Which Expands More with Temperature—Paint or Metal?
With the recent surge in popularity of painting on metal, such as aluminum composite material (ACM) and copper, some have questioned its viability in regards to the thermal expansion of metal. When an object is heated or cooled, its length changes by an amount proportional to the original length and the change in temperature. This is known as “linear thermal expansion” or the change in length of a material.
To understand whether this is a problem or not for painting on metal we can compare the thermal expansion of different metals and oil paint. The linear thermal expansion of an object can be expressed as:
Δl = L0 α (t1 – t0)
where
Δl = change in object length (m)
L0 = initial length of object (m)
α = linear expansion coefficient (m/mºC)
t0 = initial temperature (ºC)
t1 = final temperature (ºC)
The relative expansion (also called strain) divided by the change in temperature is called the material’s coefficient of linear thermal expansion and generally varies with temperature. Coefficient of linear thermal expansion is designated by the symbol α (alpha).
Here are the Linear Temperature Coefficients for common materials used in painting:
| Linear Thermal Expansion Coefficient | ||
| Material | 10-6 m/(m°C) | 10-6 in./(in.°F) |
| Aluminum | 21–24 | |
| Birch plywood, parallel to grain | 3.25–5.30 | |
| Birch plywood, perpendicular to grain | 35.11–40.29 | |
| Copper | 16–16.7 | 8.89–9.28 |
| Glass, plate | 9 | 5 |
| Marble | 5.5–14.1 | 3.06–7.83 |
| Oak, perpendicular to grain | 54 | 30 |
| Lead white linseed oil paint | 41–48 | |
| Titanium white linseed oil paint | 145–175 |
Aluminum, copper, glass and marble expand and contract with the rise and fall of temperature less than linseed oil paint. Due to its structure, wood has two coefficients of thermal expansion: one in the direction (parallel) of the grain and another perpendicular to the grain. The thermal expansion and contraction of wood is anisotropic; it expands and contracts much more across the grain than with the grain, increasing the potential for cracks in paintings parallel to the grain.
All oil paints expand and contract with the rise and fall of temperature. The coefficient of thermal expansion for oil paint largely depends on the pigment content. For example, titanium white oil paint expands and contracts nearly four times that of lead white oil paint.
Linseed oil paint expands and contracts at a higher rate than copper or aluminum. Since the middle of the sixteenth century, many oil paintings were executed on copper. By contracting with temperature decreases, copper relieves some of the stresses and strain in the oil paint layers. This is another reason that these paintings have survived in such good condition. The same may likely be true of paintings on other rigid supports, such as aluminum composite material (ACM).
Sources
Browne, F.L. (1960) Thermal Expansion of Free Films of House Paint, Forest Products Laboratory, Forest Service U. S. Department of Agriculture. Download PDF. [Accessed 2 March 2020]
Engineering ToolBox (2008) Linear Thermal Expansion. [online] View. [Accessed 2 March 2020]
Mecklenburg, Marion F., Determining the Acceptable Ranges of Relative Humidity And Temperature in Museums and Galleries, Museum Conservation Institute, p. 12. Download PDF. [Accessed 2 March 2020]
Weatherwax, Richard and Stamm, Alfred (1956) The Coefficients of Thermal Expansion of Wood and Wood Products, Forest Products Laboratory, Forest Service U. S. Department of Agriculture. Download PDF. [Accessed 2 March 2020]
What is the importance of understanding thermal expansion when painting on metal surfaces?
Understanding thermal expansion is crucial when painting on metal surfaces because it impacts the adhesion and longevity of the paint, as different metals and the paint itself expand and contract with temperature changes, potentially causing cracks or detachment.
How can the linear thermal expansion coefficient be used to assess potential issues in painted metal surfaces?
The linear thermal expansion coefficient quantifies how much a material’s length changes with temperature, allowing us to predict the extent of expansion or contraction in the painted surface, thus helping to assess the risk of mechanical stress and cracking.
Why does wood exhibit anisotropic thermal expansion, and what are the implications for painting on wood?
Wood exhibits anisotropic thermal expansion because it expands more across the grain than along it, which can lead to cracks or warping in paintings if the expansion and contraction are not accounted for, especially in large or detailed works.
How does the thermal expansion of oil paint compare to that of metals like copper or aluminum?
Oil paints, particularly those with pigments like titanium white, tend to expand and contract more than metals such as copper or aluminum, with titanium white oil paint expanding nearly four times more than lead white oil paint, which can influence the stress and durability of painted surfaces.
What historical significance does copper support have in relation to the durability of oil paintings?
Copper supports, used since the sixteenth century, provide a rigid, low-expansion backing that helps relieve stresses caused by thermal contraction in oil paints, contributing to the enduring condition of such artworks over centuries.
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