Despite what the sales brochures would have you believe, for artistic purposes, hide glues remain unsurpassed when made and applied properly. This newsletter is presented for the purpose of giving you the knowledge to make and use hide glues properly, ensuring long-lasting works of art.

Although there are quite a few good choices for various artistic uses, let's confine ourselves to just two of that number – Hide Glue and Rabbitskin Glue. Hide glue is a commercial product used in industry. It's reasonably pure but like all natural products, requires skill in making. Industrial grade glues are known as technical glues. The poorer grades smell badly, a result of allowing the raw hides to putrefy. If you've ever made glue that smelled, the reason was the hides were not handled properly. Smelly glue sells for less on the wholesale market. They might not sell for less in the art stores ...the smell being passed on instead of the savings.

Aside from hides, hide glue is often made with leather scraps as well as hides from a variety of animals. There is little uniformity from some less than scrupulous makers. Fortunately for the artist, there are a handful of tradesmen who take pride in what they produce. Those are who we go to for our glues.

In picture A, you can see the immediate difference (much hide glue is fobbed off as rabbitskin glue). This example of hide glue is good for general purpose uses as an adhesive, joining wood, sizing paper and making flexible molds (like all protein glues, adding or washing the surface with an alum solution causes it to become tough and leathery, while maintaining detail and flexibility). After cooking the hides, the glue is poured out in sheets, dried and then granulated.

The rabbitskin glue in this picture is certainly the very finest available. It is made in a small plant in Portugal, next to a larger plant that uses rabbit fur to make felt for fine hats. The hides are never allowed to sit and smell. Once the hides are boiled to remove the glue, the wet glue is sprayed into a drying chamber, drying into small grains in mid-air. This is called claro grade glue. Nothing compares ...and we import this directly.

Glue strengths are determined by standard test in which a probe is lowered onto a sample until the surface forms a crack. This is the Bloom Strength test and it measures the number of grams pressure required to cause a crack. Our hide glue has a Bloom Strength of 376 grams. Our rabbitskin glue is much harder at 510 grams. Only certain gelatins have a higher Bloom Strength. The lower the Bloom Strength, the slower it takes to set and the greater the shrinkage. In the case of gluing joint in wood, a slow-setting glue that shrinks, drawing the joints together is desirable. For sizing canvas, we want to avoid too much shrinkage. Thus, the use of a very hard glue is called for. Once dry, the harder glue is far less hygroscopic (drawing water from the atmosphere) than a glue with a lower Bloom Strength. Top quality rabbitskin glue is as, or even less hygroscopic than some synthetics like PVA (Elmer's Glue).

One of the reasons that certain synthetic glues are preferred by conservators is simple – those glues are formulated in factories where the workers read the directions and follow them. If you read these directions and follow them (with no variation) I can guarantee that you will produce a sized canvas that will last for centuries (remember, the purpose of sizing is to provide a flexible but impenetrable barrier between the oil in the paint and oil primer and the fabric of the canvas).

For any of our waterborne formulas, we strenuously recommend that you use only distilled water. Tap water varies from location to location and can range from hard to soft with great variation in the pH. Distilled water is constant and guarantees results you can duplicate from batch to batch.

To make the base from which we work, mix one part of dry glue to eleven parts of water at room temperature (picture B). Do not try to speed things up by warming the water. Stir thoroughly and remove any clumps. Allow it to sit until the powdered glue has absorbed the water and, over a few hours, forms a gel.

In picture C, you can see the quality of the gel you should expect by adding eleven parts of water to one part of dry glue. It is a soft gel but stiff enough to support a paint stick without sagging. A harder gel indicates that the mix is not dilute enough for our purposes. Again, measuring is very important. As you can see, a little dry glue goes a long way and a one pound bag will last most artists for a year.

To be useful, the gel has to be made liquid and flowing. That is done with gentle heat. Most artists have heard to cook the glue in a double boiler. The problem is that boiling water is 212F (100C) and, at that temperature, the protein strands begin to unravel, greatly weakening the glue. As shown in picture D, the correct temperature for melting glue is to place it in a container that is plunged into a water-bath holding water not above 135F (57C). That's enough to allow the glue to melt without causing it to weaken. Most hot tap water is hot enough to use in the water bath if it is circulated with running hot water from the tap. In any case, use a thermometer. It will add centuries to your canvases.

Once the base (11:1) gel is melted, it is mixed with an equal part of warm water to make a 22.5:1 dilution as seen in picture E. This dilution is called the drink and it is usually overlooked. The most common problems are 1) glue too hot and weakened, 2) not dilute enough, much too stiff a gel, 3) gel pushed through to the back of the canvas, forming little wicks. Let's learn the proper way to do it.

Dip a clean sponge in the drink and soak the canvas. This is so dilute that it simply soaks into the fibres. When the canvas dries, you can see (using a glass) that the spaces between the threads have a thin film between each of them. These are exactly halfway between the thickness of the threads and form an ideal platform to prevent the base coat from pressing through to the back of the canvas. The base coat can be applied with a broad brush or, my favorite, a large foam brush. Move quickly to avoid softening the thin platform of glue made by the drink coat.

Once dry, the canvas is perfectly protected from the oil on the surface and atmospheric moisture from behind.

For more information, please visit us at The Cennini Forum online.