The modern process of manufacturing tin plate was invented in Bohemia about the year 1620. Tin plate was first made in England about one hundred years later, and its manufacture was started, but not successfully, in the United States in 1872-73. It was not until 1892, however, that the industry was established in this country.

In 1810 Appert published his work on canning, and about the same time Durrand secured an English patent covering a process for "the preservation of fruits, vegetables, and meats in hermetically sealed containers of tin, glass, pottery, or other fit material." With the discovery of this new method of preserving foods, which has now become essentially American, the tin-plate industry received an impetus which caused the yearly production to increase enormously, until at the present time thousands of tons of plate are produced annually, of which a large proportion is used in the manufacture of containers for foodstuffs, a not inconsiderable quantity entering into the manufacture of containers for the storage and commercial distribution of fats and oils.

Various countries have enacted laws requiring that tin plate for the manufacture of food containers must be prepared from tin of the best quality and that the method of tinning be such as to yield a surface as free as possible from so-called “pin holes," scratches, and other imperfections. Even when the greatest precautions are observed in the manufacture of the plate, however, imperceptible abrasions or imperfections are at times found, these being indicated later by corrosion of the metal at the point where the coating is imperfect and discoloration of the product at this point, or in certain instances through the discoloration of the entire contents of the receptacle. An ingenious method for the demonstration of such "pin holes" or abrasions in plate of this character has been described by Walker."

In 1837 a process for covering iron with a coating of zinc-socalled "galvanized iron "-was patented by Crawford, the object being, as in the tin plate, to protect the readily oxidized iron base from deterioration. The claim was made that zinc in this respect acted even more perfectly than tin, which is electro-negative to iron; and this, together with the reduced cost of the plate as compared with tin, caused many manufacturers to use it instead of tin plate in the manufacture of vessels intended for use as containers of foods. In the case of tin plate the protection is perfect only provided the surface of the iron is completely covered, for imperfections and damage to the tin covering cause the action on the iron to proceed more rapidly than if the tin were absent, while with a zinc coating the zinc, being electro-positive to iron, would alone suffer corrosion as long

• The Journal of Industrial and Engineering Chemistry, vol. 1, No. 5, p. 295. Easton, Pa.

as any of it remains. The process of galvanizing is generally not, as its name tends to imply, an electric one, but is carried in a manner similar to that employed in the manufacture of tin plate, the annealed iron base being covered by dipping, rolling, etc., yielding a coating of zinc instead of tin.

The bright surface of the zinc coating soon becomes covered with a thin layer of zinc oxid, which is highly resistant to air and water, and, if subject only to their influences in the pure state, is extremely durable. However, imperfections in the coating, impurities in the zinc, or the presence of acids or alkalies, even though in small amounts, in substances with which the metal comes in contact, hasten corrosion. In the case of galvanized iron, therefore, the deterioration of the coating is hastened by the presence of even the weaker vegetable or flesh acids.

The "spelter" used in the galvanizing process may contain small amounts of lead, and it has been stated that in some instances tin and lead are added in small quantities, that the formation of the crystalline structure of the coating may be accentuated. It is probable that any agent which would serve to dissolve the zinc would act in a similar manner on metallic impurities, such as lead, etc., with which the zinc might be contaminated.

THE CONTAMINATION OF FATS AND OILS BY METAL CONTAINERS.

With the adaptation of pails made of metal to the permanent storage of fats, the question naturally arose as to the possibility of the absorption of metals by the fats so stored. That fats and oils possess the quality of holding in solution small quantities of metallic soaps has long been recognized. In some of the " boiled oils " of commerce their presence as a result of treatment during the process of manufacture is so uniformly constant that they are regarded as normal constituents of the oil, while in other cases they are added to the oil by the refiners to meet the requirements of the trade. In view of the solubility of metallic soaps in fats and oils and the known fact that fatty acids react with metals to form such soaps, it is reasonable to suppose that edible oils and fats, containing more or less free fatty acid, will act on metallic containers and that the resulting metallic soap may dissolve in the fat.

In those fats and oils which at ordinary temperatures remain in a more or less solid condition the products of the action between the fat and the metallic surfaces exposed would naturally be confined to that portion of the fat or oil in intimate contact with the container or to a zone immediately adjacent to it. Provided the surrounding temperature does not rise sufficiently to cause melting, diffusion of the metal throughout the mass would be retarded, and the greater portion of the fat would therefore be free from metallic contamination.

REVIEW OF LITERATURE.

A search of the literature shows that while considerable work has been done concerning the action on metals of fatty oils used in conjunction with mineral oil as lubricants, comparatively little mention is made of the action of the edible fats and oils on metals with the view of determining the suitability of such metals as containers.

Redwood in an investigation of this question concludes that the chemical examination of the oils after exposure appears to afford a most valuable guide in determining what metal is best adapted for the construction of storage tanks for the different oils used as lubricants, as in some cases a protective covering is formed by a fatty acid salt of the metal, which, being insoluble or difficultly soluble in the oil and adhering closely, prevents general absorption of the metal by the oil. In his results the percentage loss of the metal due to the action of the oil is given. The extent of surface exposed and the acidities of the oils employed are not considered.

Volney investigated the effect of some of the vegetable and animal oils on brass with a view of determining their relative value as lubricants and as protectors of metals. He found that neat's-foot oil, crude cotton-seed oil, and lard oil all acted on the brass to a greater or less extent, but as he failed to record the acidity of the oils his figures are applicable only to the particular oils used in his tests.

Watson in a paper "On the Action of Various Fatty Oils Upon Copper " finds that of the vegetable oils castor oil has the least action, while linseed oil has the greatest. The acidities of the oils used is not mentioned, and the results therefore apply only to the particular oils employed in his experiments. The loss of metal per unit area of metallic surface exposed is given.

Thompson conducted a series of extended experiments on the action of different fatty oils upon strips of metallic copper, some fully immersed in, others ony half covered by the fat. He bases his conclusions as to the action of the oils on the relative proportion of copper dissolved by the oil and the appearance of the strip. The actual acidities of the oils employed was not determined, only a relative acidity of a watery extract of the oil being secured. It is interesting to note that whale oil, a sample of which was used by this investigator, did not dissolve even a trace of the metal. In this sample the oil through oxidation was solidified to about one-third of its depth from the surface.

a Journal of the Society of Chemical Industry, vol. 5, p. 363. London, 1886. The Analyst, vol. 8, p. 68. London.

The Chemical News, vol. 36, p. 200. London.

d Ibid., vol. 34, pp. 176, 200, 213.

Engler referred to the feature of acidity as applied to the action of mineral oils, maintaining that metals are attacked by petroleum only under the joint influence of air or oxygen when acid compounds are formed, and shows that petroleum which has been freed from acid by washing with caustic alkali and then further purified by distillation in an atmosphere of carbon dioxid has no solvent action upon the metals.

Donath, in an article on the action of lubricating fatty oils on metals, states that in his opinion neutral fats and fatty acids have no direct chemical action on metals, the action taking place between the oxid of the metal and the free fatty acid in the fat, the formation of the oxid being influenced by the presence of air and moisture with the fatty acid acting as a predisposing agent promoting more rapid and energetic oxidation.

INSIDE COATINGS AND PROTECTIVE COVERINGS.

Some of the factors which may in one way or another influence the extent of deterioration of metals when acted upon by fats have been already referred to, but other factors remain to be considered, and in this connection it will not be amiss to discuss briefly another phase of this question. In recent years the subject of corrosion has been extensively investigated along scientific lines, and descriptions of these investigations are to be found freely scattered throughout the literature.

In these investigations the protection afforded by the application of a coating or insulation, if the term may be used, to the surface of the metal has been the basis of many of the articles written. In so far as it applies to metallic containers, the protective covering has usually consisted of a thin film of varnish or lacquer applied to the inner surface. If this coating remains impervious to the contents of the container, the corrosive action is of course prevented.

While the work already published on this subject has been directed more particularly to the so-called "inside coating" for cans intended for the preservation of fruit and vegetables, the conditions obtaining would in a limited sense apply equally as well to containers used as receptacles for fats and oils, especially in certain instances, as, for example, butter and oleomargarin, in which the materials necessary to the formation of a voltaic cell are to be found. In a recent article Walker and Lewis show that these inside coatings, under conditions favorable to the setting up of an electric current, accelerate instead of retard the corrosive action by the absorption and removal of the

a The Chemical News, vol. 41, p. 284.

Dingler's Polytechnisches Journal, Band 294, pp. 186-187. Stuttgart.

The Journal of Industrial and Engineering Chemistry, vol. 1, No. 2. Easton, Pa., 1909.

81268°-11--2

thin film of depolarizing hydrogen which, evolved by this electrolytic action, in a degree protects the metal when no depolarizing agent is present. These investigators were unable to find a lacquer which was nonporous and nonabsorptive, or at least one which, when subjected to the various processes necessary to the construction of the container, was not rendered permeable through the destruction in some manner of its continuity of surface. By the addition of paraffin to the lacquer, they demonstrated that the resistance of the latter could be enormously increased, thereby practically elminating the corrosive action; but they found this procedure made the lacquer so brittle as to render its use in a practical way out of the question. In the application of these coatings as protective agents to containers in which fats and oils are to be stored, the soluble action of the oil, in conjunction with the softening effect produced by the oil, causing disintegration of the film and consequent exposure of the metal, compels consideration. The ready solubility of paraffin in fats and oils has been demonstrated, and the protection afforded by its use would therefore be but temporary.

The specifications for tinned butter as promulgated in the schedule of supplies for the United States Navy, in referring to the tin plate to be used and the method to be adopted for the construction of containers for butter, specify "prime coked tin plate, weighing 90 pounds box of 112 sheets 14 by 20 inches in size," to be used, the "outside tins, tops, bottoms, and sides to be lacquered but under no circumstances to be lacquered on inside."

The use of a parchment paper lining on the interior surface of the container was suggested as a protective agent. Tests conducted along this line in which galvanized-iron receptacles were used showed a diminution in the amount of metal absorbed by the fat as compared with the amount taken up when no such protective covering was used. The action, however, was sufficiently pronounced to demonstrate the inadequacy of parchment paper as a protective agent.

EXPERIMENTAL WORK.

In the experimental work carried out by the writer, which extended over a period of several months, the method of procedure was necessarily subjected to some variations. The investigation as originally planned was intended to apply only to the action of fats upon galvanized iron. As the investigation proceeded, however, it was considered advisable to extend the scope of the work to embrace vessels made of tin plate, and later the possibility of the presence of other metals as impurities through the use of lead solder, imperfections in the coating, or the use of other metallic utensils was recognized. view of this a study of the action taking place between metals which

In