Marine glue is probably the strongest cement known, and when well made and properly applied, it is capable of uniting wood, metal, glass, leather, etc, with a strength and durability that is astonishing. It is a combination of shellac and indiarubber in proportions which vary according to the purposes for which the cement is to be used. Some is very hard, some quite soft. The degree of softness is also regulated by the proportion of naphtha used for dissolving the indiarubber and shellac. It is difficult to prepare it on the small scale. The following is the formula for the ordinary variety: - (1) Indiarubber (cut small), 1 part; coal-tar naphtha, 12; digest in a covered vessel with heat and agitation, and when the solution is complete, add of powdered shellac, 20 parts. Continue the heat and stirring until perfect liquefaction has taken place, and pour the fused mass, whilst still hot, on slabs of polished metal or stone so as to form thin sheets. When used it is to be heated to its melting-point, 248° to 250° F. (120° to 121° C), in an iron vessel, and applied in the liquid state with a brush. Great care and considerable experience are necessary to enable any one to use this cement.

If the solid cement be heated but a very few degrees above its melting - point, it crumbles and becomes useless. One may succeed by cutting it in shreds, placing these between the parts to be joined, and heating the whole until the glue can be pressed into uniform contact with the entire surfaces. Sometimes it is convenient to use a form of the glue which is more fluid, from containing more naphtha. The following formulas answer in such cases, but are not as strong as the ordinary marine glue. (2) Dissolve 3 parts shellac, and 1 of indiarubber, in separate vessels, in ether free from alcohol, applying a gentle heat. When thoroughly dissolved, mix the two solutions. Use rectified sulphuric ether that has been washed to remove alcohol and acidity, and india-rubber that has not been vulcanized. When the indiarubber has become well softened by the ether, break it up into small pieces, and stir well until a homogeneous soft mass is obtained. It will be as well to cut the rubber into small pieces before pouring the ether on, but the mass must be frequently and well stirred. Pour the solution of shellac into that of the rubber, and incorporate them thoroughly by stirring.

This is a modification of the famous marine glue, and resists the action of water, both hot and cold, and most of the acids and alkalies. If the glue be thinned by the admixture of ether, and applied as a varnish to leather, along the seams where it is sewed together, it renders the joint or seam water-tight, and almost impossible to separate. (3) The following recipe, taken from New Remedies, is said to yield a strong cement: 10 parts of indiarubber are dissolved in 120 of benzine or petroleum (?) naphtha with the aid of a gentle heat. When the solution is complete, which sometimes requires 10 to 14 days, 20 parts of asphalt are melted in an iron vessel, and the indiarubber solution is poured in very slowly, in a fine stream, and under continued heating, until the mass has become homogeneous, and nearly all of the solvent has been driven off. It is then poured out and cast into greased tin moulds. It forms dark-brown or black cakes, which are very hard to break. This cement requires considerable heat to melt it; and to prevent it from being burnt, it is best to heat a capsule containing a piece of it first on a water-bath, until the cake softens and begins to be liquid.

It is then carefully wiped dry, and heated over a naked flame, under constant stirring, up to about 300° F. (149° C). The edges of the article to be mended should, if possible, also be heated to at least 212° F. (100° C), so as to permit the cement to be applied at leisure and with care. The thinner the cement is applied, the better it binds. (4) Indiarubber, 15 to 20 gr.; chloroform, 2 fl. oz.; dissolved; powdered mastic, 1/2oz., is added. The cement must be kept well corked, and in a cool place, to prevent loss by evaporation. (5) Finely divided indiarubber, 1 part, is dissolved in naphtha oil, or crude naphtha, 40 parts. The solution is not completed in less than 10 or 12 days, and, in order to facilitate it, the mixture should be repeatedly agitated. To it, is then added gum lac, in the proportion of 2 parts by weight of lac to 1 of solution. The compound is then placed in an iron vessel over a fire, and constantly thinned till it becomes homogeneous. It is then poured on a cold surface, such as a slab of marble or a flag-stone, and left till cool, when it is broken up and put by for use. The indiarubber is sometimes omitted, in which case, the proportions will be 1 part of naphtha and 2 of lac.

When required for use, the cement is heated at a-temperature not exceeding 212° to 230° F. (100° to 110° C), in a thick vessel of copper or cast iron, and is brushed in thin and even layers on the surfaces to be joined; these are then brought into close contact, and strongly pressed. If the surfaces are so wide that the cement becomes cool before the operation is finished, it is well to pass a hot iron - say at about 140° F. (60° C.) - over it. It is valuable, not only for repairing broken wood, but also for cementing the moulds used in foundries, for caulking ships, for joining blocks of marble or granite, and for uniting wood and iron. It can be made as hard as desired, by increasing the proportion of lac. With the addition of bichloride of mercury dissolved in wood spirit, this cement might, with economy, replace the copper sheathing of ships. Wood, iron, plaster, and brick, to which it is applied, assume a varnished appearance; timber is rendered free from the attacks of insects and from liability to rot, and iron is preserved from rust.