To Choose Adhesives.—An adhesive should be selected according to the nature of the substances to be united and the use to which they are to be put. If the right cement is employed, the hardest and smoothest surfaces, as glass and polished metals, may be united so firmly that they will break anywhere rather than where the parts are cemented.
To Use Adhesives.—The object of using adhesives is to bring two sur faces into such intimate contact as to make them practically one, and not to interpose between them any perishable layer or thickness of the adhesive it self. Most adhesives are more brit tle than the substances which they unite; hence the best work is • done when the adhesive penetrates into the pores of the materials on both sides and brings the particles of both sur faces closely together, so that the strength of the materials themselves is added to that of the adhesive in a union which may be stronger than the adjacent parts.
The presence of any foreign sub stance, as dirt, grease, or bubbles of air, hinders adhesion. Heating the
surfaces to be joined promotes it by expanding the pores and thus enabling them to absorb more of the adhesive. Heating the adhesive itself also as sists.
Moreover, heat tends to drive away the air. Hence the hotter one can handle the parts and the adhesive the closer they can be brought into con tact and the less adhesive will be re quired.
To Use Glue.—To get the best re sults from glue, it should be thin and hot, and the parts should be at least warm enough to prevent the glue from being chilled by them. As glue is gummy and elastic, the parts, when possible, should be squeezed together by means of a vise or under clamps tightened by a screw, so as to squeeze out the excess of glue and bring the parts into intimate contact. The arti cles should be left in the vise until the glue is set.
To Use Cement.—Resinous cements which are used in a melted state will not do good work unless the adjacent parts are heated above the point at which the resins melt.