All the shower-proof fabrics come under this heading, as well as every cloth which is pervious to air and repulsive to water. The most time-honored recipe for proofing woollen goods is a mixture of sugar of lead and alum, and dates back hundreds of years. The system of using this is as follows: The two ingredients are dissolved separately, and the solutions mixed together. A mutual decomposition results, the base of the lead salt uniting with the sulphuric acid out of the alum to form lead sulphate, which precipitates to the bottom. The clear solution contains alumina in the form of acetate, and this supplies the proofing quality to the fabric. It is applied in a form of machine shown in Fig. 8, which will be seen to consist of a trough containing the proofing solution, C, with a pair of squeezing rollers, A, over the top. The fabric is drawn down through the solution and up through the squeezers in the direction of the arrows. At the {747} back of the machine the cloth automatically winds itself onto a roll, B, and then only requires drying to develop the water-resisting power. D is a weight acting on a lever which presses the two rollers, A, together. The water-repelling property is gained as follows:
Fig. 8
Drying the fabric, which is impregnated with acetate of alumina, drives off some of the volatile acetic acid, leaving a film of basic acetate of alumina on each wool fiber. This basic salt is very difficult to wet, and has so little attraction for moisture that in a shower of rain the drops remain in a spheroidal state, and fall off. In a strong wind, or under pressure, water eventually penetrates through fabrics proofed in this manner; but they will effectually resist a sharp shower. Unfortunately, shower-proofed goods, with wear, gradually lose this property of repelling water. The equation representing the change between alum and sugar of lead is given below. In the case of common alum there would, of course, be potassium acetate in solution besides the alumina.
| Alum. |
| Al2K2(SO4)4 + |
| Sugar of lead. |
| 4 Pb(C2H3O2)2 = |
| Lead sulphate. |
| 4 PbSO4 + |
| Potassium acetate. |
| 2 KC2H3O2 + |
| Aluminum acetate. |
| Al2(C2H3O2)6 |
Now that sulphate of alumina is in common use, alum need not be used, as the potash in it serves no purpose in proofing.