It is notorious that a large proportion of tinned goods which come to this country are injured so as to be rendered unsaleable, but it is not equally well known that a large number of those which are actually sold are really in a condition which renders them unfit for food. It very frequently happens that when tinned fruit or fish or meat goes bad the tin becomes what is technically known as "blown." This "blown" condition is the expansion or bellying outwards of the ends and sides of the tin, and is produced by the liberation of gas within the tin itself, which should have been almost in a vacuous state. In some cases the gas which causes the "blown" state is produced by the decomposition of the tinned substance, and many tins are absolutely burst by the force with which this gas escapes. But in other cases a small amount of free acid, or an acid still present in the substance itself or generated by its decomposition, acts upon the iron when it is imperfectly coated with tin, or upon the lead if the proportion of lead in the tin is too high, and the gases are liberated which at once set up an objectionable state in the contents of the tin, and bring about its destruction, either by bursting the tin by the internal pressure of the gas, or perforating it by the action of the acid upon the uncovered points, which are necessarily those most easily affected by the acids.
Accompanied by this comes another and perhaps equally serious result. The tin, some of the iron, and the lead contained in the tin are dissolved, and the contents of the can become contaminated with these metallic substances. In some cases this contamination is very serious. It is uncommon, for instance, to meet with a sample of tinned salmon or lobster which is in a really good condition, viewed from a chemical standpoint, and acid fruits such as peaches, apricots, and plums, seldom remain good for the second season. To obviate this result various methods have been proposed. In many cases the interior of the tin has been lacquered or coated with some supposed impervious varnish, but this more frequently proves injurious than beneficial. It is scarcely possible to manipulate a tin which has been varnished without cracking the varnish to some extent and producing either a slight separation of the film, or a pin hole. When this is the case, the whole of the action of any acid contained in the contents of the tin is concentrated on the one spot, and the destruction of the tin is only a question of time, and probably a short time. Another attempt was made by coating the iron with chemically pure tin by electro-deposition, in the hope that by this means any microscopical holes would be covered.
In practice this did not work any better than the other plan, because the solder still furnished a second metal, so that galvanic action was set up inside the tin to as great an extent as before. In the Paris laboratory hundreds of samples of tinned foods have been examined, and the almost universal presence of both tin and lead in all classes has been strongly remarked upon.
Of dry goods, tea is perhaps the best illustration, inasmuch as not only is the flavour of the tea itself very delicate in character, but tea appears to be remarkably prone to acquire any external odour from the air in which it is placed. It is, of course, well known that tea is always packed in cases which are lined with lead. In the case of China teas the lead is tolerably pure, cast into sheets by pouring the melted metal on to one stone and dropping another stone on the top of it. This primitive method produces a sheet of somewhat singular uniformity in thickness, weighing about 2 lb. to 3 lb. to the sq. ft. Indian teas are packed almost exclusively in lead which is sent out from this country. It is not pure. It contains an admixture in most cases of tin, and sometimes a small proportion of antimony. These are added to enable the lead to be rolled much thinner, and the weight of it is not more than 1/4 lb. to the sq. ft. Before any injury can occur to the tea itself this lead must be either destroyed or perforated, or at any rate it must not be in an air-tight condition.
It is obvious that, except in cases of neglect, all such goods would be packed in wood which was at any rate fairly well seasoned.
Assuming that the wood has been cut for a time and is well seasoned, it is evident that if it be saturated or supersaturated with moisture for days or weeks it is brought again into a condition closely approximating to green wood as regards its moisture, and also, by an inference which seems to be founded on fact, as regards its chemical liability to decomposition. Or again, supposing that this wood has been supersaturated with moisture, a new chemical state may be introduced.
The wood itself may not only be wet in the same sense as when freshly hewn, but wet in the sense of having been dipped in water, or water having been poured over it. This specially applies to the top layer of packages in the vessel's hold, because the coldness of the deck during the night almost invariably produces* considerable deposition of moisture, which is entirely evaporated again during the day. it is a matter of common knowledge that for years past numerous varieties of wood have been used for packing goods shipped here from the East, but that with few exceptions, until recently, only one kind has been used for pack-ing tea. This is a species of wood known in this country as "toon" wood, and every one who has ever seen a tea-chest made of it must be familial with its general characteristics. It is easily worked, does not require to be stacked long to season, is free from smell, and not very liable to absorb water. The cases of Injury with this wood have been of only occasional occurrence, and appear to have been determined much more by accidental circumstances than by even an occasional failure in the character of the wood itself.
But of late years the supply of "toon" wood has run short, the quantity which has been cut down for tea packages has completely destroyed some of the larger forests which used to form the leading source of supply, and the Chinese have had to resort to the use of other woods as substitutes, and in the whole of the Assam district woods are being used at random without any attempt at proper selection. Several of these woods are fairly suitable, while others are distinctly unsuitable, from their smell, and a peculiar action to which it will be necessary to draw attention.