Any substance which is employed in uniting together things of the same or of different kinds may be termed a cement. The following are some of the principal of those used for various purposes.
To unite pieces of Derbyshire spar, or other stone, take seven or eight parts of resin and one of wax; then melt them together, and mix them with a small quantity of plaster of Paris. The stone should be made sufficiently hot to melt the cement, and the pieces should be pressed so closely together that the smallest quantity possible of the cement may remain between them. It is a rule of general application, that the thinner the stratum interposed the firmer the junction will be.
Jewellers unite precious stones which have been accidentally broken, by means of gum-mastic. The parts of the gem must be previously heated to a degree sufficient to melt the cement. Cameos of white enamel or coloured glass are also in this way joined to a real stone as a ground. Mastic is likewise employed by jewellers in various ways as a cement. The jewellers in Turkey ornament trinkets and weapons with gems, by uniting them together with the following composition:—Isinglass, soaked in water till it swells up and becomes soft, is dissolved in French brandy or rum, so as to form a strong glue; then two small bits of gum galbanum, or gum ammoniacum, are dissolved in two ounces of this by trituration; and five or six pieces of mastic about the size of peas are to be dissolved in as much alcohol as will render them fluid, and mixed with the compound by means of a gentle heat. This cement must be kept in a phial closely stopped; and when used it must be liquefied by immersing the phial in warm water. It will be found to resist moisture.
As a cement for broken china, take quicklime and white of eggs, or thick old varnish; pound and temper them well together, and the composition is ready for use. Broken glass may be cemented in the following manner:—Let glass which is more easy of fusion than the parts to be united, be ground up like a pigment, and interposed between the pieces; then let these be subjected to a heat which will melt the cementing medium, and make the parts agglutinate without being themselves fused. Anything thus united will be found nearly as strong as ever.
If clay and oxide of iron be mixed with oil, they will form a cement which hardens under water. A cement insoluble in water is prepared from skimmed milk cheese. The cheese is cut into slices, the rind being thrown away, and boiled till it becomes a strong glue, which, however, does not dissolve in the water. It is to be afterwards washed in cold, and then kneaded in warm water. This process must be several times repeated. The glue is then to be put warm on a levigating stone, and kneaded with quicklime. Though this cement may be used cold, it is best to warm it; and it will unite marble, stone, or earthenware, so as to render the joining scarcely discernible.
Boiled linseed oil, litharge, red lead, and white lead, mixed together and laid on both sides of a piece of flannel, or even linen or paper, and then put between two pieces of metal before they are brought home or close together, will make a durable joint, capable of resisting boiling water, or even a considerable pressure of steam. The proportions of the ingredients are not material; but the more the red lead predominates the sooner the cement will dry, and the more of the white lead the contrary. This cement joins stones of any dimensions.
The following is an excellent cement for iron, as it ultimately unites with it into one mass:—Take two ounces of muriate of ammonia, one of flowers of sulphur, and sixteen of cast-iron filings; mix them well in a mortar, and keep the powder dry. When the cement is to be used, take one part of this mixture, twenty parts of clear iron borings or filings, pound them together in a mortar, mix them with water to a proper consistence, and apply the compound between the joints.
A cement often used by coppersmiths to lay over the rivets and edges of the sheets of copper in large boilers, in order to serve as an additional security to the joinings, and to secure cocks and the like from leaking, is made by mixing powdered quicklime with ox's blood. This cement dries soon, and accordingly must be used fresh.
Temporary cements are required in cutting, polishing, or grinding optical glasses, and various articles of jewellery, as these must be fixed to blocks or handles for the purpose. Four ounces of resin, a quarter of an ounce of wax, and four ounces of whitening made previously red hot, forms a good cement of this kind, as the articles may be united or separated by heat, though they adhere with great tenacity when cold.
The following composition is recommended as a good cement for electrical apparatus:—Five pounds of resin, one of bees' wax, one of red ochre, and two table-spoonfuls of plaster of Paris, all heated together. The following is an analysis by Sir Humphry Davy of Roman or Parker's patent cement, which has the property of hardening under water.
One hundred grains contain
| Silica | 22 | |--------|----| | Alumina | 9 | | Oxide of iron and manganese | 13 | | Carbonate of lime | 55 |
One hundred grains lost by heating = 325
An excellent artificial water cement is obtained by heating for some hours to redness a mixture of three parts clay and one part slaked lime, by measure. (Ure's Dictionary of Chemistry; Philosophical Magazine.)
Cement for Building. See Building, vol. iii., p. 742; and Masonry.