MORDANTS.

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The mordants for wool and worsted dyeing and printing are generally themost decidedly acid. Those for silk are more neutral, and in cotton dyeingand printing excess of free acid must be carefully avoided. On the otherhand, cotton can be successfully mordanted with preparations, in which thesolvent is an alkali, and the matter to be deposited on the fibre plays the partof an acid, such as the stannate and the aluminate of soda, which areadvantageously used for cotton, whilst with animal fibres their applicationsare very limited.

Mordants are either applied preparatory to the colouring matters (theordinary routine for cotton), along with the colours (the usual procedure forwool), or, though less frequently, after the colour.

The metals which enter into the composition of mordants are aluminium,tin, iron, chromium, lead, copper, zinc, arsenic, mercury, antimony, and man-ganese. Bismuth, nickel, cobalt, tungsten, molybdenum, and magnesiumhave also been applied. Even sodium performs functions which can only beregarded as those of a mordant.

The acids employed are the nitric, hydrochloric, sulphuric, tartaric, oxalic,hyposulphurous, boracic, silicic, and phosphoric.

A. Mordants of Tin, technically known as “Tin-Spirits.”

Tin is employed as a mordant in three different states of chemical com-position, the protoxide, the sesquioxide, and peroxide, or their correspondingchlorides. The protoxide of tin, known also as stannous oxide, is acompound of the metal with the smallest possible quantity of oxygen.It forms with the acids colourless salts and solutions, which have astrong affinity for a further quantity of oxygen, and are hence powerful re-ducing agents. With alkalies, such as soda and potassa, it is also capable ofplaying the part of a feeble acid, forming compounds known as stannites.

The sesquioxide of tin is a body concerning whose nature and propertieschemists are not agreed. Its compounds are of a reddish-amber colour, andare not permanent, passing, if kept for any length of time, into variablemixture of the other oxides, combined with whatever acids were present.

The peroxide of tin, known also as stannic oxide, stannic acid, binoxide oftin, contains the highest proportion of oxygen. Its salts and solutions, ifthe acids employed be perfectly pure, are colourless. Should, however, thesmallest trace of iron be present they have a very faint yellow colour. Thecompounds of the peroxide of tin have an oxidising effect upon bodies withwhich they are brought in contact.

Generally speaking the proto-salts of tin are applied to wool, and the per-salts to cotton. The sesqui-salts, the consumption of which is morelimited, are used in certain cases to both. The compounds where tin playsthe part of an acid, such as the stannites and stannates, are almost exclusivelyrestricted to cotton dyeing and printing.

The successful preparation of tin-mordants is a matter of great nicety,demanding close attention to certain minute particulars, which can scarcelybe explained in writing. Purity of the ingredients used is a matter of vital im-portance. The tin should be the finest “ grain-bar,” the “ Iamb and flag ” brandbeing generally the most satisfactory. The economy effected by the substitutionof lower qualities, containing traces of iron, copper, arsenic, antimony, &c.,