PIGMENTS 333 



time of drying. This compound when mixed in a thick paste will 

 keep 30 days without hardening. 



Zinc. Zinc white (zinc oxide specific gravity 5.3) is a white 

 loose powder, devoid of smell or taste and has a good covering power. 

 Zinc paint has a tendency to peel, and when exposed there is a tendency 

 to form a zinc soap with the oil which is easily washed off, and it 

 therefore does not make a good paint. However, when mixed with red 

 oxide of lead in the proportions of I lead to 3 zinc, or 2 lead to I zinc, 

 and ground with linseed oil, it makes a very durable paint for metal 

 surfaces. This paint dries very slowly, the zinc acting to delay harden- 

 ing about the same as lampblack. 



Iron Oxide. Iron oxide (specific gravity 5) is composed of 

 anhydrous sesquioxide (hematite) and hydrated sesquioxide of iron 

 (iron rust). The anhydrous oxide is the characteristic ingredient of 

 this pigment and very little of the hydrated oxide should be present. 

 Hydrated sesquioxide of iron is simply iron rust, and it probably acts 

 as a carrier of oxygen and accelerates corrosion when it is present in 

 considerable quantities. Mixed with the iron ore are various other in- 

 gredients, such as clay, ocher and earthy materials, which often form 

 50 to 75 per cent of the mass. Brown and dark red colors indicate 

 the anhydrous oxide and are considered the best. Bright red, bright 

 purple and maroon tints are characteristic of hydrated oxide and make 

 less durable paints than the darker tints. Care should be used in buying 

 iron oxide to see that it is finely ground and is free from clay and ocher. 



Carbon. The most common forms of carbon in use for paints are 

 lampblack and graphite. Lampblack (specific gravity 2.6) is a great 

 absorbent of linseed oil and makes an excellent pigment. Graphite 

 (blacklead or plumbago specific gravity 2.4) is a more or less im- 

 pure form of carbon, and when pure is not affected by acids. Graphite 

 does not absorb nor act chemically on linseed oil, so that the varnish 

 simply holds the particles of pigment together in the same manner as 

 the cement in a concrete. There are two kinds of graphite in common 

 use for paints the granular and the flake graphite. The Dixon 

 Graphite Co., of Jersey City, uses a flake graphite combined with silica, 



