Hide- or Scotch-type glues in cake, pearl and bead forms require preliminary soaking in water and heating in water-jacketed pots to not more than 160° F (54° C). Greater temperatures or constant reheating destroy much of the adhesive strength, and the heated glue should always be a dark honey colour, sweet smelling and liquid enough so that a slight rattle is heard as it runs from the brush. Components must be slightly warmed and pressure applied before the glue gels, for the chilled gel itself has no adhesive properties. Liquid hide glues only require slight warming in the tin in cold weather and give more latitude. All types are freely soluble in water and are not, therefore, resistant, but they can be reconstituted and, therefore, there is no waste. Their great advantages in some classes of work are their flexibility, and the strong tack or suck they give, so that freshly glued surfaces can be rubbed together and will grip without clamping pressure. Glued components should be left for 12 hours under normal workshop conditions, and longer if the workshop is cold or the components under strain. Clamps should be tightened up after the first 15 minutes, as absorption by the wood or squeezing from the joint edges may have reduced the pressure. If the clamps must be removed as soon as possible, a test-piece coated with glue should be laid alongside, and only when the glue-film cannot easily be indented with the thumb-nail should the pressure be released. The old craftsmen used a minimum of glue, but it is better to spread both surfaces thinly, and wipe off the squeezed-out surplus with clean rags soaked in warm water.
Hide glues can be used equally well for assembly-work and veneering, and will tolerate thicknesses of 1/16 in (1.58 mm) in the glue-line. They are invaluable for antique repair-work.
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