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Blackening

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Blackening 

a type of oxide coating for iron and steel products, used for corrosion protection and for decorative purposes. The most common method of blackening is to immerse the products in a concentrated alkaline solution containing sodium nitrate or sodium nitrite as an oxidizer. As the concentration of the alkali is increased (other conditions being equal), the thickness of the oxide film also increases. A thin coating (20 to 40 angstroms) does not alter the external appearance of the surface nor does it provide practical protection for the metal against corrosion. Thick coatings (2.5 micrometers), which are black or grayish black, have poor mechanical strength. The best thickness is 0.6 to 0.8 micrometer. Blackening is a special case of blueing.
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His cheeks were very white, and something like a scowl was blackening his heavy, insipid face.
The Yellow Crayon by Oppenheim, E. Phillips View in context
He felt that in letting Dunstan have the money, he had already been guilty of a breach of trust hardly less culpable than that of spending the money directly for his own behoof; and yet there was a distinction between the two acts which made him feel that the one was so much more blackening than the other as to be intolerable to him.
Silas Marner by Eliot, George View in context
Two axes lay upon the sands beside me; chips and splinters were scattered broadcast, and the ashes of the bonfire were blackening and smoking under the dawn.
The Island of Doctor Moreau by Wells, H.G. View in context
 
 
 
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