282 The Art of Dyeing . 



that the calico saturated with the solution of sulphate of 

 iron absorbs a portion of the air contained in the flask, and 

 water takes its place. The cotton which was previously 

 colourless now assumes a yellowish colour. As a piece of 

 calico saturated with a solution of sulphate of iron under- 

 goes the same change in the open air, when it is yet cold 

 and slightly moist, if the cotton has not been too rapidly 

 dried, it is evident that it is the air, which, in both cases, 

 produces the change upon the solution of sulphate of iron. 



These properties render it impossible to employ sulphate 

 of iron itself as a mordant with good effect ; as all those 

 places which remain in contact with it longer than others, 

 or are more exposed to the air, exhibit from the assistance 

 of the air more iron, and acquire a darker colour. 



The acetate of iron, prepared from iron and vinegar, or, 

 from sulphate of iron and sugar of lead, undergoes the same 

 change. Pyrolignate of iron is, however, less changeable. 



The sulphate of iron, as well as the acetate of iron mor- 

 dants which were prepared by means of sulphate of iron, 

 are not fit for mordanting calico, in order to produce a 

 colourless ground. 



On the other hand, they are very useful for first printing 

 (vordruck.) 



In order to prevent the inconveniences now mentioned, 

 the solution of iron vitriol should be saturated with air 

 before being employed as a mordant, and for this purpose 

 should be placed in a flat vessel exposed to the air, or it 

 may be poured from one vessel into another in a small 

 stream. Both methods are, however, tedious. The object 

 in view is sooner attained by roasting the sulphate of iron 

 till it becomes yellow. 



Sulphate of iron thus roasted leaves behind, when dis- 

 solved in water, a quantity of oxide of iron, and contains 

 much free sulphuric acid. It will, therefore, prove injurious 

 to the calico unless it is converted into an acetic acid salt 

 by means of sugar of lead. 



If iron alum, however, can be procured, it is best to em- 

 ploy it for preparing; the iron mordant. The object is thus 

 obtained without difficulty, and with precision. This will 

 be farther treated of afterwards.* 



* See a specimen of Iron Buff, Records, i. 17. — Edit. 



