Porous goods are made waterproof according to two very distinct systems. According to the first the articles are made absolutely impervious to water and air by having their pores filled up with some oily or gummy substance, which becomes stiff and impenetrable. Caoutchouc, paints, oils, melted wax, etc., ai*e of this kind. The other system consists in making the fabric repellent to water, while it remains quite porous and freely admits the passage of air. Goods so prepared will resist any ordinary rain, and we have seen a very porous fabric stretched over the mouth of a vessel and resist the passage of water one or two inches deep. The following recipes have been tried and found good. Most of those found in the recipe books are worthless.
1. Melt together 2 oz. of Burgundy pitch, 2 oz. of soft wax, 2 oz. of turpentine, and 1 pint of raw linseed oil. Lay on with a brush while warm.
Grind 6 lbs. English ochre with boiled oil, and add 1 lb. of black paint, which mixture forms an indifferent black. An ounce of yellow soap, dissolved by heat in half a pint of water, is mixed while hot with the paint. This composition is laid upon dry canvas as stiff as can conveniently be done with the brush. Two days after, a second coat of ochre and black paint (without any soap) is laid on, and, allowing this coat time to dry, the canvas is finished with a coat of any desired color. After three days it does not stick together when folded up. This is the formula used in the British navy yards, and it has given excellent results. We have seen a portable boat made of canvas prepared in this way and stretched on a skeleton frame.
The following is highly recommended as a cheap and simple process for coating canvas for wagon tops, tents, awnings, etc. It renders it impermeable to moisture, without making it stiff and liable to break. Soft soap is to be dissolved in hot water, and a solution of sulphate of iron added. The sulphuric acid combines with the potash of the soap, and the oxide of iron is precipitated with the fatty acid as insoluble iron soap. This is washed and dried, and mixed with linseed oil. The soap prevents the oil from getting hard and cracking, and at the same time water has no effect on it.
The following recipes are intended to be applied to woven fabrics, which they leave quite pervious to air but capable of resisting water.
1. Apply a strong solution of soap, not mere soap suds, to the wrong side of the cloth, and when dry wash the other side with a solution of alum.
2. The following recipe is substantially the same as the preceding, but if carefully followed in its details gives better results:
Take the material successively through baths of sulphate of alumina, of soap and of water; then dry and smother or calender. For the alumina bath, use the ordinary neutral sulphate of alumina of commerce (concentrated alum cake), dissolving 1 part in 10 of water, which is easily done without the application of heat. The soap is best prepared in this manner: Boil 1 part of light resin, 1 part of soda crystals, and 10 of water, till the resin is dissolved; salt the soap out by the addition of one-third part of common salt; dissolve this soap with an equal amount of good palm oil soap (navy soap) in 30 parts of water. The soap bath should be kept hot while the goods are passing through it. It is best to have three vats along side of each other, and by a special arrangement to keep the goods down in the baths. Special care should be taken to have the fabric thoroughly soaked in the alumina bath.
3. Drs. Hager and Jacobsen remark that during the last few years very good and cheap waterproof goods of this description have been manufactured in Berlin, which they believe is effected by steeping them first in a bath of sulphate of alumina and of copper, and then into one of water-glass and resin soap.