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FAQ: Black Oxide & Cold Blackening
by Ted Mooney <>
Black oxide is a finish applied to iron and steel. There is also a
' black oxide' process applied to the copper inner layers of printed
circuit boards, but that is another topic.
There are two general types of black oxide for iron and
steel: hot black oxide (or hot blackening) and room temperature
blackening (or cold blacking).
Hot black oxide can be done from generic mixtures of caustic soda,
sodium nitrite / nitrate, wetting agents and stabilizers or from
The result of the process is an attractive but very thin and
marginally corrosion-resistant, dark black iron oxide finish. This
black finish is familiar to consumers on gears and sprockets, some
brands of spark plugs, and socket wrenches and other tools. It is
also used on firearm components, such as rifle barrels.
Usually, the parts are cleaned, black oxided, and then waxed or
oiled (with intervening rinses).
While most metal finishing processes use toxic chemicals, the
black oxide process is especially hazardous, and amateurs are most
definitely discouraged from attempting hot blackening! One of the
things that makes black oxiding so dangerous is that the black oxide
bath operates at about 290 degrees F. Note that this is well above
the boiling point of water, and the difference is the biggest
problem. Water will evaporate from the black oxide tank rapidly, but
when replacement water (which turns to steam at 212 degrees) is
introduced to a 290 degree tank, it will have a strong tendency to
explosively flash into steam--'erupting' and spraying everything and
everyone with this terribly hot and terribly caustic solution. People
have been killed. It is vital that the water make-up system be
properly engineered and knowledgeably operated.
In order to reduce the hazards of hot blackening, and to save
energy, proprietary cold blackening solutions have been introduced.
These operate at about room temperature, and on a different chemical
basis, so they are substantially less hazardous. However, room
temperature blackening is not a true black oxide process; rather it
is the application of a copper selenium compound. This compound is
not always an acceptable substitute for black oxide as it does not
look as nice, and can tend to be very smutty (easily rub off onto
hands and clothes).
There are dozens of on-line letters here related to black oxide
and cold blackening. Here is a partial list of some of the early
Many more letters about black oxide and room temperature
blackening can be found by searching the site.
Where to get it done
Many jobshops offer black oxide services. Please go to our
Directory of Jobshops and search
for the term "black oxide". If this does not satisfy your needs, post
an RFQ for the service you need on our
"Looking for a Jobshop" page.
Where to buy supplies to do black oxide or cold blackening
Several national suppliers offer hot black oxide and cold
blackening processes. Please go to our
Directory of Chemicals &
Consumables page and search for the term "black oxide". If this
does not satisfy your needs, post an RFQ for the service you need on
our "Looking for Products &
Please post any technical questions about black oxide or cold
blacking in our "Hotline Letters"
Disclaimer: It is not possible to diagnose a finishing problem or the potential hazards of an operation via these pages. All information
presented here is solely for general reference and does not represent a professional opinion nor the policy of an author's employer. The
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