A greater understanding of Pissarro's factory paintings can be had by examining paintings of the same subject by other artists. One particularly interesting example is Degas.
In many of his racetrack paintings, the viewer sees silhoutted factories in the background, smokestacks protruding into the sky. Industry is clearly not the focus of these paintings, but it is intriguing to speculate as to Degas' reason for placing them so. He could be highlighting the contrast between nature and industry. In the foreground, viewers see horse and rider; in the background, a more powerful force, looming in wait to somehow make nature obselete. It seems that Degas felt this kind of conflict, and very subtly brought attention to it in some of his racetrack paintings: very different indeed from Pissarro's factory paintings on the Oise, where the plant is the clear subject of the painting, but at the same time smoothly blends with its the nature by which it is surrounded.
Armand Guillaumin, in Sunset at Ivry, also pits nature against industry, where Pissarro's work was more likely to harmonize the two. Here, he depicts one of natures most beautiful symbols, the sunset, being covered over by black clouds of smoke being emitted by the stacks of the factory. While perhaps he does acknowledge the beauty of this factory's silhouette, he certainly puts the factory in battle with its surroundings, as opposed to the peace that Pissarro creates in his work.
Images from the top:
1) Jockeys in Front of the Grandstand by E. Degas, 1869-72.
2) Sunset at Ivry by Armand Guillaumin, 1869.