III. Water, 2 quarts; isinglass, 5 ozs.; alum, 240 grains.
Water-proof Glue.—Solution of glue by itself or mixed with pigments is used in painting walls in distemper. A water-proof coat is obtained as follows: Boil 1 part of powdered gall-nuts and 12 parts of water until the mass is reduced to two-thirds of its bulk. Then strain through a cloth and apply the solution to the dry coat of distemper paint, the latter becoming thereby as solid and insoluble as oil-paint. The tannin of the gall-nuts acting only upon soft glue, the solution has to be applied so the lower layer of glue becomes thoroughly soaked through.
To render wrapping paper water-proof the following glue-solution may be used: Dissolve 24 parts of alum and 4 parts of white soap in 32 parts of water in one pot, and in another 2 parts of gum arabic and 6 parts of glue in 32 parts of water, and mix the two solutions. Heat the mixture, immerse the wrapping paper in it, and pass it through hot rolls, or dry upon twine stretched in frames.
Fabrics may be rendered water-proof with glue and tannin. The process is based on the fact that by the action of tannin or bichromates, compounds insoluble in water are formed. It is, however, of first importance that both solutions—tannin and glue—should fully penetrate the fabric. If the latter is dipped directly into strong solution of glue and then of tannin, the glue will only become insoluble on the outside, and that which has penetrated deeper into the fibre will be unchanged, having been protected by the superficial insoluble layer. Hence, the treatment is commenced with a very weak solution of glue, prepared by leaving glue broken small in hundred times its weight of water for twenty-four hours. By that time the glue will have swelled up, and the whole is boiled, whilst being constantly stirred, so as to get a perfectly clear solution in which the fabric is boiled for 10 to 15 minutes. This time is no more than necessary for complete penetration. The fabric is then well wrung between two rollers placed over the glue-bath so that excess of solution runs back into it. The fabric is then hung up, and when nearly dry is passed through a tannin solution. The latter may be made from tannin itself, or from a tannin extract, or by boiling galls or oak bark in water. The tannin solution can be used fairly strong, as only so much of it is taken up as corresponds to the glue present, and it can be used over again as long as it can supply the tannin required, and can then be reinforced with more tannin as required.
It is not necessary for the fabric to stay long in the tannin, as it reacts quickly with the glue. The tanned material is again hung up to dry, and when quite dry is washed in plain water to remove any excess of tannin. The whole process from the beginning is then twice repeated. After this second repetition there is so thick a layer of tannate of gelatine on the fabric that the dry cloth has acquired considerable solidity and a smoothness which recalls that of leather. The fabric is now passed through a stronger glue solution, using three or even four parts of glue per hundred of water, but never exceeding the latter limit. After the glue-bath the fabric goes through the tannin bath, whereby it becomes quite thickly coated with tannate of gelatine. By repeatedly treating with glue and tannin alternately, this coating can be made as thick as desired, and finally masses are obtained in which the texture of the fabric is entirely hidden, and especially after the fabric has been calendered under heavy pressure after water-proofing. The color acquired by goods thus water-proofed is a more or less dark leather-brown.
Muratori and Landry treat the fabric with a solution made in three separate operations:
1. Potash alum 100 lbs. dissolved in 10 gallons of boiling water.
2. In another vessel 100 lbs. of glue are soaked in cold water till the glue has trebled in weight. The remaining water is poured off, and the glue liquefied by the application of heat. When the glue is boiling, 5 lbs. of tannin and 2 lbs. of soda water glass are put in it.
3. The two solutions are boiled together, being stirred constantly.
When the mixture is complete, it is allowed to cool to a jelly. To water-proof the goods some of the jelly is boiled with water (1 gallon to 1 lb., or a little over) for three hours, adding water to compensate for evaporation, so as to keep the volume of the solution constant, as shown by tests of its specific gravity with a hydrometer. The bath is then allowed to cool to 176° F., and the fabric is soaked in it for half an hour and then stretched out horizontally for six hours to drain. The fabric must be kept horizontal, so that the solution remains uniformly distributed through it. The drainings are collected to be used over again. The fabric is then dried in the open air or in a drying room, still in the horizontal position. If a drying room is used the temperature of it should not exceed 122° F.