Editing Canvas (section)

== Mechanical properties in canvas conservation ==
Understanding the mechanical properties of art canvases is necessary for art conservation, especially when deciding on transporting paintings, conservation treatments and environmental specifications inside museums.<ref name=":0">{{Cite journal |last1=Janas |first1=Arkadiusz |last2=Fuster-López |first2=Laura |last3=Andersen |first3=Cecil Krarup |last4=Escuder |first4=Angel Vicente |last5=Kozłowski |first5=Roman |last6=Poznańska |first6=Katarzyna |last7=Gajda |first7=Aleksandra |last8=Scharff |first8=Mikkel |last9=Bratasz |first9=Łukasz |date=2022-10-10 |title=Mechanical properties and moisture-related dimensional change of canvas paintings–canvas and glue sizing |journal=Heritage Science |volume=10 |issue=1 |pages=160 |doi=10.1186/s40494-022-00794-3 |doi-access=free |issn=2050-7445|hdl=10251/193062 |hdl-access=free }}</ref> Canvases are layered structures made from weaving fibers together, where each layer responds differently to changes in humidity, resulting in localized [[Stress (mechanics)|stresses]] that cause [[Deformation mechanism|deformation]], [[Fracture mechanics|cracking]], and [[delamination]].<ref name=":0" /> There are two directions to the canvas: the warp direction (threads run vertically) and the weft direction (threads run horizontally). Researchers performed [[tensile testing]] to determine the effects of humidity on the strength of canvases and observed that increasing humidity decreased the effective [[elastic modulus]] (combined modulus of the weft and warp directions). For example, the effective modulus at 30% relative humidity is 180 MPa, which drops to 13 MPa at 90% relative humidity, suggesting that canvas is becoming more flexible and susceptible to deformation.<ref name=":0" /> There is an inherent anisotropy to the elastic modulus measured in the weft and warp direction as evidenced in the strain vs. load behavior of the canvas. The canvas exhibits a 0.1 [[Strain (mechanics)|strain]] in the weft direction and 0.2 strain in the warp direction before failing (thread ripping apart).<ref name=":0" /> Though, tensile testing provides an explicit measure of material strength, conservators are unable to tare a piece of painting to create the samples (required length of 250 mm), therefore the traditional methods of assessing mechanical properties have been visual cues and pH values.<ref name=":1">{{Cite journal |last=Maisey |first=Sarah |date=2023 |title=Conserving Canvas |url=https://muse.jhu.edu/pub/331/oa_edited_volume/chapter/3767941 |website=Project MUSE}}</ref>  

Art conservators have recently adopted a new method called zero-span strength analysis, nanoindentation, and numerical modelling to quantitatively evaluate the mechanical properties of painting canvases.<ref name=":1" /><ref name=":3">{{Cite journal |last1=Lee |first1=D. S.-H. |last2=Kim |first2=N.-S. |last3=Scharff |first3=M. |last4=Nielsen |first4=A. V. |last5=Mecklenburg |first5=M. |last6=Fuster-López |first6=L. |last7=Bratasz |first7=L. |last8=Andersen |first8=C. K. |date=2022-08-12 |title=Numerical modelling of mechanical degradation of canvas paintings under desiccation |journal=Heritage Science |volume=10 |issue=1 |pages=130 |doi=10.1186/s40494-022-00763-w |doi-access=free |issn=2050-7445|hdl=10251/193061 |hdl-access=free }}</ref><ref name=":2">{{Cite journal |last1=Tiennot |first1=Mathilde |last2=Paardekam |first2=Erik |last3=Iannuzzi |first3=Davide |last4=Hermens |first4=Erma |date=2020-05-13 |title=Mapping the mechanical properties of paintings via nanoindentation: a new approach for cultural heritage studies |journal=Scientific Reports |language=en |volume=10 |issue=1 |pages=7924 |doi=10.1038/s41598-020-64892-7 |pmid=32404938 |issn=2045-2322|pmc=7220919 |bibcode=2020NatSR..10.7924T }}</ref> Zero-span strength analysis measures the [[Ultimate tensile strength|tensile strength]] of materials, such as paper and yarns, by reducing the clamping distance to 0.1 mm and applying load to a particular point on the yarn.<ref name=":1" /><ref>{{Cite web |title=Experiment B: Nondestructive Estimation of Strength in Historical Papers - Paper Through Time: Non-Destructive Analysis of 14th through 19th Century Papers - University of Iowa |url=https://paper.lib.uiowa.edu/experimentb.php |access-date=2024-05-12 |website=paper.lib.uiowa.edu |language=en}}</ref> This minimizes effects from material geometry and accurately assesses intrinsic fiber strength. This also reduces the amount of material needed for samples to 60 mm.<ref name=":1" /> Using zero-span strength analysis, conservators measured tensile strength of flax, commonly used canvas material in historical paintings and correlated tensile strength to the degree of cellulose depolymerization -- cellulose is a component of flax.<ref name=":1" /> Another method for assessing canvas quality is [[nanoindentation]] utilizing a millimeter-sized cantilever with a microsphere at its end and measuring local [[Viscoelasticity|viscoelastic properties]].<ref name=":2" /> However, with the nanoindentation method, conservators can probe the composite behavior of the layers of paint on top of the canvas, not the actual strength of the canvas itself. Lastly, conservators are using finite element modeling ([[Finite element method|FEM]]) and extended-FEM ([[XFEM]]) on canvases undergoing desiccation (removal of moisture) to visualize the global and local stresses.<ref name=":3" />