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Introduction to Patinas
A patina is a surface coating on a bare metal surface formed by a chemical reaction. It is different from applied coatings like painting and powder coating in that the effect is caused by actually reacting a solution with the metal surface. Metals react with moist air on their own at a gradual pace. By using a blend of acids and/or salts, the process is greatly accelerated to a timeframe that is manageable for decorative application. Also, there are nonreactive patina products available that either modify existing patinas or splash color on bare metals. One such nonreactive product is dye oxide patina.
When one thinks of putting a patina on metal, they usually think of metals like copper, silver, and bronze. This is because these metals have the greatest range of reactive colors over the other metals. Unfortunately, the cost of these metals makes them practical for jewelry and not so much for large artworks. Steel is the most widely-used industrial metal and the most inexpensive. If diligent, one can find steel even cheaper—or even free—in scrap form. As this article implies, there is still a good selection of finishes for steel besides the traditional rust finish.
Surface Preparation for Metals
If you are applying any finish to steel, whether it is a patina or otherwise, proper surface preparation is paramount. There are several ways to prepare the surface of steel. To get the desired results any combination of the methods mentioned here may be needed. There is mechanical debris removal with abrasives and wire brushes, acid pickle, and solvent cleaning.
There are two major kinds of mild steel, cold rolled and hot rolled steel. Hot rolled steel is shaped and rolled at high temperatures. Hot rolled steel has a black oxide coating called mill scale. To patina hot rolled, the mill scale must come off. A good and inexpensive way to accomplish this is to use an acid pickle consisting of equal volumes of water and muriatic acid. This could be hazardous due to the fumes and exposure so be sure to wear gloves and eye protection. One way to mitigate the hazards of handling acid s to use smallest volume possible. One way to accomplish this is to apply the pickle with a foam brush. This is better than making gallons of corrosive acid solution for dipping. Cold rolled steel just needs to usually be wire brushed followed by a wipe down with solvent. If the cold rolled is heavily rusted then it will more than likely need an acid pickle.
When abrasives and wire brushes are insufficient to remove extensive rust or heavy millscale, an acidic pickle can be most helpful. One of the fastest acting pickle is muriatic acid. In most situations, it is best to dilute this powerful acid with water. In extreme cases though it might be best to dunk the steel in the concentrate right out of the bottle. Keep in mind this acid can blind you almost instantly, it burns skin, and the fumes are toxic with a suffocating odor. Also, when you are done pickling, you have to rinse with a lot of water to mitigate flash rusting because the steel will be in an activated state.
If fumes are a major concern, there is an acid that produces almost no fumes and converts rust into an insoluble primer base that can be painted. That acid is phosphoric acid. There are several brand name rust removers/converters on the retail market based on phosphoric acid. The principle of the reaction is the iron oxide in the steel corrosion products is converted to iron phosphate. This phosphate is relatively inert and insoluble and offers a barrier that can be painted over.
Abrasives and Wire Brushes
As far as abrasives are concerned, the most effective tool is an angle grinder with a flap wheel. Flap wheels come in a variety of grits can be had cheaply by bulk online. Another option is the sanding adapter kits sold for angle grinders. Like the flap wheels, the sand paper discs come in several grits from coarse to fine. They are beneficial as they have more abrasive surface area to work with.
There are also sanding kits available for angle grinders. These consist of a rubber pad that is screwed onto the spindle and a sanding disc applied and secured with a nut. Unfortunately, you can only sand with one side of the disc at a time.
Wire brushes also come as fixtures for angle grinders. Usually two configurations are found. One being a flat wheel for crevices and irregular surfaces, and cup brushes for flat sheet and plate.
Vinegar/Peroxide Patina Formula
Vinegar/peroxide patina is a formula derived from all easily accessible materials and is of low toxicity. Dissolve the salt in the vinegar and once dissolved, add the peroxide until total liquid volume is 16 ounces.
- 2 tablespoons of rock salt or kosher salt
- 2 ounces or 1/4 cup of 5% distilled vinegar
- 3% Drugstore Hydrogen Peroxide until total volume is 16 ounces
Solvent Cleaning of Steel
By far the most useful solvents for cleaning steel is acetone and denatured alcohol. For very warm weather conditions it is advisable to use denatured alcohol as acetone will evaporate instantly on a warm day. These are the least expensive solvents but are excellent and removing oils and grease. These liquids are flammable so make sure to keep the containers away from ignition sources like welding equipment. Also, they mix with water in all proportions. This is helpful when water rinsing steel from an acid pickle. Acetone and alcohol will remove remaining traces of moisture then evaporate leaving dry metal.
Types of Steel Patinas
|Types of Steel Patina||Materials Used||Warnings and Suggestions|
Many cheap and accessible ingredients including: Salt, vinegar,hydrogen peroxide,chlorine bleach, muriatic acid, air, water, and sun
Bleach and muriatic acid are corrosive and toxic. Only bare,cleaned and decreased steel will rust uniformly
Gun bluing and other blackeners
Gun bluing and other commercial blackeners contain copper salts and other oxidants which are toxic. Clean shiny metal works best
Just heat and air
Bare and polished steel woorks best to give vivid colors. Be mindful of heat sources and hot metal
A slightly acidified solution of copper sulfate or copper chloride
Copper salts are toxic and harmful to the environment. Adhesion can be tricky. Thoroughly clean and degrease bare steel prior to immersion.
A rust patina is exactly as it sounds, a coating of rust oxidation on steel. One of the simplist ways to accomplish this is to leave bare steel outside in the weather. After a few weeks or months, depending on moisture and humidty, the metal will start an oxidation process eventually coating the whole surface.
If time is of the essence, then we need to speed things up chemically. Thankfully, most formulas for rusting steel can be made from readily available products. One very simple method uses just salt water and warm sunshine. Just repeated application of salt water and allowing the warm sun to work takes several days instead of weeks. If it is winter or you live in a predominently cloudy conditions, then perhaps a little more active solution is needed. A cheap but very effective recipe involves a mix of salt, vinegar, and peroxide. This solution is of low toxicity and fast acting. It gives the most intense chile red color on contact with steel. The proper proportions are in the recipe below. It is best to make a fresh batch each time. If you want more information, check out the instructable.
Other Rust Patina Formulations
The above paragraphs give a description of the easiest, most cost effective, and safest route to obtain a fairly consistent rust patina. There are numerous other ways to accomplish the same results but they require corrosive and/or noxious chemicals. One method is a strong acid like hydrochloric or sulfuric acid diluted to less than 5% and a trace of copper to accelerate rusting by galvanic corrosion. Bleach with peroxide is rather effective but you have to cover your clothing and eyes to keep bleach out. Also, the mixture fizzes profusely so you can only add bleach to peroxide a little at a time.
Gun Bluing on Mild Steel
There is a product called gun bluing that is sold whereever sporting goods are sold. Typical use is to give a handsome bluish black finish to gun barrels. It is a poisonous light blue acidic liquid that turns mild steel black on contact. It's active ingredient is selenium dioxide which oxidizes the iron to magnetic black iron oxide and also deposits elemental selenium. Apply to clean steel and burnish to desired results then promptly rinse. If you do not rinse this patina off thoroughly, the metal will start rusting under the patina. Also, it should be know that gun bluing is ineffective on stainless steel alloys.
Black Patina on Cold Rolled Steel
Birchwood Casey Super Blue For Bluing Mild Steel.
Your Ideal Steel Finish
Heat Patina on steel
Heat Coloring of Steel
If you have a shiny and freshly poolished piece of steel, you can create an impressive pallet of color just by heating in air. What happens is the oxygen from the air plays upon the hot steel and starts forming oxides. The layers of oxides are thin at first and the transparent colors that result are an interference effect. The colors are difficult to control. Although you can use a torch, an even heat source is best. A ceramic support over an electric heating element or in an oven will give more consistent results. Placing a few drops of water or oil on the metal before heating can create some interesting patterns. Many of the blues, magentas and violets cannot be clear coated because of the thickness of the clear coat added to the oxidation changes it optic properties. The golden and bronze finsihes can be clear coated but will eventually darken with age. A periodic application of renaissance wax are recommended for protection but also the coating is thinner and would be helpful for preserving more delicate coatings.
Copper Patina on Steel
It is possible, with diligent care and preparation, to obtain a copper finish on bare steel. This is not the same as electroplating and is not nearly as durable. In fact the process is more like electroLESS plating or immersion plating.
Copper is less chemically active than iron, if you have copper in compound form dissolved in water, then some of it can be displaced by more active metals like the iron in steel. It is usually necessary to add a small amount of acid to the copper solution so to "activate" the steel surface by removing residual oxides, and assist in keeping the copper salt dissolved in the process.
Once applied, the copper patina solution must be rinsed with clean water and sealed by a clear coat at once unless the steel will rust from underneath. A 10% solution of copper sulfate with a few drops of sulfuric or muriatic acid added is a good place to start. There are also a few companies that market proprietary products that have been tested extensively.
Copper Patina on Mild Steel
Japanese Brown Patina
Japanese brown patina is a reactive patina that can be applied hot or cold. The appearance is based on the application. For instance, a fine light spray would produce a more even coat while dabbling with a sponge would give a more mottled and variegated appeAramco.
Questions & Answers
Question: Have you used Black Magic patina? If so, is this better than the Super Blue? Which you recommend for blackening steel?
Answer: No I have not used black magic patina. I’m sure it’s a similar chemical formula. An acidic oxidation with a touch of copper.
Question: Do you have any treatment suggestions that will reduce/eliminate corrosion of a steel structure that will be outside exposed to the Minnesota weather year round? This piece will be exposed to extreme temperatures (1000 + F) so I am doubtful that any clear coats would function well. Do you have any suggestions for the best solution to create an interesting effect and reduce corrosion?
Answer: Hopefully it is made from Corten or special weathering steel. If not, make sure no snow or water is caught in pockets or recesses. I would recommend blasting thing a torch until bright orange-yellow (2000 degrees F) to allow fire scale to form. Fire scale is just black magnetic iron oxide and forms a nonflaking oxide coating. A thinner black oxide can be had with gun bluing. Never leave bluing on steel and remove excess solution promptly or it may accelerate rusting.
Question: We are thinking about using the rust technique on rolled steel for a wood burning stove hearth. Once the desired rust level is reached, how can we stop the process or seal the surface to prevent further rusting.?
Answer: I’m assuming that the hearth will be exposed to a high temperature. If this is indoors the heat should form a protective oxide that should be fine. Rustoleum has a high temp clear coat that is good up to 2000 degrees. Spray that once a year, you should be fine.
Question: Can I do a combo of a copper finish and rust to cold pressed steel? I don't want the copper to look so clean. We were planning on just pre-sanding it to create character but I want more color variation.
Answer: You might be limited on rust due to possible adhesion issues with copper finish. Copper finish needs clean shiny steel. One idea is you could apply copper to clean steel and use a flap disc to grind it off high spots. Spray a satin clear to seal then lightly dry brush with iron oxide acrylic paints. In a pinch, you can mix any iron oxide pigment powder with an acrylic medium. You can scrape rust off a piece of rusty steel to make the paint.
Question: My project is a hot tub surround on which I’d like to create a mottled/streaked patina. I’m thinking of spraying bluing and letting it drip down. Rust will introduce itself in our wet climate and I’m worried about the bluing being poisonous. Is there an alternative to bluing or precautions I could take to protect myself and the environment?
Answer: Yes, it is quite toxic by ingestion but not so much by inhalation. I would try soaking scrap cotton fabric in the bluing and laying it over the surface in strategic spots. As long as the rags are not running the chemical everywhere, clean up should be as easy as putting used rags in a plastic bag and tying it shut.
Question: My project is a chess set (one half) made out of mild steel, turned on a lathe. They're slowly forming an undesired rust layer. I want to preserve the look and feel, and coat them in something to protect the piece from rust, and from grubby fingers whilst I play chess against my kids. I don't mind the idea of the black patina, however, you mentioned it is poisonous, and that doesn't go so well with kids. How can I preserve my steel chess set?
Answer: The black patina is only poisonous in its liquid form. After it is applied to the steel and rinsed off only a black oxide is left on the metal. For the safety of youngsters, I’d only apply the gun bluing outdoors when they are sleeping or away. Maybe store the bottle offsite somewhere else if need be.
As far as protective coatings, I’d go over it with 3 coats of lacquer. Reapply that every 6 months as needed.
Question: How hot do you need your oven?
Answer: Well, that depends on what heat patina you are after. 300-350 Fahrenheit for the gold patina. Blues and violets show up at 475 degrees and up.
Question: I’m trying to change the color of my stainless steel stove hood. Can I use these patina finishes on stainless steel?
Answer: No. There are products specifically made to change stainless steel black but patina formulas generally do not normally react with stainless steel. A faux patina paint job might be a better option here. Or you can patina thin sheets of mild steel and attach to stainless with 2 part epoxy. A sort of iron cladding.
Neha Passi on June 25, 2020:
I tried to putt copper sulfate on steel and kept it overnight : nothing happened plz suggest what should I do
Jason (author) from Indianapolis, IN. USA on April 22, 2019:
Just email to firstname.lastname@example.org
Kelsey Sweeney on April 09, 2019:
Hi Jason, is there a way i could send you a picture to get your opinion on how a patina look was obtained? Thanks
Buddy on August 23, 2018:
Forgot to add some really cool stuff can be done with sand blasting, gun bluing, then highlighting with prisms color pencils, and finally clear coating.
Buddy on August 23, 2018:
You can also sandblast then brass brush, and finally burnish steel to get a golden appearance.
SUSANM on April 05, 2018:
Hi Jason, I loved reading your article. I have never done this before. Here goes. I have 2 giant steel lions ... 4ft tall and 3 ft wide, that are outdoor landscape pieces. They have previously been green patinated. Now some rust is coming out in places and the patina is wearing off and i see the steel. I want to repatinate them to look like patinated bronze and then sell them. I want to do this in my back yard.
So if i understand correctly here are my steps.
1. Remove all patination and rust to prepare the surface. Use muriatic acid and water with eye, skin, and breathing protection. Question. I have a large side yard covered in rocks and plan to wash the runoff there. Is flushing thoroughly enough to make safe? I have German shepherds who ruin all over the place and although they won't be near when I do this, I want to be very careful.
Should I consider another method?
Sharon Proudfoot on March 14, 2018:
Hi Jason, thank you for your very helpful post on patina. I am doing a fireplace surround with hot rolled steel, but the material is much more black than I wanted. Can I treat the hot rooked steel to get it less black and more of the blue and grey coming out? I have read your info about the acid pickle and the gun bluing, but I am still a bit confused. Would it be possible to get more information? We could take it off line my email address is “email@example.com”. Thank you
Jason (author) from Indianapolis, IN. USA on October 28, 2017:
Oh that is nice. I will check it out. If it dries completely then maybe the dark rust finish could be accented as well. Maybe with an orange or red Lacquer or thinned down gold paint.
Alex on October 27, 2017:
Nice easy brush on oil treatment which seals around the rust layer and bonds to the underlying raw steel driving out all oxygen. Result: a slightly darkened rust surface which is as stable as painted surface. Its Tough stuff. I have units outside year in and out with snow and no problem after several years. The maker suggests applying it
Annually, but I think this would be savage overkill.
Not so cheap but for artwork, definitely an interesting option. You may have to experiment with the how the oil changes the look of the rust, but I love the deep dark rust look this oil makes.
Makaira on June 02, 2017:
Thank you for the article! I have been using your formula for sometime. I have very mixed results due to my climate and cold winters. In addition, through a lot of testing, I've found that the result differs greatly from repeated spraying vs. tumbling (almost semi-submerged). I love the result of the tumbling the best, as it gives that bright chili color. I do have a question that I hope you can help me with...After I've rusted my products, they become so dusty and leave residue on everything. Can you suggest a way to stop the process and avoid the rusty dust? I don't think clear coating is an option for me as I'm working with huge quantities and hundreds of small pieces. In addition, I try to eliminate or keep the toxicity to a minumum by not using spray paints or chemicals! Thanks for any input
Jason on January 04, 2017:
Thank you Bill! Still much work to be done!
BILL WORDEN from ST. GEORGE, UT on January 01, 2017:
Nice Article. I enjoyed reading it. Thanks! Bill