Thinking Glue

©Dean Torges/The Bowyer's Edge™

No one glue is perfect for all a bowyer's needs. Each glue has strengths and weaknesses which need to be considered for each particular application. If you understand these strengths and weaknesses, you can make reasoned choices for your particular need to get the results you desire.

Generally speaking, any glue that has gap filling properties requires a less than perfect mating surface for a sound bond. The important word here is "requires." In other words, those glues with gap filling properties fail when they are used on tight fits. Conversely, furniture glues created for close tolerances fail when they are asked to mate irregular surfaces or sloppy joints.

To complicate matters more, the degree of clamping pressure upon the joints can have as much to do with bonding success as do both the kind of glue used and the fit of the joint. Put another way, the kind of clamping pressure one glue type requires to succeed will cause another glue type to fail under identical circumstances.

Once you travel beyond selfbows and start playing with various backings or gluing riser handles onto backed slats, your success begins, and sometimes ends, with an appreciation of these variables and how they derive from the "gap" concept.

Catalyzed glues tend to have gap filling properties. Regular glues do not. Catalyzed glues which top my list include Urac 185, Resorcinol and Smooth-On EA40. Regular glues which I find useful are hide glue, liquid hide glue, Titebond (aliphatic resin), Franklin White Glue and Titebond II, in declining order of usefulness and importance to the bowyer.

Let me explain my rankings.

You could argue convincingly that yellow glue satisfies the needs of the bowyer in almost all applications. It can be used successfully for sinew, for snakeskins, for rawhide, to glue down hickory and bamboo backings, for billet handle splices, and even for riser handles. And you could make a case that the better choice of yellow glue is Titebond II because of its water resistance. Though Titebond does seem to be a good spectrum glue, in each instance mentioned, a better glue choice exists.

Yellow glue does not have the affinity for sinew, rawhide and snakeskins that hide or even liquid hide glue does. In the case of snakeskins, the perceived advantage of waterproofing is moot because snakeskins themselves are more impervious to moisture than the glue itself. Since it lacks the gap filling properties of hide glue and also tends to repel from the gluing surfaces of these animal products, it does not provide the adherence that the animal glue does. In addition, it does not provide the squeegee properties characteristic of the best, tightly laced sinew jobs which allow the bowyer to reconstitute the glue and resurrect the glue bed with warmth and moisture as he works the sinew until he is satisfied with the application.

Hide glue, and to a lesser extent liquid hide glue, also have a characteristic known in the furniture business as "suck." You can swab hide glue on a piece of wood, wiggle it into position on another piece of wood, and the glue holds both pieces together at the pressure which was exerted upon them without the requirement for clamping until the glue dries and cures. This characteristic coupled with their affinity for animal products makes hide and liquid hide ideally suited for applying sinew, rawhide and snakeskins. Even on rawhide, the use of a veneer hammer and a clothes iron can make the process free of bandages and wrappings otherwise required to apply pressure and hold mating pieces in place.

We use epoxies for horn nocks any more, usually of the 5 minute variety, though some bowyers use cyanoacrylates for these and tip overlays as well. But hide glue works well on English style horn nocks, too, and was the glue traditionally used for them because it allowed nock repositioning with an application of heat in the event the bowyer did not line them up properly on the first try, or nock removal and reuse in the event the bow failed and broke.

Liquid hide glue has a bad rap with bowyers. Laubin regarded it as largely worthless. Waldorf corroborated Laubin's experiences in "The Art of Making Primitive Bows and Arrows" (p. 25) with a heartbreaking account of its failure to dry properly on three prized sinewed and snake-skinned bows he wanted to display at Friendship, Indiana. Jay Massey probably was guided by their experiences in "The Bowyer's Craft" where he wrote two years later that "only the powdered variety [of hide glue] is suitable for bow-making," and "the problem with liquid hide glue is that it seems to take forever to dry out." (p. 75)

To my mind, it is the perfect glue for attaching snakeskins. It does not gather up or repel from the skin, like yellow glue does, but rather flows right into it (because it's composed of the same protein as the skin itself), and unlike quick-acting yellow glue, it sets up with just enough time to permit maneuvering the skin into perfect alignment down the bow limb while squeegeeing the air bubbles from underneath it.

Moreover, if after the skin is dry and you detect an air bubble by listening for "whispers" as you run your fingers over it lightly, all you need do for a repair is pin prick the bubble to void the air and then iron down the spot with the tip of a warm clothes iron. The iron temperature is correct when you can hold it an inch or two from your cheek without experiencing discomfort.

Liquid hide glue has a short shelf life. Laubin's and Waldorf's problems almost certainly resulted from using expired or nearly expired glue. To test liquid hide, put a dab between your thumb and forefinger and start tapping them together. If the glue remains viscous, or nearly so, it is degraded and worthless. If it starts grabbing and sticking almost immediately and begins forming whiskers when you try to pull your finger from your thumb, the glue is in good working order. Expired glue also usually has an odor to it, too, such as you would expect from a spoiled animal product.

Titebond II is touted in bows because of its moisture resistance. It is important to note that II is not waterproof. Urac and Resorcinol and Smooth-On are waterproof; Titebond II is water resistant. So is regular Titebond, only less so. However, regular Titebond forms a stronger bond than Titebond II, so if you employ Titebond II you must understand that you are giving away holding power to gain what a good finish should supply you with anyway-good moisture resistance.

Titebond works best in those applications where the joints mate closely and where the materials joined are similar in kind. It is almost impossible to cause a joint found in bowmaking to fail as a result of starving it through too much clamp pressure with Titebond, but Titebond can and does fail when the assembly time exceeds the set-up time. This probably happens often without the bowyer's awareness in the case of hickory or bamboo backing glue-ups, which suck the moisture from the glue and speed up its set time.

Consider also the long assembly time required of most bow backings-of spreading glue over both surfaces and then going through a clamping procedure-and you are almost guaranteed of assembling the joint at least on the edge of set-up time. If the glue sets or flashes off, the bond is not sufficient to prevent eventual delamination and a broken limb.

Titebond also fails when dissimilar materials are glued together, such as bamboo and wood, through a process known as joint creep. It happens this way:

No finish can keep moisture from coming and going through wooden bows, though some finishes retard this process more effectively than others. Bamboo is exceeding hygroscopic, as is hickory backing, at least compared to most of the woods it is glued to. Result is that hickory, and especially bamboo, expand and contract at different rates than the wooden slat to which they are glued. This causes something akin to plate movement in continental drift, placing great strain on the joint. At first, the outside edges of the joint will yield to the strain and through moisture absorption begin to loosen. You can observe where they break through the finish at the glue line, and your fingernail will reveal that the surface glue line is no longer flush.

Because the annual rings of an attached riser handle are seldom oriented perfectly with the grain in the slat, the riser handle and the slat will expand and contract in different directions as well, and the riser handle, which usually includes the fadeouts or dips, becomes especially vulnerable to strain and joint failure through the same moisture absorption process mentioned above. When the riser handle is flat sawn and the slat is quarter sawn (or vice versa), the contrast in expansion/contraction directions is most dramatic, one piece of wood moving north and south, the other east and west. The deterioration begins where the riser fadeouts undergo flex and stress. The weakened joint fails under compression in these thin areas first, and that failure works into the fadeouts until the tiller goes or the handle joint fails. In effect, the joint simply worries itself into collapsing.

If the joint is loose or sloppy between the riser and the slat, or between the slat and the backing, then the joint will deteriorate all the more rapidly.

A waterproof catalyzed glue circumvents these dangers altogether. The set-up time is liberal enough that you need not hurry in applying glue or in clamping up your stave for fear that the glue will flash off and the joint will not bond. In addition, if the mating surfaces are irregular, catalyzed glues take up the slack and fill gaps.

I prefer Urac over Resorcinol because Resorcinol's purple color seeps into both mated pieces of wood and thereby exaggerates the appearance of the glue line at the bond and because Resorcinol has become, at least in my neighborhood, increasingly difficult to find. On the other hand, Resorcinol seems to me to make the stronger joint on the more poorly mated and gapped gluing surfaces. Handle splices cut by the tyro are more safely made with Resorcinol therefore. Smooth-On works fine but is comparatively expensive, whereas a quart of Urac with catalyst costs about 12 dollars.

Unlike Smooth-On or G-2 epoxy, Urac's catalyst and resin parts need to be measured accurately for the strongest bond. You can only do this by weight and not by volume, at a rate of 14 parts to 100, catalyst to resin. The real downside with Urac 185, however, is that 1) the resin has a shelf life and 2) tight clamping pressure on closely mated surfaces likely results in starved glue joints. Countermeasures exist.

The powdered catalyst lasts indefinitely at room temperature. Storing the resin in a cool place like a refrigerator will extend its life by three fold. Bring it to room temperature before use. When the resin does degrade past usefulness, it resembles hide glue in that it loses its viscosity and turns watery and smelly. You can't miss it if you are aware that it happens.

A properly prepared glue surface will provide both a tight and a sound joint if you are using glues with gap filling properties (catalyzed glues). In other words, you don't have to have a sloppy or loose joint for them to work (and you don't have to worry too much if you do, either). By scuffing or scarring the mating surfaces sufficiently, you can keep them from closing upon each other and squishing out the glue, and you can still clamp them up tightly enough to create only a thin and unobtrusive glue line.

Electrical sanding on 30 grit belt sanders will provide such a sound foundation. Great care must be taken during the process to sand (or grind, in the case of bamboo) the surfaces dead flat. The greater likelihood is that mating surfaces rock slightly on the table platten and thereby round over, becoming convex. The result shows unsightly and exaggerated gaps along the edges of the glue line. Of course, a thickness sander does not create these problems, but few of us have access to one to avoid this hazard. Hand sanding with coarse paper will not get the job of surface preparation done.

A simple toothing plane blade, which is available for around 20 dollars, dragged over closely mated surfaces, will create a perfect glue bed. Scrape it the length of both work pieces at such a scraping angle that its column of sharp teeth furrow into the wood and create close parallel channels or grooves down the entire gluing surface, both faces. Work over the surface several times, until the teeth have found their full depth into the wood.

A good tool tip is to grind and hone the blade very slightly convex so that with thoughtful scraping you create a slight hollow or dish along the length of the slat and backing, or riser handle and slat, as the case may be. Clamping pressure applied to the middle of the joint or upon the crown of the bamboo then causes the joint to squat or cave, providing a tight fit across the width and a comely joint along the length with no danger of glue starvation.

A few final tips: Resinous tropical woods and oily woods (such as osage) require a "degreasing." Wipe their gluing surfaces with a rag dipped in acetone prior to the application of glue. Allow the acetone plenty of drying time after it flashes off before applying glue, ten to fifteen minutes. Wear rubber gloves both to protect from the acetone and from the catalyzed glues; they are both hazardous to your health through skin absorption. Always apply glue to both mating surfaces regardless of glue type.

Be thoughtful in your glue choices and in joint preparation and you significantly decrease the likelihood of failed joints breaking your heart.