The term "cement" is here used to denote only adhesive substances or compounds, and does not include building cement; lutes are bodies employed to make tight joints without really effecting a tenacious union between the parts.
A very comprehensive and valuable account of cements is given in a little work by John Phin, on the 'Preparation and use of Cements and Glue,' published by the Industrial Publication Co., New York. This has formed the basis of the present article, to which are added more recent recipes from the various technical journals, and other sources.
Phin has called attention to the fact that 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: -
1. 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. (93 1/2° 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.
2. 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 layer of the same cement 1/8 in. thick, 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.