cements, gum arabic - Cements, Gum Arabic; cements, gum arabic - Gum arabic is the product of various species of Acacia. It is the material from which true mucilage is made, and it forms one of the most valuable cements. Faraday says there is no cement which exceeds it in strength. Pure gum arabic is in roundish or irregular pieces of various sizes, more or less transparent, hard, brittle, and breaking with a shining fracture. It is usually white or yellowish white, but frequently presents various shades of red, and is sometimes of a deep orange or brownish colour. In powder it is always more or less purely white. It is liable to adulteration both in powder and in masses. Much of the white gum arabic of the shops, consists of the cheaper and coarser gum Senegal, bleached by what is called "Picciotto's process." The gum is dissolved in water, and sulphurous acid gas passed through the solution. The liquid is afterwards boiled to expel the sulphurous acid, a little of which, however, still remains behind. The product is very white, but lacks the peculiar toughness and adhesiveness of the best gum.
The powdered gum is frequently adulterated with dextrine, gum Senegal, starch, sugar, cherry-tree gum, etc. These substances are not difficult of detection, but where a good article is required for preparing a cement, it is best to purchase gum arabic in lump from a reliable dealer, taking care, in any case, to avoid the bleached article. Powdered gum has no advantage, except in the fact that it dissolves more quickly than when in lumps. It, therefore, forms, when in this state, a very convenient and portable cement, which may be made ready in an instant by the addition of a little water.
For preparing gummed surfaces which will adhere when moistened (such as gummed labels, etc.), there is no material superior to gum arabic. The great difficulty with gum arabic, and, indeed, with other gums and pastes, lies in the fact that when thoroughly dry, they become brittle, so that the label or other object falls off. A simple remedy for this difficulty lies in the addition of 5 to 10 drops of glycerine to each fl. oz. of mucilage or paste. Gum arabic is used not only alone, but when mixed with other matters. The following formulae produce very good cements.
Eub together, in a mortar, 2 parts nitrate of lime, 25 of water, and 20 of powdered gum arabic. This forms a transparent cement of great strength, and applicable to wood, porcelain, glass, and stone. The surfaces to be united should be painted with the cement, and firmly bound together until the drying is complete.
A white paste, adhesive to most surfaces, is said to be made as follows: A solution of 2½ oz. gum arabic in 2 qt. of warm water, is thickened with flour paste well boiled, and to this is added a solution of alum and sugar of lead, 720 gr. each, in water; the mixture is heated and stirred till about to boil, and then cooled. It may be thinned, if neccessary, with the gum solution. It will be seen that this mucilage consists of a solution of gum arabic and flour paste in acetate of alumina, coloured white with sulphate of lead.
To 250 grm. (9 oz.) of mucilage prepared by dissolving 2 parts of gum in 5 of water, add 2 grm. (30 gr.) of crystallised sulphate of aluminium dissolved in the least possible quantity of water. A solution of alum does not answer as well as the simple sulphate of alumina, which can be prepared from alum by precipitating the alumina with ammonia, washing thoroughly on a filter, and dissolving in sulphuric acid. The mucilage thus prepared does not sour or mould, and may be used as a cement for general purposes. (4) It is said that a mixture of 1 part dry chloride of calcium, or 2 parts of the same salt in the crystallised form, and 36 parts gum arabic, dissolved in water to a proper consistency, forms a mucilage which holds well, does not crack by drying, and yet does not attract sufficient moisture from the air to become wet in damp weather.
Preserving Gum-Arabic Solutions
A few drops of oil of cloves, or of alcohol, or any essential oil, will preserve a quart of the mucilage of gum arabic or gum tragacanth from turning sour.
Mucilage for Labels
Macerate 5 parts of good glue in 20 parts of water for 24 hours, adding 20 parts of rock candy, and 3 parts of gum arabic.
Artificial or British Gum
Malt, crushed small, 1 lb.; warm water, 2 gal. Mix; heat the whole to 145° F.; add of potato starch 5 lb.; raise the heat to 160° F., and mash for about 25 minutes, or until the liquid becomes thin and clear; it must then be instantly run off, and raised to the boiling point to prevent the formation of sugar; after boiling for 3 or 4 minutes, the whole, must be filtered and evaporated to dryness by a steam heat.