CHAPTER XII.

CLASSIFICATION OF CEMENTS.

The great variety of substances entering into the manufacture of cements and pastes makes a division of them extremely difficult. Stohmann divides them into the following groups:

1. _Oil cements._ 2. _Resinous cements._ 3. _Cements containing rubber or gutta percha._ 4. _Cements containing glue, or starch paste._ 5. _Lime cements._

Generally speaking, this division is correct; the only change we would suggest is to apply the term _agglutinant_ or _paste_ to bodies containing glue and starch paste.

When we attempt a division of the cements according to the bodies to be cemented, we find that the result will be a larger number of groups; as we must take into consideration whether the articles to be cemented have to be heated or not, whether they are to come in contact with water or other liquids, and other circumstances which would necessitate modifications in the composition of the cements themselves.

According to this, we might group the cements as follows:

1. _Cements for glass and porcelain, for repairing broken articles, for fastening glass letters upon show-windows, etc._

2. _Cements for metals not exposed to an increase of temperature, for instance, for tightening the joints of gas and water pipes._

3. _Cements for stoves and other articles, which have to stand an increased temperature._

4. _Cements for chemical apparatus, i. e., such as will have to resist the action of chemical agents._

5. _Cements to protect vessels of glass, porcelain, or metal against the action of fire._

6. _Cements for filling hollow teeth, for microscopical preparations, and other delicate articles._

7. _Cements for special purposes, for instance, for cementing meerschaum, tortoise shell, etc._

_Chemical nature of cements._ The different varieties of cement frequently contain substances which act chemically upon each other, or upon the bodies to be united with them. To determine the practical availability of a variety of cement for a determined purpose, it is of importance to know the reciprocal behavior of these substances towards each other, as from this we are able to judge at once whether a cement is suitable for a certain purpose or not.

_Oil cements._ The fluid fats, commonly called oils—though there are oils which remain solid at the ordinary temperature, as, for instance, palm oil and cocoanut oil—may, as regards their behavior on exposure to the air, be divided into two large groups, namely, drying and nondrying oils. As samples of these groups may be mentioned olive oil and linseed oil.

If a thin layer of olive oil protected from dust is exposed to the air, it will remain fluid for years and retain its characteristic oily consistency. The only change it undergoes is that it becomes somewhat more viscid and rancid, and acquires a darker color, but it never dries up.

Linseed oil treated in the same manner solidifies in the course of a few weeks to a hard, tough and elastic mass, resembling, as regards its physical qualities, resin or rubber.

By compounding a drying oil with a small quantity of litharge, pyrolusite, manganous borate, etc., and heating the admixture to the boiling-point, it acquires the property of drying in a few hours when exposed to the air in a thin layer. Oil so treated has been changed to a varnish.

By bringing a drying oil in contact with a body possessing strong basic properties a peculiar process takes place; the sebacic acids contained in the oil combine with the basic bodies to solid combinations which are insoluble in water, and, on exposure to the air, change gradually into masses as hard as stone. Such combinations, as regards their chemical composition, resemble ordinary soap, and for this reason are called insoluble soaps to distinguish them from ordinary soap which is soluble in water.

Burned lime, calcined magnesia, whiting, ferric oxide, litharge, and minium possess the capacity for forming insoluble soaps on coming in contact with drying oils and, still more quickly, with varnishes prepared from them.

The hardness of these soaps in time increases considerably by the oil not saponified drying in. The oil cements are principally used for tightening water and gas pipes, as they resist the action of water, steam and gas.

The only drawback connected with these cements is that they must reach a certain age before becoming entirely hard, and that, on account of the high price of drying oil or varnish which is absolutely required for their preparation, they are rather expensive. The ordinary glazier’s putty and the red lead and linseed-oil cement used in constructing water and gas conduits belong to this group.

_Resinous cements._ By resins are understood a number of constituents of plants which exude in thick viscous masses through incisions made in the trees, and on exposure to air are gradually converted into less transparent, brittle masses. When heated they melt more or less readily, forming a thick, ropy liquid, and brought in contact with an ignited body they burn with a bright flame and much sooty smoke.

By making incisions in the bark of any of the whole genus of _Pinus_ belonging to the _Coniferæ_ family, a viscous mass of a strong odor, called turpentine, is obtained. It consists of a solution of common rosin in the essential oil of turpentine, and when distilled yields from 75 to 90 per cent. of colophony or rosin, which remains in the retort, and from 25 to 10 per cent. of the essential oil, commonly called spirits of turpentine. Pure rosin is a brittle, tasteless, and almost inodorous mass of a light yellow color and a smooth, shining fracture.

The various resins found in commerce, such as shellac, mastic, elemi, copal, etc., are formed in a similar manner.

The principal points of importance for our purpose are the different degrees of hardness and brittleness and the melting-points of the various resins. While some possess but slight hardness, for instance elemi, others, such as copal and amber, excel in this respect and their brittleness and high melting-point.

To decrease the brittleness of resins, essential oils are sometimes added, or resinous cements are mixed with oil cements or a fat drying oil, or compounded with rubber cement.

Resinous cements are either softened by heating or entirely melted, or solutions of resins in volatile solvents are used, which, in evaporating, leave the resin behind.

The resinous cements possess great power of resistance, and are therefore well adapted for tightening water and gas pipes, but they have the disadvantage of not standing a high temperature and possessing a certain degree of brittleness which renders them unfit for the cementing of articles exposed to frequent shocks.

Many of these cements, especially those prepared with pitch or asphaltum, can be produced at a very low cost, and do excellent service for water-proofing vessels, water-reservoirs, brickwork, etc.

_Rubber and gutta-percha cements._ Caoutchouc, commonly called India rubber, or briefly rubber, is derived from the milky juices of certain tropical plants. It is distinguished by great elasticity and indifference to chemical agents.

Both these properties make it a valuable material for cement, and it is much used for this purpose either in the form of solution or as a constituent of other compositions. For cements which are to have a certain degree of elasticity combined with indifference toward chemical agents, it is absolutely indispensable, as no other known body possesses these properties in such a high degree.

The derivation of gutta percha is similar to that of rubber. At an ordinary temperature it forms solid and very tenacious masses, of a leather-like consistency, but at a somewhat higher temperature (below the boiling-point of water) it is converted into a very plastic, soft mass, which can be drawn into very fine threads, and rolled to very thin plates.

By itself or mixed with other substances it furnishes an excellent cement, possessing the valuable properties of tenacity and pliancy when exposed to shocks. As regards resistance to the action of water and chemical agents it is almost equal to rubber, and, for certain purposes, is frequently preferred to the latter.

_Glue and starch cements._ By itself, _i. e._, converted by boiling with water into a viscous mass which solidifies on cooling, glue cannot be classed with the cements; the same applies to paste, _i. e._, starch or flour swelled and boiled in water.

But compounded with other substances both yield excellent cements, in which a part of the properties distinguishing glue solution and paste is preserved. They both possess the property of decreasing the brittleness of many cements, but unfortunately the latter thereby lose their power of resisting the action of water; for starch as well as glue swells in water, and the latter, when moist, passes quickly into putrefaction and destroys the cement.

In a wider sense isinglass, compounds of glue and vinegar, of lime and glue, etc., must be classed with the glue cements, and ordinary flour and shoemakers’ paste with starch cements.

_Lime Cements._ Lime possesses the property of forming insoluble combinations with egg albumen or caseine, this being the reason why lime cements, of which there are a great number, are generally composed of burned lime and one or the other of the above substances. Lime compounded with a solution of water-glass forms also very solid and durable cements.

Although the cements and agglutinants mentioned in the foregoing are most frequently used, a compound of different cements is often employed, in consequence of which the composition of many cements is very complicated.

In the following we give a description of the preparation of the different kinds of cement, according to the manner of their employment.