Stratena. This well-known household cement is said to be prepared as follows: Dissolve 12 parts of white glue in 16 of acetic acid, and then add this solution to one of 2 parts gelatine in 16 of water. After mixing add 2 parts shellac varnish.
Cement for cloth. Gutta-percha 16 parts, rubber 4, pitch 2, shellac 1, linseed oil 2 pints. Dissolve the whole by heat, stirring constantly.
HOW TO USE CEMENTS.
It is unquestionably true that quite as much depends upon the manner in which a cement is applied, as upon the cement itself. The best cement that was ever compounded would prove entirely worthless improperly applied. In the foregoing a number of cements have been given which answer every reasonable demand when properly prepared and properly used. Good common glue will unite two pieces of wood so firmly that the fibres will part from each other rather than from the cementing material; two pieces of glass can be so joined that they will part anywhere rather than on the line of union; glass can be united to metal, metal to metal, stone to stone, and all so strongly that the joint will certainly not be the weakest part of the resulting mass. What are the rules to be observed in effecting these results?
The first point that demands attention is to bring the cement itself into intimate contact with the surface to be united. If 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. The same is more eminently true in regard to cements that are used in a fused state, such as mixtures of resins, 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 olden days of seals. When the seal was used, in succession, 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 generally destroyed, from the simple fact that the sealing-wax would rather part in its own substance than at the point of adhesion to the seal. Sealing-wax or ordinary so-called electric cement is a very good agent for uniting metal to glass or stone, provided the masses to be united are made so hot as to fuse the cement; but if the cement is applied to them while they are cold, it will not stick at all. This fact is well known to those itinerant venders 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 rather break at any other part than along the line of union. But although people see the operation constantly 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 junction of any two surfaces are air and dirt. The former is universally present, while the latter is due to accident or carelessness. All surfaces are covered with a thin adhering layer of air, which it is difficult to remove, and which, although it may at first sight seem improbable, bears a relation to the outer surface of most bodies different from that maintained by the air a few lines away. The reality of the existence of this adhering layer of air is well known to all who are familiar with electrotype manipulation. It is also seen in the case of highly polished metals which may be immersed in water without becoming wet. Unless this adhering layer of air is displaced, the cement cannot adhere to the surface to which it is applied because it cannot come in contact with it. The most efficient agent in displacing this air is heat. Metals warmed to a point a little above 203° F. 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 in contact with the surfaces to which they are to be applied. Another very important point is to use as little cement as possible. When the surfaces are separated by a large mass of cement we have to depend 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.
The cement forced out of the joint by pressing the surfaces together should be removed while the cement is in a fused state or liquid. This can generally be effected by wiping the surplus off, while after solidification a certain amount of force has to be used which may frequently break the joint.
Oil cements, which generally solidify slowly, have the advantage of being water-proof. In cementing with oil cements, coat the surfaces to be joined with linseed oil, or, still better, boiled linseed oil, but in working with resinous cements apply oil of turpentine, spirit of wine, or a fluid which will readily dissolve the cementing constituent of the cement.
For cleansing the surfaces from grease and dirt place the articles in strong lye and rinse off in clean water without touching the surfaces with the hands. For painted porcelain articles which cannot be placed in lye, it is recommended to brush the surfaces several times with carbon disulphide.