Material and/or Finish vs. Application

Choose the appropriate material and finish for your particular application. If a marine application, consider silicon bronze or brass. If exterior but not marine, consider a plated product, i.e., statuary bronze plated, or nickel plated. If you will be forging the heads of square head lag bolts, plain is all you will need. Black Oxide is decorative, but not weather-proof, so you may decide to use it for interiors, or, with extra clear-coatings, for some exteriors. More detail for the various materials and finishes will be found in the following paragraphs.

We sometimes will make recommendations regarding materials or finishes, but without knowing all the parameters of your project, our advice is still only worth what you pay for it (it's free). Recommendations are made without any warranty, express or implied.

In the final analysis, the responsibility for the choice of materials and finishes is yours. It is up to you to make the determination of suitability of an item, it's material and its finish.

Black Oxide is a versatile finish. Black Oxide is a conversion coating formed by a chemical reaction produced when parts are immersed in an alkaline aqueous salt solution at approximately 285 degrees F. Although this process will produce similar results on some non-ferrous metals and alloys, it is usually found on iron, steel, and stainless steels. The reaction between the iron of the ferrous alloy and the hot oxidizing bath produces a magnetite (Fe3 O4) on the surface of the parts.

When one speaks of a Black Oxide process, they are usually referring to the hot bath process. There are in fact both hot and cold processes, as well as a process using an intermediate operating temperature. The hot bath Black Oxide process works best between 285 and 290 degrees F. As this is an aqueous solution, there is considerable evaporation loss, which must be accommodated by the introduction of replacement water. As the temperature of the bath is well above the boiling point of water, the replacement water must be added very carefully, so as to avoid entrapment of water, resulting in bursts, or explosions, of steam. This is the element that makes the process dangerous for the inexperienced or under-equipped.

To avoid the danger posed by the formation of super-heated steam, both a cold process, and a "warm" process have been developed. The cold process is accomplished at room temperature. The results are uniformly disappointing, as the resulting color is not uniform, and as the final finish is almost always "smutty", i.e., it rubs off on your hands and clothing. The Birchwood-Casey Company has developed a process that operates at 190 degrees F., below the boiling point of water, and reportedly gets better results than the cold process.

Hot black oxide can be accomplished from generic mixtures of caustic soda, sodium nitrate/nitrite, wetting agents and stabizers, or from proprietary mixtures. Some have inquired regarding gun-bluing and/or steel-brown patinas. Most gun bluing solutions are identical with the hot bath black oxide formulations; some employ more sodium cyanide, which lends the bluish cast to the black. Likwise, the steel-brown is accomplished with the addition of a different salt.

There are five steps to a successful black oxide coating: Clean, Rinse, Black Oxide, Rinse, After-finish. It is most obvious that without proper cleaning, and rinsing, the black oxide coating may be less than uniform in color or coverage. What is not as obvious is that without a proper after-finish, flash-rusting almost always results. After-finishes fall into two major categories, oil, and wax. Lacquers are also used. If none is specified, the default is an oil. The preference is determined by application of the item, and the appearance desired. An oil after-finish will be more toward a glossy appearance, while a wax after-finish will be more to the matte.

The resulting black oxide finish will have a significant degree of water- and rust-resistance; however, black oxide coatings cannot be expected to perfom well in long-term exterior, or harsh marine environments. Water-resistant, yes; water-proof, no.

Occasionally, it may be necessary to remove the black oxide coating from a few parts. This can be accomplished with Muriatic Acid (a 10% solution of Hydrochloric Acid). Be sure to take all necessary precautions for personal, and environmental, protection if and when such removal becomes necessary. The black oxide coating will also be removed by high heat. The after-finish (oil or wax) will fail between 200 and 300 degrees F., then the color will become mottled and finally "burn off" as the temperature approaches 900 or 1000 degrees F. If the parts are to be hot-forged to achieve a decorative effect, there is no need for the acid bath, as the heat of the forge will accomplish the removal of the black oxide.

I know, I know, our Third Rule is NO ZINC. We hate the look of shiny bright zinc. However, there are times when you need the black color, and the resistance to rust that only a true plating will provide.

Black Zinc will do both, and look good, too. We haven't included an example in our illustration above because, well, it looks just like Black Oxide.

We can't afford to carry all the different sizes in Black Oxide, Plain, and Black Zinc, too, so it will necessarily have to be a custom finish. The minimum charge will be $85, so a batch size of about 75 pounds will meet the minimum. This is to say that it costs $1+ per pound, with a minimum of 75 pounds of fasteners, for a minimum cost of $85. If you have less than 77 lbs. to process, it will cost correspondingly more per pound, and per piece, to plate them.

More information about the Black Zinc finish option will be found, in the left menu, under Miscellaneous >> Services >> Black Zinc...

Statuary Bronze Plate refers to an electro-plated bronze applied to steel. The carriage bolt in the picture above has a chocolate brown color darker than the bronze wood screws. We have only a few sizes of carriage bolts in this finish, and a variety of Slotted Oval Head Wood Screws. The appearance of the wood screws varies from light to dark, just as with the solid bronze fasteners. The advantage of this finish over black oxide is that Bronze Plate is water-proof, while black oxide is only water-resistant.

The "Bronze Wood Screws" we stock are Silicon Bronze (the material), Plain finish.

Brass, an alloy of copper and zinc. There are many different brass alloys, each according to properties necessary for its application. A nice yellowish color in appearance. Brass the alloy might be confused with brass the finish, because there is the possibility to electro-plate an item with a brass finish.

At this time, we only have a few brass items, and they are all solid brass, plain finish, NOT Brass-plated. You will find a few brass wood screws and some brass rivets in our store.

Dark Nickel happens when you think an item has a zinc coating, but it really is nickel-plated, and you send it out to be stripped and black-oxided, but it comes back with a smokey, dark nickel finish. Accidental, well, yes, but still, it has an intriguing appearance. We only have a couple different sizes of woods screws with this finish; you might have just the right application for this finish, where black oxide would be too dark. Dark Nickel is a product of patinating an electro-plated finish.

Plain is the finish without patination or plating. It may be plain and oiled; a light oil to prevent rusting while in storage and transport. Or it may be plain, with, or without, a bit of rust. Rust is not a finish, but it may affect the final finish. Light rust may be converted during black oxiding, while more serious rust will need to be dealt with by sand- or bead-blasting, before the final finish is applied.

We try to accurately describe our products, alerting you to the situations where there is some rust present on the items.

Bronze (Silicon Bronze) or Brass fasteners are usually furnished Plain.