Glues and Sizes
Size is the first layer in the preparation of painting support. A size is not a coating and not an independent layer; it is a penetrating liquid used to seal, to fill the fibers and to make the painting surface nonabsorbent. If the next coat is composed of substance that can damage or deteriorate the painting surface like oils, it is necessary to seal it, the surface will be nonabsorbent and it will protect it. For this reason, we apply a coat of glue sizing (with neutral pH); solutions of animal glue, casein, synthetic resin and so on are used as materials for sizing. Protein materials such as gelatine glue, bone glue, rabbit skin glue and casein can cause mold growth, that's why it is recommended to brush or spray directly onto the dry glue film a 4% formaldehyde (formaline). The formaldehyd will protect and harden the size.
Delamination of the ground (with the paint film) from the canvas support could occur if the size layer will not penetrate the fabric structure. That's why it's important to apply the size in its liquid and not jelly state. If the size is applied in its jelly state it will make a discrete glue size layer that will coat the canvas support. If the size layer will loose its adhesive strength someday, the ground layer will release from the canvas.
Animal glue made from skins, bones, or cartilage is sold as a powder, granules or sheets. When dry, they harden to tough leathery films but do not change chemically. Thus they can be dissolved again in water, which means it is the obvious choice for repairs which have to be reversible. The main component of the glue is the collagen. Collagen is a protein that is the main support of skin, tendon, bone, cartilage and connective tissue. Conservation scientists caution painters that animal glue absorbs atmospheric moisture on damp days and swells; gives off moisture on dry days and shrinks. This movement of the size layer can cause aged oil paintings to crack. But if the size is used on a good prepared panel that is varnished after the paint layer cured, there is a smaller chance for the moisture to penetrate and cause the movement. It is very important to apply the glue size hot and not in his cold jelly state, when it's hot it penetrates the fabric structure, thereby locking the yarns and reducing the canvas shrinkage and paint cleavage that could occur. When animal glue dries it acquire very strong surface tension, the fabric gets tight, if a fabric is glued to a panel with such glue it can bend the panel.
Casein is a protein found in milk. It has been used as a glue and binder since earliest recorded periods but it is not popular as a size.
The dried casein is insoluble in water and alcohol but is soluble in carbonates, ammonia and other alkaline solutions. Casein is too brittle therefore it is better to use the casein for sizing rigid panels like wood-panels fiberboards. Or you can glue the edges of wood pieces for making a panel because casein has a very strong adhesive power. But there are some problems: casein is irreversible and new casein doesn't stick to an aged one. So, when your panel will need a restoration, it will be necessary to remove some wood grain till you'll get a clear surface.
Casein paints have been used for panel paintings, wall paintings and as architectural paints. They stand very good in aging tests. There are three processes in commercial production of casein known as: self-soured, acid and rennet caseins. The first two are used for art as adhesives and paint-binders. The usual commercial casein is the acid casein, made by adding special acids to a fresh skim milk, because of the acid environment the casein get together and turns to curds, the curds are carefully washed and dried.
Casein is sold as yellowish granular powder, and it is better when it's fresh. The glue is prepared by adding distilled water to the powder and an alkaline substance, like ammonia water, ammonium carbonate and ammonium compounds. Those kinds of alkali substances are used for grounds and permanent art works, because they are volatile and doesn't leave alkali environment. The casein contains acidic particles from the commercial process, when you mix the water with the powder the solution becomes acidic. It is important for glue to be with neutral pH, therefore an alkali is added. The excess ammonia can be discerned by its odor or by immersing in the solution a strip of red litmus paper, which will turn blue if ammonia is present. The excess ammonia can disappear by warming the solution in a double boiler (water bath), casein should never be boiled. Or it can pass with the drying, but be aware that there are no other ingredients in the solution that can be affected by the excess of the ammonia.
For the more coarse sorts of use like commercial coatings/paints, alkali like soda, borax or lime is added. There are a lot of recipes for casein that you can make at home by using milk, vinegar as an acid and soda (or borax) as an alkali. But it is very unwise to use it as a substitute for the commercial one, because the homemade casein contains butterfat and milk sugar, which will impair the final product. Occasionally a plasticizer, such as glycerol or sorbitol and a preservative, such as formaldehyde, are added to the solution. You can Casein is a water base natural product and will spoil if kept in a wet state. Store unused casein solution in the refrigerator-this should preserve it for two weeks.
Cellulose - carbohydrate which forms the chief component of plant cell walls. It is not recommended to use it for gluing canvases to panels because its adhesion power is not strong. As a size it is not popular because it forms a continuous film even if it's diluted, and the size must be not an independent layer but a penetrating liquid; this size layer can be a problem for the ground (primer); it will not stick well to the sized canvas.
PVA (Polyvinyl Acetate)
PVA is a synthetic resin, discovered in 1912. It is sold in liquid form, as an adhesive for porous materials such as wood, paper and cloth.
It is mostly used by carpenters as "white glue" and yellow "carpenters glue", but all of them are acidic or alkali. The PVA glues sold in different stores are slightly acidic ore alkali too. The conservation scientists recommend painters to use neutral pH PVA size on canvas (linen, cotton…) instead of animal glue.
PVA provides a good size layer that seals the fabric but does not re-absorb atmospheric moisture, swell and shrink like animal glue does. PVA Size doesn't tighten fabric like animal glue, so it will not curve the panel to which the canvas is glued.
There are hundreds of different formulae of PVA, there for it is better to use the ones that are meant for this purpose. The most popular neutral pH size is the one that makes Gamblin Artists Colors it is already diluted with distilled water; it is a contemporary size for fabrics. The other one is Lineco Inc. their "Neutral pH Adhesive" is reversible and can be used for sizing and gluing canvas to a board because it is not as diluted as Gamblings'. Those glues are permanent, does not emit harmful volatiles; they will not yellow and will not become brittle with ages. They are easy to use, doesn’t require any preparation.
Beva 371 Glue
Beva is an adhesive developed by Gustav A. Berger. It is based on ethylene vinyl acetate, and used by the art conservators because it is fully reversible by heat.
Beva is widely used for lining oil paintings and it is the best adhesive used for gluing canvases to a board; it is totally compatible with the size used on canvases. It can be used for gluing paper to a board or gluing a new canvas to the back of an old one.Beva sticks to almost every material except silicone. It causes no contractions, expansion or softening of the materials to which it is applied, making its application possible on even the most delicate surfaces.
Beva comes as a "Film", as a "Solution", and as a "Dry Resin Mix" for making the solution; it is thinned with petroleum solvents. The "Film" version comes sandwiched between a white, silicone-coated release sheet and a Mylar release sheet. Beva 371 "Film" is the best choice because it is easier to use, it can be cut to the needed size, and you'll get a uniform and smooth glue film; it will be hard to achieve this by using the Beva "Solution". Available as 1mil (25.4micron) thickness that is suitable for mounting of lightweight papers and fabrics, and 2.5mil (63.5micron) thickness is for use on canvas and heavy weight materials.Although designed for use on the vacuum hot table, it has been successfully applied utilizing a hand iron or a hot air blower. Beva is removable with petroleum solvents, mineral spirits, and/or heat (150°F=65.5°C).
Acrylic is a synthetic resin, it can be diluted with water but become water insoluble. It can be used as a size or as glue, it doesn’t swell and contract like the animal glue does. It is an excellent barrier for oils and it dries to a flexible film. The acrylic is approved by the conservation scientists. There are a lot of companies that make acrylic products, but be aware, lot of them are not for permanent use. The most popular is GOLDEN Artist Colors Inc.; they are experts in the acrylic field and do a lot of researches. Their "Special Purpose Polymers" and "Soft Gels" can be used for preparing an archival surface. Their Soft Gel Gloss is used as glue for collage work; and the Special purpose Polymer-GAC100 as a size because it will block oil penetration to the canvas or surface. It is not recommended by the company to use on top of the GAC100 alkyd base ground, because the fresh acrylic layer may have substances which migrate into the wet alkyd film and retard its ability to dry correctly. They have a very informative site, where more information can be obtained.
Another good products are made by Lascaux®; they have permanent and reversible glue "Lascaux Adhesive 498HV"and "Lascaux Adhesive 498-20x" the second one is more suitable for non-porous surfaces. They produce glues like the Beva 371 "Lascaux Heat-Seal 375", and they have permanent "Lascaux Acrylic Sizing" as well.