The yield point defines the upper limit of the elastic (reversible) range of a material's mechanical performance. The value of the slope for the large drying change in relative humidity is RH. The initial strain of 0.005 shown in the figure is the initial yield strength and defines the upper limit of the wood's elastic or reversible range.
The modulus of any material is the ratio of stress to strain in the elastic region only. The yield points used for all plots are indicated by an arrow at a strain of 0.005. It is interesting that these cracks are formed in the gesso layers in both paintings and that the cracks are primarily perpendicular to the grain of the wood panels.
One is that there is some cracking in the paint layers, despite the gesso layers. The yield point used for all plots is indicated by the arrow at a strain of 0.005. That is, the restriction in grain direction and the release of stresses and strains perpendicular to the grain.
In most cases, it can be seen that the paint is damaged by the wood, as the cracks in the paint layer are parallel to the grain of the wood. Consider the case where there is complete restriction of support in all directions and large changes in relative humidity. This is due to the wide variety of materials used and their complex response to the environment.
One of the most misunderstood features of canvas supported paintings is the support itself. This can be illustrated by looking at the individual layers of the painting as they are held and exposed to changes in relative humidity. The tension of the hide glue at the maximum force per width of these samples was 3920 psi.
On the other hand, the cause of the high forces developed with drying is the addition of a coarse size of skin adhesive to the painting. In the middle of the painting and in the horizontal direction, the stresses are 1/8 inch. In the center of the painting and in the vertical direction, the stresses are 1/8 inch.
One of these reasons is the excessive humidification if the interior spaces of the building in winter.
RH Cycling
One of the most common types of damage to paintings, both on canvas and wood, is the result of exposure to high levels of humidity. In older historic buildings, moisture can condense on the inside of exterior walls for a number of reasons. In such cases, the exterior walls of older buildings can become very cold to the point where interior surfaces reach the dew point.
Conversely, in the summer the outside walls become hot and the space behind the painting is warmer than the interior space of the gallery where the painting is exhibited.
Stretcher expansion
Conversely, the outer walls become warm in the summer, and the room behind the painting is warmer than the interior room in the gallery where the painting is exhibited. relative humidity can be as low as 35%. This occurs on many of the monuments in Washington, D.C. One of the less frequently considered conditions occurs on very hot, humid days in. Many of the traditional covering techniques with skin glue and paste covering contribute to increasing the moisture content of the painting.
This increases the possibility of causing massive shrinkage of the original canvas of the painting and weakening the original size of the sticker. Condensation of water on paintings often tends to flow to the bottom of the painting and usually causes damage along the bottom of the painting as shown in Fig. In the case of the painting presented in Fig. 46, there was enough water on the canvas that it completely broke the adhesive bond of the animal glue size layer.
Now there isn't enough room to get the broken pieces of paint back into proper alignment. It is clear that there has been a complete separation of the color and ground layers from the canvas. The moisture level was sufficient to cause cracks in the molding layers and bond failure in the adhesive layer.
The moisture was sufficient to completely cause the adhesive bond of the animal glue layer to fail. Warm walls effectively reduce the relative humidity of the ambient air in the vicinity of the walls. Looking at the mechanical and dimensional properties of the various paints discussed above, one would expect that the white lead paint, because of its strength and resistance to moisture, would survive wide fluctuations in relative humidity.
This painting was damaged by wet walls, and the selective damage is due to the low strength and high dimensional response of earth colors to moisture. They typically have only half the dimensional response to moisture compared to tangentially cut wood. Pastes such as Japanese wheat starch pastes are actually quite strong, about half of the skin glues.
