As part of a large scale patination project in which I patinated a steel roof surface 24 by 48 feet on both sides I performed some 40 experiments to find out how to patinate the steel which was a requirement for structural reasons. In doing so I also experimented with paint, buying over $350.00 worth of spray paint, eventually finding one single color which for all intents and purposes is green patina. When placed in recesses and the high areas are rubbed off it is indistinguishable from a cupric nitrate patina.
It is a car paint: GM 42, 1980 Chevrolet Medium Green. While this is ideal for smaller surfaces my paint experiments did not produce the surface effects I required on the large scale work. I reasoned that if I could plate the steel with copper and then convert the copper to patina in a fume not only would the job be easier but it would also be safer than dealing with solvents or corrosive patination techniques (such as a cupric nitrate patination) over large surface areas. I was dealing with 4 x 8 foot sheet steel to be equally patinated on both sides simultaneously. Other types of objects might be easier to deal with. 'Tents' of polyethylene plastic sheeting stapled to a framework of 'economy' studs were built. The construction of such a tent requires that it be sealed (draped onto the floor from the frame and then weighted down). The object inside is positioned on supports of some kind so that it is suspended off the floor in the air inside the tent. Then pans of household ammonia are placed underneath the object. The fumes attack copper or copper based alloy surfaces. Under normal conditions one can activate a copper containing surface with a dilute salt solution to speed up the procedure and obtain a blue patination but this proved too corrosive for dealing with steel. The final procedure chosen was as follows:

l) The steel was cleaned well. Sandblasting would be ideal but was impractical for the project. Solvents were also out for safety reasons on such large surfaces without good ventilation. We ended up using Fantastic® cleaner. Two scrubbings with Fantastic® on large sponges and good rinsings in between and after were adequate most of the time. The surfaces were then left damp with the rinse water. Only the edges were handled to avoid contamination of the cleaned surfaces.

2) A contact plating solution for copper plating was prepared (see below) and this was applied to the steel using paint rollers (goggles/gloves!) Brushes work also but the paint roller is a bit more gentle. Plating occurs instantly. Several passes may be made over the same area, without pressing hard, which can remove the delicate plating. The surface was then rinsed very well. If areas of the plating lifted grease residues were the cause and a further local Fantastic® sponging and good rinsing sufficed to allow plating to take place. After final rinsing the steel (held by the edges) was taken to the tent. One moves fast to retain the surface moisture.

3) The steel was then placed in the tent and pans of ammonia enclosed under it. The tent was sealed. The centers of the thin sheet sagged causing pooling, therefore we built a wooden support with a single nail pointing upwards to support the sheet. More stable objects would not need support, though pooling may be factor to consider depending upon the surface relief. The time required to convert the copper plating was optimal at about 1-1 and a half hours.

4) The steel was removed and gently rinsed as scrubbing or hard spraying can remove the delicate patina surface. It will be a mixture of blues, greens and hints of brownish red where pooling has occurred and the surface dried. In my case I chose to re-introduce pink spatter marks to the surface by spattering droplets of the contact plating solution onto the patina surface where they instantly went pinkish-brown. The steel was then dried with fans and immediately sealed using clear automobile enamel paint. I then went back with stencils and gold spray paint to further modify the surface.
The steps in the procedure are then: Clean, rinse,plate, rinse, fume, rinse, dry and seal.

The conversion process
The copper on the surface is attacked by ammonia liquid, not as much by the fume which has a different chemical composition than the liquid. The water dampened surface slowly takes in ammonia fumes where they are converted to ammonia liquid in solution so that they can attack the copper. The purpose of the pans of ammonia below the object is to provide a constant vapor pressure which replenishes the ammonia on the surface at a constant rate as it is used up in converting the copper to patina. This system therefore ensures better overall constant dilution control than beginning with ammonia on the surface.

Control Factors:
Resists: Resists may be used to prevent plating or to prevent the plating from being converted to patina by the fumes. Resists to plating may be a greasy material (litho-crayon, oil) or thinned rubber cement. Other resists require too much cleaning time and may need solvents for removal. Resists to patination may be a protective spray through stencils (Pam®) or thinned rubber cement. Pattern control through resists is easy. Time: Time is a factor in all fumings. Experiment with various times on sample pieces to have a palette of process marks (colors, tones, effects) to choose from. Pooling: Where pooling occurs variations in color will result. Pooling can be encouraged and controlled by local application of greases before or during patination and by the position and shape of the object. Various liquid thicknesses cause surface variations. Sealers: Sealers will each have a characteristic effect on the surface. I recommend making a palette of various sealing options over a patinated surface. Examples of sealers include waxes, oils, lacquers, transparent acrylics, enamels, varnishes and so on. They often have a tendency to darken the colors on the surface. I prefer clear auto enamel or Spray-Lac number 1473 professional Finish Clear Dead Flat lacquer. It is available from Star Chemical based in Hinsdale Illinois, Deerfield Beach, Florida and Dallas Texas. It is an industrial quality spray and requires good ventilation. It is very unobtrusive on a surface. With any spray the surface chosen can be glossy, like paint (in which case why not use paint?) or shortly after spraying can be matted down with a cloth pad for better surface control. Other Chemicals: I mentioned dilute salt solutions earlier. Many chemicals will modify surfaces. (Remember never to mix bleach and ammonia). Experimentation and sample making will offer the user control choices. Suggestions for initial investigations include salt, vinegar, baking soda and local heating. There are a number of patination books available including one I sell on patinas for small studios.

Contact Plating Solution Recipe
All safety warnings apply. Always add Acid to Water!! Goggles/Gloves!

250 grams copper sulfate (CuSO₄) Technical grade chemicals for this solution is fine.
42 cc sulfuric acid
Distilled water to the 1000 ml level.

Put about 800cc water into plastic or glass container after marking the 1000cc level on it. Add the copper sulfate and stir to dissolve. Slowly pour a thin stream of acid into the swirling water. Heat is evolved-be aware of this. Rinse the acid container with distilled water and top up the mixture with it to the 1000 ml level. This solution can also be used as an electroforming solution for growing copper. Remember, acids are dangerous. A dust mask is suggested around chemicals. Work cleanly. Copper salts are toxic and irritant and should be handled with care. Dispose of properly.