PHYSICAL LABORATORY PAINT TESTS 81 



fication of a paint may easily ho seen through the checking of 

 oil paintings, which, as Ostwalt has pointed out, is due to the 

 unequal coefficients of expansion between the ground and the 

 paint. This is particularly noticeable in the alligatoring of many 

 enamels which contain large percentages of zinc. 



Curves have been prepared having pressure as an abscissa 

 and elasticity as ordinate. These curves show remarkable 

 differences in different pigments. For instance, in the case of 

 white lead, the curve takes a steep upward trend when it ap- 

 parently reaches a maximum, the curve then flattening out and 

 finally taking another steep upward trend just before breaking. 

 This may be construed as follows: That under low pressures 

 the white lead film is perfectly elastic, when a maximum is 

 obtained, beyond which elasticity does not extend. This point 

 is the maximum point of the upward trend. From here on 

 pressure may be applied without any increase in stretch, this 

 being represented by the flat part of the curve, while the steep 

 upward trend just before breaking shows where the paint begins 

 to tear, finally culminating in breaking. In the case of asbestine, 

 however, the curve is more of a straight line up to the breaking 

 point, which would go to prove that elasticity is proportionate 

 to pressure in the case of this pigment. 



Moisture Absorption. The structure of certain pigments is 

 such that when they are ground in linseed oil and painted out, 

 films are produced which are very water-resistant. This action 

 is possibly due to the filling of the voids in the oil, thus making 

 a compact and water-resistant film. Pigments which are coarse 

 and which present an angular crystalline structure, often produce 

 films which contain a relatively large number of voids and are 

 less waterproof. Certain pigments are chemically active and 

 tend to produce, when ground in oil, metallic soaps which act 

 for a time more or less as varnish gums, in keeping out moisture. 

 Later on, however, such films are apt to break down and admit 

 moisture in quantity. The tests herein described were designed 

 by the author to determine the water-excluding value of a number 

 of typical pigments when ground in linseed oil and painted out 

 into films. Unfortunately, no method has been devised by 

 which films of the same gauge could be prepared. The variations 

 in the thickness of the films used in these experiments, however, 

 are not very great. 



