Canvas analyses: Tensile tests

At the Royal Danish Academy – Conservation, we are performing several tests and analysis of 15 different canvases, to understand the variety of shrinkage potential in linen canvases used for canvas paintings.

This is important in the process of validating the data for canvas that the computational model of canvas paintings is relying on. In the blogpost from July, we explained cover factor and spinning degree analyses; this time we will explain the tensile testing of the canvases, that we are performing at the moment.

Tensile testing of canvas samples is carried out to test the canvases’ response to fluctuating relative humidity. In other words, we test how the canvases are reacting or behaving when humidity in the surrounding air is changing or fluctuating. We are conducting two types of tensile tests: Iso Strain test and Free Swelling test.

The Iso-strain test shows how the degree of tension is changing in a restrained canvas as a result of fluctuating ambient humidity. This test simulates a tensioned painting (such as paintings mounted on a stretcher) subjected to changes in ambient humidity.

The Free Swelling test shows how the canvas is swelling and shrinking when the ambient humidity is fluctuating. This test makes it possible to quantify potential for shrinkage in a canvas at different humidity levels.

The resulting data from the tensile tests provides input on for example at which relative humidity levels the canvases may shrink, or how the canvases are taking up moisture and thus how much they swell and loose tension. These inputs are important in the validation process of our computational canvas painting model, as data in the model can thus be compared with the behavior of the tested canvas samples.

By testing several canvases, we study the varieties of the shrinking and swelling behaviors of different canvases and make it possible to make more precise assumptions in the model. 

The two types of tensile tests are performed on equipment designed and constructed for this exact purpose by Marion F. Mecklenburg, senior research scientist at Smithsonian Museum Conservation Institute.