Ill 



A lead paint will be opaque since its refractive index is greater 

 than that of the oil, whilst a silica paint in turpentine or linseed oil 

 will be practically transparent OAving to close equality in the refractive 

 indices of pigment and medium. 



The opacity varies inversely with the amount of oil absorbed by 

 the pigment, but the durability is improved by the presence of more 

 oil. 



The refractive indices of silica, barytes, zinc oxide, white lead, and 

 zinc sulphide are 1-55, 1-6, 1-9, 2-0, and 2-37 respectively. 



In hthopone a mixture of the components fails to give the same 

 opacity as when prepared in contact. Between the limits of 28 per 

 cent, and 38 per cent, zinc sulphide the covering power is best. It 

 is probable that surface adsorption of the zinc sulphide by the barium 

 sulphate occurs. 



Rapidity of precipitation, strength of solution and temperature 

 control, are factors which aid in the production of fine grained particles 

 giving the greatest opacity. 



The phenomenon of surface adsorption shown by certain lake 

 bases in the presence of colouring matters is of interest and explains 

 why the highly coUoid pigments are often preferred. A measure of 

 the degree of dispersion might be based on their colour. 



Bingham and Green {Am. Soc. Testing Materials, 1919) distinguish 

 between the viscosity of true liquids and the rigidity of plastic solids. 



The appUcation of the generaUsations drawn from the study of 

 other classes of colloid bodies to problems of the paint industry are 

 referred to by Bancroft (" Theory of Emulsification," V., Jour. Phy. 

 Chem., 17, 501, 1913). The use of sodium silicate to give an emulsion 

 with Unseed oil to prevent the paint from setting or hardening in the 

 package has been known from 1865. Generally 2 per cent, water is 

 the limit, although 4 per cent, may be employed to prevent settling, 

 provided the emulsification of oil with water is assured. Instead of 

 water as combining medium a rosin oil mixing may be employed. 

 An alkaline water fluid is not desirable, and the addition of zinc oxide 

 to the lead white is useful in maintaining the suspension in the linseed 

 oil. 



E. E. Ware and Christman {Jour. Ind. Eng. Chem., 8, 879, 1916) 

 recommend that a non-aqueous protective coUoid, e.g., aluminium 

 palmitate or oleate should be added to mixed paints to which small 

 quantities of water have been added containing a protective colloid to 

 prevent settling of the pigment. 



The same authors have investigated the skinning, puttying, and 

 livering of mixed paints. Livering is dependent on the acidity of 

 the pigment, and in the case of enamels must be connected with the 

 gelatinisation of the colloid resin due to reduction of its acidity. The 

 coagulation depends on many factors of composition and the presence 

 of foreig ; substances. Such gels would absorb oil and thinner with 

 separation of the pigment. Skinning would seem to be caused by 

 the oil acids acting on the pigment. 



In an oil paint containing rosin the formation of resLnates increases 

 the viscosity and the further formation of zinc soap separating from 

 the viscous solution of zinc resinates gives a gel occluding pr adsorbing 



