_Alum._ Alum (1/4 weight of cotton) is dissolved in hot water with carbonate of soda crystals, or other alkali (1/4 weight of alum); work cotton in the solution, steep for several hours or overnight. Then well wash. Aluminium acetate solution as for silk (page 56) may be used. After drying, the cotton may be passed through a fixing solution of some alkali, for examples see page 50. Before mordanting with alum, the cotton is often prepared with tannic acid. _Iron._ Iron is usually employed as a "saddening" agent, i.e. the cotton after dyeing is steeped in a cold solution of the mordant. A further use is in dyeing black, when the cotton, after being prepared with tannin, is steeped in a cold solution of Iron. This process by itself gives a dark colour before any dye is used. _Tin._ Tin is rarely used alone as a mordant for cotton but brightens the colour in combination with other mordants. _Chrome._ Chrome is used for browns and other colours with Catechu. After boiling in a solution of the dye stuff, boil a short time in chrome solution, this oxidizes the colouring matter of the Catechu. _Copper._ Copper is sometimes added in small quantities to the dye bath for brown or yellow to vary the shade. _Tannin (Tannic acid)._ Cotton and linen strongly attract tannin and when prepared with it they are able to retain dyes permanently. Cotton saturated with tannin attracts the dye stuff more rapidly, and holds it. Tannic acid is the best tannin for mordanting as it is the purest and is free from any other colouring matter; it is, therefore, used for pale and bright shades. But for dark shades, substances containing tannic acid are used, such as _sumach_, _myrobalans_, _valonia_, _divi-divi_, _oak galls_, _chestnut_ (8 to 10 per cent tannin), _catechu_. Cotton and linen are prepared with tannin after they have been through the required cleansing, and, if necessary, bleaching operations. A bath is prepared with 2 to 5 per cent of tannic acid of the weight of the cotton, and a sufficient quantity of water. For dark shades, 5 to 10 per cent should be used. The bath is used either hot or cold. It should not be above 60 deg.C. The cotton is worked in this for some time, and then left to soak for 3 to 12 hours, while the bath cools. It is then wrung out and slightly washed. The following gives the relative proportions of the various substances containing tannin:--1 lb. tannic acid _equals_ 4 lbs. sumach, 18 lbs. myrobalans, 14 lbs. divi-divi, 11 lbs. oak galls. _Examples from various recipes_: For 10 lbs. cotton use 12 ozs. tannic acid. " 50 " " " 10 lbs. sumach. " 40 " " " 10 lbs. " " 20 " " " 2 lbs. yellow (or black) catechu. " 20 " " " 3 lbs. catechu with 3 ozs. blue vitriol. Some recipes soak the cotton 24 hours, others 48. RECIPES FOR DYEING (1) INDIGO VAT