Humidity and Temperature

Why worry about relative humidity and temperature?

Relative humidity and temperature are two of the environmental factors which can contribute to the deterioration of our valued collections.

A bark painting—split after dying out in a low relative humidity environment. Photograph courtesy of Artlab Australia, reproduced with the permission of the Museum of Victoria.

Extremes of temperature and relative humidity—and rapid fluctuations in these—can lead to a range of problems. The risks of physical damage, such as warping, cracking and splitting, chemical deterioration, and insect or mould attack are all increased when temperature and relative humidity are too high or too low.

The discolouration of the cotton proceeded much more rapidly in a damp environment. Photograph courtesy of Artlab Australia.

Insects generally like a warm damp environment. Photograph courtesy of Artlab Australia.

The effects of extremes and fluctuations in temperature

Extremes and fluctuations in temperature are potentially less harmful than extremes or fluctuations in relative humidity; but it is difficult to separate the two because they are closely interrelated. Some independent effects of high temperature include:

• increased biological activity. Most insects and moulds thrive and reproduce readily in warmer conditions; and

• acceleration of chemical deterioration processes. Temperature affects the rate at which chemical reactions take place. For example, a temperature rise from 20–30^oC may double the rate of some degradation reactions. And this worsens if light, water or pollution also contribute to these chemical reactions.

Fluctuations in temperature cause:

• expansion and contraction. If this is uneven and/or rapid, it can cause physical damage and distortion. This can be hazardous for objects made of composite materials; and

• some types of plastic, for example, vinyl records, shrink and warp in high temperatures.

The most important effect of temperature is the effect it has on altering relative humidity levels.

The effects of extremes and fluctuations in relative humidity

In high relative humidity conditions insects and moulds thrive and reproduce readily, metals corrode, dyes and textiles fade and deteriorate more quickly, organic materials such as wood and leather swell or change shape, and gelatine emulsions and adhesives become sticky.

High relative humidity conditions promote the corrosion of metals. Photograph courtesy of Sarah Jane Rennie

The bolts are corroding in high relative humidity conditions. The corrosion products are staining the wood, and will eventually cause splitting. Photograph courtesy of Artlab Australia, reproduced with the permission of the History Trust of South Australia.

Organic materials absorb water. This is particularly noticeable in thinner materials, such as paper, vellum and parchment, textiles, leather and bark paintings. As materials absorb water, they swell and change shape, for example, stretched vellums and mounted textiles sag.

The effects of humidity on organic materials are not always immediately noticeable. But after a while, extensive damage eventually occurs.

For example, a large block of wood may take weeks or even months to transfer water from its surface into its bulk, leading to different parts of the wood having different water-contents. The consequence this has on the wood is to make it swell by different amounts, which will have the effect of splitting and warping the material.

Wood also swells more across the grain than along the grain; and by an amount which varies according to the type of wood. This makes life complicated when caring for furniture.

Textiles can display what seems to be the opposite response to changes in relative humidity. A multi-strand thread shortens in length when the relative humidity goes up. This is because the individual threads expand in diameter more than they expand in length. The result is that the strands wrap around each other more tightly, which causes the overall length to decrease. Often this process does not reverse when the relative humidity drops again.

A canvas responds in the same way as textiles; however, the paint layer on the canvas does not contract. Rather, it will compress, leading to cracking or separation between canvas and paint layer.

Different components of single objects absorb moisture at different rates and swell by different amounts. This can cause problems, such as paint layers splitting and separating from timber panels.

Composites of metal and wood are affected also. As the metal corrodes, the wood starts to split in order to accommodate the corrosion products.

Adhesives that absorb water become sticky and are an attractive food source for moulds and insects.

Gelatine emulsions on photographs also swell in humid conditions and can readily stick to the glass in their frames or, if they are stacked, they can stick together.

As for papers which are stuck down at the edges, they will increase in size in humid conditions and thus expand in the middle as their edges are restricted. This can lead to creasing.

In very low relative humidity conditions, such as in arid areas:

• insects can still survive;

• organic materials give out the moisture they contain. This can cause materials to dry out and become brittle or to distort and split;

• thicker materials lose moisture much more rapidly from their surface. This can cause warping;

• different components of single objects release moisture at different rates, which can cause the bonds between them to loosen; and

• adhesives dry out and crack, and can fail as a result.

If fluctuations are occurring constantly, the materials are being subjected to constant movement which is usually not uniform and often results in cracking, splitting and warping. Some examples of extreme damage caused by fluctuations are:

• bark paintings expand and contract as they absorb water and release it. This leads to the bark warping and splitting, and they can lose paint;

• bone and ivory are very susceptible to damage caused by fluctuations; and they warp and split. This is especially a problem for very thin ivory sheets, such as those used for miniature painting;

• furniture with veneers can be damaged severely, because the thin, veneer layer is likely to curl and pop off the surface of the furniture if it repeatedly expands and contracts; and

• fluctuations in relative humidity can also alter the chemical composition of some minerals, so that they become another mineral.

What happens in extreme, but stable environments?

When conditions are extreme but constant, damage can still occur. Experience shows, however, that many materials become conditioned to an extreme environment.

An object in constantly high or low relative humidity does not absorb and lose water repeatedly; and it is not subject to the enormous stresses of the cycles which affect objects in fluctuating environments. Such an object is likely to be preserved longer and in better condition than a similar object in a fluctuating environment.

Remember, the emphasis should be on stability.