3. The third point is the necessity for cleanliness, both in the preparation and in the application of the cements. It may be safely laid down as a positive rule that every extraneous substance that is mixed with the material of a cement is an injury to it. Glue prepared in a greasy pot cannot be expected to make a strong joint, and the presence of dust and dirt tends to weaken all cements. So, too, in the application of cements. If it be attempted to glue together two surfaces of wood that are covered with dirt, the substances that are to be united are not wood to wood, but dirt to dirt, and the joint, instead of possessing the strength of wood, united by means of good glue, will have simply the strength of dirt. Moreover, it must be remembered that the different cements do not adhere with equal force to substances of different kinds. Thus, glue adheres powerfully to wood and paper, but not at all to metal or glass. Shellac, if properly applied, adheres readily to earthenware, glass, and metal, but not to some other substances. If, then, glue be applied to a greasy surface, it will not stick. Hence the necessity for great cleanliness. All surfaces should be kept as clean as possible, or, if they should get accidentally soiled, they should be carefully cleaned.
The mere rubbing of two wooden surfaces with a dirty hand will weaken the subsequent glue joint by at least 10 per cent.
The most common case in which this rule is violated by the inexperienced is in mending articles which have been formerly glued, and have been again broken at the old place. Such articles when first mended, frequently last for a long time, but when a second attempt is made to glue the pieces together, the joint seems almost to fall to pieces of itself. Here it is attempted to glue together, not two pieces of wood, but two pieces of old glue, and the result is failure. Soak off all the old glue (do not cut or scrape it, or the pieces will no longer fit accurately together), wash the surfaces with a sponge dipped in boiling water, and when they are dry and warm, glue them together in the usual manner, and you will be surprised at the strength of the joint.
4. See that the opposing surfaces make a close, neat joint, before you attempt to cement them. Two pieces of wood that are to be glued together should be planed up so true that they are in contact at every point, and where an article has been broken, the surfaces to be joined should be preserved from being broken or battered. This is par-ticularly the case when articles of glass or earthenware are accidentally broken, and it is not convenient to mend them at the instant. They should be carefully wrapped up in separate pieces of paper, and laid away where they will not be soiled, and where the edges will not be chipped. In such cases the joint will be greatly disfigured, and considerably weakened if the edges are chipped and broken by careless handling, or by being needlessly and frequently fitted together. Keep the pieces from contact with each other and with foreign substances until you are ready to join them, and the joint will then be not only strong, but almost invisible.
5. Plenty of time should be allowed for the cement to dry or harden, and this is particularly the case with oil cements, such as copal varnish, boiled oil, white lead, etc. These cements are said to dry, but they do not dry by evaporation. Instead of losing anything, they actually gain in weight by absorbing oxygen from the air, and this process of oxidation is a very slow one, except as regards the very thin layer that is in immediate contact with the air. Thus when two surfaces, each 1/2 in. across, are joined by means of a layer of white lead placed between them, 6 months may elapse before the cement in the middle of the joint has become hard. In such cases, a few days or weeks are of no account; at the end of a month, the joint will be weak and easily separated, while at the end of 2 or 3 years it may be so firm that the material will part anywhere else than at the joint. Hence, where the article is to be used immediately, the only safe cements are those which are liquefied by heat and which become hard when cold. A joint made with marine glue is firm an hour after it has been made. Next, in rapidity of hardening, to cements that are liquefied by heat, are those which consist of substances dissolved in water or alcohol.
6. Where neatness as well as strength is an object, it will often be advisable to use a cement of a colour as nearly like that of the materials to be united as possible. Thus a white porcelain cup, mended with black cement, would show some very ugly lines. If, however, a white cement be used, the lines of fracture will be invisible. The same rule applies to other articles, and it is always easy to colour a cement to any desired tint. (Phin.)
(1) A solution of indiarubber in twice its weight of raw linseed oil, heated, and mixed with an equal weight of pipeclay, yields a plastic mass, which will long remain soft under cover, and never completely hardens, so that it may be easily removed at pleasure. It resists most acids, and bears the heat at which sulphuric acid boils. (2) Melted india-rubber alone answers well for securing joints against chlorine and some acid vapours. (3) A mixture of China-clay and boiled linseed oil, in the proportions needed to produce the right consistence. (4) Quicklime and linseed oil, mixed stiffly together, form a hard cement, resisting both heat and acids. (5) A stiffly mixed paste of pipeclay and coal tar. (6) A cement which, according to Dr. Wagner, is proof against even boiling acids, may be made by a composition of indiarubber, tallow, lime, and red lead. The indiarubber must first be melted by a gentle heat, and then 6 to 8 per cent, by weight of tallow is added to the mixture while it is kept well stirred; next dry slaked lime is applied, until the fluid mass assumes a consistence similar to that of soft paste; lastly, 20 per cent, of red lead is added, in order to make it harden and dry. (7) A concentrated solution of silicate of soda, formed into a paste with powdered glass. (8) 1 part rosin, 1 sulphur, 2 brickdust; the whole is melted after careful mixing.