As will be speedily observed in the great number of recipes for mending which will be given in this book, there are many which occur frequently in different combinations; therefore it will be advisable and indispensable for those who wish to master mending as an art to indicate these as a basis.

As Sigmund Lehner has observed in his valuable work on Die Kitte- und Klebemittel, there have been such vast numbers of recipes published of late years for adhesives in various technological works, that the combination of the usual materials depends almost on the judgment of the experimenter, and every practical operator will soon learn to make inventions of his own. These materials, according to Stohmann, may be classified as follows:—

I.Those in which Oil is the basis.
II.Resin or pitch.
III.Caoutchouc (indiarubber) or gutta-percha.
IV.Gum or starch.
V.Lime and chalk.

Lehner extends the list as follows into adhesives, or cements:—

I.For glass and porcelain in every form.
II.For metals not exposed to changes of temperature.
III.For stoves and furnaces, or objects exposed to heat.
IV.For chemical apparatus and objects exposed to corrosive liquids.
V.Luting or cements, to protect glass or porcelain vessels from the action of fire.
VI.Cements for microscopic preparations, for filling teeth and similar work.
VII.Those for special objects, such as are made of tortoise-shell, meerschaum (ivory), &c.

Oils are divided into those (such as olive) which never become hard, and the linseed, which in time dries into a substance like gum. The latter combined with a great variety of mineral substances, such as plumbago, calcined lime, magnesia, chalk, red oxide of iron, soapstone, or with varnishes, forms insoluble “soaps,” which, as cements, resist water. They require a long time to set or become hard.

Resins and Gums include a great number of substances, such as resin or hard pitch, which is distilled from pine-trees; shellac, mastic, elemi, copal, kauri gum, amber, gum arabic, dextrine made from flour, the gum of the peach and cherry, and of many other trees. To these may be added frankincense and tragacanth, which is less an adhesive than a stiffener and dresser. Gums are generally rather brittle; this is remedied by combination with oily substances, volatile oils, or caoutchouc. With these gums Lehner includes asphaltum. The defect of such adhesives is, as he also remarks, that they will not resist high temperatures. This, however, will apply to most objects.

Varnish.—This belongs properly to the gums, but is technically regarded as a separate material. It is gum in solution in turpentine or spirits. For details vide Die Fabrikation der Copal- Terpentinöl und Spiritus-Lacke, by L. E. Andés; Leipzig, price 5 m. 40 pf.

Caoutchouc and Gutta-Percha are gums which when hard are still elastic, and resist the action of water. I have read that a perfect imitation or substitute for them has been made of turpentine, but have not seen it, though I have met with glue made with oil and turpentine, which very much resembled them in elasticity or flexibility. Reduced to a liquid form with ether, benzine, &c., these gums can be kept in a liquid state for a long time, and then hardened in any form by exposure to the air. They enter into a very great variety of cements, such as are meant to be tough or waterproof. Indiarubber is, on the whole, the best, and gutta-percha the cheapest, for cements.

Glue.—This is made, by boiling, from horns and bones; it is essentially the same as gelatine. It is the most generally known of all adhesives, and may be modified by certain admixtures to suit almost any substance. It has the peculiarity that it must always be boiled in a balneum mariæ, or in a kettle in hot water in another kettle. Its strength is vastly increased by admixture with nitric acid or strong vinegar. On the subject of glue in all its relations, the reader may consult Die Leim- und Gelatine-Fabrikation, or “The Manufacture of Glue and Gelatine,” by F. Dawidowsky; Vienna, price 3s.