Effects of Humidity & Temperature Changes on our Possessions & Products
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 possessions.
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 mold attacks are all increased when temperature and relative humidity are too high or too low.
The Owner stored some decorative bamboo poles in the garage over one winter. She noticed this summer that all of the poles had split down the center. This damage is a good example of what can occur to wood and other natural fiber products due to insufficient levels of moisture.
The Effects of Extremes & 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.
Fluctuations in Temperature
Temperature fluctuations causes 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.
Fluctuations in Relative Humidity
In high relative humidity conditions, insects and molds 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 gelatin emulsions and adhesives become sticky.
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.
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 or wooden musical instruments.
This antique vanity dresser was stored in non-climate controlled storage for more than 9 months. You’ll note the elongated cracks in the lower right-hand of the photograph on the top of the vanity dresser caused by the extremes of a Montana winter.
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.
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 molds and insects. Additionally, they lose their adhesive ability causing joints on furniture to loosen and separate. Cold temperatures cause adhesives to dry out and crack. The results are the same: adhesive properties are destroyed.
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.
- 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 that can be caused by fluctuations:
- 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.
- Fluctuations in relative humidity can also alter the chemical composition of some minerals, so that they become another mineral.
- 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 paintings.