Timber containing a high percentage of moisture cannot be glued satisfactorily with any type of adhesive, although casein is more tolerant than other glues and synthetic resins more critical. A moisture content of 20 per cent is about the limit, and higher values lower the viscosity of the glue to such an extent that starved joints may result, and the excess of moisture may cause the glue to harden as separate particles instead of a continuous film, thus reducing the ultimate strength. For good results, therefore, the wood should be in equilibrium with the average air conditions within the range 8 to 16 per cent, and only up to 20 per cent when that happens to be the normal environmental content.
Contrary to the widely held belief, there is little to be gained by sanding or toothing the wood surfaces prior to gluing, and in fact such treatment may positively interfere with the bond. Ideally, the wood should be glued immediately after it has been worked, and there will be no appreciable difference in adhesion between planed and fine-sawn surfaces provided that blunt cutters have not burnished the wood. Where gluing has to be delayed the surfaces should be only lightly sanded, and then well dusted to dislodge the greasy dust which any unprotected surface will gradually accumulate. Teak and certain other oily and notoriously difficult woods are quite straightforward with synthetic resin and even hide glue. provided the surfaces are glued within a few hours of working, for it is the seepage of natural oils to the surface which prevents the bond. If necessary oily wood can be degreased with any suitable solvent (cellulose thinners, etc.), but never with carbon tetrachloride which is sometimes advocated, for in conjunction with cigarette smoke it can produce a lethal gas (phosgene). End grain of wood rarely permits an effective bond, and therefore sufficient side grain should always be available, but if for any reason end-grain wood must be glued it should be repeatedly sized with well-diluted hide glue (not decorators' powder glue size). Synthetic resins can also be diluted with water or methylated spirit for sizing purposes, and each successive coat should penetrate and harden before another is applied. The usual dilution for size is from three to five parts water or solvent to one part glue, dependent on the viscosity of the latter.
Glues can be coloured to match the wood, using flake-white for light-coloured woods, and gas-black, mineral-earth colours or aniline dyes for ebony, blackwood, etc. Alternatively, Salisbury or rabbit skin glue, often employed in water gilding and gesso-work, can be used for delicately coloured inlays, while PVA emulsion gives a translucent glue-line. A method often adopted by the writer in gluing up light-coloured woods—cherry, sycamore, holly. etc.—was to rub the edges of the joints with ordinary blackboard chalk before applying the glue, and this worked well with highly absorbent butt-jointed veneers. Hide glue will accept most colouring agents without appreciable loss of strength, but tests should be made with resin glues as the acid/alkali reaction is critical.
Resin glues are not so tolerant of abuse as animal glues, and all weighing and measuring must be in accordance with the manufacturers' instructions. Where the quantities of glue required are small, hand-mixing is sufficient, but mechanical mixers are imperative for large batches, for if the separate ingredients are not evenly dispersed throughout the body of the glue, patchy setting and weak glue-lines will result. In particular, scrupulous cleanliness must be preserved for resin glues, as they will not tolerate oil, grease, acid or alkaline impurities, while liquid acid hardeners must never be stored in metal containers or spread with wire-bound brushes. Separate sticks or brushes must be kept for resin and hardener, for even the slightest trace of hardener on a stick carelessly dipped into a can of resin can spoil the whole batch. A photographic rubber print roller is excellent for spreading resin glues in thin films for veneering, as they do not flow out so easily as hide glues, and bald spots must be avoided.
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