UNDERSTANDING THE BINDER IN GESSO
In order to understand the binder in gesso it is necessary to understand the purpose and content of gesso. Gesso is used to create a surface for painting and gilding. Gesso is employed in gilding to create a surface that makes an object seem as if it were made of solid gold. Gesso is a mixture of binder and filler prepared for application to a surface. There are many different types of binder: rabbit skin glue, fish glue, gelatin, parchment glue, acrylic binders and casein, to mention a few. There are different types of filler which, for making gesso, are usually given the general name of whiting. Some materials used as whiting are precipitated chalk, powdered gypsum, marble dust or some form of calcium carbonate. There are different types of gilding and many different types of surfaces that can be gilt.
The choice for binder and filler in gesso is directly related to the surface to be covered and the desired finishing technique one wishes to execute. The most temperamental gesso for framers is gesso for water gilding wood. The water gilding technique consists of applying a binder to the wood surface; applying layers of gesso to the binder; applying layers of bole (clay and binder) to the gesso surface; and gold to the bole surface. This particular gilding technique is chosen when a highly reflective quality is desired from a gold finish on a wooden object. In this gilding process the gold finished surface may be burnished creating the most reflective gold finish possible.
The water gilding technique requires that the binder in the gesso possess properties that make burnishing possible and allow the surface to respond to climatic conditions in a manner similar to the wood. It is the binder in gesso that gives it most of it's properties. The filler, or whiting, is inert. Most acrylic binders for example, dry to a non-absorbent state. This eliminates them as a binder for water gilding wood frames. All of the processes for water gilding require that all of the materials involved be absorbent. Acrylic binders do not allow for burnishing. Acrylic gessos are a mixture of acrylic binder and whiting.
Rabbit skin glue has been the binder of choice for water gilding wood frames for about as long as there have been frames. The reasons are numerous but there are three very important factors. First, rabbit skin glue is strong. It is in fact one of the strongest adhesives know to man. When gilding wooden chair and table legs, gesso made with rabbit skin glue is used because it can take a kick and still live to tell about it. Rabbit skin glue has the unique ability to distribute pressure through out it's structure when it receives pressure instead of cracking. This renders the materials it makes burnishable. Secondly, rabbit skin glue is hygroscopic, it reacts with moisture. The gesso it makes will react to climatic changes along with the wood insuring a durable bond. Third, rabbit skin glue reacts to heat. All mixtures made with rabbit skin glue must be applied to a surface warm. When diluted rabbit skin glue is blood temperature it is a thin liquid that can penetrate the wood surface. When a warm aqueous mixture is applied to a wood surface the fibers of the wood swell and open aiding the penetration of the glue into the surface creating a wonderful bond between the object and the finish.
The fact that rabbit skin glue reacts to heat also effects the application of gesso to a surface. Gesso can be air brushed but normally the warm gesso is brushed onto a surface by hand. As rabbit skin glue cools it gels and this permits a thick application of material to the surface. While the gesso is still wet on the surface the brush stoke flows out forming a smooth surface. If the room where gesso is being applied is too cold the gesso will cool and set before the brush stroke has a chance to flow out. This is important when gilding due to the reflective property of gold which causes light to catch at edges. If the gesso is not smooth and flawless, light will bounce off the gold surface accentuating every imperfection in the gesso. The bole surface that is applied to gesso in water gilding is not intended to, nor does it have the body to fill or otherwise hide flaws in the gesso surface. The gesso will need to be resurfaced by sanding or scraping if it's application is not smooth enough for the desired gilt finish.
Rabbit skin glue's reaction to heat effects the brushing technique used during application. It is necessary to apply gesso quickly before it cools and sets. Often it is suggested to add boiled linseed oil to a gesso mixture to aid the flow factor of the gesso while it is drying. The brush strokes will lay down better and resurfacing can be
mineralized or avoided all together. This is an issue that is in debate. One school of thought is that the oil will be absorbed by the filler and cause no adverse effect. Another school of though is that the oil will migrate causing delamination and prevent proper maintenance of the gilt surface in the future. These craftsmen advocate perfecting a brushing technique that delivers the gesso to the surface warm enough so that the brush stroke will flow out without the assistance of an additive. This brushing technique is dependent upon the sensitivity of the hand of the craftsman and his or her patience to perfect it.
The strength of a gesso surface is the ratio of filler to binder. The key to understanding this is to imagine that the binder is a mesh, a structure that is asked to hold particles, meaning the filler. If too much filler is added it weakens the binder. If too little filler is added the mixture will be too strong and shrink excessively when it dries. Gesso that is too strong will shrink, crack and delaminate from the surface it is applied to. Fillers, like whiting, prevent glues from shrinking too much. To manipulate the strength of the glue in gesso some craftsmen like to keep the quantity and ratio of the glue to water the same and change the quantity of the whiting added. Other craftsmen like to keep the quantity of the whiting constant and change the ratio between the glue and water.
The strength of the rabbit skin glue is the structural strength of the gesso it makes. It is important to understand how and when the strength changes. When applying layers of gesso to a surface a strong layer of gesso cannot be applied to a weak layer of gesso. When pressure is applied the weaker layer gives way causing the stronger layer to collapse. When the strength of the glue of the gesso layers is incorrect a fault is created that will result in future delamination.
Remembering that the thickness or thinness of a gesso effects how much whiting is added and the whiting to rabbit skin glue ratio effects the strength of the gesso, let's consider the wood surface a moment. Wood has been a material of choice for framers because it is strong and can easily be shaped to the forms required for framing. Frames made with wood could be smooth and flat or they may be carved. Gesso likes to pool in recesses which obscures detail in carved wood surfaces. The means to compensate for this factor is to adjust the consistency of the gesso. Either
thin coats of gesso are applied to the detailed carving or the detail is carved into the gesso surface instead of the wood surface. In order to carve or cut a sharp edge into the gesso surface the glue strength needs to be properly balanced. If the gesso is too weak it will crumble under the pressure of cutting and if it is too strong it will chip. Both instances result in an unclean line.
Rabbit skin glue reacts to moisture and heat therefore it reacts to climatic changes during application. If you learned how to make gesso for water gilding away from home or you are following a recipe from someone who is from another town beware. You may be properly preparing the gesso but in application the gesso is acting very strangely. This is may be due to the weather. Generally speaking in a hot, humid climate a weak glue mixture performs well and when one is gilding in a dry, cold climate a stronger glue moisture works. You will need to understand glue strength manipulation if you live in a climate that has a habit of changing.
The binder for water gilding wood frames is rabbit skin glue because it is a strong binder and it reacts with the wood to moisture changes in the atmosphere but only to an extent. The excessive movement at joints in a wooden object during climatic changes is desirable to arrest. For this reason fibrous material has been used to cover joints before the application of gesso. This is done in the hope of isolating the movement of the joint behind the fiber to keep the gesso from cracking from the stress. Rabbit skin glue is strong but it is does not make a gesso that is very flexible. The gesso made for manuscript illumination is created to bend along with the parchment or paper it is applied to. When illuminated parchment pages are turned the gesso applied to them will not crack or delaminate. This is accomplished by changing the binder of the gesso to fish glue, using slaked plaster as the filler, and adding other materials like lead white and sugar for additional flexibility.
It is the binder that gives gesso its properties. The key to understanding how to manipulate a gesso lies in understanding how to manipulate it's binder. The key to creating a great finish of any kind lies in knowing how to maintain the finishes' structural integrity.
Baggot Leaf Company