CHAP. IV. OF TINNING COPPER-TIN-PEWTER. UNHAPPILY for mankind, the fatal accidents attending the use of copper vessels, in the preparation of food and physic, are too common, and too well attested, to require a particular enumeration or proof: scarce a year passes, but we hear of some of them, especially in foreign countries; and many slighter maladies, originating from the same source, daily escape observation, or are referred to other causes, in our own. In consequence of some representations from the College of Health, the use of copper vessels in the fleets and armies of Swe. den was abolished in the year 1754; and tinned iron was ordered to be substituted in their stead*. The Swedish government de serves the greater commendation for this proceeding, as they have great plenty of excellent copper in the mines of that country, but no tin. An intelligent surgeon suggested, in 1757, the probability of the use of copper vessels in the navy, being one of the causes of the sea scurvy, and recommended the having them changed for ves. sels of iron; he remarked, that of the 200 sail of ships which went to sea from Scarborough, most of them used iron pots for boiling their victuals, and that the symptoms called highly scorbutic, were never seen, except in some few of the larger ships in which copper vessels were used +. Notwithstanding this hint, and the example of Sweden, I do not know that any other European state has prohibited the use of copper vessels for the dressing of food on board their ships; but many of them have shewn a laudable attention to prevent its malignity, by inquiring into the best manner of covering its surface with some metallic substance, less noxious, or less liable to be dissolved than itself. This operation is usually called tinning, because tin is the principal ingredient in the metallic mixture, which is made use of for that purpose; and, indeed, since the year 1755, • Mem. de l'Acad. de Prusse, par M. Paul, vol. IV. Dis. Prel. p. 63. + Medical Observ, by a Society of Phys. in Lond, vol. II. p. 1. it has been frequently, in this country at least, used alone. In that year, The Society for the Encouragement of Arts, Manufac tures and Commerce, thought it an object deserving their attention, to offer a premium for the tinning copper and brass vessels with pure tin, without lead or any other alloy. There were several candidates for the premium; and since that time, the tinning with pure tin, and hammering it upon the copper, has become very general in England. But this mode of tinning does not appear to have been known, or at least it does not appear to have been adopted in other countries; for in the Memoirs of the Royal Aca. demy at Brussels, for the year 1780, M. l'Abbé Marci recom. mends, as a new practice, the tinning with pure block-tin from England; though, he says, block tin is a compound body, even as it is imported from England; but he thinks it a much safer co. vering for copper than what is ordinarily used by the braziers; and he gives some directions as to the manner of performing the operation. The Lieutenant General of the Police at Paris, gave it in commission to the College of Pharmacy, in 1781, to make all the experiments which might be necessary for determining-whether pure tin might or might not be used for domestic purposes, without danger to health? The researches which were made, in consequence of this commission, by Messieurs Charland and Bayen with great ability, were published by order of the French govern. ment; and they have greatly contributed to lessen the apprehen. sions relative to the use of tin, which had been generally excited by the experiments of Marggraf, published first in the Berlin Memoirs for 1747. That gentleman, in pursuing an experiment of Henckel, who first discovered arsenic in tin, shewed, that, though there was a sort of tin which being fluxed from an ore of a particular kind, contained no arsenic, the East India tin, which is generally esteemed the purest of all others, contained a great deal of arsenic. M. Bosc d'Antic, in his works, which were published at Paris, 1780, sets aside the authority of Marggraf, Cramer, and Hellot, relative to the existence of arsenic in tin; and is not only of opinion, that the Cornish tin does not conceal any arsenic in its substance, but that its use as kitchen furniture is not dangerous. Messieurs Charland and Bayen found that neither the East India, nor the purest sort of English tin, contained any arsenic; but that the English tin, usually met with in commerce, did contain arsenic; though in so small a proportion that it did not amount, in that species of tin which contained the most of it, to more than one grain in an ounce; that is, it did not constitute more than one five hundredth and seventy sixth part of the weight of the tin, there being 576 grains in a French ounce. This proportion of arsenic is so wholly inconsiderable, that it is very properly concluded, that the internal use of such small portions of tin, as can mix themselves with our food, from being prepared in tinned vessels, can be in no sensible degree dangerous on account of the arsenic which the tin may contain. But though tin may not be noxious, on account of the arsenic which it holds, it still remains to be decided, whether it may not be poisonous of itself; as lead is universally allowed to be, when taken into the stomach. The large quantities of tin, which áre sometimes given in medicine with much safety, and the con. stant use which our ancestors made of it in plates and dishes, before the introduction of china or other earthen ware, without experi encing any mischief, render all other proof of the innocent nature of pure tin superfluous. And hence it may be proper to add a few observations concerning the purity of tin. The ores of metallic substances often contain more substances than that particular one from which they receive their denomina. tion. M. Eller, of Berlin, had in his collection an ore, which con. tained gold and silver, and iron and quicksilver, closely united together in the same mass. Lead ore, it has been remarked, so often contains silver, that it is seldom found without it; it is often also mixed with a sulphureous pyrites, which is a sort of iron ore, and with black jack, which is an ore of zinc; so that lead, and silver, and iron, and zinc, are commonly enough to be met with in the same lump of lead ore. Tinore, in like manner, though it is sometimes unmixed, is often otherwise; it frequently contains both tin, and iron, and copper. The fire with which tin ore is smelted, is sufficiently strong to smelt the ores of the metals which are mixed with it; and hence the reader may understand, that, without any fraudulent proceeding in the tin smelter, there may be a variety in the purity of tin, which is exposed to sale in the same country; and this variety is still more likely to take place, in spe cimens of tin from different countries, as from the East Indies, from England, and from Germany. This natural variety in the purity of tin, though sufficiently discernible, is far less than that which is fraudulently introduced. Tin is above five times as dear as lead; and as a mixture consisting of a large portion of tin with a small one of lead, cannot easily be distinguished from a mass of pure tin; the temptation to adulterate tin is great, and the fear of detection small. In Cornwall, the purity of tin is ascertained, before it is exposed to sale, by what is called its coinage: the tin, when smelted from the ore, is poured into quadrangular moulds of stone, containing about 320 pounds weight of metal, which, when hardened, is called a block of tin; each block of tin is coined in the following manner:-" The officers appointed by the Duke of Cornwall assay it, by taking off a piece of one of the under corners of the block, partly by cutting, and partly by breaking; and if well purified, they stamp the face of the block with the impression of the seal of the Duchy, which stamp is a permission for the owner to sell, and at the same time an assurance that the tin so marked has been purposely examined, and found merchantable *." This rude mode of assay, is not wholly improper; for if the tin be mixed with lead, the lead will by its superior weight sink to the bottom, and thus be liable to be discovered, when the bottom cor. ner of the block is examined. But though the seal of the Duchy may be some security to the original purchasers of block tin, it can be none at all to those foreigners who purchase our tin from Holland; for, if we may believe an author of great note-" in Holland every tin founder has English stamps, and whatever his tin be, the inscription, block tin, makes it pass for English t." This foreign adulteration of English tin may be the reason that Musschenbroeck, who was many years professor of natural philosophy at Utrecht, puts the specific gravity of what he calis pure tin equal to 7320, but that of English tin, and he has been followed by Wallerius, equal to 7471 ; for it will appear presently, that such sort of tin must have contained near one-tenth of its weight of lead. Borlase's Nat Hist. of Cornw. p. 183. + Newman's Chem. by Lewis, p.89. Musschen. Ess. de Phys. 1739. French Trans. Wallerii Min. vol. I. p. 154. There is a very good Table of Specific Gravities, published in the second vo lume of Musschenbroeck's Introductio ad Philosophiam Naturalem, 1763, in which the author does more justice to English tin, putting the weight of a cubic foot of the purest sort equal to 7295; avoir. oun. One specimen of the purest sort of Malacca tin gave 7331, and another 6125 ounces a cubic foot, which is the lightest of all the tins which he examined. Weight of a cubic foot of English tin, according to different From the following experiments it may appear probable, that not one of these authors, in estimating the specific gravity of tin, has used the purest sort, but rather a mixture of that with lead, or some other metal. A block of tin, when it is heated till it is near melting, or after being melted, and before it becomes quite fixed, is so brittle that it may be shattered into a great many long pieces like icicles, by a smart blow of a hammer*: tin in this form is called by our own manufacturers grain tin, by foreigners virgin tin, or tears of tin; and they tell us, that its exportation from Britain is prohibited under pain of death. The tin which I used in the following experiments, was of this sort, but I first melted it, and let it cool gra. dually; a circumstance, I suspect, of some consequence in de. determining the specific gravity not only of tin, but of other metals. I have put down in the following table, the specific gravity of this tin, and of the lead I mixed with it by fusion, and of the several mixtures when quite cold; the water in which they were weighed was 60o. Weight of a cubic foot of lead, tin, &c. This property is not peculiar to tin; I have seen masses of lead which, under similar circumstances, exhibited similar appearances; and it has been observed, that zinc, when heated till it is just ready to be fused, is brittle. Ency. Fran. and Mr. Baumé calls it "étain en roche, à cause que sa forme |