The lead of which the linings of tea chests are made is never quite pure - such a thing would be commercially impossible. The consequence of this is that the same result takes place as in ordinary white-lead manufacture. The action of the dilute acetic acid vapour is most energetic at places where some infinitesimal particle of foreign metal offers a point of vantage for the dissolution or change. Examination of a large number of chests shows that this conjecture is true, for instead of the pitting which is invariably found in crates taken out from white-lead stacks, we have not only pitting but absolute perforation in the lead taken off from tea chests in which this action has taken place. Perforation takes the form of minute pinholes scattered all over the lead, but showing to a much greater extent on the upper surface of the package than on the lower one, thus confirming the idea that it is partly due to the condensation of moisture on the top of the packages. This gives a new result. The package is no longer air-tight, or anything approaching it, but, on the contrary, there is ample ingress for any fumes which may be given off from the wood or from any general cargo, and ample exit for the peculiar odour of the goods packed in the lead lining.
First, this perforation. causes the goods packed inside the lead to be pervaded with any peculiar odour that may come from other portions of the same cargo, but obviously when the damage produced in this way is slight, it is a matter of great difficulty to tell what goods were packed in the immediate neighbourhood of the damaged packets, and in 9 cases out of 10 the cause would probably never be properly traced out. But in the other case something definite may occur. Acetic acid may be produced in minute quantities; it may act on the lead, and in so acting on the lead it is certain it would act also on the wood. Every one who has been inside a white-lead factory must be familiar with the fact that a peculiar smell is produced in the stack, and this smell, or one very closely analogous to it, is also produced by the action of this dilute acetic acid on the wood. This odour is not a general one pervading the hold of the vessel, but a special and peculiar one produced inside each chest or case, and in actual contact with the goods which the chest or case was intended to preserve.
The generation of acetic acid has made the perforations, and the same generation of acetic acid will or may cause the production of a certain amount of objectionable odour, and thai odour may or may not pass into the goods which are supposed to be carefully enclosed inside the wooden envelope. If ever such a small dose pass in, it is obvious that the goods may be tainted and injured. It must be borne in mind here that the objectionable smell is not that of acetic acid pure and simple, but that of acetic acid acting on wood, which produces decomposition products far more unpleasant in character.
We now come to two more effects which may be the result of the same action. White-lead having been produced, and perforation of the lead having been effected, it is not at all necessary, even assuming that damage has taken place, that the smell should have been produced by the action of the acetic acid. In the first place, some other cargo might have caused the injury; but this, although not impossible, is unlikely, because of the care which is universally taken in stowing delicate cargoes of this kind. Secondly, the wood, which has been the cause of the damage so far, may by its own inherent smell damage the contents of the cases of which it is made as to render it impracticable to use or sell them. There have been instances in which Indian tea had been packed in cases, part of the wood of which was of such a character that it was impossible that any delicately-scented article could be in contact with it for several weeks without acquiring an odour. It is, however, difficult to identify these woods accurately in this country - in fact, several of the samples already obtained arc not to be found in the Herbarium at Kew; but among the woods which have been used for the manufacture of tea chests are Manjifera sylvatica, or "mango wood" (which is notorious for its sour or vinegary smell), Buchaniana, Holigarna longifolia, Erythrina indica, and Derris robusta.
Every one of these is either a poisonous or a stinking wood. A poisonous wood per se has no effect upon the subject, except that it is much more likely to change in character and so do some injury. Again, the Pinus longifolia was tried for some time in Upper India, in the Assam district, but that failed, because of its resinous smell. It is scarcely credible that tea growers, familiar as they presumably must have been with the general character of these woods, should have used such utterly unsuitable specimens for the manufacture of tea chests.
The action on the lead differs in different pieces of wood, and differs to some extent according to the character of the wood itself; and the action is in all cases more energetic on the top of the chest or package than on the lower part. It seems to follow from this, that although an external source of acetic acid would be quite sufficient to account for some damage occurring to chests packed in this way, it could not possibly account for the local damage which occurs, nor for the peculiar smell derived from the wood, which does permeate the contents of the package. From all this it is clear that the object of the foreign producers has been, while bringing their goods up to the highest quality, to pack them as cheaply as possible, and having exhausted their supply of suitable woods they have been using any unsuitable wood within easy reach, allowing importers in this country to bear the damage which has been caused.
The summary of the whole matter is this: - Goods such as tea must be packed in envelopes, the substance must be wholly covered, and as far as practicable it must be preserved from external effect; this substance must practically be sheet lead; and this sheet lead must be protected in some way from any such action as that pointed out. A wooden casing is suitable and right as regards mechanical protection; but if the question of chemical action arises at all, the wood must be selected with such care that it is impossible that the lead lining should be injured.
For many substances, metallic cases might be replaced by waterproof paper, as described on another page.
Possibly woods unsuitable for packing -cases, where they would come into contact with metal, may be rendered harmless by treatment with a potash, soda, or lime solution, to remove all traces of free acid.