Picture varnish adhesion

The first synthetic polymers to be used as paint varnishes were acrylic and vinylic resins. Poly(vinyl acetate) (PVAc), commercialized under the name Mowilith by Hoechst and Vinylite by Union Carbide, has been used in conservation as an adhesive since 1932 and in 1937 it was proposed as a picture varnish by Stout and Cross [63]. PVAc was soon rejected as a varnish because, despite its light stability and good solubility in organic solvents, it demonstrated poor optical properties in terms of colour saturation and the tendency to pick up dirt due to its low glass transition temperature. 

Properties Yel. brittle amorphous lumps or powd. turpentine-like fresh resinous odor sol. in alcohol, acetone insol. in benzene, aryl and aliphatic hydrocarbons, water sp.gr. 1.05-1.09 m.p. 135-150 C soften, pt. 100-130 C iodine no. 112-141 sapon. no. 145-157 ref. index 1.545 Uses Binder in lacquers and paper/picture varnishes emulsion stabilizer in cosmetics natural flavoring agent in alcoholic beverages in food-pkg. adhesives in food-contact coatings... 

Coatings are applied to and act on surfaces that are visible at the time of application, such as picture varnishes or sealants of porous edges before applying adhesives. 

Role of polymer Adhesive/coating/consoli-dant/moulding material A brief description of the tested conservation application is valuable Cellulose acetate was rejected as a picture varnish... 

Physical properties Tensile strength, hardness, extensibility, glass transition temperature These data (derived from manufacturer s literature or practical experience) indicate the limits of use PVAL coatings form strong films but have little adhesion to picture varnish... 

Egg white has long been used as an adhesive (Theophilus, 1963) and as a picture varnish this is known to cause problems (Woodhuysen-KeUer, 1999). Calcium albumate, made from white of egg, has also been used as an adhesive (Standage, 1931). 

Venice turpentine has been used in picture varnishes and as a tackifying resin in adhesives partly because it inaeases the flexibility of the film, though this effect will diminish as the monoterpenes evaporate. Canada balsam was widely used for mounting specimens on microscope shdes and as a glass adhesive, though again the evaporation leads to shrinkage and distortion. 

Xylene is the most important aromatic solvent used in the paint, varnish and adhesives industries, and is also a starting material in the production of plastics. It enters the human organism by the same routes as benzene and toluene, and the clinical picture is similar to that for toluene. 

Epoxy resins have been used in canvas picture conservation. An unfortunate use as a varnish caused problems in identification and removal (Keck and Feller, 1964). Epoxy resin adhesives have been used for sticking threads across tears in canvases and also as a filler (Berger and Russell, 1993). The formulations need to be of sufficiently high viscosity to prevent absorption into the canvas, but must be flexible enough to respond to movement in the fabric. 

---