Isoperms–Relative Humidity

Commission on Preservation and Access

Isoperms

Effect of Relative Humidity

Water is important as a reactant or catalyst in many chemical reactions. As already stated, the rate of deterioration of cellulose by hydrolysis is directly related to the moisture content of the paper which in turn is directly related to the percent relative humidity (% RH) of the atmosphere in which the paper is placed. We may summarize the effect of % RH as: the greater the % RH of the environment, the greater the moisture content of the paper, the higher the moisture content of the paper, the greater the hydrolysis deterioration rate of the paper, the faster the paper deterioration, the shorter the life expectancy (permanence) of the paper. These ideas can be summarized mathematically in the equation:

    r2     H2O concentration2     RH2     P1

    __  =  __________________  =  ___  =  __

    r1     H2O concentration1     RH1     P2

                                            (2)

where RH is the percent relative humidity of the environment in equilibrium with the paper.[1] Equation 2 provides a good quantitative approximation for deterioration rates (and permanence) in the practically important range of 30-65% RH; outside this range papers show increasing deviations from Equation 2 behavior. Thus, by controlling the relative humidity we can affect the paper permanence and, within the middle of percent relative humidity values, we can quantitatively estimate the effects with good accuracy.