the Chromate t)/ Iron, &€* 2! 



Tlie importance, however, of the oxide oF chrome, on 

 account of its beauty and solidity as a green pigment f'of 

 'Carlhenware, and in forming imitations of emerald, added 

 to the discovery of large quantities of chromate in the de- 

 partnient of the Var, determined me to resume the subject, 

 and to study the properties of chrome at greater length. In 

 the experiments about to be detailed I was assisted hj 

 M. Robiquet, an eminent chemist of Paris. 



Process for decomposing the Chromate of Iron. 



Chromate of iron is generally employed in order to pro- 

 cure a large quantity of the oxide of chrome : this ore has 

 for its matrix a kind of steatite, which, from its colour and 

 some other physical properties, might be confounded to a 

 certain extent with chromate itself, the more easily becausa 

 these two substances at first sight seem to form one and ih© 

 «ame mass : after a little attention, however, we find that 

 the matrix is composed of long and pearl-like laminae, 

 whereas chromate of iron is very fine grain^, shining, and 

 denser than the matrix. 



I had formerly employed, in preparing the oxide of 

 chrome, three parts of nitre to one of the ore reduced to 

 fine powder; but I have since found this proportion was 

 by far too large : indeed, as we can isolate the matrix but 

 very im(3erfectly, it happens that the nitre, by means of 

 the alkali which it sets free, attacks not only the chron)ate 

 of iron, but also the aUnnine and silex, which are there in 

 large proportions ; and thus the chromate of potash is mixed 

 with an alkaline solution of all these earths, from which 

 two inconveniences result : in the first place, we are under 

 the necessity of employing, in the separation of these 

 earths and the saturation of the excess of alkali, a great 

 quantity of nitric acid, and if it happens that we go beyond 

 the quantity of acid necessary to the exact saturation of the 

 potash, we redissolve a portion of silex, but principally of 

 alumine : in the second place, these earths, on bemg pre- 

 cipitated, carry down with them chromate of potash, which 

 cannot be freed from them by washing: a third incon- 

 venience occurs, particularly when we operate on a large 

 scale, and when the heat is necessarily long continued, 

 — this excess of alkali attacks the crucible and melts it. 

 Thus the oeconomy and success of the operation require that 

 we should employ only one half part of nitre to one of chro- 

 mate : by these means the mass does not enter into fusion, 

 »nd the chromate is well attacked. It has often happened 



B 3 tha( 



