cements and lutes - cements and lutes; cements and lutes - See also glue; sealing wax; shellac.
A cement is a natural substance, or a compound which will act as a strong adhesive between either two similar substances, or two substances of quite dissimilar character. Lutes are employed to make tight joints without being adhesive.
The success of a cement depends quite as much upon the manner in which it is used as upon the cement itself. It is especially necessary to understand the characters and properties of the cement. Every cement may be assigned to one of four classes, according as it (1) Dries by evaporation; (2) Congeals by cooling; (3) Hardens by oxidation; or (4) "Sets" by chemical changes. To the first class belong pastes, mucilages, alcoholic and other solutions of gums and resins, and, to a certain extent glue. To the second belong such cements as sealing-wax, turner's cement, shellac, etc. The third class includes gold-size, drying oil, white and red lead, etc.; and the fourth class covers plaster-of-Paris, the so-called iron cement, and others of that kind.
If the best results would be attained, the following rules must be rigorously adhered to:
The cement must be brought into intimate contact with the surface to be united. Thus, when glue is employed the surface should be made so warm that the melted glue will not be chilled before it has time to effect a thorough adhesion; a drop of melted glue allowed to simply fall on a surface of dry, cold wood and solidify there, will often fail to adhere at all, while if the same drop had been rubbed in, it would have attached itself to it with wonderful power of adhesion. The same is more eminently true in regard to cements that are used in a fused state, such as mixtures of resin, shellac, and similar materials. These matters will not adhere to any substance unless the latter has been heated to nearly or quite the fusing-point of the cement used. This fact was quite familiar to those who used sealing-wax in the old days of seals. When the seal was used rapidly, so as to become heated, the sealing-wax stuck to it with a firmness that was annoying, so much so that the impression was in general destroyed, from the simple fact that the sealing-wax would rather part in its own substance than at the point of adhesion to the stamp. Sealing-wax, or ordinary electrical cement, is a very good agent for uniting metal to glass or stone, providing the masses to be united are made so hot as to fuse the cement, but if the cement be applied to them while they are cold, it will not stick at all. This fact is well known to the itinerant vendors of cement for uniting earthenware. By heating two pieces of delf so that they will fuse shellac, they are able to smear them with a little of this gum, and join them so that they will break at any other part rather than along the line of union. But although people constantly see the operation performed, and buy liberally of the cement, it will be found that in nine cases out of ten the cement proves worthless in the hands of the purchasers, simply because they do not know how to use it. They are afraid to heat a delicate glass or porcelain vessel to a sufficient degree, and they are apt to use too much of the material, and the result is a failure.
The great obstacles to the absolute contact of any two surfaces are air and dirt. The former is universally present, the latter is due to accident or carelessness. All surfaces are covered with a thin adhering layer of air, which is difficult to remove, and which, although it may at first sight seem improbable, bears to the outer surface of most bodies a relation different from that maintained by the air a few lines away, and until this layer or film of air has been removed, it prevents the absolute contact of any other substance. The reality of the existence of this adhering layer is well known to all who are familiar with electrotype manipulation, and it is also seen in the case of highly polished metals, which may be immersed in water without becoming wet. Thus the surface of a needle retains this film of air so strongly, that it will float on the surface of water rather than give it up.
Unless this adhering layer of air is displaced, it will be impossible for any cement to adhere to the surface to which it is applied, simply because it cannot come into contact with it.
The most efficient agents in displacing this air are heat and pressure. Metals warmed to a point a little above 200° F. (93½° C.), become instantly and completely wet when immersed in water. Hence for cements that are used in a fused condition, heat is the most efficient means of bringing them into contact with the surfaces to which they are to be applied.
When it is intended to unite two pieces of earthenware or glass together, or a piece of glass or other substance to metal, by means of a cement that is to be used in a fused state, the surfaces that are to be united should always be made so hot that the cement will become perfectly liquid when brought into contact with them.
In the case of glue, the adhesion is best attained by pressure and friction, combined with moderate warmth. In large establishments, where good glue joints are an important item, a special room, carefully warmed, is set aside for this operation.
A very important point is that as little cement as possible should be used. When the united surfaces are separated by a large mass of cement, everything depends upon the strength of the cement itself, and not upon its adhesion to the surfaces which it is used to join; and, in general, cements are comparatively brittle. At first sight one would suppose that the more cement is used, the stronger will be the joint, and this is an error into which most inexperienced persons fall. Two pieces of earthenware, joined together by a layer of shellac as thin as possible, will adhere together and will be as strong at the junction as at any other part, while the same pieces united by means of a thick layer of the same cement, would fall apart on receiving the slightest jar. The rule which directs us to use as little cement as possible, admits of no exceptions, and as a general thing the only way to obtain thin layers of cements that are to be used in a fused state, is to heat thoroughly the pieces that are to be united, press them forcibly together, and keep them under pressure by means of weights, screws, or cords until the cement has hardened.
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.
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 particularly 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. 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.
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 any thing, 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 ½ 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. A glue joint sets firmly in 24 hours; a joint made with shellac varnish becomes dry in 2 or 3 days. Oil cements (boiled oil, white lead, red lead, etc.), take months.
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.)