Water

It should be noted that the percentages of water shown in these analyses vary considerably by reason of variations in the humidity, temperature, and pressure of the atmosphere to which the different papers had been exposed just before the analyses were made. There are, however, slight permanent peculiarities in samples made from different fibres or sized in different ways; inconsequence the moisture-absorbing and moisture-retaining properties of different papers are not precisely identical under identical atmospheric conditions. This hygroscopic moisture does, indeed, vary inversely with the temperature, and directly with the amount of water-vapour in the air; it is increased also by an increased barometric pressure. There is no doubt that if it could be wholly excluded, the larger number of changes which occur in the pigments of a water-colour drawing would be prevented. It is most injuriously active when a framed drawing is exposed to considerable ranges of temperature. Under these conditions the moisture of the paper is first partly turned into vapour, then condensed on the glass, and, lastly, is re-absorbed by the paper, and, for a time, especially by the pigments lying on its surface.

This temporary condensation of an excess of moisture upon the coloured surface does much injury before hygroscopic equilibrium is once more re-established. Much less harm would accrue were the vapourized water allowed to escape.