RELATIVE HUMIDITY IN A CLOSED PACKAGE

Problems of packaging which have received attention up to now are mainly connected with weight and mechanical strength. Materials for packaging have been studied fairly well, and many advances have been nlade in this field. Only of late has moisture in the package been taken into consideration, and it has become common practice to insert some drying agent into a package accommodating iron goods or groceries. Art objects are, in general, fragile and may lose much of their value through damage occurring in transport. The exchange of art objects in the past took place nlainly within limited domains -for instance, in Europe or at most between Europe and America-and mostly between districts of similar climate. We have had many opportunities recently, however, of sending art objects from one temperate district to another through a tropical zone or a cold zone, or from a temperate zone to a tropical zone, and so on. If a Japanese art exhibition is held in Europe, for example, the articles must pass through hot or cold climatic regions, whether they are sent by sea or by air. Because of this, various failures have been experienced. A bronze Buddha which was sent from Japan to America was damaged by surface corrosion caused by condensation, losing its lustre at the places affected. This is thought to have been caused when the ship went through a cold zone following the great circle route. Conversely, the same result might have taken place with a bronze statue sent from Japan to a tropical zone. In fact, a bronze Buddha which was sent last year from Japan to Thailand was damaged on its surface by condensation just like the one mentioned above. We know of many other accidents caused in travel, including increased flaking of paint and tearing of surface paper on sliding doors, etc. In 1959 the Matsukata collection was sent to Japan from France; one painting in this collection, 'Douleur au pays. de la mer' painted by Cottet, has lost mu.ch of its value from deformation of the wooden box and consequent generation of mould, as I reported in lIe News, Vol. 1, No.2, pp. 18, 19. I think we Japanese have more experience of such events caused by humidity changes than most in the world for we are accustomed to great temperature variations during relatively short journeys. A temperature variation generally gives rise to a great change in relative humidity in a closed package, but this form of package is adopted in Japan as a general rule for art objects. I believe that warnings against such events are necessary not only for Japanese people, but for all in charge of travelling art objects, especially since there are likely to be more travelling exhibitions in the future. Now we must consider the 'humidity' which acts on art objects. As is known, there are two kinds of expression for humidity. Absolute humidity is expressed by the water vapour contained in a given quantity of air, whilst relative humidity means the percentage ratio of the weight of water vapour in the air to the weight which would be contained in the same volume of saturated air at the same temperature. The effect of humidity on an art object is related to relative humidity, not absolute. As can be seen from the above definition, air of a definite absolute humidity-for instance, of 8 g/m3-can have different relative humidities at different temperatures. This air will have a relative humidity of about 85 per cent at 10°C because a little over 9 g of water vapour is the most that can be contained in air at that temperature, while at 20°C, at which the maximum quantity is about 17 g, a relative humidity of about 47 per cent is realized. From this it is easily understood that air of a certain absolute humidity and enclosed in a closed vessel varies its relative humidity greatly if the vessel is made to travel through districts of various temperatures. Consider now the reactions of materials in the closed package when temperature and relative humidity vary in this way. Wood has been used as a packing material since ancient times. It is also a common material for art objects themselves. The reactions of wood to temperature and relative humidity changes are, however, very complicated and those in charge of packing must know the nature of wood. We can attribute many failures to ignorance of the nature of wood. The following concept about wood has become a commonplace, though it is responsible for the failures. If we put wood in air of a ~efinite temperature and relative humidity for a sufficiently long time, it reaches a condition in which no water is emitted from or absorbed into the wood-namely, an equilibrium between the wood and the air is created. We then put this wood in a box made of a material that has no reaction against humidity, and seal the box. Of course, some of the air in equilibrium with the wood will be sealed in the box. Let us then make the air in the box drier by some means (this can be accomplished by making the temperature of the whole systeITIrise, as mentioned above). When the air in the box thus becomes drier, water contained in the wood gradually evaporates into the air. Soon a new state of equilibrium is reached and the transposition of water stops. By this reaction the humidity change of the air in the box is mitigated, and the actual humidity variation of the air is smaller than when the box contains no wood. This fact indicates the effectiveness of the presence of wOQdin a box ~s a means of mitigating relative humidity variations when the box contains, say, art objects, for which large humidity changes are unfavourable. Everyone has these concepts in mind, and they are true in one way. But, as there are few who take the other properties of wood into consideration, failures often do take place. The cause of transposition of water from or into wood does not lie only in relative humidity change. Variation of temperature is another very effective cause. By this I do not mean the indirect action of temperature in which the temperature change invokes a change in relative humidity of the enclosed air which, in turn, causes transference of water. Water contained in wood is driven out from the wood directly by a rise of temperature, and the reverse transference takes place also by a drop of temperature. If we try to keep the air enclosed in a wood package at a constant relative humidity by raising the temperature of the package as mentioned before, this rise makes the water contained in the wood evaporate into the air, and the air generally becomes more humid than before. This increase in humidity is, of course, related to the quantity of wood, and it can generally