Silver Bars—How Assayed

Mark Twain

Virginia City Territorial Enterprise/February 17, 1863

We propose to speak of some silver bars which we have been looking at, and to talk science a little, also, in this article, if we find that what we learned in the latter line yesterday has not escaped our memory. The bars we allude to were at the banking house of Paxton Thornburgh, and were five in number; they were the concentrated result of portions of two eight-day runs of the Hoosier State Mill, on Potosi rock.

The first of the bricks bore the following inscription, which is poetry stripped of flowers and flummery, and reduced to plain common sense: “No. 857; Potosi Gold and Silver Mining Company; Theall & Co., assayers; 688.48 ounces, gold, 020 fine, silver, 962 fine; gold $572.13, silver $1,229.47.” Bars No. 836 and No. 858 bore about the same inscription, save that their values differed, of course, the one being worth $1,800, and the other a fraction under $1,300. The two largest bars were still in the workshop, and had not yet been assayed; one of them weighed nearly a hundred pounds and 1 was worth about $3,000, and the other, which contained over 900 ounces, was worth in the neighborhood of $2,000. The weight of the whole five bars may be set down in round numbers at 300 pounds, and their value, at say, $10,000. Those are about the correct figures.

We are very well pleased with the Hoosier State mill and the Potosi mine—we think of buying them. From the contemplation of this result of two weeks’ mill and mining labor, we walked through the assaying rooms, in the rear of the banking house, with Mr. Theall, and examined the scientific operations there, with a critical eye. We absorbed much obtuse learning, and we propose to give to the ignorant the benefit of it.

After the amalgam has been retorted at the mill, it is brought here and broken up and put into a crucible (along with a little borax,) of the capacity of an ordinary plug hat; this vessel is composed of some kind of pottery which stands heat like a salamander; the crucible is placed in a brick furnace; in the midst of a charcoal fire as hot as the one which the three Scriptural Hebrew children were assayed in; when the mass becomes melted, it is well stirred, in order to get the metals thoroughly mixed, after which it is poured into an iron brick mould; such of the base metals as were not burned up, remain in the crucible in the form of a “sing.”

The next operation is the assaying of the brick. A small chip is cut from each end of it and weighed; each of these is enveloped in lead and placed in a little shallow cup made of bone ashes, called a cupel, and put in a small stone-ware oven, enclosed in a sort of parlor stove furnace, where it is cooked like a lost sinner; the lead becomes oxydized and is entirely absorbed by the pores of the cupel—any other base metals that may still linger in the precious stew, meet the same fate, or go up the chimney.

The gold and silver come from the cupel in the shape of a little button, and in a state of perfect purity; this is weighed once more, and what it has lost by the cooking process, determines the amount of base metal that was in it, and shows exactly what proportion of it the bar contains—the lost weight was base metal you understand, and was burned up or absorbed by the cupel.

The scales used in this service are of such extremely delicate construction that they have to be shut up in a glass case, since a breath of air is sufficient to throw them off their balance—so sensitive are they, indeed, that they are even affected by the particles of dust which find their way through the joinings of the case and settle on them. They will figure the weight of a piece of metal down to the thousandth part of a grain, with stunning accuracy. You might weigh a musquito here, and then pull one of his legs off, and weigh him again, and the scales would detect the difference.

The smallest weight used—the one which represents the thousandth part of a grain—is composed of aluminum, which is the metallic base of common clay, and is the lightest metal known to science. It looks like an imperceptible atom clipped from the invisible corner of a piece of paper whittled down to an impossible degree of sharpness—as it were—and they handle it with pincers like a hair pin. But with an excuse for this interesting digression, we will return to the silver button again.

After the weighing, melting and re-weighing of it has shown the amount of base metal contained in the brick, the next thing to be done is to separate the silver and gold in it, in order to find out the exact proportions of these in the bar. The button is placed in a mattrass filled with nitric acid (an elongated glass bottle or tube, shaped something like a bell clapper) which is half buried in a box of hot sand—they called it a sand bath—on top of the little cupel furnace, where all the silver is boiled out of said button and held in solution (when in this condition it is chemically termed “nitrate of silver”).

This process leaves a small pinch of gold dust in the bottom of the mattrass which is perfectly pure; its weight will show the proportion of pure gold in the bar, of course. The silver in solution is then precipitated with muriatic acid (or something of that kind—we are not able to swear that this was the drug mentioned to us, although we feel very certain that it was) and restored to metal again. Its weight, by the mosquito scales, will show the proportion of silver contained in the brick, you know.

Now just here, our memory is altogether at fault. We cannot recollect what in the world it is they do with the “dry cups.” We asked a good many questions about them—asking questions is our regular business—but we have forgotten the answers. It is all owing to lager beer. We are inclined to think, though, that after the silver has been precipitated, they cook it a while in those little chalky-looking “dry cups,” in order to turn it from fine silver dust to a solid button again for the sake of convenient handling—but we cannot begin to recollect anything about it.

We said they made a separate assay of the chips cut from each end of a bar; now if these chips do not agree—if they make different statements as to the proportions of the various metals contained in the bar, it is pretty good proof that the mixing was not thorough, and the brick has to be melted over again; this occurrence is rare, however. This is all the science we know. What we do not know is reserved for private conversation, and will be liberally inflicted upon any body who will come here to the office and submit to it.

After the bar has been assayed, it is stamped as described in the beginning of this dissertation, and then it is ready for the mint. Science is a very pleasant subject to dilate upon, and we consider that we are as able to dilate upon it as any man that walks—but if we have been guilty of carelessness in any part of this article, so that our method of assaying as set forth herein may chance to differ from Mr. Theall’s, we would advise that gentleman to stick to his own plan nevertheless, and not go to following ours—his is as good as any known to science. If we have struck anything new in our method, however, we shall be happy to hear of it, so that we can take steps to secure to ourself the benefits accruing therefrom.

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