Mixed Media Fine Arts

BURN-JUNKIE COLOR LOUNGE: An Introduction to Chemical Patination

All living matter - all organic material - is carbon-based; so naturally, we attribute a premium to carbon-based matter. Almost instinctually we know that organic materials, such as wood, need to be cared for and nourished. A piece of sheet metal doesn’t necessarily arouse the same kind of TLC. We relate differently to inorganic material, especially once it has undergone processing. While organic and inorganic matter are elementally different, the distinction becomes obscured in a more general ecological framework. After all, inorganic matter provided the raw ingredients of life. Water is inorganic – so is air, as is most of the earth’s constitution (i.e. oxides and sulfides). Further, organic materials will eventually convert back into inorganic matter as they petrify into fossils, via years of water and mineral exchange. Anyway, the point here is that inorganic materials also respond to care.

It is a mistake to view metal as completely inert. Metals are reactive - they undergo complex chemical reactions when oxygen, hydrogen and acids are introduced. These reactions completely reorganize the structure of the metal surface. To me, this reactivity is bewitching. It is one of the reasons metal provides such a beautiful medium for art. There are hundreds of ways to introduce and layer chemical patination that will intensify, alter and otherwise transform the appearance of the base metal. In this way, the medium is alive.

Patination is the process of oxidation on the surface of metal. This “patina” occurs naturally, as witnessed when weathered copper becomes green (or verdigris); but patinas can also be intentionally created through the use of various chemicals. Artistic patination is a kind of controlled corrosion - an introduction of specific chemicals to the metal in order to stimulate a particular reaction and then, just-in-time, a suppression of the reaction.

When enamel is painted onto metal, it is applied over the top of the metal as a wholly separate coating. The metal surface is left to oxidize or rust beneath. A patina is not applied to top of the metal. Patination is a process whereby the applied chemicals bind with the metal surface and promote a transformation. The effect may be either transparent or opaque and there are myriad textures that consequently develop.

Chemical patination allows effects to be produced in a relatively short period time that, with practice, can have a predictable appearance. The color palette for artistic patination is substantial - from deep reds and oranges to lush blues and greens – from varied hues of whites and browns to numerous shades of distinctive blacks. Patinas can be carefully layered to achieve a rich and complex summation. While there are many basic patinas and teachable techniques, the execution of a successful patina is somewhat challenging. Everything can impact the patina’s outcome: composition of the base metal, purity of the utilized chemicals, constitution of companion water, surface preparation, method of application, intensity of heat, ambient temperature, light, humidity and duration of elements. The protective finish chosen to apply over the patina will also affect the overall appearance of your piece. This blog about the process of patination is an introduction. In later blog posts I’ll provide details regarding specific application processes.

Prepare your studio

Before you get started with artistic patination, remember that you are using chemicals– acids, chlorides, oxides and sulfates. The first priority in a metal studio really has to be safety. Any green or blue patination chemical likely contains copper or nickel salts, which are irritants – plus they are toxic. Darkening and oxidizing agents can let off noxious gases. In other words, patinas are often corrosive when handled and harmful to an unprotected respiratory system. Even low-level exposure can assault your nasal passages and lungs. So, be smart! Work only in a well-ventilated space and make sure that you seal up chemicals when they are not in use. Get yourself a quality respirator and use it (a respirator is far less effective when left hanging on a peg in the corner). Remember, you only have one set of eyes. Wear goggles and keep eye-flush available just in case. I always have eye-flush (saline), burn cream and a first aid kit nearby on a red go-to shelf (it’s easy to see and always in the same place). Handling patinas is not a good idea either – many chemicals are irritants and can be absorbed through the skin, so wear gloves when you’re working with patinas. You know, the basics here are pretty much the same as in chemistry lab: wear protective clothing, don’t mix chemicals together when you don’t know how they interact and don’t eat or drink in the studio.

Prepare your metal

Prior to the application of patina chemicals, you must clean the metal surface to remove all rust, dirt and traces of oils. Anything left behind will impact the outcome of the patination. Sometimes you want to keep a bit of a natural patina – other times a pristine surface is desirable. Surfaces are best fully cleaned by bead blasting immediately prior to the patina application but this is not always possible (or desirable). Start with a good detergent wash with a bristle brush and a through rinse in cool water followed by wiping the metal with a solvent like denatured alcohol or acetone. There are also metal degreasers specifically made for artistic patination. Keep in mind that your choice of cleaning method will often affect the way the patina develops. Anything used on the surface of the metal may accelerate or impede the chemical process. Sometimes you’ll need to sand the metal – sometimes you’ll just want to lightly polish the metal in order to even the surface and to help the patina adhere to the surface. For sanding, I use compressor tools or a flap disc on my grinder; for light polishing I like white scotch-brite pads which are not terribly rough and they don’t leave a residue behind. Wipe down your surface after cleaning and make sure it’s completely dry. Use a clean cloth and a heat gun on low setting (or a hair dryer). Don’t use sustained heat that is above 200°. Follow  the metal preparation immediately with a primer, base coat and/or patina application. Steel begins to oxidize within a couple of hours after cleaning and the surface will be affected.

Base Coats

Coatings can be used that contain various ground metals (e.g. copper, bronze) suspended in an acrylic binder. These metal coatings can be used to create a reactive finish and patination chemicals for those specific metals can be used thereafter. If you are applying a coating to your metal prior to patination or in lieu of patination, you’ll want to use a primer to prepare your metal. Red oxide primer is a specially formulated coating to be used as a base coat for ferrous metals to stop corrosion. Use a brush to apply two coats of the primer. Each coat will be dry to the touch in about 2 hours, but not fully cured for about 6 hours. Red oxide primer will be ready for the metal coating after 24 hours.

Hot Patinas

Hot patinas are applied to base metal (or to a reactive base coat) that has been heated to around 200° as opposed to cold patinas which are applied to the metal at room temperature. A wide variety of colors and effects can be achieved. Heating the metal opens its pores, which allows the chemical to bond more tightly to the surface. After cleaning the piece with degreaser, heat the metal to around 200-250°F. To test this spray a few drops of distilled water on the metal. If the water sizzles or steams but does not ball up, it is the right temperature. If the water balls up and bounces off, it is too hot. If the water runs off without steaming, it is too cold. Apply the patina to the hot metal. Using a spray (bottle or compressor) will provide more even coloring. Stippling with a stiff, short-haired brush will provide marbled, mottled effect.  Continue heating as needed to keep the metal hot during patination.

Cold Patinas

Cold patinas mean that chemicals are applied to metal that has not been heated. After cleaning with degreaser, chemicals are applied to the surface of your metal by spraying, brushing, sponging or by dipping. Cold Patinas take longer to develop and are less durable than hot patinas.

Sealing the Patina

Patination on metal must be sealed. Left unprotected, moisture and ambient chemicals will alter the color of the patina and rust and/or pitting is likely to occur. A textured surface will oxidize more quickly than a smooth one. There are many final coat options for metal that produce varied finishes (e.g. shiny, satin, glossy, matte) including lacquer, acrylic, waxes and oils. Topcoats should always be applied in thin layers. Don’t forget to use your respirator! Solvents are often used to thin topcoats and for cleaning up (e.g. acetone; xylene; methyl ethyl ketone) and the vapors can be pretty nasty (toxic and flammable).

This introduction to chemical patination is just the beginning of a longer conversation. There is a learning curve associated with even basic patina application. So, I’ll be writing more posts about patination: specific methods for chemical application; how different metals receive various chemicals; and tips for putting final luster on your metal art. Have fun.