EXTENDING OPEN TIME, TACK LIFE
For those specific glue operations requiring an appreciable period of time for assembly before pressure can be applied, as in hand gluing of irregular pieces, the gluing of mortise and tenon joints, dowel work, and case goods assembly, the speed of set and period of tack of the adhesive may be varied at will through the incorporation of gel-depressant chemicals. These chemicals, commercially available, are added direct to the warm hide glue in amounts sufficient to obtain the working characteristics desired – retarding the rate of jelling of the adhesive and prolonging the period of tack. If sufficient gel-depressant is added, a cold liquid glue is produced which may be applied at room temperatures in the range of 70-85o F. The preferred gel-depressant chemicals are those which only physically depress the rate of set, have no adverse chemical effect upon the structure of the glue, and produce a joint strength indistinguishable from that obtained in using a normal type warm hide glue.
Cold liquid hide glues are particularly well suited as the adhesive in assembly work employing the newer-type compression dowels and mortise and tenons. In the use of these special assembly aids, the liquid glue furnishes sufficient moisture to expand the compressed wood to fill the joint rigidly and firmly, and the set of the glue is retarded to give a period of tackiness long enough to insure permanent adhesive anchorage at the joint. The following cold liquid hide glue formula is suggested for this specialized application:
10 parts by weight 283-315 gram test hide glue
20 parts by weight water
2 parts by weight gel-depressant (urea – a white granular material, commercially available).
The dry glue is swollen in the cold water until soft (30-45 minutes); melted in a jacketed container at 145o F; the dry gel-depressant chemical added and dispersed in the warm glue solution; the adhesive cooled to room temperature is ready for use. It is also possible to prepare liquid hide glue by simply adding the glue and gel depressant to cold – warm water, stir occasionally until a smooth liquid is obtained.
VENEERING WITH HIDE GLUE
Hide glues find wide acceptance in the production of high quality veneered panels both for core construction and laying of the highly prized veneer. In this operation a thin layer or veneer of expensive wood is applied to a solid core assembly of lower priced wood. This procedure permits wide utilization of our forest products and broadens the availability of attractive furniture of the office and home at reasonable cost. Inlays of various designs, shapes, grain markings, and color are frequently used for decorative purposed on panels, table-tops, and the like, and are closely associated with veneers in their use and application. All grades and types of hide glues may be used for adhesive purposes in this broad field. Depending on the specific application and required length of tack, the warm hide glues, cold liquid hide glues, or modified warm hide glues partially treated with gel-depressants find use. While extreme strength is not required in this field, the relatively non-bleeding qualities of properly prepared hide glue adhesives, their neutral color, permanence, and non-staining properties are qualities of interest and value to the user.
The use of hide glue sizes in the finishing of quality furniture surfaces is not commonly known by the public at large. In this process a dilute warm hide glue solution at approximately one pound of glue per gallon of water is applied to the wood surface and let dry. The compression grain is raised and the glue fills the porous exposed wood structure. On sanding, a glass-like surface is obtained, which is stable against moisture changes and which takes a lasting final stain or finish.
MOISTURE RESISTANT JOINTS
There are many instances in which, through trade demands, a moisture-resistant joint is desired. Hide glue is readily made moisture-resistant through a simple, practical method which has been employed for many years. The need for moisture-resistant joints, except for a few tropical and sub-tropical areas, has been greatly exaggerated. Furniture and musical instrument assemblies using unmodified hide glues have for many decades successfully withstood all usual climatic conditions the world over.
Where added moisture resistance is desired, the following procedure is recommended:
- Select the type and test grade of glue normally required.
- Prepare the warm glue solution following standard procedures.
- To one face only of the wood joint to be assembled, apply with sponge or brush a solution prepared from one pint of commercial formalin and nine pints of water in which has been dissolved ½ pound of borax. Allow the excess moisture to be absorbed from the surface of the wood.
- The glue solution is applied to the matching face of the assembly.
- The formalin-treated and glue-treated members of the assembly are then brought together and placed under pressure.
- The glued assembly, after removal from the clamps, is allowed to season for from five to seven days, to permit a gradual even development of moisture-resistant properties.
This simple procedure effects a controlled tannage of the glued bond, providing moisture-resistance while still maintaining the strength and shock-resistant properties so characteristic of hide glue joints. Note this tannage of the glue film is not reversible. So the hide glue characteristic of being reactivated with moisture and heat is completely eliminated.
GLUE BOND FAILURES
In all adhesive work there are occasional joint failures. With hide glue and its inherent margin of safety factor and a tensile strength far in excess of wood, such joint failures as may occasionally be experienced are more properly defined as gluing failures. Among the more common causes of gluing failure are:
- Improperly seasoned wood. A kiln-dried wood is desirable wherein the excess moisture has been properly reduced to the moisture content equilibrium under which it will be used (8 to 12% moisture), with freedom from case hardening and internal stresses.
- Poorly dressed or machined wood. Glue is not a filler – the joints should be true.
- Insufficient clamp pressure. Sufficient pressure should be applied to bring the individual members into alignment, squeeze out air and excess glue, and hold the members in rigid position during development of adhesive strength. Pressure of 100 to 200 pounds per square inch are recommended.
- Excessive clamp pressure. Pressures over 150 to 200 pounds per square inch are generally unnecessary, and are apt to squeeze out glue essential to formation of a sound joint.
- Removal of assembly from clamps before sufficient development of joint strength.
- Machining or surfacing of joint before full development of joint strength and equilibrium moisture content of joint have been attained.
- Improper glue solution for specific assembly.
If the above listed variables are properly under control, the glue solution at 140o F in case of warm hide glue solutions, and the room and wood temperatures at 72-80o F, the proper glue concentration will provide:
- Adequate wetting-out of the wood surface.
- Proper transfer of glue applied to the one side of the joint to the adjacent face after pressure is applied.
- A tacky, slightly stringy glue film at the joint just as pressure is to be applied.
- Extrusion of a small amount of tacky, fairly heavy-bodied excess glue from the joint as pressure is applied.
Careful examination of a few test joints, as being processed, and with the above conditions in mind will enable the operator to evaluate the conditions present and modify the basic glue concentration as required. If the glue concentration is too heavy, imperfect results under a) and b) will be found. If excessive glue is extruded under d), excess glue was applied to the joint, or the glue solution was too thin, or pressure was being applied too soon. If relatively no glue is extruded under d), insufficient glue was applied, or the glue was chilled or dried before application of pressure. “Starved”, “chilled” and “dried” joints may be eliminated through proper variation in glue concentration and gluing technique to insure the condition under c).
All hide glue suppliers are in a position to offer more detailed information and technical service concerning the use of hide glue as an adhesive, and with particular reference to its use in woodworking, to those who are interested.