In the year 1779 the Society of Emulation in Paris proposed as a prize question, "To discover a composition fit for making kitchen utensils which should be free from the disadvantages attending copper, lead, tinned vessels, glazed earthenware, &c., which should be as strong as possible, less costly than the vessels used at present, and which should be able to bear the highest degree of kitchen fire, and the most sudden changes from heat to cold."
In consequence of this proposal, Mr. Sven Rinman of the Royal Academy of Stockholm, without any intention of being a candidate for the premium offered by the Society of Emulation, instituted a set of experiments on small vessels of copper and hammered iron, with the view of giving to them a coating of what may properly be called enamel, which should not have the defects of tinning, and which, when applied to iron, should take from it the inconvenience of rusting, and of blackening many sorts of victuals when they are dressed in it. These experiments he submitted to the academy of which he was a member; and as we think them important, we shall lay the substance of them before our readers.
The most common, and the cheapest, kind of white enamel that is to be met with in the shops (which is an opaque white glass, composed of powdered quartz, of glass of lead, and of calx of tin), was tried for coating kitchen-utensils; and he found that it was excellent for the purpose, as it produced a coating, which was not only clean and agreeable in its appearance, but possessed likewise all the power of resisting the action of fire and of acids that could be desired. But, as it is very difficult to apply, is very dear for common use, and is besides considered as not being capable of resisting violent blows or falls, he made various experiments with substances of less price; of which the following are certainly worthy of being related.
1. The white semi-transparent fluor spar was reduced into a fine powder, with an equal quantity of unburnt gypsum, and afterwards calcined in a strong fire with a white heat; the whole being, from time to time, carefully stirred. The vessel which he intended to coat, having first been wetted by dipping it in water, had as much of the aforesaid powder applied to its inside, by means of a very fine silk sieve, as would adhere to it of itself, or could be made to do so by pressing it with the finger. After this vessel had been dried and gradually heated, it was exposed to a sudden and violent heat, partly in a coal-fire, kept up by a pair of bellows (the vessel being at the same time covered, so that no coals or ashes could fall into it); and partly in an assaying furnace.
In the coal-fire, and with a heat as violent as is commonly used to make copper-folder run, the mixture was melted, in about the space of a minute, into an opaque white enamel, which evenly covered the surface of the copper, and fixed itself pretty firmly to the metal; it also bore hard blows without breaking, and resisted the trials made by boiling things in it, and by applying acids to it. The aforementioned mixture was also reduced into a fine powder in a glass mortar, and made into a sort of thin paste with water; it was then applied to the vessel with a small brush, an operation as easy as that of applying any other wet colouring matter. He likewise tried this paste, by covering vessels with it in the same way the potters apply their common glazing for stone-ware. By both the above-mentioned processes he obtained a very smooth coating, particularly by the latter, which is more quickly performed. When the paste is applied, the vessel should be made a little warm, so also should the paste itself.
If the constituent parts of these two substances be considered (that is to say, that gypsum is composed of calcareous earth saturated with vitriolic (sulphuric) acid, and fluor spar of a particular acid united to siliceous earth; also, that the whole, when put into the fire without the addition of any other substance, is, of all earthy or stony mixtures, that which the most easily melts into an opaque white glass, not very brittle), and if, on the other hand, the action of acids be attended to—we shall easily conceive these substances must attach themselves strongly to copper, and that the varnish formed by them cannot afterwards be dissolved or acted upon by acids.
The greatest difficulty attending on this simple mixture is, the strong and sudden heat necessary to apply it with effect, that heat being greater than is commonly to be obtained in an assaying furnace. On that account, M. Rinman endeavoured to render it more fusible by the addition of some other substance.
Of his experiments made with this view, some failed, and others succeeded. We shall record only such as were successful, and at the same time attended with such moderate expense as not to preclude them from common use.
2. With the substances employed in his first experiment, which, with the author, we shall henceforth call n° 1, he mixed an equal quantity of what is called fusible glass (vitrum fusibile), composed of six parts of lime, four of flour spar, two of quartz reduced into a fine powder, and one tenth of a part of manganese; the whole having been calcined, and ground with water, in the manner colours are ground, he spread it on the vessel with a brush. This mixture ran pretty well upon the copper in the coal fire; it also attached itself very strongly to it, and produced an enamel which was firm and hard, and seemed likely to bear wear; but it was of a dark grey colour, and without any brilliancy. Two parts of n° 1, with one part of the fusible glass, and a quarter of a part of manganese, had nearly the same effect. This last mixture, indeed, was rather more easily melted, but it had a darker colour.
3. Eight parts of n° 1, with one half of a part of borax, one quarter of a part of nitre, and half a part of manganese, were melted, in the space of ten minutes, into a brown liver-coloured glass; which, in the assaying furnace, produced upon the copper vessel a black enamel, which had a dull surface. In other respects it was firm, even, and hard; but it did not sufficiently cover the vessel by a single application, nor was it capable of resisting the action of acids.
4. One part of the brown glass mentioned in the last experiment, with three parts of n° 1, became, in the assaying furnace with a red heat, almost as fluid as the last, and had an even and smooth surface; but it was of a dark colour, and had not any brilliancy. It was not sensibly acted upon by vitriolic (sulphuric) acid.
5. Four parts of n° 1, mixed with one half of a part of litharge, were melted in a crucible, with the help of the bellows, in five minutes, so as to become as fluid as water. This mixture, during the fusion, emitted a smell of sulphurous acid, and formed an opaque glass of a straw colour; which, after being ground, as usual, and spread upon a copper vessel, produced an enamel which covered the vessel very evenly, and was without bubbles. It was likewise, perhaps, the hardest of all, but could not be melted in the assaying furnace, requiring a stronger fire kept up by the bellows. It preserved its straw colour, but without any lustre, and resisted the action of acids better than the common glazing of the potters.
6. Mr. Rinnan mixed together equal quantities of gypsum, fluor spar, and what the potters call white lead (a), and which serves for the basis of their glazing. This mixture, after being calcined, melted in five minutes, with the assistance of a pair of bellows, into a very white, hard, and opaque enamel, which was very easily poured out of the crucible. This enamel, treated like the others, ran very freely, equally, and without bubbles, by the heat of the assaying furnace. It was also pretty hard and strong, but without any lustre, and had green and yellow spots, occasioned by the acids of the gypsum and fluor spar, which had acted upon the copper during the fusion of the enamel. It, however, bore melting two or three times, and then appeared of a white colour; it was but very little affected by other acids.
7. Equal parts of fluor spar, of gypsum, of litharge, and of pure flint glass, powdered and mixed together, melted in five minutes, by the help of a pair of bellows, and produced a white and hard glass, very like that of the last experiment, but rather harder. After being applied on the vessel in the usual manner, it formed, with the greatest heat of an assaying furnace, an enamel of a yellowish white colour, firm and hard, but without lustre. In order to avoid the formation of bubbles, care was taken as ought always to be done in enamelling, to remove the vessel from the fire as soon as it had acquired a brilliant appearance therein, or as soon as the enamel was completely melted.
8. Twelve parts of glass of lead, or of litharge, with eight parts of flint glass, and two of flowers of zinc, were melted, in the space of seven minutes, into a clear yellow glass, which, when used for enamelling, was disposed to form bubbles; but, by continuing the heat for a longer time, the bubbles were dispersed, and he obtained a pretty good enamel, of a yellow brown colour with a greenish cast, very hard and firm. It resisted the action of the vegetable acids, like the enamels already spoken of, but it was a little attacked by the mineral acids.
9. He powdered and mixed together five parts of fluor spar, five parts of gypsum, two parts of minium, one half of a part of borax, two parts of flint glass, one half of a part of calx of tin, and only one twenty-fifth of a part of calx of cobalt. This mixture was melted in a crucible in six minutes, by help of the bellows, and produced an opaque glass of a pearl colour, a little inclining to blue, on account of the calx of cobalt. It was pretty hard, and, after being ground with water in the usual way, it became of a very good consistence, so as to be very fit for spreading over vessels, to which it adhered very strongly. If any bubbles formed on the vessel during its drying, they might be rubbed down with the finger, and the whole surface rendered smooth and even. After being warmed, and gradually heated, it was put into an assaying furnace made very hot with birch charcoal, which had been just kindled under the muzzle. After a minute it melted, and began to appear brilliant; so that he found it necessary to take out the vessel very quickly, which was already very evenly coated with a thick, and sufficiently hard, enamel, the surface of which, however, had no brilliancy.
The colour remained always inclining to green, because the copper had been a little attacked by the acids of the gypsum and fluor spar during the fusion; but in other respects this enamel was very firm, was very little hurt by slight blows, and bore very well sudden changes of heat and cold. Weak acids had no action upon it; but he had some reason to think that it would, in length of time, have been acted upon, to a certain degree, by vitriolic (sulphuric) acid. Its colour, except the aforementioned shade of green, was white, with a dull, and rather changeable, surface.
The calx of cobalt which has been just mentioned, and which Mr. Rinnan made use of merely with the intention of obtaining a fine colour, was prepared by saturating a solution of cobalt in aquafortis (nitric acid) with common salt, and evaporating to dryness; by which means he obtained a fine rose-coloured calx. A very small quantity of this calx, when mixed with any fusible glass, gives it a beautiful blue colour.
Of the various species of enamel, which have been described in the course of these experiments, and which may be all applied, with more or less advantage, to kitchen utensils, the least expensive are n° 1, 2, and 5; but they are also those which require the greatest heat. On the other hand, n° 9 may be recommended.
(a) This substance is itself a mixture, being composed of four parts of lead and one of tin. as the most easy of fusion, and, at the same time, very durable when used for coating vessels in which victuals are to be dished, which is here the principal object, and is of far greater importance than the brilliant appearance resulting from the enamel generally used by artists, which however may be employed when the saving of expense is not regarded.
The enamels hitherto described are not applicable to vessels made of iron, though they may be employed to cover copper with great advantage. Iron will not indeed bear the common practice of enamellers, namely, to be put into the fire and taken out again several times; for the sparks which fly from iron, when in a hot fire, detach and carry off the enamel from the parts contiguous to those where the sparks are formed. The acids, too, of the gypsum and fluorspar, made use of in the enamels already mentioned, acted upon the iron during the fusion of the enamel, from which resulted bubbles and bare spots, which entirely spoiled the appearance of the work. Our author therefore continued his experiments with a view to discover a proper enamel for vessels made of this metal.
10. He reduced into a very fine powder, and ground together, nine parts of raminum (red oxyd of lead), six parts of flint glass, two parts of pure potash, two parts of purified nitre, and one part of borax. This mixture was put into a large crucible, which it only half filled; he covered the crucible so that no coals could fall into it, and gradually increased the fire under it. When the effervescence had entirely ceased, he caused the mixture to melt, by using the bellows for four or five minutes; by these means he obtained a clear and compact glass, which he poured out of the crucible upon a piece of marble. Having quenched it in water, and reduced it to a very fine powder in a glass mortar, he ground it with water to the consistence of a very thin paste. He then covered an iron vessel with it on both sides, which, after having dried and heated it by degrees, he put under a muffle well heated in an assaying furnace. The enamel melted very readily in the space of half a minute, and with a very brilliant appearance. He immediately withdrew the vessel, and let it cool: it was found to be entirely coated with a beautiful enamel of a black colour; which colour appeared to be caused by a thin layer of calcined iron, which might be seen through the transparency of the enamel.
A copper vessel having been covered with the same enamel, the fine colour of the copper was visible through the thin coat of glass; and it was as well defended from rust by this coating as it would have been by an enamel of a stronger kind.
11. To hinder the colour of the metal from being seen through the coating he added to the mixture, used in the preceding experiment, only one hundredth part of the calx of cobalt described in no. 9. The whole was melted into a beautiful blue glass; it was prepared for enamelling, and applied, in the manner before described, upon another iron vessel. The enamel proved to be smooth, thick, and brilliant, like the preceding, but it covered the vessel more perfectly; it was of a fine blue colour, with some black spots in those parts where it had been most thinly applied.
12. The glass of no. 10, reduced into powder, and ground with potters white lead, of which mention has already been made, melted with the same facility; it produced a very smooth enamel, of a grey colour, but more firm and hard than the former, and, on account of the addition made to it, of a still less price. By mixing with the same glass a small quantity of crocus martis, he obtained a very fine enamel, of a dark red colour, not to mention other colours in still more beautiful. The crocus martis he used in this experiment was prepared from a solution of iron in aqua regia (nitro-muriatic acid), which was evaporated to dryness, and the matter thus edulcorated and calcined.
13. In order to render the forementioned enamel more solid, and to give it what is called body, he melted together a mixture of twelve parts of flint glass, eighteen parts of minium, four parts of potash, four parts of nitre, two parts of borax, three parts of calx of tin, and one eighth part of calx of cobalt, observing always the usual precautions. He obtained a glass of a light blue colour, which, after having been ground with water, and spread upon small iron basins, or tea cups, produced, by means of a brisk fire in an assaying furnace, an enamel which was smooth and even, and of a pearl colour. The coating was of a proper thickness, to obtain which requires a certain degree of dexterity and practice. He also tried to paint upon this enamel with what is called mineral purple (purpura mineralis), which he used with a little powdered quartz, nitre, and borax; it produced a very beautiful red colour.
Though this last mentioned composition is more beautiful when applied upon iron, and more even than the preceding, it has the disadvantage, on account of the salts which it contains, of not resisting the action of the stronger vegetable acids, and still less that of the mineral ones. But as a vessel when coated with this enamel bears, without any injury, sudden changes of heat and cold, and also to any greasy mixtures baked or boiled in it (even those which are of a caustic alkaline nature, or those which contain the usual weak acids which are used in the preparation of our food), it may be applied to vessels of various kinds, among others to tea cups; particularly as it is neither brittle nor subject to crack, provided it is not exposed to violent blows. It is hardly necessary to say, that this enamel can only be applied upon vessels made of hammered iron, and not upon those of cast iron, these last being always too thick to be heated with sufficient quickness; for the greater is the space of time necessary to make the vessels red hot, the greater is the quantity of scales formed upon them, and, of course, the enamel becomes more injured.
Our author makes some other judicious observations on the enamel for iron, of which he has described the composition, and says, that, independent of its use for coating kitchen utensils, it might be made to serve many other purposes, such as preserving things made of that metal, not only from rust, but also, as he proved by experiment, to a certain degree, from calcination.