170 Mr. Couper on the Chemical Composition of Pottery. 



IV. The colours used in producing the dipt ware are of a very cheap 

 kind, as it is only for common purposes that they are employed. 

 The colours when used for dipt ware are put on the ware before it is 

 burned. The following are some of those colours : — 



(1.) A black dip is made from manganese, ironstone, and clay slip. 



(2.) A drab dip, by nickel and clay slip. 



(3.) A sage, or a greenish blue dip, by green, chrome, and slip. 



(4.) A blue dip, by cobalt and clay slip. 



(5.) A yellow dip, by yellow clay alone, or a compouud of white and 

 red clay, natural, which produces the same results. 



(6.) A red dip is produced from the red or brown clay, but it is not 

 every quality of this clay that will answer, as it requires to bum red. 



The first four of these dips are prepared by mixing a little of the colour- 

 ing agent with a quantity of clay slip, while the two last mentioned dips 

 are mixed with water to produce the slip state, in which condition they are 

 employed. 



V. There are several kinds of bodies manufactured, but they may be 

 all classed under two heads, viz., porcelain and earthenware. 



(1.) Porcelain or China, is a rich, very smooth, and transparent ware, 

 and is the finest quality that has yet been manufactured. It is a fused 

 body, and owes its transparency to this circumstance ; it also requires a 

 very high temperature to burn it, and is manufactured in this country from 

 flint, Cornish stone, (granite,) China clay, and bone earth; the phosphate 

 of lime employed acting as a flux partly fusing it. By analysis of two 

 pieces of china from different manufactories in Staffordshire, I found them 

 to be differently composed. The last of these species was also analysed 

 by Mr. William Crichton; the three analyses being as follows : — 



Silica, 39-88 40-60 39*685 



Alumina, 21*48 24-15 24-650 



Lime 1006 14-22 14176 



Protoxide of iron,") ik.qqa 



™ , . -,. \ 26*44 15-32 15d8t> 



Phosphate oi lime,) 



Magnesia, — '43 "311 



Alcali and loss, 2-14 5-28 5.792 



100- 100- 100- 



No. 1, by R. A. C. ; No. 2, by R. A. C, ; No. 3, by W. C. 



(2.) Foreign manufacturers do not employ bone earth ; but instead of 

 it they use feldspar, the alcali of which supplies the place of the phosphate 

 of lime ; the Germans make the best porcelain for chemical purposes, as 

 that body is more vitrified and less liable to be acted upon by acids, as 

 well as being capable of standing a very strong heat ; hence it is exten- 

 sively used by chemists. By the analysis of some specimensjof foreign 

 porcelain, I obtained the following results : — 



