piece.
Working with such small amounts of the sample, exactness and skill are extremely necessary. The balances used are necessarily very delicate—just as delicate as were the scales upon which the jeweler weighed your diamonds—you remember, of course. On these balances we can weigh an inch-long mark made by an ordinary lead pencil.
Dissolving in Acids
This is done under a hood that the irritating fumes given off may be kept from the room.
As the results of the analysis have to be known inside of three or four hours that the cars may be quickly unloaded in order to avoid demurrage, which is the penalty for holding cars longer than the allowable time, separate portions of each sample are weighed out for determination of the silicon, manganese, sulphur, phosphorus, graphitic carbon, and combined carbon. These are necessary in order to determine that the iron is up to the quality specified in the purchase contract and also to provide for its most efficient use in the manufacture of iron castings.
The exactly weighed portions are put into clean, numbered beakers, which are small pieces of high grade glassware that will stand sudden changes of heat and cold. Some of these portions are dissolved in nitric acid, some in hydrochloric acid, others in combinations of acids. In each case the drillings go into solution in the acids, and after various treatments of boiling, evaporating, filtering, etc., well known to those of the chemical profession, the desired results are obtained. In some cases it is by actually weighing a constituent which has been filtered out and burned to ash of a constant known composition, in others it is by comparison of color with standards of known composition, and sometimes it is by other means.
Filtering Silicons
After evaporating the excess acid, baking dry, cooling, and redissolving in weaker acid, the silicon compound formed may be filtered out. The iron and other soluble constituents, now in solution, pass through the filter, which is of pure, porous, unglazed paper.
In all of this analytical work the chemist must take care to lose not one drop of the solution or one grain of the ash from the burned “precipitate,” as the “filtered out” constituent is called.
The pig iron is always bought upon guarantee that it will contain a certain percentage of silicon—the element which in cast iron is known as a “softener.” But this is not the only thing necessary in the iron that is purchased. It must also show proper specified quantities of manganese, phosphorus and carbon, which also are very desirable elements in iron castings, and as little of that undesirable element, sulphur, as possible. Therefore they pay in proportion to the content of silicon, manganese, phosphorus, and carbon—and penalize the seller for sulphur.